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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.
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).
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.