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Rubber is widely used as sealing material in various applications. In many fields the function of seal materials at low temperatures is necessary. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling, due to the glass transition the material changes from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour, that allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition. But as the minimum working temperature limit of elastomers cannot be defined globally and precisely, the lower operation temperature limit of rubber seals should be determined in dependence of the application conditions and the most relevant material properties.
In this paper, wesummarize results of our temperature dependent investigation of seal material properties by classical thermal analysis as Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA), combined with measurements of standardized tests as compression set and the seal performance determined in component tests. To reduce the test time of compression set tests a faster technique was developed and applied.
To study the influence of dynamic events on the seal performance and to enhance the understanding of occurring seal failure, a setup for a fast partial seal release was designed.
Recovery is an important measure for seal applications representing to which extent the elastomer regains its initial shape after deformation and release of an applied force. Compression set (CS) indicates the degree of recovery. Ethylene propylene diene rubber (EPDM) was aged at 75 ◦C, 100 ◦C, 125 ◦C and 150 ◦C for different ageing times up to five years and compression set measurements were performed at different times after disassembly and after additional tempering. Short- and long-term recovery up to one year after release for samples aged at 125 ◦C and 150 ◦C was also studied. To assess the curvature in the Arrhenius diagram that may occur due to non-sufficiently aged specimens, a degradation-rate based model was fitted to the CS data after tempering. For each ageing temperature, two decay fit functions were proposed, each with an activation energy and a corresponding degradative process. The influence of ageing on the leak-tightness after fast small partial release is investigated and estimated through the analysis of the shift factors from time temperature superposition (TTS) of CS measurements at different times after disassembly. Shift factors of CS measurement after 1 s and after additional tempering are in good agreement.
Rubbers are widely used as main sealing materials for containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. The save encapsulation of the radioactive container inventory has to be guaranteed according to legislation and appropriate guidelines for long term storage periods as well as down to temperatures of -40 °C during transport.
Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling the material changes from rubber-like entropy-elastic to stiff energy-elastic behaviour, that allows nearly no strain or retraction, due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely, what can cause problems during application. Therefore the lower operation temperature limit of rubber seals should be determined in dependence of the material properties.
The results of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) are combined with the results of standardized measurements as the compression set according to ISO 815. To reduce the test time of the standard tests a faster technique was developed.
Additionally, the breakdown temperature of the sealing function of complete O-ring seals is measured in a component test setup to compare it with the results of the other tests. The experimental setup is capable of measuring the leakage rate at low temperatures by the pressure rise method.
A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. Some materials (seals and test sheets) were purchased from a commercial seal producer and some materials were compounded and cured at BAM in form of rubber sheets.
In Germany, the concept of dry interim storage in dual purpose metal casks before disposal is being pursued for spent nuclear fuel (SF) and high active waste (HAW) management. However, since there is no repository available today, the initially planned and established dry interim storage license duration of up to 40 years will be too short and its extension will become necessary. For such a storage license extension it is required to assess the long-term performance of SF and all safety related storage system components in order to confirm the viability of extended storage.
The main safety relevant components are the thick-walled dual purpose metal casks. These casks consist of a monolithic cask body with integrated neutron shielding components (polymers, e.g. polyethylene) and a monitored double lid barrier system with metal and elastomeric seals. The metal seals of this bolted closure system guarantee the required leak-tightness whereas the elastomeric seals allow for leakage rate measurement of the metal seals.
This paper presents an update on running long-term tests on metal seals at different temperatures under static conditions over longer periods of time. In addition, first results of our approach to understand the aging behavior of different elastomeric seals and neutron radiation shielding material polyethylene are discussed.
Elastomers are widely used as the main sealing materials for containers for low and intermediate level radioactive waste and as an additional component to metal seals in spent fuel and high active waste containers. The safe encapsulation of the radioactive container inventory has to be guaranteed according to regulation and appropriate guidelines for long term storage periods as well as for temperatures as low as -40°C during transport. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance. It is known that the material properties of elastomers are strongly temperature dependent. At low temperatures, this is caused by the rubber-glass transition (abbreviated: glass transition). During continuous cooling, the material changes from a rubber-like entropy elastic to a stiff energy elastic behaviour, which allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition, but the minimum working temperature limit is not defined precisely; this can cause problems during the above noted applications. Therefore, the lower operation temperature limit of elastomer seals must be determined in dependence of the material properties. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are combined with standardised measurements to determine the compression set according to ISO 815. To reduce the test time of the standard tests, a faster technique than normally used was developed. Additionally, the breakdown temperature of the sealing function of complete O ring seals was measured in a component test set-up to compare it with the results of the other tests. The experimental set-up is capable of measuring the leakage rate at low temperatures by the pressure rise method. A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. The materials (seals and test sheets) were purchased from a commercial seal producer.
Ultra high molecular weight polyethylene (UHMW-PE) - due to its high hydrogen content - is used as neutron shielding material in casks for storage and transport of radioactive materials. Besides this, UHMW-PE - as a high-performance polymer - has been used for several years in medical technology due to its excellent slip and wear properties. Due to the special properties of UHMW-PE which result basically from its extreme chain lengths and its high degree of crystallinity, it is predestinated for the aforementioned applications.
In both cases, irradiation and its impact on the molecular structure of polyethylene play an important role. In the first case, irradiation exists as a by-product of inserting the radioactive material in the cask. Hence PE has to withstand any type of degradation affecting safety relevant aspects to be applicable for long term radiation shielding purposes for instance over a period of 40 years. In the second case irradiation is applied deliberately for purposes such as sterilization and crosslinking, leading to partial improvement of the mechanical properties (e.g. fracture toughness, crack propagation resistance, wear resistance) and better Chemical stability.
Specifically concerning their use in the field of medical technology, different types of UHMW-PE have been objects of numerous publications. It is generally accepted that two parallel and competitive processes, based on chain scission and reactions of Ccentered radicals and molecular fragments, occur in PE as a consequence of radiation: radical recombination accounts for crosslinking, together with some disproportionation, formation of low molecular weight fragments, and recrystallization. Furthermore, formation of oxygenated structures in the presence of traces of oxygen is an antagonist of the C-centered radicals. Radiation induced scission preferentially takes place in the amorphous phase and noncrystalline surface of the crystals. It is followed by folding of molecular fragments, crystallinity growth and increased perfection in the crystal lamellae [4, 5]. Chain scission results in shorter polymer molecules, fewer entanglements, and consequently, increased molecular mobility.
The scope of our investigation comprises an estimation of the radiation and thermal impact on the molecular and supra molecular structure of the two types of PE used for neutron shielding cask components. A further point which is worth to explore is to what extent these changes are detectable by thermo-analytical (TA) methods, such as Differential Scanning Calorimetry (DSC), Thermo Mechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA) and Thermogravimetry (TG). Additionally FT-IR and solubility measurements have been performed.
In our studies we focused on the UHMW-PE material GUR 4120 which was y irradiated at RT and compared to the untreated material.
THz and Mid IR Fourier transform spectroscopy on polyethylene irradiated with Gamma- Co60 radiation
(2014)
THz and mid IR spectroscopy of high molecular PE (HMW) and ultra high molecular PE (UHMW) reveals modifications of the molecular structure induced by γ-Co60 radiation. UHMW-PE and HMW-PE can be distinguished. The integral absorption in the B1u THz-region can quantitatively describe the amorphization of crystallinity. The formation of trans vinylene unsaturations and the decay of vinyl can be followed by mid IR spectroscopy
The thermal expansion of polymeric parts can be an issue in many applications where the available space is limited, or exact dimensions of the part are required. For this study, a device was designed and built that allowed measuring the thermal expansion simultaneously in all three spatial directions on cubic samples with real-scale dimensions (78 mm edge length). The results are shown between 25 °C and 125 °C for two PE materials, one HMWPE and one tempered UHMWPE, for non-irradiated samples as well as cubes that have been irradiated with 100 and 400 kGy. The results measured with the new device were very similar to those measured with conventional thermo-mechanical analysis equipment and to literature data of UHMWPE. The HMWPE material shows a much larger thermal expansion coefficient in one direction compared to the other two directions during the first heating due to frozen stresses from the pressing step during material manufacturing. These stresses are mostly released by the expansion during the first heating, so that the expansion during the second heating is more uniform. The overall volumetric expansion is the same for both heating runs. By contrast, the tempered UHMWPE material shows no significant difference between first and second heating run, as the stresses from processing could already relax in the tempering step. The irradiation treatment does not affect the values significantly for the given test set-up.
Due to delays in the siting procedure to establish a deep geological repository for spent nuclear fuel and high level radioactive waste as well as in construction of the already licensed Konrad repository for low and intermediate level radioactive waste, extended periods of interim storage become more relevant in Germany. BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues.
The long-term performance of elastomer seals for lid systems of transport and storage casks, whether used as auxiliary seals in spent fuel casks or as primary seals for low and intermediate level waste packages, is an important issue in this context. The polymeric structure of these seals causes a complex mechanical behavior with time-dependent sealing force reduction.
The results of a comprehensive purpose-designed test program consisting of basic compression and tension tests as well as relaxation tests on unaged specimens of representative types of elastomers (fluorocarbon rubber (FKM) and ethylene propylene diene rubber (EPDM)) at different temperatures and strain rates are presented. They were used to identify the constitutive behavior and to obtain parameters for finite element material models provided by the computer code ABAQUS®. After estimating the influence of uncertainties such as Poisson’s ratio and friction coefficient by sensitivity analyses, the chosen parameters had to prove their suitability for the finite element simulation of the specimen tests themselves.
Based on this preliminary work the simulation of a specific laboratory test configuration containing a typical elastomer seal with circular cross section is presented. The chosen finite element material model and the implemented parameters had to show that they are able to represent not only the specimen behavior under predominantly uniaxial load but also the more complex stress states in real components. Deviations between the measured and calculated results are pointed out and discussed.
For the consideration of long-term effects in the simulation of elastomer behavior, test results of aged specimens are needed. First information about a new test program, started recently and planned to provide these data, are given.
Testing and numerical simulation of elastomeric seals under consideration of time dependent effects
(2016)
Due to delays in the siting procedure to establish a deep geological repository for spent nuclear fuel and high level waste and in construction of the already licensed Konrad repository for low and intermediate level waste, extended periods of interim storage will become more relevant in Germany. BAM is involved in most of the cask licensing procedures and is responsible for the evaluation of cask-related long-term safety issues. Elastomeric seals are widely used as barrier seals for containers for low and intermediate level radioactive waste. In addition they are also used as auxiliary seals in spent fuel storage and transportation casks (dual purpose casks (DPC)).
To address the complex requirements resulting from the described applications, BAM has initiated several test programs for investigating the behavior of elastomeric seals. These include experiments concerning the hyperelastic and viscoelastic behavior at different temperatures and strain rates, the low temperature performance down to -40°C, the influence of gamma irradiation and the aging behavior.
The first part of the paper gives an overview of these tests, their relevant results and their possible impact on BAM’s work as a consultant in the framework of approval and licensing procedures.
The second part presents an approach of the development of a finite element model using the finite element code ABAQUS®. The long-term goal is to simulate the complex elastomeric behavior in a complete lid closure system under specific operation and accident conditions.
A device has been designed, built and tested that allows the investigation of effects of dynamic loads on the leak tightness of elastomer seals at low temperatures. With the test setup, the compression of the tested seal can be reduced by a defined degree in a time period of less than 1 s. For the evaluation of the leak tightness, leakage rates can be determined.
It was shown that the rapid partial release of an elastomer seal leads to its failure already at the beginning of the rubber–glass transition and, therefore, at significantly higher temperatures than the loss of leak tightness observed under static conditions.
It is common practice in the application of finite element analysis to model compression set (CS) of elastomers during aging with two different material models according to the two-network theory of Tobolsky. The theory relies on the existence of two networks. The first one represents the original network after vulcanization and is sensitive to chain scission. The second network accounts for the formation of additional crosslinking during aging. Besides the use of user subroutines to describe the two-network model, an element overlay technique is also needed as the full set of both material behaviors did not exist for assignment to a single element. This element overlay technique is valuable for research and developmental purposes but makes extension to industrial usage quite challenging. Our goal is to simulate the CS of elastomers after long-term aging in a commercial finite element software with no need for extra subroutine codes or mesh superposition.
Ethylene propylene diene (EPDM) O-rings were aged in a compressed state at 75 °C, 100 °C, 125 °C and 150 °C for up to 183 days. Investigations of the experimental test results were used to identify material models and their parameters to develop a finite element model to simulate CS. The model was implemented in the finite element software ABAQUS/Standard® with a sequential temperature-displacement coupling.
Regarding the influence of temperature, the Arrhenius equation is adopted for the time-temperature relationship. The activation energy value that is required for the simulation is firstly determined from shifting the experimental CS results with the time-temperature superposition technique and plotting the shift factors in an Arrhenius diagram. The experiments were compared with the simulation results. Afterwards different activation energies were used in the simulation and discussed. A suitable choice of the activation energy value with regard to the reference temperature and the test temperature is presented. With the chosen activation energies, the match between numerical CS values after long-term aging and the experimental results was improved.
Elastomers are susceptible to chemical ageing, i.e., scission and cross-linking, at high temperatures. This thermally driven ageing process affects their mechanical properties and leads to limited operating time. Continuous and intermittent stress Relaxation measurements were conducted on ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) samples for different ageing times and an ageing temperature of 125 °C. The contributions of chain scission and cross-linking were analysed for both materials at different ageing states, elucidating the respective ageing mechanisms. Furthermore, compression set experiments were performed under various test conditions. Adopting the two-network model, compression set values were calculated and compared to the measured data. The additional effect of physical processes to scission and cross-linking during a long-term thermal exposure is quantified through the compression set analysis. The characteristic times relative to the degradation processes are also determined.
Both chain scission and crosslinking reactions occur during ageing of EPDM rubber. However, with many conventional polymer analysis methods such as hardness and DMA, it is hardly possible to obtain information about the contribution of each reaction type to the measured data. For example, hardness and Tg both increase during ageing of EPDM, indicating crosslinking during ageing, but it is not clear whether this is partly counterbalanced by chain scission reactions which would lower hardness and Tg. An indication that chain scission reactions probably counteract the hardness or Tg increase by crosslinking is given by Compression Set (CS) measurements. CS exhibits a higher change than hardness or Tg, as CS increases additively through both chain scission and crosslinking reactions. In order to elucidate the share of chain scission reactions in the total degradation, a method testing relaxation and recovery behaviour using DMA equipment was applied. The method revealed the strong influence of chain scissions, leading to more pronounced relaxation and higher residual strain after compression.
Safety of long term containment and storage of radioactive waste is an essential issue, which is increasingly gaining international attention. Extending interim storage beyond initial license periods is one of the major challenges worldwide.
BAM has been involved in all kinds of radioactive waste container safety evaluation for storage and transportation including their long term performance from the very beginning. Today, all work related to the safe management of radioactive waste is linked by the activity field “Nuclear Waste Management” within the Focus Area “Energy” at BAM and in conjunction with a research strategy and agenda. Most recently, the working group “Long Term Safety of Container Systems and Components” was established to launch and perform research projects related to safety issues concerning the extended interim storage of radioactive waste.
For example, within our project LaMEP, we investigate ageing effects of metal and elastomer seals as well as polymeric neutron shielding materials, which are container components that fulfil major safety functions. The results of the experiments help to understand the long term behaviour of these components. Likewise, our project BRUZL started at BAM in 2018 with the aim of developing a fracture mechanics approach to understand and describe a potential brittle failure mechanism of fuel cladding during long-term interim storage. BAM also contributes to the GRS project “Development of methods and models and international exchange for safety evaluation in the frame of interim storage of radioactive waste”. Additionally, BAM is involved in several international research programmes dealing with extended interim storage, such as the European Joint Programme on Radioactive Waste Management (EURAD), the Extended Storage Collaboration Program (ESCP) and IAEA Coordinated Research Projects.
The present contribution provides a comprehensive overview of BAM research activities contributing to national and international gap analyses and to filling knowledge gaps regarding the safe long term management of radioactive waste.
Elastomer seals are widely used as barrier seals in containers for low and intermediate level radioactive waste and in some countries for spent fuel transportation casks. Depending on the area of use, the rubber materials have to demonstrate proper sealing performance with regard to mechanical, thermal and environmental conditions as well as irradiation during the entire operation period.
In Germany the low and intermediate level radioactive waste is planned to be disposed in the Konrad repository in standardized containers. Many of these containers use an elastomer seal between lid and container body. Depending on the radiactive inventory and a possible containment of the waste in inner barrels and/or concrete structures, such containers may require a specific leak tightness. In such cases, specified leakage rates are shall not be exceeded by the the seals to keep potential activity releases from the inventories well below defined limits.
Generally, all waste containers have to fulfill the waste acceptance criteria for the Konrad repository as defined by the reports BfS SE-IB-29/08 and SE-IB-30/08. From these criteria several requirements for seal performance are derived. The seals have to:
- ensure full functionality until final container disposal,
- maintain its function in case of a thermal or mechanical accident,
- ensure encapsulation also at -20 °C.
This combination of requirements is challenging especially as a seal exchange, which is typical for many other applications, is hardly possible and thus a sound understanding of how to perform safety evaluation of containers is required. Therefore BAM has initiated investigations on elastomer seal behaviour. This work focuses on low temperature behaviour and sealing performance concerning static conditions as well as partial loss of compression and ageing of elastomer seals. Concerning ageing, property changes caused by long-term thermooxidative conditions as well as gamma irradiation are investigated.
This contribution presents an overview and explanation of the aims of the ongoing research and gives an update of the most relevant results.
Elastomer seals are widely used as barrier seals in containers for low and intermediate level radioactive waste and in some countries for spent fuel transportation casks. Depending on the area of use, the rubber materials have to demonstrate proper sealing performance with regard to mechanical, thermal and environmental conditions as well as irradiation during the entire operation period.
In Germany the low and intermediate level radioactive waste is planned to be disposed in the Konrad repository in standardized containers. Many of these containers use an elastomer seal between lid and container body. Depending on the radiactive inventory and a possible containment of the waste in inner barrels and/or concrete structures, such containers may require a specific leak tightness. In such cases, specified leakage rates are shall not be exceeded by the the seals to keep potential activity releases from the inventories well below defined limits.
Generally, all waste containers have to fulfill the waste acceptance criteria for the Konrad repository as defined by the reports BfS SE-IB-29/08 and SE-IB-30/08. From these criteria several requirements for seal performance are derived. The seals have to:
- ensure full functionality until final container disposal,
- maintain its function in case of a thermal or mechanical accident,
- ensure encapsulation also at -20 °C.
This combination of requirements is challenging especially as a seal exchange, which is typical for many other applications, is hardly possible and thus a sound understanding of how to perform safety evaluation of containers is required. Therefore BAM has initiated investigations on elastomer seal behaviour. This work focuses on low temperature behaviour and sealing performance concerning static conditions as well as partial loss of compression and ageing of elastomer seals. Concerning ageing, property changes caused by long-term thermooxidative conditions as well as gamma irradiation are investigated.
This contribution presents an overview and explanation of the aims of the ongoing research and gives an update of the most relevant results.
The temperature dependences of dielectric permittivity and elastic modulus of poly(vinylidene fluoride - hexafluoropropylene) (P(VDF-HFP)) are compared and explained by use of structural data obtained by differential scanning calorimetry and X-ray analysis. Special emphasis is put on the effect of uniaxial stretching which renders the polymer ferroelectric. It is shown that dielectric and mechanical relaxations at the glass transition (aa relaxation) are closely related and not significantly affected by stretching. It is further confirmed that stretching destroys the non-polar spherulitic crystalline structure and produces relatively small-sized polar crystallites. This manifests in the disappearence of the so-called ac relaxation which is associated to molecular motions within the spherulitic crystalline lamellae. However, there are discrepancies between dielectric and mechanical losses above the glass transition which point towards a still not described structural transition.
The special properties of high molecular weight polyethylene (HMW-PE) and ultra high molecular
weight polyethylene (UHMW-PE) result basically from their extreme chain lengths and their high
degree of crystallinity. As high-performance polymers, they are used for a variety of applications.
UHMW-PE in particular is often utilized for endoprothesis (due to its excellent slip and wear
properties) and due to its high hydrogen content as a neutron moderator in casks for storage and
transport of radioactive materials.
To prepare the material for instance for its use as a total joint replacement, it is exposed to radiation
for several reasons, such as sterilization and crosslinking, leading to partial improvement of the
mechanical properties (e.g. fracture toughness, crack propagation resistance, wear resistance) and
better chemical stability.
To be applicable for long term radiation shielding purposes for instance over a period of 40 years,
PE has to withstand any type of degradation affecting safety relevant aspects.
The scope of our investigation comprises an estimation of the radiation impact on the molecular and
supra molecular structure of two types of PE and to what extent these changes are detectable by
thermo-analytical (TA) methods, such as Differential Scanning Calorimetry (DSC), Thermo
Mechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA) and Thermo Gravimetric
Analysis (TGA). Additionally FT-IR spectroscopy as well as density and gas sorption
measurements were carried out.
Due to the poor solubility of HMW-PE and UHMW-PE, some classical analytical techniques are
not applicable. But TA-methods represent a feasible approach to detect structural and
morphological features of these materials as well as changes caused by external influences, such as
thermal treatment and/or irradiation. With the combination of the applied TA-techniques it is
possible to distinguish between crosslinking and degradation.
Polymer Testing - Editorial
(2016)
Polymer Testing - Editorial
(2016)
Elastomer seals are widely used as barrier seals in containers for low and intermediate level radioactive waste and for spent fuel transportation casks. In addition, they are also used for spent fuel storage and transportation casks (dual purpose casks (DPC)) as auxiliary seals to allow leakage rate measurements of metal barrier seals for demonstration of their proper assembling conditions. Depending on the area of use, the rubber materials have to demonstrate proper sealing performance with regard to mechanical, thermal, and environmental conditions as well as irradiation during the entire operation period. Concerning DPC, degradation effects should be limited in a way that, for example, effects from potentially released decomposition elements may not harm e.g. metal barrier seals. Leakage rate measurements should be possible also after long interim storage periods prior to subsequent transportation.
Because of the complex requirements resulting from the various applications of containers for radioactive waste and spent nuclear fuel, BAM has initiated several test programmes for investigating the behaviour of elastomer seals. In this contribution the current status is described and first results are discussed.
During an extensive test programme at the Bundesanstalt für Materialforschung und prüfung, material property changes of EPDM O-rings were investigated at different ageing times and two ageing temperatures of 125∘C and 150∘C. To exclude possible diffusion-limited oxidation (DLO) effects that can distort the data, IRHD microhardness measurements were taken over the cross section of compressed O-rings. Continuous stress relaxation measurements were taken on samples free of DLO effects. The additional effect of physical processes to irreversible chemical ones during a long-term thermal exposure is quantified by the analysis of compression set measurements under various test conditions. By combining the different experimental methods, characteristic times relative to the degradation processes were determined. On the basis of experimental data, a microphysically motivated model that takes into account reversible and irreversible processes was developed. The parameter identification strategy of the material model is based on our experimental investigations on homogeneously aged elastomer O-rings. The simulated results are in good agreement with the experiments.
Overview of ongoing research and future prospects on polyethylene neutron shielding materials at bam
(2023)
The extension of the interim storage period of radioactive waste before disposal will cause additional challenges for the nuclear waste management in Germany, so that an extensive knowledge of the long-term performance of casks, including their components and inventories, will be required for future extended storage licenses.
Ultra-high and high molecular weight polyethylenes ((U)HMW-PE) are used for neutron shielding purposes in casks for storage and transport of spent fuel and high-level waste due to their extremely high hydrogen content. During their service life of several decades as cask components, the PE materials are exposed to neutron and gamma radiation from the radioactive inventory of the casks, mechanical assembling stresses and temperature. All these combined effects affect the material properties of such components which in turn may be crucial for some possible accident scenarios.
At the Bundesanstalt für Materialforschung und -prüfung (BAM), the effects of high temperature exposure in combination with subsequent or previous irradiation were investigated with a comprehensive aging program including thermal aging at 125 °C for different aging periods up to 5 years and irradiation with doses ranging from 50 to 600 kGy.
This contribution provides an overview of the ongoing research related to the structural changes of (U)HMW-PE induced by gamma irradiation and high temperature exposure and focuses on current research perspectives at BAM with regard to the prediction of the dynamic behavior of the material during extended interim storage in case of an accident scenario. First results of the coupled effect of temperature, radiation and mechanical loading will be presented. The effect of microstructural changes induced by gamma irradiation and high temperature on the mechanical behavior of (U)HMW-PE will be assessed.
At the Bundesanstalt für Materialforschung und -prüfung (BAM) in Division 3.4 Safety of Storage Containers, one of our tasks is to evaluate the safety of containers designed for disposal of low and intermediate radioactive waste. As such containers might be transported before and stored until disposal, safe enclosure of the radioactive inventory is important for this time span. Elastomer O-rings are widely used as barrier seals in these containers. Thus, as for many other applications, an understanding of the practical effects of ageing degradation on elastomer seals during long-term exposure is mandatory for predicting the lifetime of such components. According to a long-term test programme on three kinds of rubbers (EPDM and FKM (relevant for application), HNBR (for comparison)), over several years we have studied the degradation and the change of mechanical properties (e.g. hardness, strain at break) at four different ageing temperatures (75 °C, 100 °C, 125 °C and 150 °C) as well as the change of sealing properties. Continuous and intermittent compression stress relaxation (CSR) measurements were performed in order to investigate the respective contribution of crosslinking and chain scission to the observed degradation effects. Thus, the degradation kinetics and mechanisms could be resolved more clearly. For assessing the seal performance, compression set (CS) and leakage rate measurements were conducted. The experimental results showed that the O-rings remained leak-tight under purely static conditions even when CSR, CS and mechanical properties already indicated far advanced degradation. For this reason, a modified leakage test involving a small and rapid partial decompression of the seal was developed that enabled determining an end-of-lifetime criterion for O-rings with a safety margin for thermal shrinkage and vibrations.
The investigation of the long-term performance of sealing systems employed in containers of radioactive waste and spent nuclear fuel is one research focus area for division 3.4 \Safety of Storage Containers" at the Bundesanstalt für Materialforschung und -prüfung. The behaviour of metallic seals, which are employed in interim storage casks for spent nuclear fuel and high active waste, over time is of high importance for ensuring safe enclosure. Therefore, investigations on these systems were started at BAM to get a general understanding of the relevant processes. Our investigations comprise investigations on different parameters which influence the seal Performance and the main part is focussed on the time and temperature dependent behaviour. Our aim is to understand the longterm behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation. In this contribution an overview of the performed tests and their respective results is given. A focus lies on a comparison between seals with large torus diameter in comparison to small diameters.
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung. The behaviour of metallic seals, which are employed in interim storage casks for used fuel and high active waste, over time is of high importance for ensuring safe enclosure. Therefore, investigations on these systems were started at BAM to get a general understanding of the relevant processes. Our investigations comprise investigations on different parameters which influence the seal performance and the main part is focussed on the time and temperature dependent behaviour.
In this contribution an overview of the performed tests and their respective results will be given. A focus will lie on a comparison between seals with large torus diameter in comparison to small diameters.
Our aim is to understand the long-term behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation.
Optical and thermal analysis of PVB encapsulant polymer functionalized with luminescent organic dyes
(2018)
This work focused on the technology of luminescent down shift (LDS), with a primary aim to identify and investigate a methodology to introduce the luminescent organic dye into PVB polymer encapsulant as emergent material for photovoltaic application. For this goal, we propose to study the feasibility to implement the LDS functionality and to identify suitability of available luminescent to be incorporated into the host polymer encapsulant material. The first step to this direction was through a comprehensive optical study of Violet 570 (V) organic dye in ethanol solvent. The methodology and experimental conditions such as laboratory polymer preparation and luminescence dye concentration were presented. Also, the emergent polymer encapsulant sheets were characterized by using optical and thermal analysis techniques. The absorption spectrum of the prepared PVB material shifts towards longer wavelengths, with increasing organic dye concentration.
Due to delays in the site-selection procedure to establish a deep geological repository for spent nuclear fuel and high level waste and in construction of the already licensed Konrad-repository for low and intermediate level waste without heat generation, extended periods of interim storage become more relevant in Germany. BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues.
The long-term performance of elastomer seals for lid Systems of transport and storage casks whether used as auxiliary seals in spent fuel caslcs or as primary seals for low and intermediate level waste packages is an important issue in this context (Jaunich, 2013; Jaunich, 2014; Kömmling, 2015). The polymeric structure of these seals causes a complex mechanical behaviour with time-dependent elasticity reduction and loss of elastic recovery.
The paper presents first results of a comprehensive test Programme consisting of several static and dynamic mechanical short- and long-term tests which have been carried out at BAM on specimens made of representative types of elastomers, fluorocarbon rubber (FKM) and ethylene propylene diene rubber (EPDM). The investigation of the test results used to identify material models and their Parameters as well as the development of two finite element models for the numerical Simulation of tension and compression tests using the finite element code ABAQUS® are described. The calculation results are presented in comparison to the test results. The influence of important material and test parameters was investigated and discussed in sensitivity analyses.
Elastomer seals are used in many industrial applications due to their excellent mechanical properties at a wide range of temperatures. Their high versatility and recovery potential under several load conditions make them well suitable for the application in containers designed for transport, storage and/or disposal of radioactive materials. In containers for low and intermediate level radioactive waste, elastomer seals are used as barrier seals, and as auxiliary seals in storage and transportation casks (dual purpose casks) for heat generating radioactive waste, such as spent fuel and high-level waste. While a seal exchange at defined intervals is typical in many conventional applications, it is impossible or at least hard to perform when principles of minimization of radiation exposure have to be considered and prohibit an unnecessary cask handling. An extensive knowledge of the change of the elastomer’s properties during aging and the availability of reliable end-of-lifetime criteria to guarantee the permanent safe enclosure of the radioactive material is mandatory. As BAM is involved in most of the national cask licensing procedures and in the evaluation of cask-related long-term safety issues, great efforts have been already made and are still planned to scientifically support this task.
Compression stress relaxation and compression set were identified as key indicators of elastomer long-term performance and quantitatively investigated in comprehensive test programs. Among other representative types of elastomers, specimens made from ethylene propylene diene rubber (EPDM) were tested before, during and after aging to capture the most important of their complex mechanical properties.
In the presented study, exemplary results were used to simulate the compression stress relaxation and the compression set of elastomer O-rings during aging. Regarding the influence of temperature, the time-temperature superposition principle is applied in the relaxation analysis of elastomer O-rings. The proposed model is implemented in the commercial finite element software ABAQUS/Standard® [1] with a sequential temperature displacement coupling. Numerical results match the experimental compression stress relaxation measurements well. The prediction of compression set values after long-term aging shows a relatively good agreement with the experimental results. Nevertheless, all input parameters derived from the specimen tests, additional assumptions concerning boundary conditions and modeling strategy are discussed with regard to the identified slight discrepancies.
The possibility to extend the finite element model to represent the O-ring seal’s ability to recover after a (fast) partial release is taken into account.
It is important to evaluate the effect of thermal ageing on the sealing performance of metal gaskets under high temperature for long-term usage. Therefore, in order to gain representative data for this kind of metal-sealed lid System, BAM is currently performing laboratory tests of different gasket types with aluminum and silver jackets at three different temperatures under static conditions up to four years so far, using test flanges for gaskets with full scale cross section diameter but much smaller outer diameter. On the other hand, in order to investigate the applicability of the numerical methodology to evaluate the long-term behavior of the metal gaskets, such as a correlation between seal pressure force and holding time, CRIEPI is developing a modeling method including material tests (tensile and creep tests) at high temperature. In this paper, the applicability of the finite element method (ABAQUS) to predict the recovery displacement and residual seal pressure force of the gasket complex was verified by comparing the calculated values with BAM’s laboratory test results under the joint research agreement between BAM and CRIEPI.
In consideration of the influence of the curing reaction on the material properties of elastomers, it is very important to control the vulcanization process properly.
As shown previously [1], the effects of the vulcanization reaction of an elastomer can be monitored by use of ultrasonic sound waves. This technique has to a certain degree a high similarity to the standard curemeter test according to ISO 6502 but can be applied also inside a production tool.
So far, the method was only applied to the vulcanization of one compound at a given temperature and sample thickness. This is now complemented with measurements on another compound system at different temperatures and sample thicknesses.
The expected effects of temperature and thickness on the curing behaviour were found by use of the ultrasound online control.
Monitoring the vulcanization of elastomers: Comparison of curemeter and ultrasonic online control
(2009)
The vulcanization of elastomeric materials has a high impact on the properties of the final product. Therefore, it is important to monitor and control this crosslinking process. A common technique to attain the necessary curing time is the use of a curemeter in accordance with ISO 6502 in order to determine the time for full cure of a sample with a standardized geometry. Based on this result and a lot of practical experience, the required curing time for a given product geometry is estimated. Within the scope of this work, a new analysis technique will be compared with the standard procedure. The ultrasonic online control employs ultrasound waves to measure the changes in material properties caused by vulcanization. For this study, a natural rubber compound with a conventional curing system was investigated by both techniques. It was found that the results of the ultrasonic technique show good agreement with the results of the curemeter.
Today rubber is due to its unique property profile one of the most important materials for the production of bearings, dampers or also tires and tubes. In the production of rubber goods a chemical cross-linking reaction (vulcanisation) generates the final material properties. This process takes place during the shape giving of the rubber compound in the mould and can proceed for several minutes. The cross-linking process is influenced by the mixture of the basic polymer, the cross-linking agent, the influence of catalysts and inhibitors and especially the processing conditions. The quality of every ingredient, the reproducibility of the mixing ratio, the homogeneity of the mixture and the accurateness in temperature control can have a direct influence on the cross linking reaction. To control the quality of a rubber compound and verify the processing conditions in industry moving die rheometers, named curemeter, are involved in the production process. Starting from our experience in cure monitoring of thermosetting materials by measuring ultrasound velocity in the mould we extended the measuring principle to rubber vulcanisation monitoring. After first investigations using a compression mould we dealt here with on-line monitoring of vulcanisation in an injection moulding process which is one of the most used techniques in rubber industry.
In Germany spent nuclear fuel is stored and transported in casks that possess bolted lid systems equipped with double jacket metal seals of Helicoflex® type in order to enable safe enclosure of the radioactive inventory. A dry interim storage period of 40 years was planned and the casks are licensed for that time span. However, due to political reasons and delayed disposal projects this time span is expected to increase significantly. It is therefore necessary to evaluate the long-term sealing behavior of the casks exceeding 40 years of operating time. In this paper, we discuss our approach to investigate the aging behavior of such metal seals, and to predict the long-term sealing behavior.
Accelerated aging component tests with seals are performed at temperatures up to 150°C to investigate the course of seal force, useable resilience and leakage rate. Even though these tests have already shown a significant decrease in seal force and useable resilience after aging times of less than 1 year, for most seals the leakage rate stayed better than the specified value. However, observation of the leakage rate has to be continued to determine the aging effect on the seal properties. As a main influence on the seal long-term behavior the increasing permanent deformation of the outer jacket has been recognized. It is made of aluminum or silver which means that the temperatures that are relevant for the aforementioned application in casks lead to creep deformation.
For further investigation of seal behavior a comprehensive research program concerning the material characteristics and aging behavior of the individual seal components, with an emphasis on the outer jacket material, has been launched. The aim is to get a better understanding of the time and temperature dependent creep mechanisms and deformation. The examinations are separated into tests on seal segments and tests on standardized specimen.
In order to be able to study the aging process, seal segments are compressed in flanges and stored at temperatures ranging from room temperature to 150°C. After defined time intervals the segments are analyzed by using standardized tests including structure investigation and creep tests. The results are compared with the behavior of the specimen basic raw material. This is done to ensure comparability of the thin jacket material and additional sheet material used for standardized tests. Thus, the prediction of component changes after different aging times at different temperatures can be made. As a result forecasts on the long term seal behavior are intended.
Rubbers are widely used as sealing material in various applications. In many fieldsthefunction of seal materials at lowtemperatures is required. Therefore the understanding of faiiure mechanisms that lead to leakage at low temperatures is of high importance. Therefore the lower Operation temperature limit of rubber seals should be determined in dependence of the material properties.
The results of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) are combined with results of standardized measurements as the compression set according to ISO 815. To reduce the test time of the Standard tests a faster and more efficient technique was developed and applied. In addition the breakdown temperature of the sealing function of O-ring seals at lowtemperatures is measured in a component test Setup in dependence of the material and the degree of compression.
Rubber is widely used as sealing material in various applications. In many fields the sealing function at low temperatures is necessary. Therefore the understanding of failure mechanisms is of high importance. Rubbers are normally used above their glass-rubber transition region but the minimum working temperature limit is not defined precisely. Therefore the lower operation temperature limit of rubber seals should be determined in dependence of the material properties. The results of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) are combined with the results of the standardized compression set according to ISO 815 respectively a modified test using the DMA equipment.
Understanding the failure mechanisms that lead to leakage of rubber seals at low temperatures is important. Rubber properties are highly temperature dependent. Changes in behaviour at low temperatures can result from the rubber-glass transition and for some rubbers by partial crystallisation. Experimental results on the low temperature behaviour of different rubber seals were discussed to help understand seal failure processes.
Rubber is widely used as sealing material in various applications. In many fields the function of seal materials at low temperatures is necessary.
Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling the material changes from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour, that allows nearly no strain or retraction, due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely, what can cause problems during application. Therefore the lower operation temperature limit of rubber seals should be determined in dependence of the material properties.
In the past we investigated the material properties and sealing behaviour of elastomeric seals at low temperatures. Additionally we started to study the effect of a small partial release of elastomer seals at low temperatures. To study the influence of dynamic events on seal performance and to enhance the understanding of occurring seal failure, a setup for a fast partial seal release was designed.
In this contribution we focus on the correlation of material properties and component behaviour under different application conditions. It is desired to widen the understanding of the process of seal failure at low temperature.
Rubber is widely used as sealing material in various applications. In many fields the function of seal materials at low temperatures is necessary. Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance. It is known that the material properties of rubbers are strongly temperature dependent. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling the material changes from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour, that allows nearly no strain or retraction, due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely, what can cause problems during application. Therefore the lower operation temperature limit of rubber seals should be determined in dependence of the material properties. In the past we investigated the material properties and sealing behaviour of elastomeric seals at low temperatures. Additionally we started to study the effect of a small partial release of elastomer seals at low temperatures. To study the influence of dynamic events on seal performance and to enhance the understanding of occurring seal failure, a setup for a fast partial seal release was designed. In this contribution we focus on the correlation of material properties and component behaviour under different application conditions. It is desired to widen the understanding of the process of seal failure at low temperature.
Low temperature performance of rubber seals - Influence of crystallite formation on seal performance
(2018)
Rubbers are widely used as sealing materials in various applications. In many fields the function of seal materials at low temperatures is necessary. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that rubbers possess strongly temperature dependent material properties. At low temperatures high changes in the material properties are observed which can be caused by the rubber-glass transition (abbr. glass transition) or for some rubbers also by partial crystallisation at lower temperatures. During continuous cooling the material changes during rubber-glass transition from rubber-like entropy-elastic behaviour to stiff energy-elastic behaviour. In case of (thermal-) crystallisation the formed crystallites act as additional (physical) crosslinks within the material but the molecules in the amorphous phase can still behave like a rubber. Hence, rubbers are normally used above rubber-glass transition and (thermal-) crystallization temperature but the minimum working temperature limit is heavily depending on the application conditions. Therefore, the lower operation temperature limit of rubber seals is under investigation.
In the past we have investigated material properties and sealing behaviour of rubber seals at low temperatures. As the initial work was dedicated to purely static sealing conditions we started to investigate the effect of a small partial release of rubber seals at low temperatures to study the influence of dynamic events on leak tightness. In addition, other groups report on the influence of counterpart surface and adhesion.
In this contribution we focus on the difference between purely amorphous elastomers and elastomers which become partially crystalline at low temperatures. The influence of partial crystallization on the low temperature performance of rubber seals is discussed.
Elastomers show a high versatility which makes them ideal materials for sealing applications in various fields. Especially under changing application conditions the high recovery potential of this class of material is beneficial to compensate temperature or pressure fluctuation, and geometrical changes resulting from mechanical loads in e.g. accident conditions. Out of these reasons elastomers are also used in containers for low and intermediate level radioactive waste and for spent fuel transportation casks. In cask designed for low and intermediate level waste elastomer seals can act as primary seal responsible for the containment function whereas in spent fuel storage and transportation casks (dual purpose casks (DPC)) elastomer seals are used as auxiliary seals to allow leakage rate measurements of metal barrier seals. An inherent prerequisite for this kind of application is the long time-scale of operation without or with limited possibility of seal replacement. In Germany an interim storage license for DPC`s is typically issued for 40 years, a timeframe which might increase in future due to challenges of the final repository siting procedure. For low and intermediate level waste, also long time periods are required before final disposal can be achieved. Therefore, the performance of elastomer seals over extended time periods is, as for other applications, of high importance.
A typical approach to ensure long-term functionality is to perform accelerated aging tests to calculate an estimated lifetime by assuming e.g. Arrhenius like equations for the time-temperature relationship. This approach requires a suitable end of life criterion considering the application of interest. This often can represent a challenge on its own.
As BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues we initiated several test programs for investigating the behavior of elastomer seals. Experiments concerning the low temperature performance down to -40°C and the influence of gamma irradiation have been started first. Currently the thermal aging behavior of elastomer seals, which is the topic of this contribution, is examined.
For our aging investigations we use a broad approach to first determine the property changes in different elastomer materials due to thermo-oxidative aging at elevated temperatures and secondly we test how the typical methods of lifetime extrapolation can be applied to these results. This approach enables us to detect and exclude undesired side effects which very often influence lifetime estimations. In this contribution, our recent results are discussed.
Elastomers show a high versatility which makes them ideal materials for sealing applications in various fields. Especially under changing application conditions the high recovery potential of this class of material is beneficial to compensate temperature or pressure fluctuation, and geometrical changes resulting from mechanical loads in e.g. accident conditions. Out of these reasons elastomers are also used in containers for low and intermediate level radioactive waste and for spent fuel transportation casks. In casks designed for low and intermediate level waste elastomer seals can act as primary seal responsible for the containment function whereas in spent fuel storage and transportation casks (dual purpose casks (DPC)) elastomer seals are used as auxiliary seals to allow leakage rate measurements of metal barrier seals. An inherent prerequisite for this kind of application is the Long time-scale of operation without or with limited possibility of seal replacement. In Germany an interim storage license for DPC`s is typically issued for 40 years, a timeframe which might increase in the future due to challenges of the final repository siting procedure. For low and intermediate level waste, also long time periods are required before final disposal can be achieved. Therefore, the performance of elastomer seals over extended time periods is, as for other applications, of high importance.
A typical approach to ensure long-term functionality is to perform accelerated aging tests to calculate an estimated lifetime by assuming e.g. Arrhenius like equations for the timetemperature relationship. This approach requires a suitable end of life criterion considering the application of interest. This often can represent a challenge on its own.
As BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues we initiated several test programs for investigating the behavior of elastomer seals. Experiments concerning the low temperature performance down to -40 °C and the influence of gamma irradiation have been started first.
Currently the thermal aging behavior of elastomer seals, which is the topic of this contribution, is examined. For our aging investigations we use a broad approach to first determine the property changes in different elastomer materials due to thermo-oxidative aging at elevated temperatures and secondly, we test how the typical methods of lifetime extrapolation can be applied to these results. This Approach enables us to detect and exclude undesired side effects which very often influence lifetime estimations. In this contribution, our recent results are extended. The results show that lifetime estimation based on single material properties can be misleading and therefore a combination of several methods is recommended.
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung (BAM). Our investigations comprise investigations on metallic and elastomeric seals and covers experimental investigations to get a database on the component/material behaviour, work on analytical descriptions and numerical analysis. Our aim is to understand the long-term behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation.
The investigation of the long-term performance of sealing systems employed in containers for radioactive waste and spent nuclear fuel is one research focus area for division 3.4 “Safety of Storage Containers” at the Bundesanstalt für Materialforschung und -prüfung (BAM). Our investigations comprise investigations on metallic and elastomeric seals and covers experimental investigations to get a database on the component/material behaviour, work on analytical descriptions and numerical analysis. Our aim is to understand the long-term behaviour of the sealing systems for evaluation of their performance during possible extended interim storage and subsequent transportation.
The long time-frame of the necessary safe enclosure of radioactive waste in containers gains interna-tionally an increasing attention. As well extended storage periods as a usually necessary subsequent transportation after storage have to be evaluated.
In Germany, the concept of dry interim storage in dual purpose metal casks before disposal is being pursued for spent nuclear fuel (SNF) and high level waste (HLW) management. The initially planned and established dry interim storage license duration of up to 40 years will be too short and its exten-sion will become necessary since there is no repository available in due time.
The main safety relevant components are the thick-walled dual purpose metal casks. These casks consist e.g. of a monolithic cask body with integrated neutron shielding components (polymers, e.g. polyethylene) and a monitored double lid barrier system equipped with metal and elastomer seals. The metal seals of this bolted closure system guarantee the required leak-tightness whereas the elastomer seals allow for leakage rate measurement of the metal seals.
At BAM we focus on the long-term performance of the sealing system and neutron shielding materials as their application conditions have to be concerned as rather demanding.
Therefore, BAM began to investigate the overall function and long-term behavior of elastomer and metal seals and also the irradiation influence on neutron shielding materials.
Recently a new project was initiated to expand our investigations to get more and in some context better applicable data.
This paper describes the approach of our new research project with the aim of further closure of existing knowledge gaps and presents an overview of investigations concerning the low temperature behavior of elastomer seals, of running long-term tests on elastomer seals at different temperatures under static conditions and the influence of gamma irradiation on neutron shielding materials.
In technical applications, static seals are sometimes also subjected to dynamic loadings. Therefore, the leakage behaviour under dynamic conditions has to be evaluated as well. For this purpose, FKM elastomer seals have been tested by using newly designed equipment that allows for rapid partial release of the seal and simultaneous leakage rate measurement at a wide range of test temperatures. Furthermore, material characterisation was done by using Dynamic Mechanical Analysis, Differential Scanning Calorimetry and Compression Set. It was shown that, under static conditions, the leakage rate increased significantly during cooling at temperatures around 18 K lower than the glass transition range. On reheating, the seal’s functionality was restored in the high temperature region of the glass rubber transition. In the subsequent dynamic release tests, that comprised a reduction of the seal compression within 1 s from 25 % to 23 %, increased leakage rates were observed in the high temperature region of the glass transition range. It was shown that the temperature that is critical for increased leakage is significantly lower under static conditions compared to dynamic conditions. The obtained leakage rates for static tests and dynamic release tests at different temperatures were analysed with reference to results of the material characterisation.
As a Federal institute for materials research and testing it is part of our tasks to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the investigation of elastomeric seals and polymeric neutron shielding materials used in the containers.
In this contribution we would like to present our research focused on the long term performance and associated changes of material properties of elastomeric seals (e.g. EPDM, FKM) and neutron shielding materials ((Ultra) High Molecular Weight Polyethylene) employed in storage casks. Our approach comprises accelerated ageing investigations at elevated temperatures and to some extent investigations of irradiated materials.
As the requirements for the extended use of cables in nuclear power plants and for the long term use of cask components are to a certain extent similar, our work might be of interest for the workshop on “Cables ageing in nuclear power plants; R&D current status and forecast”.
For many sealing applications rubbers are applied due to their special material properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to low temperatures during operation. Therefore, it is important to know the material behaviour at those low temperatures to determine the temperature range that allows a safe operation of the seal.
In this work, we focus on the behaviour of elastomer seals at low temperatures with regard to potential decrease of leak-tightness or catastrophic seal failure of O-ring rubber seals. This is required as material properties of rubbers are strongly temperature dependent but their temperature application range is not always clearly defined.
Based on previous investigations which considered the physical material properties and the seal behaviour under purely static conditions we widen the focus on the sealing performance after a fast partial relief of compressed seals and additional materials.
For the investigations, different typical rubber seal materials were used as e.g. fluorocarbon (FKM), ethylene propylene diene (EPDM) and hydrogenated acrylonitrile-butadiene (HNBR) rubber.
The correlation of the physical material properties with seal performance is studied. This includes as well purely static sealing applications as the Performance during/after fast partial relief of seals at low temperatures.
In Germany, spent nuclear fuel and high active waste from reprocessing is stored in transport and storage containers with double lid systems that are equipped with metal seals completing the primary sealing barrier. The tasks of the Bundesanstalt für Materialforschung und -prüfung (BAM) within the interim storage licensing procedures ruled by the German Atomic Energy Act include the long-term safety evaluation of the container design regarding the permanently safe enclosure of the inventory. In order to generate a knowledge base for the safety evaluation, research regarding the long-term behavior of the critical components is performed. So far, the containers are licensed for an interim storage period of 40 years. However, due to significant delays in establishing a final repository, the required time span for interim storage is expected to increase significantly. Thus, a widespread investigation program is run to gain systematic data on the long-term behavior of the seals and to develop prediction models.
Long-term seal investigations consider the development of their restoring seal force, their useable resilience and their achievable leakage rate caused by aging at temperatures ranging from room temperature up to 150 °C. This year, the total time span of the tests reaches 10 years. Furthermore, seal segments are aged at the selected temperatures for up to 300 days. From these segments additional information on the sealing behavior, changes of the seal contact and the material behavior is gained.
This contribution deals with the current results of the long-term seal investigations at BAM. Furthermore, insights of the more in-depth component and material investigations of the metal seals with focus on the seal contact development are discussed and the ongoing work aiming for an analytical description of the thermo-mechanical aging effects on metal seals are presented.
Due to their simple and reliable application, elastomers are used for many tasks. Many fields of application like transport, aviation and process technology, entail low temperatures where proper functionality of the employed seals has to be ensured. Therefore, understanding the low temperature behavior of elastomeric seals is important for safe seal operation to restrict the temperature application range.
For all areas of application, the question of seal lifetime is also important for replacement or maintenance intervals. The lifetime is especially important in applications that allow no seal replacement. Since the seal can determine the lifetime of storage in such cases, reliable lifetime estimation is necessary for these applications, and requires an extensive knowledge of the specific aging behavior.
The storage of spent fuel and high level radioactive waste in Germany is performed in interim storage containers with double lid systems. The lids are bolted and equipped with metal seals (e.g. Helicoflex®) to ensure the safe enclosure of the inventory. The used metal seals have a layered structure consisting of three components as can be seen schematically in the cross-sectional view in Fig. 1.
In the center a helical spring is positioned that is surrounded by two C-shaped jackets and is mainly responsible for generation of the required restoring force. The inner jacket is made of stainless steel and homogenizes the restoring force of the helical spring. The outer jacket is made of silver or aluminium which both are soft metals in comparison to the contact partners (lid and container body). During bolting of the lid to the container body the seal is compressed. The generated restoring force of the helical spring causes a plastic deformation of the outer jacket and adapts to the surfaces of the lid and the container body. Hence, leakage paths are closed and the sealing function is generated.
Typical durations for existing interim storage licenses in Germany are 40 years, but it can be expected that they have to be extended to longer periods as a final repository will not be available before the end of the running licence periods. This extension of license periods requires a solid understanding of the long-term behaviour of the seals under storage conditions. To meet this challenge long-term investigations have been started at Bundesanstalt für Materialforschung und –prüfung (BAM) in 2009. These tests focus on seals assembled in test flanges which are stored at temperatures ranging from room temperature to 150 °C for accelerated ageing. The aged seals are tested repeatedly after certain ageing steps and the leakage rate as indicator for sealing performance, the remaining seal force, and the useable resilience upon decompression are determined.
In the poster an update on the performed investigations in respect to earlier publications (Grelle et al. 2019, Goral et al. 2023) will be given and the implications of the results for resilient long term safety will be presented. Additionally, a focus will be laid on the currently planned further investigations and the question “What is additionally needed for evaluation of an interim storage period extension in regard to the used metal seals?” will be addressed.
In Germany, for the transport and storage of spent nuclear fuel, casks with double closure lid systems are used, which are equipped with Helicoflex® metal seals. The original interim storage period for these casks was planned to be 40 years. However, recent political developments indicate that a storage time of more than 80 years might be necessary. Therefore, the current storage licenses have to be renewed in due course, which requires extended knowledge of the long-term behaviour of all cask components.
At the Bundesanstalt für Materialforschung und -prüfung (BAM), metal seals have been investigated after ageing at temperatures between room temperature and 150 °C for up to 7 years. At regular intervals the seals have been tested for leakage rate as well as for their mechanical behaviour. From these tests pronounced influence of ageing could be shown which results in the decrease of the seal force and the useable resilience. The main reason attributed to these performance changes is the increasing permanent deformation of the outer layer of the seal due to creep. So far, an explicit analytical description of the long-term behaviour that could be used for predictions exceeding the tested ageing times and temperatures was not suitable.
In addition to the component tests that were already conducted, a comprehensive investigation programme has been started to describe the mechanical behaviour of the individual seal components.
The individual seal components are tested regarding different material characteristics including creep, deformation and microstructural properties. The comparability of the material characterization is ensured by comparing the basic raw material of the specimen with the actual component material.
Furthermore, detailed investigations of the time and temperature dependent deformation behaviour are realized by ageing seal segments at multiple temperatures for different periods of time and employing standardized tests.
By analysing the acquired data in combination with the ongoing long-term component tests a dataset for analytical prediction of the long-term behaviour of the seals is intended for an extended period of time under a certain temperature regime.
The Bundesanstalt für Materialforschung und –Prüfung (BAM) runs an investigation program on the long-term behavior of multi-component metal seals. Such seals are used in a wide area of applications including transport and storage casks for spent nuclear fuel and high level radioactive waste.
The seal function is mainly based on the compression of the inner helical spring, which generates the necessary seal force to keep the sealing surfaces in close contact. This in turn leads to a plastic deformation of the outer jacket of the seal, comprised of highly ductile aluminum or silver that adapts to the sealing surfaces of cask body and bolted lid, thus providing high Level leak tightness. In Germany, those casks are licensed for Interim storage periods of up to 40 years or more if extended Interim storage would become necessary before a final repository is available. Thus, the sealing performance has to be evaluated, including factors like elevated temperature due to decay heat or mechanical loads due to transport under normal as well as accident conditions. Long-term investigations at BAM have been running over the last nine years to identify and evaluate the seal performance by measuring the remaining seal force, the useable resilience and the leakage rate after various time intervals at temperatures ranging from room temperature up
to 150 °C. It was found that the seal force and useable resilience decrease with time and temperature, caused by creep deformation of the outer jacket. In order to obtain an analytical description for the seal behavior and to achieve more information on the material behavior under application conditions a comprehensive investigation program with Focus on aluminum as outer jacket material was launched. The program includes material investigations such as compression and tension creep tests with representative basic materials. An additional test setup allows for the continuous measurement of the remaining seal force at temperatures of up to 150 °C. Furthermore, seal segments are compressed and stored in heating chambers, thus producing segments at different stages of the aging process. The segments are investigated regarding the development of the contact area width, jacket thickness and microstructural changes. This data will be used to develop material models and an analytical description of the time and temperature dependent long-term sealing behavior. This paper explains the current status of gained test results and modelling approaches and closes with an outlook to the future Project plans.
For many sealing applications rubbers are applied due to their special material properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to low temperatures during operation. Therefore, it is important to know the material behaviour at those low temperatures to determine the temperature range that allows a safe operation of the seal.
In this work, we focus on the behaviour of elastomer seals at low temperatures with regard to potential decrease of leak-tightness or catastrophic seal failure of O-ring rubber seals. This is required as material properties of rubbers are strongly temperature dependent but their temperature application range is not always clearly defined.
Based on previous investigations which considered the physical material properties and the seal behaviour under purely static conditions we widen the focus on the sealing performance after a fast partial relief of compressed seals and additional materials.
For the investigations, different typical rubber seal materials were used as e.g. fluorocarbon (FKM), ethylene propylene diene (EPDM) and hydrogenated acrylonitrile-butadiene (HNBR) rubber.
The correlation of the physical material properties with seal performance is studied. This includes as well purely static sealing applications as the performance during/after fast partial relief of seals at low temperatures.
The cure behaviour of a specific ethylene vinyl acetate material as used for encapsulation of photovoltaic modules was analysed by rheometer, differential scanning calorimetry and Fourier transform infrared spectroscopy to test for a suitable replacement for the laborious determination of gel content. The results show that all applied methods are capable of describing the effects of the cross-linking process. Some provide results very similar to those yielded by analysis of the insoluble content, but the question remains as to whether indirect methods should be preferred over the direct measurement of physical properties, e.g. as performed by the curemeter. A material stored for one year was also tested to demonstrate the effect of extended storage on cure behaviour and how this is detected by different methods. This complements the other methods, which were clearly able to detect the different cure behaviour of the aged EVA, whereas determination of the gel content could not.
EVA is a widely used material for the encapsulation of photovoltaic modules. It melts at elevated temperatures, and seals the module before it is crosslinked at temperatures above 130 °C by a peroxide-initiated crosslinking reaction. EVA has good optical properties necessary for application in solar modules. For process optimization and quality management, a method for the quick and reliable characterization of EVA crosslinking behaviour is of great value. Here, the practicability of ultrasound for online crosslinking monitoring is demonstrated. A sound velocity increase of about 8 m/s during the crosslinking reaction is found. The ultrasound results are compared with rheometer measurements performed with a curemeter typically used for the investigation of rubber crosslinking.
Elastomers are widely used as the main sealing materials in Containers for low- and intermediate-level radioactive waste and as an additional component fo metal seals in spent-fuel and high-level waste containers. According to appropriate guidelines and regulations, safe enclosure of the radioactive container contents has to be guaranteed for lengthy storage periods of at least 40 years. Therefore, the understanding of seal ageing behaviour is of high importance and has to be considered with regard to possible dynamic events taking place during transport after storage. An accelerated ageing approach for compressed seals is presented, as well as some first results.
Two types of commercially applied Ethylene/Vinyl Acetate Copolymers (EVA) for encapsulation of photovoltaic modules were investigated by the thermal analysis methods of Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) in the temperature range from -150 °C to 200 °C. Glass transition, crystal melting and cross-linking were analyzed. The aims of the investigations were to gain more information for incoming goods control and to get information about the whole temperature dependent material properties in the investigated temperature range, starting at very low temperatures up to the crosslinking temperature region.
According to the relevant guidelines and regulations, the safe enclosure of radioactive Container contents has to be guaranteed for long storage periods as well as down to temperatures of -40 °C during transportation. As rubbers are widely used as the main sealing materials for Containers for low- and intermediate-level radioactive waste and as additional component to metal seals in spent fuel and high active waste Containers, a comprehensive eiastomer investigation Programme is in place at BAM. This Programme comprises investigations of overall eiastomer performance, durability, influence of gamma irradiation and low temperature behaviour.
In this paper we focus on the behaviour of eiastomer seals at low temperatures with regard to the potential increase in leakage as material properties of rubbers are strongly temperature-dependent. During continuous cooling, the material behaviour of elastomers changes in the rubber-glass transition temperature ränge from rubberlike entropy-elasticity to stiff energy-elasticity, allowing for almost no strain or retraction. Hence rubbers are normally used above their rubber-glass transition but the minimum limit at which they work has not been defined precisely. For the investigations, fluorocarbon (FKM) and ethylene-propylene-diene (EPDM) rubbers were selected, as they are often used in radioactive waste Containers. The elastomers were studied using several thermoanalytical methods and compression set. The results were compared with component tests to determine the temperature at which the sealing function of O-rings made of eiastomer breaks down. In addition to these results, under static conditions we designed a new measurement set-up which allows for a small deflection of the flange and simultaneous leakage rate measurement at low temperatures.
Rubbers are widely used as main sealing materials for containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. According to appropriate guidelines and regulations safe enclosure of the radioactive container contents has to be guaranteed for long storage periods as well as down to temperatures of -40 °C for transportation. Therefore the understanding of seal behavior is of high importance.
In this paper we focus on the behavior of elastomer seals at low temperatures with regard to poten-tial decrease of leak-tightness. In addition, changes in material properties due to aging effects over long periods of time and their influence on the seal performance is investigated.
It is known that material properties of rubbers are strongly temperature dependent. At low tempera-tures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cool-ing, the material changes from rubber-like entropy-elastic to stiff energy-elastic behavior, that al-lows nearly no strain or retraction due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely.
Aging of elastomer seals is important, as possible dynamic loads may have to be considered during the whole interim storage period (so far approved in Germany for up to 40 years) and for transporta-tion after storage.
For the investigations, fluorocarbon (FKM) and ethylene-propylene-diene (EPDM) rubbers were selected as they are often used in radioactive waste containers. Some materials were purchased from a commercial seal producer and some materials were compounded and cured at BAM. The elastom-ers where studied by several thermo-analytical methods and compression set to characterize the material behavior at low temperatures. Additionally component tests were performed to determine the breakdown temperature of the sealing function of complete elastomer O-rings.
Rubbers are widely used as main sealing materials for Containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. The save enclosure of the radioactive container inventory has to be guaranteed according to legislation and appropriate guidelines for long term storage periods as well as down to temperatures of -40 °C for transportation. Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is necessary to ensure a proper material choice which is certainly also influenced by additional factors as e.g. the aging behavior.
To achieve the required tightness levels of Containers for low and intermediate level radioactive wastes rubbers are widely applied as main sealing materials. The save encapsulation of the radioactive Container contents has to be guaranteed according to legislation and appropriate guidelines for long storage periods as well as down to temperatures of-40 °C during transportation.
Therefore the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance.
It is known that the material properties of rubbers are strongly influenced by temperature. At low temperatures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cooling the material changes from rubber-like entropy-elastic to stiff energy-elastic behaviour, that allows nearly no strain or retraction. Therefore, rubbers are normally used above their glass transition but the minimum working temperature limit is not defrned precisely, what can cause problems during application. The temperature ränge where full functionality is possible is strongly dependent on the application conditions and the material.
For this investigation mainly ethylene propylene diene (EPDM) and fluorocarbon rubbers (FKM) were selected as they are often used for radioactive waste Containers.
Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) are typically used for the determination of the temperature ränge of the glass transition process. The standardized compression set measurement according to ISO 815 is common for investigation of rubber sealing materials as the test simulates the seal behaviour after release. To reduce the test time of the Standard tests a faster technique giving the same information was developed.
Additionally, the breakdown temperature of the sealing function of complete O-ring seals is measured in a component test Setup to compare it with the results of the other tests. The experimental Setup is capable of measuring the leakage rate at low temperatures by the pressure rise method.
A model was developed that allows calculating the minimum working temperature limit of a seal by combining the results of the applied methods.
O-rings made of HNBR, EPDM and FKM were aged in the compressed and uncompressed state at 150 °C, 125 °C, 100 °C, 75 °C, 60 °C and 23 °C for aging times of up to five years. Hardness was measured and increased with aging time and temperature for HNBR and EPDM, but it remained practically constant for FKM. Indenter modulus measurements were performed on the lateral O-ring surface (that was free of DLO effects) to assess an influence of the compression during aging, but none was detected. The equilibrium compression set (CS) exhibited faster and stronger degradation than hardness and was used for lifetime predictions using the time-temperature superposition (TTS) principle. With an end-of-lifetime criterion of 70 % CS, lifetimes of 4.5 years, 50 years and 526 years at 75 °C were estimated for HNBR, EPDM and FKM, respectively. The activation energies derived from an Arrhenius plot of the shift factors from the TTS were 85 kJ/mol, 99 kJ/mol and 78 kJ/mol for HNBR, EPDM and FKM, respectively, revealing that a higher activation energy does not necessarily mean that the material has a higher lifetime at lower temperatures. Furthermore, the measured lifetime of EPDM O-rings at 100 °C (5 years) was compared to that predicted on the basis of the lifetime at 150 °C as well as 125 °C using the corresponding shift factors. The error of the prediction was only ± 4 %. However, this precise prediction could only be achieved using the five-year long-term aging data. When using only data from aging times up to 0.5 years and 2 years, the lifetime of EPDM O-rings at 100 °C was underestimated by 31 % and 22 %, respectively.
For many sealing applications rubbers are applied due to their special material properties and easy use. In many applications, e.g. in aviation, traffic or process technology the material can be exposed to low temperatures during Operation. Therefore it is important to know the material behaviour at those low temperatures to determine the temperature range that allows a safe Operation of the seal. As rubbers are used as main sealing materials for Containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste Containers, a comprehensive elastomer investigation program is run at BAM. This program comprises investigations of elastomer overall performance, durability, influence of gamma irradiation and low temperature behaviour.
In this paper we focus on the behaviour of elastomer seals at low temperatures with regard to potential decrease of leak-tightness as material properties of rubbers are strongly temperature dependent. But additionally to the investigation presented in the past which considered the material properties of new materials and the seal behaviour under purely static conditions [1, 2] we now focus on irradiated materials and on the sealing performance after a fast partial release of the compressed seal.
For the investigations different materials were used, e.g. fluorocarbon (FKM) and ethylenepropylene-diene (EPDM) rubber. To determine the effect of gamma irradiation some samples were irradiated and studied by several thermo-analytical methods and compression set. For the investigation of the seal performance under non static conditions we decided to design a new measurement setup which allows for a small flange deflection and simultaneous leakage rate measurement at low temperatures.
For many sealing applications rubbers are applied due to their special material properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to low temperatures during operation. Therefore it is important to know the material behaviour at those low temperatures to determine the temperature range that allows a safe operation of the seal. As rubbers are used as main sealing materials for containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high acitve waste containers, a comprehensive elastomer investigation program is run at BAM. This program comprises investigations of elastomer overall performance, durability, influence of gamma irradiation and low temperature behaviour.
In this paper we focus on the behaviour of elastomer seals below ambient temperatures after a fast partial release. Despite the primarily static application conditions of many seals their behaviour after partial compression release is of interest with regard to potential decrease of leak-tightness.
For this investigation a special setup was designed which allows the partial release of compressed seal systems at different temperatures accompanied by leakage rate measurements.
The results for FKM seals are presented and discussed.
In many fields the function of seal materials is required at low temperatures. Therefore the understanding of failure mechanisms at low temperatures is of high importance. In some applications elastomeric seals are used in environments with increased irradiation backgrounds, as e.g. in containers for radioactive waste. This paper reports on the influence of gamma irradiation on the low temperature properties of different fluorocarbon rubber (FKM) materials.
The samples were irradiated with gamma doses of up to 600 kGy and the induced changes of material properties were tested by methods sensitive to the rubber-glass transition, like DSC and DMA. Additionally, compression set measurements were performed to give information about recovery behaviour at low temperatures.
At BAM, which is the federal institute for materials research and testing in Germany, it is one of our tasks to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes assessment of elastomeric seals applied in the containers. Besides examining the low-temperature behaviour and irradiation effects of elastomeric seals, it is our goal to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. Therefore, we started an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) which are aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) up to 1 year. In order to assess sealability, O-rings are aged in compression by 25 % (corresponding to the compression during service) between plates as well as in flanges that allow leakage rate measurements. For comparison, uncompressed O-rings are aged as well. Further methods characterising seal performance are compression stress relaxation (CSR) reflecting the loss of sealing force of a compressed seal over time, and compression set (CS) which represents the recovery behaviour of a seal after release from compression. Additionally, hardness is measured for information about the change of mechanical properties. The experimental results indicate that while hardness, CSR and CS show considerable degradation effects, the leakage rate stays constant or even decreases until shrinkage and the loss of resilience of the aged seal leads to the formation of a leakage path. This indicates that static leakage rate, which is the only available direct seal performance criterion, has only limited sensitivity for the degradation of the seal material.
At BAM, which is the federal institute for materials research and testing in Germany, it is one of our tasks to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the assessment of the service lifetime of elastomeric seals that are part of the container lid system with regard to the requirements for long-term safety (40 years and more) of the containers. Therefore, we started an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) which are aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) up to 1.5 years. In order to assess sealability, O-rings are aged in compression by 25 % (corresponding to the compression during service) between plates as well as in flanges that allow leakage rate measurements. For comparison, uncompressed O-rings are aged as well. Further methods characterising seal performance are compression stress relaxation (CSR) reflecting the loss of sealing force of a compressed seal over time, and compression set (CS) which represents the recovery behaviour of a seal after release from compression. Additionally, hardness is measured for information about the change of mechanical properties. The experimental results indicate that while hardness, CSR and CS show considerable degradation effects, the leakage rate stays relatively constant or even decreases until shrinkage combined with the loss of resilience of the aged seal leads to leakage. This demonstrates that static leakage rate, which is the only available direct seal performance criterion, has only limited sensitivity towards the degradation of the seal material. CS data is extrapolated using time-temperature shifts and Arrhenius graphs. An exemplary CS of 50 % would be reached after approx. 1.2, 17 and 29 years at 60 °C for HNBR, EPDM and FKM respectively.
The indication-oriented Dental Bone Graft Substitutes (DBGS) selection, the correct bone defects classification, and appropriate treatment planning are very crucial for obtaining successful clinical results. However, hydrophilic, viscoelastic, and physicochemical properties’ influence on the DBGS regenerative potential has poorly been studied. For that reason, we investigated the dimensional changes and molecular mobility by Dynamic Mechanical Analysis (DMA) of xenograft (cerabone®), synthetic (maxresorb®), and allograft (maxgraft®, Puros®) blocks in a wet and dry state. While no significant differences could be seen in dry state, cerabone® and maxresorb® blocks showed a slight height decrease in wet state, whereas both maxgraft® and Puros® had an almost identical height increase. In addition, cerabone® and maxresorb® blocks remained highly rigid and their damping behaviour was not influenced by the water. On the other hand, both maxgraft® and Puros® had a strong increase in their molecular mobility with different damping behaviour profiles during the wet state. A high-speed microscopical imaging system was used to analyze the hydrophilicity in several naturally derived (cerabone®, Bio-Oss®, NuOss®, SIC® nature graft) and synthetic DBGS granules (maxresorb®, BoneCeramic®, NanoBone®, Ceros®). The highest level of hydrophilicity was detected in cerabone® and maxresorb®, while Bio-Oss® and BoneCeramic® had the lowest level of hydrophilicity among both naturally derived and synthetic DBGS groups. Deviations among the DBGS were also addressed via physicochemical differences recorded by Micro Computed Tomography, Scanning Electron Microscopy, Fourier Transform Infrared Spectroscopy, X-ray powder Diffractometry, and Thermogravimetric Analysis. Such DBGS variations could influence the volume stability at the grafting site, handling as well as the speed of vascularization and bone regeneration. Therefore, this study initiates a new insight into the DBGS differences and their importance for successful clinical results.
For many sealing applications low temperature performance is required. The overall understanding of the relevant influences on low temperature performance of rubber seals are summarized and the currently running investigations described.
The correlation of the physical material properties with seal performance is studied. This includes as well purely static sealing applications as the performance during/after fast partial relief of seals at low temperatures.
To improve the predictive capability of long-term stress relaxation of elastomers during thermo-oxidative ageing, a method to separate reversible and irreversible processes was adopted.
The separation is performed through the analysis of compression set after tempering. On the Basis of this separation, a numerical model for long-term stress relaxation during homogeneous ageing is proposed. The model consists of an additive contribution of physical and chemical relaxation.
Computer simulations of compression stress relaxation were performed for long ageing times and the results were validated with the Arrhenius treatment, the kinetic study and the time-temperature superposition technique based on experimental data. For chemical relaxation, two decay functions are introduced each with an activation energy and a degradative process. The first process with the lower activation energy dominates at lower ageing times, while the second one with the higher activation energy at longer ageing times. A degradation-rate based model for the evolution of each process and ist contribution to the total system during homogeneous ageing is proposed. The main advantage of the model is the possibility to quickly validate the interpolation at lower temperatures within the range of slower chemical processes without forcing a straight-line extrapolation.
Seals used in containers for dangerous goods need to maintain leak tightness for several decades. For investigating the change in seal material properties during aging, a comprehensive aging program on both uncompressed and compressed EPDM and HNBR seals was started. In order to obtain results closely related to practical conditions, O-rings with a full-scale cord diameter of 10 mm were aged. However, this set-up can lead to heterogeneous aging caused by diffusion-limited oxidation (DLO) effects. These effects depend on material, dimensions, time and temperature. Heterogeneous aging results in distorted bulk properties such as compression stress relaxation and compression set (CS) suggesting that HNBR has better performance than EPDM at 150 °C but which is not the case at 100 °C. The presence of heterogeneous material properties was shown by hardness measurements across the seal cross-section. If DLO-affected data is excluded, extrapolations of CS data are possible using time-temperature shifts and Arrhenius graphs. Exemplary CS values of 50 % and 80 % would be reached after approx. 10 years and 29 years, respectively for HNBR and after approx. 400 years and 1100 years, respectively for EPDM.
High and ultra-high molecular weight polyethylenes were gamma-irradiated with doses up to 600 kGy. The changes in the material properties were analysed using DSC, DMA, IR spectroscopy, as well as measurements of density and insoluble content. The irradiation led to an increase of the degree of crystallinity because of chain scissions during irradiation, leading to shorter and thus more mobile chains. Both the plateau value of the shear modulus G′ and the insoluble content increased with Irradiation dose, indicating the formation of additional crosslinks. Furthermore, IR spectroscopy revealed irradiation induced oxidation and the formation of double bonds, indicating that some of the hydrogen atoms responsible for the neutron shielding capability have been released.
Editorial
(2019)
Dear Readers, a new year started for all of us (as I write these lines, possibly it was already some time ago when these lines are finally available) and this is the time for the so-called New Year’s resolutions. Setting goals and changing our behavior as well in a private as in a professional context.
For Scientists the resolutions may comprise tasks like finalizing a longago started publication, picking up the loose ends of an application for funding or just bringing the running projects in a greater structure.
Epoxy carbon-fibre prepreg, Hexcel Type 6376 HTS, was investigated using Dynamic Mechanical Analysis (DMA). The DMA characteristic parameters are storage modulus E', loss modulus E' and loss factor tanδ. These parameters are ideally suited to observe the vitrification, referred to as glass transition, resulting from the cross-linking reaction. Detection of the cure state may also be achieved by determining the momentary glass transition temperature of partially cured samples. The consequent use of a multi-frequency measuring regime was used to derive the apparent activation energy for the glass transition process. Different temperature programs were also applied to monitor the curing process directly, as well as to investigate the different states of incomplete cure reached in preceding curing steps. The intention was to provide better understanding of the consequences of an interrupted autoclave curing process and to use DMA to detect the cure state achieved. With DMA, the continuation of an incomplete curing process also can be monitored. DMA measurements up to 300 °C showed, furthermore, that the final glass transition temperature was reduced by thermal degradation at high temperatures.
Discoloration Effects of High-Dose gamma-Irradiation and Long-Term Thermal Aging of (U) HMW-PE
(2017)
Two polyethylene types with ultra-high (UHMWPE) and high molecular weight (HMW-PE) used as neutron radiation shielding materials in casks for radioactive waste were irradiated with doses up to 600 kGy using a 60Co gammasource. Subsequently, thermal aging at 125∘C was applied for up to one year. Degradation effects in the materials were characterized using colorimetry, UV-Visspectroscopy, IR spectroscopy, and DSC. Both materials exhibited a yellowing upon irradiation.The discoloration of UHMW-PE disappeared again after thermal aging.Therefore, the yellowing is assumed to originate fromannealable color centers in the formof free radicals that are trapped in the crystalline regions of the polymer and recombine at elevated temperatures. For the antioxidantcontaining HMWPE, yellowing was observed after both irradiation and thermal aging. The color change was correlated mainly to decomposition products of the antioxidant in addition to trapped radicals as in UHMW-PE. Additionally, black spots appeared after thermal aging of HMW-PE.
Carbon-fibre prepregs have found widespread use in lightweight applications. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. Prepreg materials are generally pre-cured so that they have a higher molecular weight than typical resins in order to reduce resin flow, which facilitates storage and later processing properties.
The measurements were carried out using commercially available materials and follow the published DMA investigations of the same material. TMDSC was used to find the correlation between curing conditions, the degree of cure and glass transition temperature. TMDSC has the advantage over standard DSC that it enables better determination of the glass transition temperature, which is often accompanied by an exothermic curing reaction, and thus overshadowed. The influence of the amplitude of temperature modulation was tested in preliminary experiments. For non-cured material a glass transition temperature of approximately 0 °C was determined; whereas for the totally cured material it was approximately 230 °C. The changes in degree of cure, temperature of actual glass transition and post-reaction are given as a function of curing time at 180 °C. The correlation between actual glass transition temperature and degree of cure is derived.
In Germany spent nuclear fuel and high level radioactive waste is stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. The used metal seals consist of three components as can be seen in the cross-sectional view in Figure 1. The innermost part is a helical spring that is surrounded by an inner jacket made of stainless steel. The outer jacket that is made of a softer material which in case of assembly in the aforementioned storage containers is silver or aluminum (i.e. Al 99.5). During application the seal is compressed and due to the restoring force of the helical spring, the outer jacket is plastically deformed and adapts to the sealing surface. Hence, leakage paths are closed and the sealing function is generated. In Germany the above-mentioned containers are licensed for up to 40 years of interim storage, which in case extended storage becomes necessary before a final repository is available will have to be extended to even longer periods. Therefore, the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads as well.
At Bundesanstalt für Materialforschung und –prüfung (BAM) long-term investigations are being conducted in which seals are assembled in test flanges and aged at temperatures ranging from room temperature to 150°C for accelerated aging. The aged seals are tested semi-annually (after the first 6 months in which the seals are tested more frequently) regarding the sealing performance, the remaining seal force, and the useable resilience upon decompression. Results of these investigations have been published over the past years (e.g. Grelle, Wolff, Probst, Jaunich, & Völzke, 2017; Völzke, Wolff, Probst, Nagelschmidt, & Schulz, 2014). It was found that the seal force and the useable resilience decrease with time and temperature, which is in agreement with the result of other studies (Sassoulas et al., 2006; Wataru et al., 2016) as well. Geometry change of the outer jacket has been identified as the main reason for this seal behavior. At the prevailing operating temperatures and stresses the aluminum is subjected to creep deformation leading to a thinning of the outer jacket. Since the seal groove depth remains unchanged the helical spring expands, which in turn leads to a decrease of the generated spring and seal force.
Although the main reason for the change of seal parameters over time and temperature is known, a detailed characterization of the seal behavior and a reliable prediction of the parameter development for aging times that exceed the experimental time frame have not been possible, yet.
For deeper understanding of the aging processes, an Investigation program, which is covered in this contribution, is conducted at Bundesanstalt für Materialforschung und –prüfung (BAM) that focusses on the behavior of the aluminum jacket and its influence on the long-term sealing performance. The program investigates properties of material samples as well as the behavior of the seal as a component.
Original sheet material of the same aluminum that is used for manufacturing of the seals is investigated in compression creep tests. For this, a DMA (dynamic mechanical analysis) machine is employed (here used for static tests) that allows for a measurement of the specimens deformation under forces of up to 500 N. The advantage of this method is that the original material can be tested in the same shape as used for the seals which is 0.5 mm thick sheet material. For investigation of tensile creep standard specimens are used, that were machined from surrogate material of the same composition and annealing condition.
Furthermore, aluminum seals that are cut into smaller segments are assembled in flanges and placed in heating chambers at temperatures ranging from 23°C to 150°C. After different periods of time from 3 days to 300 days the segments are taken out of the flanges and are investigated, thus giving information on different states of aging. Measurements of the development of the seal contact width and the aluminum jacket thickness are done with an optical microscope. Further investigations on the segments will include metallography and hardness measurements.
From the detailed material and component behavior including the results of the long-term seal force and useable resilience investigations a better understanding of the overall seal behavior can be gained. The aim is to contribute to the development of material models and analytical approaches for the prediction of the sealing behavior in dependence of time and temperature.
Fracture mechanics approaches are increasingly applied for the characterization of epoxy resin and adhesive mechanical properties. Therefore, the fracture toughness and crack resistance under static load ISO 13586 [1] are often regarded as state of the art to analyse material improvements. However, experimental investigations on fatigue behaviour, thus the crack propagation according to ISO 15850 [2], seem to be much more sensitive to characterize the materials for in service loading conditions. Firstly, an efficient testing concept was developed at BAM. In this framework, the geometry for a modified single edge notched tensile specimen (SET) was developed in order to assure appropriate resolution in measuring the crack length via a CCD-camera [3]. In the next step, the influence of the cure temperature on the fracture-mechanical properties was investigated.
A Memorandum of Understanding (MOU) between Argonne National Laboratory (Argonne) and the German Bundesanstalt für Materialforschung und -prüfung (BAM) was signed in October 2014. Its objectives are to promote cooperation among scientists and specialists at Argonne and BAM and establish a framework for collaboration in advanced technologies associated with the back end of the nuclear fuel cycle. Collaborative activities involving Argonne and BAM may be implemented through the promotion of joint research activities and scientific workshops and conferences; exchange of technical information; and visits by scientists, specialists, and graduate, postgraduate, and Ph.D. students. The MOU was renewed in January 2020 for another five years. Highlights of the progress and achievements are provided in identified topical areas for collaboration on ageing management guidance, storage, transportation and disposal R&D, joint conference activities, and conclude with the benefits of the MOU in fostering Argonne and BAM collaboration.
Component and material investigations on metal seals for high level radioactive waste containers
(2018)
In Germany casks for interim storage of spent nuclear fuel and high level radioactive waste are equipped with Helicoflex® metal seals as main sealing barrier of the double lid system. The long-term behaviour of those seals is investigated at Bundesanstalt für Materialforschung und –prüfung (BAM) in order to evaluate the safety function of the containment at different temperatures over storage periods of 40 years or more in case extended interim storage becomes necessary.
Long-term investigations have been done for ageing times of up to 8.5 years at temperatures ranging from room temperature to 150 °C. It was found that the seal force and useable resilience decrease over time, which is mainly caused by creep deformation of the aluminium (or silver) outer jacket of the seals. This effect becomes stronger with increasing temperature.
The ageing processes of the seal material and the overall seal behaviour is under investigation in order to derive analytical descriptions for the long-term seal performance. Thus, standardized tests on the basic seal materials, with focus on aluminium, and additional investigations on the seals as a component are conducted. The current investigations include compression and tension creep measurements as well as tensile testing. Furthermore, ageing of seal segments provides information on the development of the contact area width, jacket thickness and microstructural changes in dependence of time and temperature. The obtained data are used for the development of material models and an analytical approach to describe and predict the time and temperature dependent sealing behaviour in the long-term.
Component and material investigations on metal seals for high level radioactive waste containers
(2018)
Long-term investigations performed at BAM look to extend the state of knowledge on safety-related components of interim storage containers. Metal seals act as the primary sealing barrier in the bolted double lid closure system of the containers. The behaviour of metal seals has been investigated for ageing times up to 8.5 years and for various temperatures. The main cause for reduction in useable resilience overtime was due to creep deformation of the outer jacket of the seal.
The method for the determination of compression set values with a Dynamic Mechanical Analysis (DMA) setup at low temperatures, which was presented previously, allows a much faster and readily automated procedure than the standardized compression set test according to ISO 815-2. This method is applied to a series of different elastomeric materials that are commonly used for sealing applications. The results of the compression set test are compared with results from thermal analysis to allow an in depth comparison of the material behaviour at low temperatures.
Furthermore, a comparison between two EPDM materials is presented. These materials show very similar properties determined by thermo analytical methods such as Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) but differ clearly in their compression set behaviour. This comparison shows the importance and value of information of the compression set test in addition to thermal analysis to judge the behaviour of sealing materials and confirms the relevance of the new compression set test method for the investigation of low temperature properties of elastomers.
Elastomeric seals are essential components in the infrastructure which prevent leakage of gas and proper function of technical devices and are therefore highly safety relevant. For proper function a remaining resilience and tolerance to pressure changes is required.
The ageing of elastomers is an undesirable but inevitable process leading to a limited lifetime of rubber products such as seals. Independent of the application, as e.g. automotive, piping or container applications, a long lifetime or at least a timely exchange of a seal before occurrence of critical failure is desired.
Hydrogen is a key element for the energy turnaround and therefore the compatibility of seals with hydrogen is essential. In addition to the effect of hydrogen uptake and release, which might also cause destructive effects such as rapid gas decompression, ageing at high temperature under hydrogen atmosphere is of interest too. Most of the existing work is addressing the performance of new materials and comprises only very limited investigations concerning long-term use and the behaviour of aged materials in contact with hydrogen. As ageing can lead to substantial changes of material properties, it must be evaluated whether these changes are beneficial or deteriorating for the component function.
In this work we present and compare results of the characterisation of three sealing materials (EPDM, HNBR, FKM) after ageing at high temperature under hydrogen and air. Despite the common assumption that ageing under hydrogen atmosphere should be less severe for the material in comparison to air ageing, this is not the case for every material.