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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.