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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.
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.
Eine Aufgabe des Fachbereichs 3.4 Sicherheit von Lagerbehältern der Bundesanstalt für Materialforschung und -prüfung (BAM) ist es, die Sicherheit von Behältern zur Lagerung von radioaktivem Abfall mit vernachlässigbarer Wärmeentwicklung zu bewerten. Da solche Behälter vor der Entsorgung transportiert und gelagert werden, ist während dieser Zeit ein sicherer Einschluss des radioaktiven Inhalts von größter Bedeutung. Zur Abdichtung dieser Behälter sind Elastomer-O-Ringe weit verbreitet. Um für solche, wie auch für viele andere Anwendungen, eine Voraussage über die Lebensdauer dieser Komponenten treffen zu können, ist es unerlässlich zu verstehen, welchen Einfluss der durch Alterung hervorgerufene Abbau auf Dauer auf die Elastomerdichtungen hat. In einem Langzeittest wurden drei Kautschukarten (EPDM und FKM, die für die Anwendung relevant sind, und HNBR als Vergleich) über mehrere Jahre untersucht. Dabei wurden die Veränderungen der mechanischen Eigenschaften (z. B. Härte und Bruchdehnung) bei vier verschiedenen Alterungstemperaturen (75 °C, 100 °C, 125 °C und 150 °C) sowie das Dichtungsverhalten betrachtet. Um den Einfluss von Vernetzung und Kettenspaltung auf die beobachteten Alterungseffekte zu bestimmen, wurden Messungen zur kontinuierlichen und intermittierenden Druckspannungsrelaxation durchgeführt. Dadurch konnten Erkenntnisse über Kinetik und Mechanismus der Abbaureaktionen
gewonnen werden. Zur Einschätzung des Dichtungsverhaltens wurden Durckverformungsrest und Leckagerate gemessen. Die Ergebnisse zeigen, dass die O-Ringe unter statischen Bedingungen auch dann dicht halten, wenn die Werte für Druckverformungsrest, Druckspannungsrelaxation und mechanische Eigenschaften schon auf einen fortgeschrittenen Alterungsabbau hindeuten. Aus diesem Grund wurde ein modifizierter Dichtigkeitsversuch mit einer geringen und schnellen teilweisen Entlastung der Dichtung entwickelt, mit dessen Hilfe sich ein Lebensdauerkriterium bestimmen lässt, das eine Sicherheitsspanne für thermische Schrumpfung und Vibrationen beinhaltet.
Abstract Hydrogenated nitrile butadiene rubber (HNBR) elastomer was thermo-oxidatively aged at different temperatures up to 150 °C. Fourier transform infrared spectroscopy (FTIR), compression stress relaxation (CSR) and international rubber hardness degree (IRHD) microhardness were used to characterise the chemo-mechanical changes of HNBR O-rings during thermo-oxidative ageing. FTIR shows the development of carbonyl, methyl and ester groups but the nitrile content was not affected by ageing. The effect of sample geometry during CSR was investigated. CSR data were converted through integrated kinetic laws. The conversion has proven its sensibility to detect heterogeneous ageing. This was confirmed by the IRHD measure-ments across the section of O-rings. The influence of compression during ageing was assessed through IRHD measurements across the section of compressed and uncom-pressed aged O-rings. The DLO effect was more pronounced in compressed O-rings. By applying the model of Wise et al., theoretical IRHD and oxidation profiles were determined on the basis of IRHD experimental data of compressed O-rings. Good agreements between the experimental and the theoretical IRHD profiles in the core region were obtained. However, near the edge, the theoretical IRHD values were overestimated.
EPDM, HNBR and FKM materials were exposed at 150 ◦C to air under atmospheric pressure and to hydrogen at 50 bar for different ageing times. All measurements after hydrogen exposure were conducted on samples in degassed condition to assess irreversible effects resulting from that exposure and to compare them to those after ageing in air. Density, hardness, tensile properties, compression set, and hydrogen permeability of all samples were analysed. In both ageing environments, HNBR exhibited the most significant changes of material properties.
However, for both EPDM and HNBR, considerably less severe ageing effects were observed under hydrogen in comparison to ageing in air. On the other hand, FKM showed about the same low level of deterioration in both ageing environments but exhibited poor resistance against damage due to rapid gas decompression in hydrogen environment that can lead to seal failure. The obtained results may serve as a guidance toward a better understanding for design and utilisation of elastomeric materials in future hydrogen infrastructure components.
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 mounted as barrier seals in lid systems of containers designed for transport and disposal of negligible heat generating radioactive waste and as auxiliary seals in spent fuel storage and transportation casks (dual purpose casks (DPC)). When the behavior of mounted seals under normal and hypothetical accident conditions of disposal and transport is to be simulated, a comprehensive knowledge of their complex mechanical properties at every state of aging is necessary. In previous works, BAM’s efforts in experimental investigations on specimen artificially aged at different temperatures and times and the implementation of the found results in finite element material models were presented. Additionally, our approaches to reproduce the aging process itself and to extrapolate the results of artificially accelerated aging to longer times were presented. Numerical simulations have shown that the behavior of the seal during mounting and one-sided pressurizing and the resulting performance values such as leakage rate strongly depend on the coefficient of friction (COF) between flange and seal. The friction coefficient, in turn, depends on the aging state of the elastomer material as several publications suggest (see below).
Dynamic COF between an exemplary ethylene propylene diene rubber (EPDM) material and a stainless steel ball were determined by using a self-designed linear oscillation tribometer. Unaged and artificially aged EPDM specimen stored for 30 days and 100 days at a temperature of 150 °C were tested. A stainless steel ball (d=10 mm) is brought in contact with the specimen’s surface und loaded by normal forces of 2.5 N, 5 N, 10 N and 20 N. During a reciprocating movement of the EPDM sheet, the horizontal force/friction force is continuously measured, and the COF can be derived.
It is well known that friction is a complex phenomenon especially in soft materials. It cannot be excluded that the measured friction force is influenced by additional force components, resulting from the ball’s grooving through the elastomer’s surface. This force depends on the penetration depth of the ball and on the resistance of the elastomer in its different states of aging. The latter results from microstructural changes i.e., chain scission and additional crosslinking that occur during aging which in turn influence the softening or hardening of the material.
A finite element (FE) ABAQUS® model was developed to reproduce the measurement process. It should help to better understand the physical mechanisms and to quantify the percentage of measured forces resulting from real friction on the one hand and forces resulting from unintended side effects that could falsify the result on the other hand. The behavior of the elastomer in its different states of aging is reproduced by a FE material model already presented in previous works of BAM.
High and ultra-high molecular weight polyethylenes are high performance materials, which are predestined for a wide range of applications due to characteristics like high density, low weight, good dimensional stability, high chemical resistance, and high hydrogen content. These polymers are used in two demanding areas: as implant material in medical technology (only ultra-high molecular weight polyethylenes) and as a component for neutron shielding purposes in casks for storage and transport of radioactive waste. In the medical field (joint replacements) as well as during neutron radiation shielding application, high and ultra-high molecular weight polyethylenes are exposed to gamma irradiation: in the first case requested as sterilization process and for surface-crosslinking and in the second case existing as a side effect of inserting the radioactive material in the cask.
Given that polyethylene as shielding material 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, the durability of the material is of special interest. High molecular weight polyethylene (HMW-PE; LUPOLEN 5261Z; Lyondell Basell) and ultra-high molecular weight polyethylene (UHMW-PE; GUR 4120; Ticona) were subjected to gamma radiation and afterwards thermally treated. The gamma doses used are in the range of 50 to 600 kGy and irradiation takes place at RT using a Co-60 source. The planned thermal treatment will take place at a temperature of 125 °C for periods of minimum 30 days and maximum of five years. With the applied conventional analytical techniques it is possible to detect structural changes of both types of polyethylene induced by gamma irradiation and certainly of thermal treatment. Through gamma irradiation melting temperature, crystallinity, and density, respectively increased. Furthermore both polyethylenes get oxidized and cross-linked. With regard to the special application as neutron radiation shielding material in casks for storage and transport of radioactive materials, the impact of irradiation lead to changes of material properties. A consolidated view indicates that the detected changes of the irradiated (U)HMW-PE are not safety relevant for long-term neutron radiation shielding purposes over a period of 40 years in Germany.
In this study, bulked adhesives were examined regarding changes in properties for their use in the restoration of stained-glass windows. Two epoxy adhesives commonly used in the restoration of flat glass, Araldite® 2020 and Hxtal NYL-1™, were investigated. The adhesives were mixed with various silica-based micro- and nano-particles as fillers. Different tests were carried out on the adhesives with and without fillers in order to characterise their chemical and mechanical properties. The tests included colorimetry, dynamic-mechanical methods of analysis and tests on bending strength. No property changes could be ascertained in the bulked adhesives that would be negative for the restoration of delicate flat glass.
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.
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.
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.
Elastomeric seals are used in many containers, including casks for radioactive waste. However, like all polymers, elastomers are prone to aging, which leads to a loss of sealing force and the ability for recovery which can ultimately result in leakage. Therefore it is important to be able to define an end-of-lifetime criterion and to judge the lifetime of elastomeric seals. For this reason, we started an aging program on three kinds of rubbers (HNBR, EPDM, FKM), monitoring the change of properties at four different aging temperatures over extended periods up to five years. The measured data is used for lifetime prediction by applying a suitable model.
BAM is the federal institute for materials research and testing in Germany. One of our tasks is to evaluate the safety of casks designed for transport and/or storage of radioactive waste. As elastomeric seals are used in the containers as safety-relevant parts, it is our goal to be able to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. For this reason, an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) was started. Ageing was performed at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) for up to 2 years. For assessing properties related to the sealability, O-rings were aged in compression by 25 % (corresponding to the compression during service) between plates. For comparison, uncompressed O-rings were aged as well. The aged materials were characterized with conventional polymer analysis methods such as hardness and tensile tests, but also with more seal-specific methods such as compression stress relaxation (CSR, reflecting the loss of sealing force of a compressed seal over time), and compression set (CS, representing the recovery behaviour of a seal after release from compression). CS is chosen as the property for lifetime prediction as it is both sensitive to degradation and related to the seal performance. CS data is extrapolated to 60 °C, which yields lifetimes of approximately 5 years for HNBR and 64 years for EPDM for a criterion of 85 % CS respectively, and approx. 40 years for FKM for a criterion of 65 % CS (the highest value measured so far).
Unsere Aufgabe im Fachbereich 3.4 der Bundesanstalt für Materialforschung und prüfung (BAM) ist die sicherheitstechnische Begutachtung von Transport- und Lagerbehältern für radioaktiven Abfall, die für langfristige Zwischenlagerung ausgelegt sind. Ein Arbeitsschwerpunkt ist die Bewertung der Lebensdauer von Metall- und Elastomerdichtungen in den Behältern im Hinblick auf die Anforderungen für langfristige Behältersicherheit (min. 40 Jahre in Deutschland). Um die Eigenschaftsänderungen der Elastomerdichtungen während einer beschleunigten Alterung über lange Zeiträume (bis zu fünf Jahre) untersuchen zu können, wurde ein Alterungsprogramm an ausgewählten Elastomeren (HNBR, EPDM, FKM) begonnen.
Im Gegensatz zu Normalterungsverfahren altern wir keine Normteile, sondern O-Ringe im unverpressten sowie verpressten Zustand. Diese Vorgehensweise ermöglicht die Bewertung des Einflusses der Verpressung auf die Alterung sowie die Untersuchung des Bauteilversagens mittels Leckageratenmessungen unter anwendungsnahen Bedingungen. Weitere Charakterisierungsmethoden sind Härtemessung, dynamisch-mechanische Analyse (DMA), Thermogravimetrie (TGA), Druckspannungsrelaxation, Druckverformungsrest (DVR), Dichtebestimmung und Zugversuch. Das Ziel ist, die Leckagerate mit anderen Eigenschaften bzw. Methoden zu korrelieren, um so ein aussagekräftiges und leichter zu bestimmendes Lebensdauerkriterium für elastomere O-Ringe abzuleiten.
Während der Alterung können in Abhängigkeit von Material sowie Alterungsdauer und –temperatur Inhomogenitäten aufgrund von diffusionsbegrenzter Oxidation auftreten, durch die sich unterschiedliche Eigenschaften an der Oberfläche und im Volumen der Dichtung ergeben, was zu falschen Lebensdauerabschätzungen führen kann. Mittels Mikrohärtemessung über den Querschnitt werden diese Effekte charakterisiert und bei der Extrapolation mittels Zeit-Temperatur-Verschiebung und Arrheniusgraph berücksichtigt. Bisher liegen Daten von bis zu einem Jahr bei vier verschiedenen Temperaturen (75 °C, 100 °C, 125 °C und 150 °C) gealterten Proben vor. Nach 100 Tagen bei 150 °C weisen HNBR und EPDM schon einen DVR von über 80 % auf, aber noch keinen signifikanten Anstieg der Leckagerate. Diese sinkt zunächst während der Alterung, da die Permeabilität aufgrund von Vernetzungsreaktionen sinkt. Bei HNBR ist dies aufgrund der dominanteren Vernetzungsreaktionen während der Alterung noch stärker ausgeprägt als bei EPDM.
At BAM Federal Institute for Materials Research and Testing in Germany, it is our responsibility to audit the safety of casks designed for transport and/or storage of radioactive material. With regard to the requirements on long-term safety of the containers, it is necessary to evaluate the service lifetime of the elastomeric seals used in the containers. With ageing, the elastomers will gradually lose their elasticity and their ability for recovery, which might result in a leakage above the allowed level or in a release of radioactivity during an incident. It is important to know the rate of degradation and which property can be used as an practical and easily measurable end-of-lifetime criterion. For this reason, we devised ageing experiments on different kinds of elastomers, namely FKM, EPDM and HNBR. The former two are actually used in containers for radioactive wastes – either as auxiliary seal in casks containing high activity waste, or as main seal in casks for medium or low activity waste. The latter is an often used seal material that is tested for comparative reasons. In our ageing program, these three materials are aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) in order to extrapolate the data to service temperature using a suitable model. They are examined at logarithmic time intervals of several days up to 5 years. The samples are aged in their undeformed O-ring state as well as compressed between flanges with a deformation corresponding to the actual compression during service. Thus we can compare the ageing between relaxed and stressed sample. Additionally, we are ageing samples in flanges that allow leakage rate measurements. Other measurements include monitoring of the recovery force of the seal and the compression set. Furthermore, the samples are examined by thermal and dynamic-mechanical analysis for detecting changes in the glass transition temperature due to crosslinking or chain scission in the material. Besides, hardness is measured as a practical macroscopic indicator. Until now, we have analyzed samples aged up to 100 days. At the highest ageing temperature of 150 °C, the compressed EPDM has already reached a compression set of 95 %, while HNBR and FKM have reached 80 % and 30 %, respectively. Furthermore, HNBR has reached a Shore D hardness of 85, which is an immense increase after the initial value of 80 Shore A. However, when looking at the cross-section of the sample, it was obvious that only a layer of about 1 mm thickness has become so hard, while the inner part remained rubbery. This shows that there has been intense crosslinking mostly near the surface of the HNBR which forms a kind of oxygen diffusion barrier, inhibiting the thermooxidation of the inner part of sample which can retain elastic properties. In EPDM, the oxygen permeability is much greater, which leads to a more homogeneous degradation across the whole sample and thus resulted in a higher compression set. The low compression set of FKM shows the outstanding high-temperature properties of this material.
At BAM Federal Institute for Materials Research and Testing in Germany, it is our responsibility to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the investigation of elastomeric seals applied in the containers. Besides examining the low-temperature behavior 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. Furthermore, we deem it necessary to exceed the requirements given in the ageing standard DIN 53508 and take into account diffusion-limited oxidation (DLO) effects and non-Arrhenius behavior when making lifetime predictions. Therefore, we started an ageing programme with selected rubbers (HNBR, FKM and EPDM) which are oven-aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) in order to extrapolate the data to service temperature using different models and check which one is appropriate. Samples are examined at different times up to 5 years. In order to be able to compare between compressed and relaxed rubber, they are aged in two conditions: the undeformed O-ring state as well as compressed between plates with a compression of 25 % corresponding to the actual compression during service. Additionally, we are ageing samples in flanges that allow leakage rate measurements which is the central performance criterion. Analysis methods include hardness as a quick indicator, compression set and compression stress relaxation as measures reflecting the actual properties of a compressed seal. Additionally, we are applying classical polymer analysis methods like dynamic mechanic analysis and thermogravimetric analysis which show changes in the polymeric structure due to chain scission and crosslinking. Furthermore, we are testing the leakage rate of the O-rings in order to correlate the changes in physical properties to the actual performance of the seal.
First results of samples aged up to 100 days show a strong increase in hardness for HNBR, a moderate increase for EPDM and hardly a change for FKM. A similar result is seen in the compression set of samples aged 100 d at 100 °C in compression as HNBR has reached 60 %, EPDM 25 % and FKM 15 % compression set. However, after ageing at 150 °C, EPDM shows a worse performance with 95 % compression set compared to HNBR with only 80 %. This is probably due to DLO effects in HNBR that appear because of the fast ageing and low oxygen permeability in HNBR. This leads to a deficit of oxygen in the center of the sample which is thus protected from ageing and can retain elastic properties. The full paper shall discuss the changes in material properties observed to date and the impact on the performance of elastomeric seals.
Characterisation of degradation and determination of end-of-lifetime criterion for O-ring seals
(2017)
The ageing of polymers, including 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. in automotive, piping or container applications, a long lifetime or at least a timely exchange of a seal before occurrence of critical failure is desired.
In order to investigate the degradation of material properties and to determine the lifetime of elastomers used as seals, an ageing programme was started with hydrogenated acrylonitrile butadiene rubber (HNBR), ethylene propylene diene rubber (EPDM) and fluorocarbon rubber (FKM) [1-3]. Both O-ring seals (uncompressed and compressed by 25 %) with a cord diameter of 10 mm as well as sheets with a thickness of 2 mm were aged at 75 °C, 100 °C, 125 °C and 150 °C for up to two years. The changes of material properties during ageing were characterised for each material using samples from the sheets by means of e.g. Dynamic Mechanical Analysis and tensile tests, while sealing properties such as leakage rate were determined on O-rings. The experimental results indicate that while material properties show considerable degradation effects, the static leakage rate stays constant or even decreases before failure occurs quite suddenly at advanced degradation levels. This reveals that static leakage rate has only limited sensitivity for the degradation of the seal material.
Our approach to determine a suitable end of lifetime criterion, which involves a partial decompression of the seal during the leakage test, is presented and discussed.
At BAM, which is a 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 casks. Besides examining the low-temperature behaviour and irradiation effects of elastomeric seals, it is our goal to estimate 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 five years. In order to assess sealability, O-rings were 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 were 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, material properties such as hardness, elastic modulus, glass transition temperature and viscoelastic loss factor as well as relaxation and recovery behaviour are examined in order to understand the underlying ageing mechanisms in each material. For obtaining 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, resulting in distorted bulk properties such as compression stress relaxation and compression set. However, if DLO-affected data is excluded, extrapolations of CS data are possible using time-temperature shifts and Arrhenius graphs.
For selecting an appropriate end-of-lifetime criterion, leakage rate measurements were performed, since leakage rate is the only characteristic directly correlated to the performance of the sealing system. A significant increase in leakage rate was considered as the end of the lifetime. However, the O-rings remained leak tight under static conditions and even exhibited an improved, i.e. decreased leakage rate while other properties already indicated strong deterioration. These tests were extended by seal tests with fast partial decompression.
Overall an update will be given on the current investigations and results and the planned activities.
Aging of elastomer O-rings and PE neutron shielding materials for radioactive waste containers
(2021)
Our institution BAM in Berlin, Germany is concerned with research and testing of materials in the context of safety in chemistry and technology. Our working group is involved in the licensing procedures of casks for radioactive waste. Besides, we’re doing research on aging and lifetime prediction of elastomer O-rings and investigate degradation and thermal expansion of PE neutron shielding materials.
At BAM, which is a federal institute for materials research and testing in Germany, it is our responsibility to evaluate the safety of casks designed for transport and/or storage of radio-active material. This includes assessment of the elastomeric seals applied in the containers. Besides examining the low-temperature behaviour 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 ageing programme with selected rubbers (HNBR, EPDM and FKM) which are oven-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. Many other properties, e.g. hardness or glass transition temperature, are influenced in opposite directions by crosslinking and chain scission reactions during ageing. By contrast, CSR and CS are influenced additively by each reaction type, as crosslinking leads to the formation of network chains that are in equilibrium with the compressed geometry, and broken chains lose their recovery potential and their contribution to the sealing force. The experimental results indicate that while 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 sample leads to the formation of a leak path.
BAM is the federal institute for materials research and testing in Germany. One of our tasks is to evaluate the safety of casks designed for transport and/or storage of radioactive waste. As elastomeric seals are used in the containers as safety-relevant parts, it is our goal to be able to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. For this reason, an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) was started. Ageing was performed at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) for up to 2 years. For assessing properties related to the sealability, O-rings were aged in compression by 25 % (corresponding to the compression during service) between plates. For comparison, uncompressed O-rings were aged as well. The aged materials were characterized with conventional polymer analysis methods such as hardness and tensile tests, but also with more seal-specific methods such as compression stress relaxation (CSR, reflecting the loss of sealing force of a compressed seal over time), and compression set (CS, representing the recovery behaviour of a seal after release from compression). CS is chosen as the property for lifetime prediction as it is both sensitive to degradation and related to the seal performance. CS data is extrapolated to 60 °C, which yields lifetimes of approximately 5 years for HNBR and 64 years for EPDM for a criterion of 85 % CS respectively, and approx. 40 years for FKM for a criterion of 65 % CS (the highest value measured so far).
At BAM, which is a 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 in 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. In order to understand the underlying ageing mechanisms in each material, the change in material properties is examined using thermal and mechanical analysis. The experimental results indicate that while material properties, 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 most relevant criterion for seal performance, has only limited sensitivity for the degradation of the seal material.
Two polyethylene types with ultra high (UHMWPE) and high molecular weight (HMWPE), which are used as neutron radiation shielding materials in storage casks for radioactive waste, were subjected to gamma irradiation doses up to 600 kGy and subsequent thermal ageing at 125 °C for up to one year. One material was a medical grade UHMWPE and the other a HMWPE containing an antioxidant. Degradation effects in the materials were characterized using colorimetry, UV-Vis spectroscopy, MIR and FTIR measurements, DSC and, in the case of HMWPE, insoluble content determination. Both materials exhibited a yellowing upon irradiation. The discoloration of UHMWPE disappeared again after thermal ageing, which is why it was attributed to annealable color centers in the form of free radicals entrapped in the crystalline regions of the polymer that recombine during thermal ageing. Furthermore, oxidation species were observed with MIR and FTIR spectroscopy. For HMWPE, the yellowing occurred during both irradiation and thermal ageing and was correlated to antioxidant decomposition. Additionally, black spots were observed after thermal ageing of HMWPE that were attributed to reaction products of antioxidant derivatives and catalyst residues. While only little evidence of oxidation species was found in the light material parts, oxidation is expected to concentrate in the black spots as the catalyst residue promotes hydroperoxide decomposition and thus radical formation that initiate polymer oxidation
The ageing of polymers, including elastomers, is an undesirable but inevitable process leading to a limited lifetime of rubber products such as seals. In order to investigate the degradation of material properties of elastomers used as seals, an ageing programme was started with hydrogenated acrylonitrile butadiene rubber (HNBR), ethylene propylene diene rubber (EPDM) and fluorocarbon rubber (FKM) [1-3]. Sheets of 2 mm thickness were aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) for time periods of 1 day up to 1 year. The aged samples were examined using standard analytical methods such as hardness, dynamic-mechanical analysis (DMA), thermogravimetric analysis (TGA), mass loss, density and tensile tests. Additionally, DMA equipment was used to investigate relaxation and recovery behaviour which is more sensitive to degradation effects and can help to correctly interpret data from hardness and DMA measurements. The results show e.g. an increase in hardness and glass transition temperature for HNBR and, less pronounced, for EPDM. This confirms ageing mechanisms described in the literature, e.g. dominant crosslinking reactions in HNBR and both chain scissions and crosslinking for EPDM. FKM, which is very heat resistant, exhibits only minor degradation. Where possible, shift factors for time-temperature data sets were determined. The values obtained from different methods are compared and discussed.
BAM is the federal institute for materials research and testing in Germany. One of our tasks is to evaluate the safety of casks designed for transport and/or storage of radioactive waste. As elastomeric seals are used in the containers as safety-relevant parts, it is our goal to be able to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. For this reason, an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) was started. Ageing was performed at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) for up to 2 years. For assessing properties related to the sealability, O-rings were aged in compression by 25 % (corresponding to the compression during service) between plates. For comparison, uncompressed O-rings were aged as well. The aged materials were characterized with conventional polymer analysis methods such as hardness and tensile tests, but also with more seal-specific methods such as compression stress relaxation (CSR, reflecting the loss of sealing force of a compressed seal over time), and compression set (CS, representing the recovery behaviour of a seal after release from compression). CS is chosen as the property for lifetime prediction as it is both sensitive to degradation and related to the seal performance. CS data is extrapolated to 60 °C, which yields lifetimes of approximately 5 years for HNBR and 64 years for EPDM for a criterion of 85 % CS respectively, and approx. 40 years for FKM for a criterion of 65 % CS (the highest value measured so far).
At BAM Federal Institute for Materials Research and Testing in Germany, it is our responsibility to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the investigation of elastomeric seals applied in the containers. Besides examining the low-temperature behavior 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. Furthermore, we deem it necessary to exceed the requirements given in the ageing standard DIN 53508 and take into account diffusion-limited oxidation (DLO) effects [1] and non-Arrhenius behavior [2] when making lifetime predictions. Therefore, we started an ageing programme with selected rubbers (HNBR, FKM and EPDM) which are oven-aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) in order to extrapolate the data to service temperature using different models and check which one is appropriate. Samples are examined at different times up to 5 years. In order to be able to compare between compressed and relaxed rubber, they are aged in two conditions: the undeformed O-ring state as well as compressed between plates with a compression of 25 % corresponding to the actual compression during service. Additionally, we are ageing samples in flanges that allow leakage rate measurements which is the central performance criterion. Analysis methods include hardness as a quick indicator, compression set and compression stress relaxation as measures reflecting the actual properties of a compressed seal. Additionally, we are applying classical polymer analysis methods like dynamic mechanic analysis and thermogravimetric analysis which show changes in the polymeric structure due to chain scission and crosslinking. Furthermore, we are testing the leakage rate of the O-rings in order to correlate the changes in physical properties to the actual performance of the seal.
First results of samples aged up to 100 days show a strong increase in hardness for HNBR, a moderate increase for EPDM and hardly a change for FKM. A similar result is seen in the compression set of samples aged 100 d at 100 °C in compression as HNBR has reached 60 %, EPDM 25 % and FKM 15 % compression set. However, after ageing at 150 °C, EPDM shows a worse performance with 95 % compression set compared to HNBR with only 80 %. This is probably due to DLO effects in HNBR that appear because of the fast ageing and low oxygen permeability in HNBR. This leads to a deficit of oxygen in the center of the sample which is thus protected from ageing and can retain elastic properties. The full paper shall discuss the changes in material properties observed to date and the impact on the performance of elastomeric seals.
References:
1. Wise, J., K.T. Gillen, and R.L. Clough, Quantitative model for the time development of diffusion-limited oxidation profiles. Polymer, 1997. 38(8): p. 1929-1944.
2. Gillen, K., et al., Evidence that Arrhenius high-temperature aging behavior for an EPDM o-ring does not extrapolate to lower temperatures, 1997, Sandia National Labs., Albuquerque, NM (United States).
Elastomeric seals are used in many containers, including casks for radioactive waste. However, like all polymers, elastomers are prone to aging, which leads to a loss of sealing force and the ability for recovery which can ultimately result in leakage. Therefore it is important to be able to define an end-of-lifetime criterion and to judge the lifetime of elastomeric seals. For this reason, we started an aging program on three kinds of rubbers (HNBR, EPDM, FKM), monitoring the change of properties at four different aging temperatures over extended periods up to five years. The measured data is used for lifetime prediction by applying a suitable model.
At BAM Federal Institute for Materials Research and Testing in Germany, it is our responsibility to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes investigating the elastomer seals used in the containers. Our current goal is a deeper understanding of ageing processes and examination of service lifetime of compressed O-ring seals with regard to interim storage periods which might be longer than the currently licensed 40 years in Germany. For this reason, we started an ageing programme with selected rubbers (HNBR, EPDM, FKM) for investigating the change of properties during accelerated ageing over long periods of time (up to five years). In contrast to standard procedure which recommends the ageing of standard test pieces, we are ageing O-rings, both uncompressed and compressed [1]. This approach takes into account the specific geometry and stress state of compressed O-rings. Additionally, it allows leakage rate measurements as the most important property of the sealing system under conditions relevant to the application. Other investigated properties and applied methods include hardness, dynamic mechanic analysis (DMA), thermogravimetric analysis (TGA), compression stress relaxation (CSR, loss of sealing force over time), compression set (CS, increases if resilience decreases), density and tensile tests. The aim is to correlate leakage rate with common material properties in order to identify a suitable and easily measurable end-of-lifetime criterion for elastomer O-ring seals. Furthermore, the appearance of diffusion-limited oxidation (DLO) effects is investigated, as these effects can lead to heterogeneous ageing and thus distorted bulk properties, resulting in overestimated lifetimes [2].
Extrapolations are made using time-temperature shifts and Arrhenius graphs. For hardness and compression set data the extrapolation leads to different lifetimes. This is because crosslinking and chain scission reactions influence hardness in opposite directions [3], whereas both effects add up to increase compression set [4]. After 0.5 years of ageing, EPDM exhibits only slight hardness increase except at 150 °C, while CS increases substantially at all ageing temperatures. On the other hand, for HNBR, both hardness and CS increase strongly. This confirms that crosslinking is the major degradation mechanism in HNBR [5], while in EPDM both crosslinking and chain scission are apparent [6]. Consequently, hardness is not a suitable measure for extrapolation, as a slight hardness increase can be accompanied by significant chain scission, resulting in severe degradation in other properties, such as CS. NMR, TGA, DMA and density measurements can give further information about the contribution of crosslinking or chain scission in the materials at different ageing stages. The experiments are in progress and results will be presented at the conference.
References
[1] Kömmling A, Waste Management Symposia 2015.
[2] Gillen KT, Polym. Eng. Sci. 1989:29:1.
[3] Snijders EA, Polym. Degrad. Stab. 2005:89:2.
[4] Andrews RD, J. Appl. Phys. 1946:17:5.
[5] Bhattacharjee S, Polym. Degrad. Stab. 1991:31:1.
[6] Bender H, Kautsch. Gummi Kunststoffe 2001:54.
At BAM Federal Institute for Materials Research and Testing in Germany, it is our responsibility to audit the safety of casks designed for transport and/or storage of radioactive material. With regard to the requirements on long-term safety of the containers, it is necessary to evaluate the service lifetime of the elastomeric seals used in the containers. With ageing, the elastomers will gradually lose their elasticity and their ability for recovery, which might result in a leakage above the allowed level or in a release of radioactivity during an incident. It is important to know the rate of degradation and which property can be used as an practical and easily measurable end-of-lifetime criterion. For this reason, we devised ageing experiments on different kinds of elastomers, namely FKM, EPDM and HNBR. The former two are actually used in containers for radioactive wastes – either as auxiliary seal in casks containing high activity waste, or as main seal in casks for medium or low activity waste. The latter is an often used seal material that is tested for comparative reasons. In our ageing program, these three materials are aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) in order to extrapolate the data to service temperature using a suitable model. They are examined at logarithmic time intervals of several days up to 5 years. The samples are aged in their undeformed O-ring state as well as compressed between flanges with a deformation corresponding to the actual compression during service. Thus we can compare the ageing between relaxed and stressed sample. Additionally, we are ageing samples in flanges that allow leakage rate measurements. Other measurements include monitoring of the recovery force of the seal and the compression set. Furthermore, the samples are examined by thermal and dynamic-mechanical analysis for detecting changes in the glass transition temperature due to crosslinking or chain scission in the material. Besides, hardness is measured as a practical macroscopic indicator. Until now, we have analyzed samples aged up to 100 days. At the highest ageing temperature of 150 °C, the compressed EPDM has already reached a compression set of 95 %, while HNBR and FKM have reached 80 % and 30 %, respectively. Furthermore, HNBR has reached a Shore D hardness of 85, which is an immense increase after the initial value of 80 Shore A. However, when looking at the cross-section of the sample, it was obvious that only a layer of about 1 mm thickness has become so hard, while the inner part remained rubbery. This shows that there has been intense crosslinking mostly near the surface of the HNBR which forms a kind of oxygen diffusion barrier, inhibiting the thermooxidation of the inner part of sample which can retain elastic properties. In EPDM, the oxygen permeability is much greater, which leads to a more homogeneous degradation across the whole sample and thus resulted in a higher compression set. The low compression set of FKM shows the outstanding high-temperature properties of this material.
At BAM Federal Institute for Materials Research and Testing in Germany, it is our responsibility to evaluate the safety of casks designed for transport and/or storage of radioactive material. This includes the investigation of elastomeric seals applied in the containers. Besides examining the low-temperature behavior 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. Furthermore, we deem it necessary to exceed the requirements given in the ageing standard DIN 53508 and take into account diffusion-limited oxidation (DLO) effects [1] and non-Arrhenius behavior [2] when making lifetime predictions. Therefore, we started an ageing programme with selected rubbers (HNBR, FKM and EPDM) which are oven-aged at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) in order to extrapolate the data to service temperature using different models and check which one is appropriate. Samples are examined at different times up to 5 years. In order to be able to compare between compressed and relaxed rubber, they are aged in two conditions: the undeformed O-ring state as well as compressed between plates with a compression of 25 % corresponding to the actual compression during service. Additionally, we are ageing samples in flanges that allow leakage rate measurements which is the central performance criterion. Analysis methods include hardness as a quick indicator, compression set and compression stress relaxation as measures reflecting the actual properties of a compressed seal. Additionally, we are applying classical polymer analysis methods like dynamic mechanic analysis and thermogravimetric analysis which show changes in the polymeric structure due to chain scission and crosslinking. Furthermore, we are testing the leakage rate of the O-rings in order to correlate the changes in physical properties to the actual performance of the seal.
First results of samples aged up to 100 days show a strong increase in hardness for HNBR, a moderate increase for EPDM and hardly a change for FKM. A similar result is seen in the compression set of samples aged 100 d at 100 °C in compression as HNBR has reached 60 %, EPDM 25 % and FKM 15 % compression set. However, after ageing at 150 °C, EPDM shows a worse performance with 95 % compression set compared to HNBR with only 80 %. This is probably due to DLO effects in HNBR that appear because of the fast ageing and low oxygen permeability in HNBR. This leads to a deficit of oxygen in the center of the sample which is thus protected from ageing and can retain elastic properties. The full paper shall discuss the changes in material properties observed to date and the impact on the performance of elastomeric seals.
BAM is the federal institute for materials research and testing in Germany. One of our tasks is to evaluate the safety of casks designed for transport and/or storage of radioactive waste. As elastomeric seals are used in the containers as safety-relevant parts, it is our goal to be able to evaluate the service lifetime of the seals with regard to the requirements for long-term safety (40 years and more) of the containers. For this reason, an accelerated ageing programme with selected rubbers often used for seals (HNBR, EPDM and FKM) was started. Ageing was performed at four different temperatures (75 °C, 100 °C, 125 °C and 150 °C) for up to 2 years. For assessing properties related to the sealability, O-rings were aged in compression by 25 % (corresponding to the compression during service) between plates. For comparison, uncompressed O-rings were aged as well. The aged materials were characterized with conventional polymer analysis methods such as hardness and tensile tests, but also with more seal-specific methods such as compression stress relaxation (CSR, reflecting the loss of sealing force of a compressed seal over time), and compression set (CS, representing the recovery behaviour of a seal after release from compression). CS is chosen as the property for lifetime prediction as it is both sensitive to degradation and related to the seal performance. CS data is extrapolated to 60 °C, which yields lifetimes of approximately 5 years for HNBR and 64 years for EPDM for a criterion of 85 % CS respectively, and approx. 40 years for FKM for a criterion of 65 % CS (the highest value measured so far).
Unsere Aufgabe im Fachbereich 3.4 der Bundesanstalt für Materialforschung und prüfung (BAM) ist die sicherheitstechnische Begutachtung von Transport- und Lagerbehältern für radioaktiven Abfall, die für langfristige Zwischenlagerung ausgelegt sind. Ein Arbeitsschwerpunkt ist die Bewertung der Lebensdauer von Metall- und Elastomerdichtungen in den Behältern im Hinblick auf die Anforderungen für langfristige Behältersicherheit (min. 40 Jahre in Deutschland). Um die Eigenschaftsänderungen der Elastomerdichtungen während einer beschleunigten Alterung über lange Zeiträume (bis zu fünf Jahre) untersuchen zu können, wurde ein Alterungsprogramm an ausgewählten Elastomeren (HNBR, EPDM, FKM) begonnen.
Im Gegensatz zu Normalterungsverfahren altern wir keine Normteile, sondern O-Ringe im unverpressten sowie verpressten Zustand. Diese Vorgehensweise ermöglicht die Bewertung des Einflusses der Verpressung auf die Alterung sowie die Untersuchung des Bauteilversagens mittels Leckageratenmessungen unter anwendungsnahen Bedingungen. Weitere Charakterisierungsmethoden sind Härtemessung, dynamisch-mechanische Analyse (DMA), Thermogravimetrie (TGA), Druckspannungsrelaxation, Druckverformungsrest (DVR), Dichtebestimmung und Zugversuch. Das Ziel ist, die Leckagerate mit anderen Eigenschaften bzw. Methoden zu korrelieren, um so ein aussagekräftiges und leichter zu bestimmendes Lebensdauerkriterium für elastomere O-Ringe abzuleiten.
Während der Alterung können in Abhängigkeit von Material sowie Alterungsdauer und –temperatur Inhomogenitäten aufgrund von diffusionsbegrenzter Oxidation auftreten, durch die sich unterschiedliche Eigenschaften an der Oberfläche und im Volumen der Dichtung ergeben, was zu falschen Lebensdauerabschätzungen führen kann. Mittels Mikrohärtemessung über den Querschnitt werden diese Effekte charakterisiert und bei der Extrapolation mittels Zeit-Temperatur-Verschiebung und Arrheniusgraph berücksichtigt. Bisher liegen Daten von bis zu einem Jahr bei vier verschiedenen Temperaturen (75 °C, 100 °C, 125 °C und 150 °C) gealterten Proben vor. Nach 100 Tagen bei 150 °C weisen HNBR und EPDM schon einen DVR von über 80 % auf, aber noch keinen signifikanten Anstieg der Leckagerate. Diese sinkt zunächst während der Alterung, da die Permeabilität aufgrund von Vernetzungsreaktionen sinkt. Bei HNBR ist dies aufgrund der dominanteren Vernetzungsreaktionen während der Alterung noch stärker ausgeprägt als bei EPDM.
Elastomeric O-ring seals are used in a wide range of applications due to their excellent elasticity. However, like all polymers, elastomers degrade under the influence of e.g. time, temperature, oxygen, radiation and mechanical stress. Especially in applications where a replacement of seals after a certain time is hard or almost impossible, it is important to know the seal lifetime. Therefore, an extensive ageing programme with elastomers made of EPDM, HNBR and other materials was started with the aim of determining suitable methods for accelerated ageing and lifetime prediction.
In order to determine the lifetime of polymeric parts, the time-temperature superposition principle is commonly used to shift property changes obtained by accelerated ageing at higher temperatures to lower temperatures. If the shift factors yield a straight line in an Arrhenius diagram, a corresponding activation energy can be determined. However, we have found that the shift factors and thus the predicted lifetime depends on the test method that yielded the shifted data. For example, the shift factor between 125 °C and 150 °C ageing temperature was roughly the same (5/5/4.5 respectively) for hardness, density and maximum of loss factor tan measured on HNBR, but different for elongation at break (8) and compression set (2.2, excluding DLO-affected data). A possible explanation might be that while the different oxidation reactions proceed with a fixed activation energy, they have differing impact on the measured properties. For example, hardness is lowered by chain scission reactions, and increases by crosslinking reactions during ageing. As usually both chain scission and crosslinking reactions occur during ageing, the measured hardness increase reflects only the net effect of both reaction types. On the other hand, compression set is influenced additively by both reaction types: chain scission leads to an increase of the remaining deformation, as broken chains lose their recovery potential, and crosslinking reactions during ageing fix the compressed geometry, which also leads to less recovery. Thus, compression set reflects the total number of changes in the network and shows degradation effects much faster, which results in lower shift factors and lower activation energy. This phenomenon was observed for EPDM as well.
In order to verify our hypotheses and to gain further insights, temperature-dependent oxygen consumption measurements are currently being performed and results will be presented at the conference. The shift factors and activation energy determined by oxygen consumption measurements are expected to be close to the values for compression set.
At BAM, which is a 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 casks. One of the aims is to identify a suitable method for estimating 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) for up to five years. In order to assess sealability, O-rings were 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 were aged as well. In order to understand the underlying ageing mechanisms in each material, material properties such as hardness, density and tensile properties were examined. Additionally, compression set (CS), which represents the recovery behaviour of a seal after release from compression, is measured. For obtaining 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 ageing caused by diffusion-limited oxidation (DLO) effects especially for HNBR, resulting in distorted bulk properties such as compression set. However, if DLO-affected data is excluded, extrapolations of CS data are possible using time-temperature shifts.
For selecting an appropriate end-of-lifetime criterion, leakage rate measurements were performed, since leakage rate is the only characteristic directly correlated to the performance of the sealing system. A significant increase in leakage rate was considered as the end of the lifetime. However, the O-rings remained leak tight under static conditions even though material properties had already degraded strongly. For this reason, a modified, more demanding leakage test involving a fast small decompression of the seal was developed that allowed determining a more conservative end-of-lifetime criterion with a safety margin for EPDM seals. FKM, which is a very heat and oxidation resistant material, exhibited only little degradation, even though it had the smallest activation energy.
Für Anwendungen, bei denen Elastomerdichtungen ihre Dichtheit über sehr lange Zeiträume (z. B. Jahrzehnte) behalten müssen, ohne dass ein Austausch praktikabel wäre, ist es nötig, die Lebensdauer solcher Dichtungen zuverlässig bestimmen zu können. Aus diesem Grund wurden Alterungsversuche an O-Ring-Dichtungen sowie Flachproben auf Basis von hydriertem Acrylnitril-Butadien-Kautschuk (HNBR), Ethylen-Propylen-Dien-Kautschuk (EPDM) und Fluorkautschuk (FKM) über bis zu zwei Jahre bei mehreren Temperaturen durchgeführt. Zur Untersuchung der auftretenden Eigenschaftsveränderungen wurden u. a. Härtemessungen, Zugversuche, dynamisch-mechanische und thermogravimetrische Analysen durchgeführt. Zur Beurteilung des Relaxations- und Rückstellverhaltens der alternden Dichtungen wurden außerdem Druckspannungsrelaxations- und Druckverformungsrestmessungen durchgeführt. Darüber hinaus wurde die Leckagerate gemessen, um ein Versagen der Dichtung zu detektieren. Durch die Untersuchung der gealterten Proben wurden sowohl die auftretenden Eigenschaftsveränderungen charakterisiert wie auch die damit zusammenhängenden Alterungsmechanismen diskutiert. Anhand der großen Datenmenge aus Ergebnissen von verschiedenen Untersuchungsmethoden, die für mehrere Alterungszeiten und -temperaturen durchgeführt wurden, sollte zusätzlich die günstigste Vorgehensweise für Lebensdauervorhersagen ermittelt werden. Dies beinhaltete auch die Bestimmung eines Lebensdauerkriteriums, das mit dem Auftreten von Leckage und damit dem Versagen der Dichtung zusammenhängt. Hinsichtlich der Eigenschaftsänderungen und Alterungsmechanismen wurde beim HNBR eine starke Versprödung aufgrund dominanter Vernetzungsreaktionen während der Alterung festgestellt. Diese resultierten auch darin, dass die Sauerstoffpermeabilität abnahm, so dass der Sauerstofftransport ins Innere der Probe (hauptsächlich bei den dickeren O-Ringen) behindert wurde. Dies führte dazu, dass diffusionsbegrenzte Oxidationseffekte (DLO-Effekte) auftraten, d. h. dass das Probeninnere weniger stark alterte als oberflächennahe Bereiche. Während der Alterung von EPDM traten sowohl Kettenspaltungen wie auch Vernetzungsreaktionen auf, was sich vor allem in einer Verschlechterung der Zugeigenschaften und einer Abnahme der Druckkraft sowie in einem steigenden Druckverformungsrest äußerte. Bei sehr stark gealterten Proben dominierten ebenfalls Vernetzungsreaktionen, was sich auch in deutlichen DLO-Effekten äußerte. Aufgrund seiner ausgeprägten Oxidations- und Hochtemperaturbeständigkeit wies FKM vergleichsweise geringe Degradationserscheinungen auf. Nur mit Druckspannungsrelaxation und Druckverformungsrest waren nennenswerte Änderungen ersichtlich. Messungen der statischen Leckagerate ergaben, dass die O-Ringe erst bei sehr weit fortgeschrittener Degradation undicht wurden und bis zu diesem Zeitpunkt kein allmählicher Anstieg der Leckagerate beobachtet werden konnte. Aus diesem Grund wurden Leckagemessungen mit einem modifizierten Versuchsaufbau durchgeführt, bei dem die Dichtung während der Messung leicht entlastet wird. Da dieser Test eine dynamische Rückstellung der Dichtung erfordert und damit höhere Ansprüche an die Dichtung stellt, wurde am geprüften EPDM-O-Ring ein früheres Versagen beobachtet. Der entsprechende Alterungszeitpunkt wurde als Versagenszeitpunkt definiert. Dieser wurde anhand der Daten verschiedener Messmethoden mit dem Degradationszustand korreliert und entspricht beispielsweise einem Druckverformungsrest von 80 %. Auf der Grundlage dieses Kriteriums konnten Lebensdauern mittels Zeit-Temperatur-Verschiebung bestimmt werden. Im Rahmen dieser Arbeit wurden zahlreiche Erkenntnisse darüber gewonnen, worauf bei beschleunigten Alterungsversuchen und Lebensdauerprognosen von O-Ringen zu achten ist und welche Fehlerquellen vermieden werden sollten. Dies wird zusammen mit Empfehlungen für eine bestmögliche Versuchsdurchführung ausführlich diskutiert.
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 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.
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 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.
Langzeitversuche über 10 Jahre an Federkern-Metalldichtungen mit Aluminium- bzw. Silberummantelungen
(2023)
Federunterstützte Metalldichtungen mit Aluminium(Al)- oder Silber(Ag)-Ummantelung werden u.a. in Behältern für Wärme entwickelnde radioaktive Abfälle eingesetzt, da diese Dichtungen eine sehr gute Dichtheit gewährleisten sowie Langlebigkeit und Beständigkeit gegenüber erhöhten Temperaturen und radioaktiver Strahlung aufweisen. Auch wenn die Sicherheit solcher Behälter und der verwendeten Dichtungen vielfach belegt wurde, sind sie Gegenstand andauernder Forschung, etwa im Hinblick auf eine absehbar benötigte verlängerte Zwischenlagerdauer. Aus diesem Grund werden an der Bundesanstalt für Materialforschung und -prüfung (BAM) im Fachbereich 3.4 „Sicherheit von Lagerbehältern“ seit über 20 Jahren Versuche an solchen Metalldichtungen durchgeführt. Dabei sollen zusätzliche Erkenntnisse hinsichtlich der Sicherheitsreserven der Dichtungen in unterstellten Störfallszenarien (axiale Bewegung des Deckelsystems bzw. Aufweitung der Nutgeometrie) und insbesondere detailliertere Erkenntnisse zum Langzeitverhalten gewonnen werden.
To determine a polymer’s spectral response of photo degradation, two main methods have been established. The so-called “filter technique” uses polychromatic radiation, which is filtered by a set of various sharp cut-on filters. This procedure provides separated reading points of an activation spectrum of the investigated material. With the "spectrographic technique” a specimen is irradiated with pectrally dispersed radiation, locally quasi monochromatically, and the local property changes.are related to the radiant exposure at the respective wavelengths. A third method, the irradiation of specimens with quasi monochromatic radiation behind interference filters, is rarely used.
These methods have advantages and disadvantages and therefore, there are limits for conclusions from these spectral data. Concerning the quasi monochromatic Irradiation of specimens the development of new UV radiation sources looks promising.
With the rapid development of inexpensive UV Light-emitting diodes (LEDs), new : sources of nearly monochromatic radiation in the UV ränge are available, even down to about 250 nm. To determine the spectral response, the use of such UV-LEDs in TO-18 ) and TO-39 metal cases combines the advantages of low cost and energy efficiency, stable spectral irradiance, temperature.and humidity control, larger specimen areas, and an easy and stable analysis.
Combinations of different UV LEDs are also discussed as replacement for xenon arc ' radiation sources, see patent application EP01528388A1.