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.
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.
HNBR, EPDM and FKM O-rings were aged uncompressed and compressed at 75 °C, 100 °C, 125 °C and 150 °C for up to 1 year. HNBR exhibited the strongest ageing effects with high increases of hardness and glass transition temperature. Furthermore, heterogeneous ageing caused by diffusion-limited oxidation effects had a significant influence for HNBR at ageing temperatures of 125 °C and 150 °C. EPDM showed similar property changes as HNBR, but less pronounced. FKM displayed only minor ageing effects. O-rings aged in compression exhibited considerable compression set (CS). CS data was used for a time-temperature shift and resulting master curve construction. Leakage rate measurements showed that O-rings can remain leak tight under static conditions even if material properties have already deteriorated strongly.
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.
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.
Seals used in containers for dangerous goods need to maintain leak tightness for several decades. For investigating the change in seal material properties during aging, a comprehensive aging program on both uncompressed and compressed EPDM and HNBR seals was started. In order to obtain results closely related to practical conditions, O-rings with a full-scale cord diameter of 10 mm were aged. However, this set-up can lead to heterogeneous aging caused by diffusion-limited oxidation (DLO) effects. These effects depend on material, dimensions, time and temperature. Heterogeneous aging results in distorted bulk properties such as compression stress relaxation and compression set (CS) suggesting that HNBR has better performance than EPDM at 150 °C but which is not the case at 100 °C. The presence of heterogeneous material properties was shown by hardness measurements across the seal cross-section. If DLO-affected data is excluded, extrapolations of CS data are possible using time-temperature shifts and Arrhenius graphs. Exemplary CS values of 50 % and 80 % would be reached after approx. 10 years and 29 years, respectively for HNBR and after approx. 400 years and 1100 years, respectively for EPDM.
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.