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Component and material investigations on metal seals for high level radioactive waste containers
(2018)
In Germany casks for interim storage of spent nuclear fuel and high level radioactive waste are equipped with Helicoflex® metal seals as main sealing barrier of the double lid system. The long-term behaviour of those seals is investigated at Bundesanstalt für Materialforschung und –prüfung (BAM) in order to evaluate the safety function of the containment at different temperatures over storage periods of 40 years or more in case extended interim storage becomes necessary.
Long-term investigations have been done for ageing times of up to 8.5 years at temperatures ranging from room temperature to 150 °C. It was found that the seal force and useable resilience decrease over time, which is mainly caused by creep deformation of the aluminium (or silver) outer jacket of the seals. This effect becomes stronger with increasing temperature.
The ageing processes of the seal material and the overall seal behaviour is under investigation in order to derive analytical descriptions for the long-term seal performance. Thus, standardized tests on the basic seal materials, with focus on aluminium, and additional investigations on the seals as a component are conducted. The current investigations include compression and tension creep measurements as well as tensile testing. Furthermore, ageing of seal segments provides information on the development of the contact area width, jacket thickness and microstructural changes in dependence of time and temperature. The obtained data are used for the development of material models and an analytical approach to describe and predict the time and temperature dependent sealing behaviour in the long-term.
A Memorandum of Understanding (MOU) between Argonne National Laboratory (Argonne) and the German Bundesanstalt für Materialforschung und -prüfung (BAM) was signed in October 2014. Its objectives are to promote cooperation among scientists and specialists at Argonne and BAM and establish a framework for collaboration in advanced technologies associated with the back end of the nuclear fuel cycle. Collaborative activities involving Argonne and BAM may be implemented through the promotion of joint research activities and scientific workshops and conferences; exchange of technical information; and visits by scientists, specialists, and graduate, postgraduate, and Ph.D. students. The MOU was renewed in January 2020 for another five years. Highlights of the progress and achievements are provided in identified topical areas for collaboration on ageing management guidance, storage, transportation and disposal R&D, joint conference activities, and conclude with the benefits of the MOU in fostering Argonne and BAM collaboration.
Rubbers are widely used as main sealing materials for containers for low and intermediate level radioactive waste and as additional component to metal seals in spent fuel and high active waste containers. According to appropriate guidelines and regulations safe enclosure of the radioactive container contents has to be guaranteed for long storage periods as well as down to temperatures of -40 °C for transportation. Therefore the understanding of seal behavior is of high importance.
In this paper we focus on the behavior of elastomer seals at low temperatures with regard to poten-tial decrease of leak-tightness. In addition, changes in material properties due to aging effects over long periods of time and their influence on the seal performance is investigated.
It is known that material properties of rubbers are strongly temperature dependent. At low tempera-tures this is caused by the rubber-glass transition (abbr. glass transition). During continuous cool-ing, the material changes from rubber-like entropy-elastic to stiff energy-elastic behavior, that al-lows nearly no strain or retraction due to the glass transition. Hence rubbers are normally used above their glass transition but the minimum working temperature limit is not defined precisely.
Aging of elastomer seals is important, as possible dynamic loads may have to be considered during the whole interim storage period (so far approved in Germany for up to 40 years) and for transporta-tion after storage.
For the investigations, fluorocarbon (FKM) and ethylene-propylene-diene (EPDM) rubbers were selected as they are often used in radioactive waste containers. Some materials were purchased from a commercial seal producer and some materials were compounded and cured at BAM. The elastom-ers where studied by several thermo-analytical methods and compression set to characterize the material behavior at low temperatures. Additionally component tests were performed to determine the breakdown temperature of the sealing function of complete elastomer O-rings.
The Bundesanstalt für Materialforschung und –Prüfung (BAM) runs an investigation program on the long-term behavior of multi-component metal seals. Such seals are used in a wide area of applications including transport and storage casks for spent nuclear fuel and high level radioactive waste.
The seal function is mainly based on the compression of the inner helical spring, which generates the necessary seal force to keep the sealing surfaces in close contact. This in turn leads to a plastic deformation of the outer jacket of the seal, comprised of highly ductile aluminum or silver that adapts to the sealing surfaces of cask body and bolted lid, thus providing high Level leak tightness. In Germany, those casks are licensed for Interim storage periods of up to 40 years or more if extended Interim storage would become necessary before a final repository is available. Thus, the sealing performance has to be evaluated, including factors like elevated temperature due to decay heat or mechanical loads due to transport under normal as well as accident conditions. Long-term investigations at BAM have been running over the last nine years to identify and evaluate the seal performance by measuring the remaining seal force, the useable resilience and the leakage rate after various time intervals at temperatures ranging from room temperature up
to 150 °C. It was found that the seal force and useable resilience decrease with time and temperature, caused by creep deformation of the outer jacket. In order to obtain an analytical description for the seal behavior and to achieve more information on the material behavior under application conditions a comprehensive investigation program with Focus on aluminum as outer jacket material was launched. The program includes material investigations such as compression and tension creep tests with representative basic materials. An additional test setup allows for the continuous measurement of the remaining seal force at temperatures of up to 150 °C. Furthermore, seal segments are compressed and stored in heating chambers, thus producing segments at different stages of the aging process. The segments are investigated regarding the development of the contact area width, jacket thickness and microstructural changes. This data will be used to develop material models and an analytical description of the time and temperature dependent long-term sealing behavior. This paper explains the current status of gained test results and modelling approaches and closes with an outlook to the future Project plans.
In Germany, for the transport and storage of spent nuclear fuel, casks with double closure lid systems are used, which are equipped with Helicoflex® metal seals. The original interim storage period for these casks was planned to be 40 years. However, recent political developments indicate that a storage time of more than 80 years might be necessary. Therefore, the current storage licenses have to be renewed in due course, which requires extended knowledge of the long-term behaviour of all cask components.
At the Bundesanstalt für Materialforschung und -prüfung (BAM), metal seals have been investigated after ageing at temperatures between room temperature and 150 °C for up to 7 years. At regular intervals the seals have been tested for leakage rate as well as for their mechanical behaviour. From these tests pronounced influence of ageing could be shown which results in the decrease of the seal force and the useable resilience. The main reason attributed to these performance changes is the increasing permanent deformation of the outer layer of the seal due to creep. So far, an explicit analytical description of the long-term behaviour that could be used for predictions exceeding the tested ageing times and temperatures was not suitable.
In addition to the component tests that were already conducted, a comprehensive investigation programme has been started to describe the mechanical behaviour of the individual seal components.
The individual seal components are tested regarding different material characteristics including creep, deformation and microstructural properties. The comparability of the material characterization is ensured by comparing the basic raw material of the specimen with the actual component material.
Furthermore, detailed investigations of the time and temperature dependent deformation behaviour are realized by ageing seal segments at multiple temperatures for different periods of time and employing standardized tests.
By analysing the acquired data in combination with the ongoing long-term component tests a dataset for analytical prediction of the long-term behaviour of the seals is intended for an extended period of time under a certain temperature regime.
In Germany, spent nuclear fuel and high active waste from reprocessing is stored in transport and storage containers with double lid systems that are equipped with metal seals completing the primary sealing barrier. The tasks of the Bundesanstalt für Materialforschung und -prüfung (BAM) within the interim storage licensing procedures ruled by the German Atomic Energy Act include the long-term safety evaluation of the container design regarding the permanently safe enclosure of the inventory. In order to generate a knowledge base for the safety evaluation, research regarding the long-term behavior of the critical components is performed. So far, the containers are licensed for an interim storage period of 40 years. However, due to significant delays in establishing a final repository, the required time span for interim storage is expected to increase significantly. Thus, a widespread investigation program is run to gain systematic data on the long-term behavior of the seals and to develop prediction models.
Long-term seal investigations consider the development of their restoring seal force, their useable resilience and their achievable leakage rate caused by aging at temperatures ranging from room temperature up to 150 °C. This year, the total time span of the tests reaches 10 years. Furthermore, seal segments are aged at the selected temperatures for up to 300 days. From these segments additional information on the sealing behavior, changes of the seal contact and the material behavior is gained.
This contribution deals with the current results of the long-term seal investigations at BAM. Furthermore, insights of the more in-depth component and material investigations of the metal seals with focus on the seal contact development are discussed and the ongoing work aiming for an analytical description of the thermo-mechanical aging effects on metal seals are presented.
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.
Elastomers show a high versatility which makes them ideal materials for sealing applications in various fields. Especially under changing application conditions the high recovery potential of this class of material is beneficial to compensate temperature or pressure fluctuation, and geometrical changes resulting from mechanical loads in e.g. accident conditions. Out of these reasons elastomers are also used in containers for low and intermediate level radioactive waste and for spent fuel transportation casks. In casks designed for low and intermediate level waste elastomer seals can act as primary seal responsible for the containment function whereas in spent fuel storage and transportation casks (dual purpose casks (DPC)) elastomer seals are used as auxiliary seals to allow leakage rate measurements of metal barrier seals. An inherent prerequisite for this kind of application is the Long time-scale of operation without or with limited possibility of seal replacement. In Germany an interim storage license for DPC`s is typically issued for 40 years, a timeframe which might increase in the future due to challenges of the final repository siting procedure. For low and intermediate level waste, also long time periods are required before final disposal can be achieved. Therefore, the performance of elastomer seals over extended time periods is, as for other applications, of high importance.
A typical approach to ensure long-term functionality is to perform accelerated aging tests to calculate an estimated lifetime by assuming e.g. Arrhenius like equations for the timetemperature relationship. This approach requires a suitable end of life criterion considering the application of interest. This often can represent a challenge on its own.
As BAM is involved in most of the cask licensing procedures and especially responsible for the evaluation of cask-related long-term safety issues we initiated several test programs for investigating the behavior of elastomer seals. Experiments concerning the low temperature performance down to -40 °C and the influence of gamma irradiation have been started first.
Currently the thermal aging behavior of elastomer seals, which is the topic of this contribution, is examined. For our aging investigations we use a broad approach to first determine the property changes in different elastomer materials due to thermo-oxidative aging at elevated temperatures and secondly, we test how the typical methods of lifetime extrapolation can be applied to these results. This Approach enables us to detect and exclude undesired side effects which very often influence lifetime estimations. In this contribution, our recent results are extended. The results show that lifetime estimation based on single material properties can be misleading and therefore a combination of several methods is recommended.
In Germany spent nuclear fuel is stored and transported in casks that possess bolted lid systems equipped with double jacket metal seals of Helicoflex® type in order to enable safe enclosure of the radioactive inventory. A dry interim storage period of 40 years was planned and the casks are licensed for that time span. However, due to political reasons and delayed disposal projects this time span is expected to increase significantly. It is therefore necessary to evaluate the long-term sealing behavior of the casks exceeding 40 years of operating time. In this paper, we discuss our approach to investigate the aging behavior of such metal seals, and to predict the long-term sealing behavior.
Accelerated aging component tests with seals are performed at temperatures up to 150°C to investigate the course of seal force, useable resilience and leakage rate. Even though these tests have already shown a significant decrease in seal force and useable resilience after aging times of less than 1 year, for most seals the leakage rate stayed better than the specified value. However, observation of the leakage rate has to be continued to determine the aging effect on the seal properties. As a main influence on the seal long-term behavior the increasing permanent deformation of the outer jacket has been recognized. It is made of aluminum or silver which means that the temperatures that are relevant for the aforementioned application in casks lead to creep deformation.
For further investigation of seal behavior a comprehensive research program concerning the material characteristics and aging behavior of the individual seal components, with an emphasis on the outer jacket material, has been launched. The aim is to get a better understanding of the time and temperature dependent creep mechanisms and deformation. The examinations are separated into tests on seal segments and tests on standardized specimen.
In order to be able to study the aging process, seal segments are compressed in flanges and stored at temperatures ranging from room temperature to 150°C. After defined time intervals the segments are analyzed by using standardized tests including structure investigation and creep tests. The results are compared with the behavior of the specimen basic raw material. This is done to ensure comparability of the thin jacket material and additional sheet material used for standardized tests. Thus, the prediction of component changes after different aging times at different temperatures can be made. As a result forecasts on the long term seal behavior are intended.
It is important to evaluate the effect of thermal ageing on the sealing performance of metal gaskets under high temperature for long-term usage. Therefore, in order to gain representative data for this kind of metal-sealed lid System, BAM is currently performing laboratory tests of different gasket types with aluminum and silver jackets at three different temperatures under static conditions up to four years so far, using test flanges for gaskets with full scale cross section diameter but much smaller outer diameter. On the other hand, in order to investigate the applicability of the numerical methodology to evaluate the long-term behavior of the metal gaskets, such as a correlation between seal pressure force and holding time, CRIEPI is developing a modeling method including material tests (tensile and creep tests) at high temperature. In this paper, the applicability of the finite element method (ABAQUS) to predict the recovery displacement and residual seal pressure force of the gasket complex was verified by comparing the calculated values with BAM’s laboratory test results under the joint research agreement between BAM and CRIEPI.