3.4 Sicherheit von Lagerbehältern
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For many sealing applications rubber O-rings are applied due to their special properties and easy use. In many applications as e.g. in aviation, traffic or process technology the material can be exposed to a variety of conditions during operation as e.g. low temperatures, elevated temperatures and different media. Therefore, it is important to know the material behaviour and the seal performance under those conditions. Additionally ageing of seals is an important factor which has an influence on the long term reliability.
In this contribution we would like to discuss different aspects affecting the seal performance, lifetime estimation and address open questions which should be addressed to come to a deeper understanding of these seals commonly understood as typical machine elements.
Results of running investigations at the Bundesanstalt für Materialforschung und -prüfung (BAM), Division 3.4 “Safety of Storage Containers” are presented and discussed.
Compared with the original time-temperature superposition (TTS) principle, an extended non-linear time-temperature equivalent (TTE) principle is suggested in this work. Results shown us, all application of TTS and most of the TTE parameters, for example, the Orr-Sherby-Dorn, Manson-Succop, Larson-Müller, Manson-Haferd, Mendelson-Roberts-Manson parameters and the more general ‘single metamodel’ from Haque–Stewart are special cases of the new suggested principle. The merits of this new method are discussed.
The excellent mechanical properties of elastomer seals at a wide range of temperatures as well as their high versatility and recovery potential under several load conditions make these materials well suitable for the application in containers designed for transport and disposal of negligible heat generating radioactive 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 which prohibit an avoidable cask handling. An extensive knowledge of the change of the elastomer properties during aging and the availability of reliable end-of-lifetime criteria to guarantee the safe enclosure of the radioactive material for the required time are 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 already been made and are still ongoing to scientifically support this task.
Among other representative types of elastomers, specimen made from ethylene propylene diene rubber (EPDM) were tested before, during and after aging to capture the with respect to application most important of their complex mechanical properties.
Exemplary results of these investigations were used to calibrate material models implemented in the commercial finite element software ABAQUS/Standard®. The finite element model already presented in previous works uses a sequential temperature displacement coupling. The calculated compression stress relaxation (CSR) and compression set (CS) values do satisfactorily match the experimental results.
In many investigations performed at BAM both values (CSR and CS) were identified as key indicators of elastomer’s long-term performance.
However, the possibility to correlate these equivalent indicators with performance values such as tightness and leakage rate, measurable in the mounted state, is an important goal of our future work. In the presented study the ABAQUS® feature of “pressure penetration” is introduced in the suggested finite element model for this purpose. It provides the possibility to simulate the penetration of a gas into a possible gap between flange and O-ring causing an opening of a leakage path. Three dimensional and axis-symmetric finite element models were generated to represent flat and grooved flanges of different dimensions.
The sensitivity of the feature to several input parameters is investigated and the observed behavior of the O-ring is correlated with the results of performed leakage tests.
The excellent mechanical properties of elastomer seals at a wide range of temperatures as well as their high versatility and recovery potential under several load conditions make these materials well suitable for the application in containers designed for transport and disposal of negligible heat generating radioactive 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 which prohibit an avoidable cask handling. An extensive knowledge of the change of the elastomer properties during aging and the availability of reliable end-of-lifetime criteria to guarantee the safe enclosure of the radioactive material for the required time are 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 already been made and are still ongoing to scientifically support this task.
Among other representative types of elastomers, specimen made from ethylene propylene diene rubber (EPDM) were tested before, during and after aging to capture the with respect to application most important of their complex mechanical properties.
Exemplary results of these investigations were used to calibrate material models implemented in the commercial finite element software ABAQUS/Standard®. The finite element model already presented in previous works uses a sequential temperature displacement coupling. The calculated compression stress relaxation (CSR) and compression set (CS) values do satisfactorily match the experimental results.
In many investigations performed at BAM both values (CSR and CS) were identified as key indicators of elastomer’s long-term performance.
However, the possibility to correlate these equivalent indicators with performance values such as tightness and leakage rate, measurable in the mounted state, is an important goal of our future work. In the presented study the ABAQUS® feature of “pressure penetration” is introduced in the suggested finite element model for this purpose. It provides the possibility to simulate the penetration of a gas into a possible gap between flange and O-ring causing an opening of a leakage path. Three dimensional and axis-symmetric finite element models were generated to represent flat and grooved flanges of different dimensions.
The sensitivity of the feature to several input parameters is investigated and the observed behavior of the O-ring is correlated with the results of performed leakage tests.
Im Forschungsvorhaben KoBrA werden Anforderungen an Endlagerbehälter für hochradioaktive Abfälle erstmals systematisch und unter Berücksichtigung des internationalen Standes von Wissenschaft und Technik hergeleitet. Parallel werden die behälterrelevanten Randbedingungen und Beanspruchungsgrößen für Endlagerbehälter in den drei potenziellen Wirtsgesteinen Steinsalz, Tonstein und Kristallingestein ermittelt. Darauf aufbauend werden die zur Erfüllung der Anforderungen unter den gegebenen Einwirkungen und Beanspruchungen notwendigen Behälterfunktionen bestimmt und erste Konzeptideen für zukünftige Endlagerbehälter diskutiert bzw. entwickelt.
In this presentation, the history and actual situation of the German nuclear waste management policy of spent fuel (SF) and high-level waste (HLW) is reported. Beside the description of the restructuration of responsible authorities and organizations in Germany, the principle of dry interim storage of SF and HLW in dual purpose casks is explained. The need for extended interim storage towards disposal implicates additional challenges for the nuclear waste management strategy in Germany. (Extended)interim storage, subsequent transportation, and final disposal are closely linked. Integrated approaches concerning waste package designs and operations are supposed to be beneficial for the establishment of efficient long-term SF and HLW management strategies. Knowledge management and staff recruiting, education and training during phase out and beyond nuclear power plant operation are major issues.
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.
In this work we present results for three typical sealing materials (EPDM, HNBR, FKM) after up to 100 days ageing at high temperature under hydrogen. To allow for a better evaluation of the occurring changes the results are compared with effects of ageing in air on the material properties. Despite the common assumption that ageing under hydrogen atmosphere is expected to be less severe for the material in com-parison to air ageing, this is not seen for every material.
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 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.
Der Vortrag illustriert den Stand und die Herausforderungen im Rahmen der Behälter-Bauartprüfungen für das Endlager Konrad. Ausgehend von den Grundlagen der Bauartprüfung und die aktuelle Situation bei den Antragsverfahren werden die fachlichen Begutachtungsschwerpunkte sowie Maßnahmen zur Optimierung Prozess- und Verfahrenssteuerung erläutert.
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 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 measurements across the section of O-rings. Oxidation profiles were determined by applying the model of Wise et al. to the IRHD experimental results.
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.
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.
Determining a suitable and reliable end-of-lifetime criterion for O-ring seals is an important issue for long-term seal applications. Therefore, seal failure of ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) O-rings aged in the compressed state at 125 °C and at 150 °C for up to 1.5 years was analyzed and investigated under static conditions, using both non-lubricated and lubricated seals. Changes of the material properties were analyzed with dynamic-mechanical analysis and permeability experiments. Indenter modulus measurements were used to investigate DLO effects. It became clear that O-rings can remain leak-tight under static conditions even when material properties have already degraded considerably, especially when adhesion effects are encountered. As a feasible and reliable end-of-lifetime criterion for O-ring seals under static conditions should include a safety margin for slight dimensional changes, a modified leakage test involving a small and rapid partial decompression of the seal was introduced that enabled determining a more realistic but still conservative end-of-lifetime criterion for an EPDM seal.
Bei der Auswahl eines Standortes für ein Endlager für hochradioaktive Abfälle (Wärme entwickelnde radioaktive Abfälle und ausgediente Brennelemente) in Deutschland werden auf Grundlage des Standortauswahlgesetzes von 2017 die drei potenziellen Wirtsgesteine Steinsalz, Tonstein und Kristallingestein in Betracht gezogen. Die Auswahl eines konkreten Endlagerstandortes steht dabei in engem Zusammenhang mit den für die jeweiligen Wirtsgesteine geeignet erscheinender Endlagersystemen. Zentraler Bestandteil dieser Endlagersysteme sind die Endlagerbehälter, deren Gestaltung und Auslegung maßgeblich die Randbedingungen für die Transport- und Einlagerungstechnik vorgibt und zudem die Nachweise zur Betriebs- und Langzeitsicherheit entscheidend prägt. Daher ist es für die Behälterentwicklung erforderlich, die an Endlagerbehälter zu stellenden Anforderungen systematisch herzuleiten. Auf dieser Grundlage können anforderungsgerechte Endlagerbehälter zielgerichtet für die jeweils zu untersuchenden Endlagerstandorte in unterschiedlichen Wirtsgesteinen entwickelt werden.
Dieser Beitrag fasst die wesentlichen Ergebnisse des von BAM und BGE TECHNOLOGY GmbH im Zeitraum 2017 bis 2020 bearbeiteten und vom Bundesministerium für Wirtschaft und Energie (BMWi) geförderten Forschungsvorhabens KoBrA zusammen, die in einem Abschlussbericht und vier zugehörigen Teilberichten umfassend dokumentiert sind.
Bei der Auswahl eines Standortes für ein Endlager für hochradioaktive Abfälle (Wärme entwickelnde radioaktive Abfälle und ausgediente Brennelemente) in Deutschland werden auf Grundlage des Standortauswahlgesetzes von 2017 die drei potenziellen Wirtsgesteine Steinsalz, Tonstein und Kristallingestein in Betracht gezogen. Die Auswahl eines konkreten Endlagerstandortes steht dabei in engem Zusammenhang mit den für die jeweiligen Wirtsgesteine geeignet erscheinender Endlagersystemen. Zentraler Bestandteil dieser Endlagersysteme sind die Endlagerbehälter, deren Gestaltung und Auslegung maßgeblich die Randbedingungen für die Transport- und Einlagerungstechnik vorgibt und zudem die Nachweise zur Betriebs- und Langzeitsicherheit entscheidend prägt. Daher ist es für die Behälterentwicklung erforderlich, die an Endlagerbehälter zu stellenden Anforderungen systematisch herzuleiten. Auf dieser Grundlage können anforderungsgerechte Endlagerbehälter zielgerichtet für die jeweils zu untersuchenden Endlagerstandorte in unterschiedlichen Wirtsgesteinen entwickelt werden.
Dieser Beitrag fasst die wesentlichen Ergebnisse des von BAM und BGE TECHNOLOGY GmbH im Zeitraum 2017 bis 2020 bearbeiteten und vom Bundesministerium für Wirtschaft und Energie (BMWi) geförderten Forschungsvorhabens KoBrA zusammen, die in einem Abschlussbericht und vier zugehörigen Teilberichten umfassend dokumentiert sind.
The long-term sealing behavior of metal seals, assembled in the lid system of casks for transportation and storage of radioactive materials, has been investigated. For that purpose, phenomenological models describing the time- and temperature dependent behavior have been introduced by BAM since 2016, e.g. by using the time-temperature superposition principle. Results have shown that these models describe the relaxation behavior adequately and are suitable for extrapolations. In this work, the applicability of these models is checked by analyzing the necessary scope of tests, which must be carried out to get sufficient information about the long-term behavior of metal seals based on short-term tests.
The paper focuses on the preliminary determination of the bar length and the experimental performance of the 1m-puncture test in compliance with the guidelines of the IAEA under the given boundary conditions. Following aspects have to be considered concerning the determination of an appropriate length of the bar to obtain maximum damage to the specimen: the design of the package, its drop orientation and the impact point as well as a pre-damage of the package resulting from a previous 9m drop test. According to the Regulations the minimum length of the bar is defined as 20 cm, which has to be adapted to the outer surface of the package, especially to packages with large impact limiters.
In this context, two main aspects are important: First, the realisation of a maximum puncture load applied to the package while no other component of the package has contact with the impact target. Second, the reduction of the risk of buckling and maximising the stiffness of the bar by a length which is as short as possible. In order to optimize the bar length, finite element calculations are often done by simulating the global behaviour of the package during the puncture test. The evaluation of the conducted puncture test regarding IAEA compliance is done by analysing deceleration measurements. The paper presents a possible approach to determine the length of the bar and the construction of a form-fitted connection to the target showing various examples of puncture tests.
Abgeleitet aus den in Teil 3 der „Systemanalyse Konrad“ definierten Lastannahmen für den „untertägigen Anlagenbereich“ müssen Verpackungen, die der Abfallbehälterklasse I (ABK I) mit dem zusätzlichen Merkmal der störfallfesten (sf) Verpackung oder der ABK II zugeordnet werden, gemäß den „Endlagerungsbedingungen, Endlager Konrad“, nach einem „…Fall aus 5 m Höhe auf eine unnachgiebige Unterlage…“ bestimmten Anforderungen genügen.
Eine nähere Beschreibung der Prüfeinrichtung sowie die Definition detaillierter Anforderungen an das Aufprallfundament erfolgen in der „Produktkontrolle, Endlager Konrad“: „Die Prüfeinrichtung, bestehend aus einer Hebeeinrichtung und einem Fundament ist so zu gestalten, dass der Fall des Prüfmusters in jeder beliebigen Aufprallorientierung aus 5 m Höhe, gemessen von der Unterkante des Prüfmusters zum Fundament, möglich ist. Das Fundament ist aus Beton der Mindestgüte B 35 herzustellen und muss so beschaffen und verankert sein, dass keine Zerstörung in zahlreiche kleinere Bruchstücke erfolgt und keine horizontale Verschiebung des Fundamentes oder seiner Teile auftritt. Maße und Masse des Fundaments müssen so beschaffen sein, dass der Einfluss des Baugrundes unter dem Fundament auf den Aufprallwiderstand vernachlässigbar ist.“
Im Rahmen der Bauartprüfung für Behälter zur Endlagerung radioaktiver Abfälle mit zu vernachlässigender Wärmeentwicklung im Endlager Konrad wird die Bundesanstalt für Materialforschung und -prüfung (BAM) von der Bundesgesellschaft für Endlagerung (BGE) als Sachverständige hinzugezogen und ist in der Regel auch mit der Durchführung der notwendigen Fallprüfungen auf dem Testgelände Technische Sicherheit der BAM (BAM TTS) einschließlich der Bereitstellung der benötigten Prüfeinrichtungen sowie des regelkonformen Aufprallfundamentes beauftragt. Der Prozess, beginnend von der Gewährleistung einer von allen Beteiligten als gültig bewerteten Fallprüfung bis hin zur abschließenden Bewertung und Interpretation der Versuchsergebnisse erfordert dabei u.a. eine sorgfältige Planung und Fertigung der für das Endlager Konrad repräsentativen Fundamentplatte und deren Ankopplung an einen Untergrund im Sinne der „Regulations for the Safe Transport of Radioactive Material“ wie er zum Beispiel auf den beiden Fallversuchsanlagen des BAM Testgelände Technische Sicherheit (BAM TTS) zur Verfügung steht.
Um diese Anforderungen zu erfüllen muss zum einen der Erhalt der Integrität der Fundamentplatte beim Fallversuch durch eine geeignete Auswahl und kraftflussgerechte Anordnung der Stabstahl-Bewehrung sowie deren Verankerung garantiert und damit eine unzulässige Energieaufnahme durch Risse sowie eine unzulässige „… Zerstörung (der Fundamentplatte) in zahlreiche kleinere Bruchstücke …“, wie in [3] gefordert, verhindert werden. Außerdem ist zu gewährleisten, dass der Beton die Anforderungen an die vorgeschriebene Mindestgüte bzw. -festigkeit zum Zeitpunkt der Fallprüfung erfüllt, die Betonfestigkeit jedoch zur Vermeidung unnötig verschärfter Prüfrandbedingungen eine zu definierende Obergrenze nicht überschreitet.
Ausgehend von dem 2009 im Rahmen der KONTEC veröffentlichten Wissensstand präsentiert die BAM im vorliegenden Beitrag die wesentlichen Ergebnisse der zwischenzeitlich bei der Planung und Durchführung von zahlreichen Fallprüfungen gewonnenen Erfahrungen bzgl. Spezifikation und Fertigung der für die Fallversuche notwendigen Fundamentplatten sowie deren Anbindung an den Untergrund. Neben den immer wieder notwendigen Anstrengungen zur Gewährleistung einer im Rahmen der Definition korrekten Betondruckfestigkeit am Tag der Fallprüfung wird die Anpassung der Bewehrungsführung an verschiedene Fallpositionen diskutiert.
Zur Ergänzung des gültigen Regelwerks entsteht parallel zu diesem Dokument eine Fachnotiz, in der BGE und BAM die Vorgaben und Spezifikationen zur regelwerkskonformen Herstellung und Montage von Fundamentplatten für Fallprüfungen im Rahmen der Behälterbauartprüfungsverfahren für das Endlager Konrad zusammenfassen und veröffentlichen werden. Diese Fachnotiz soll detaillierte Vorgaben und Spezifikationen zur Planung, Beauftragung, Herstellung und abschließenden Prüfung eines solchen Fundamentes und dessen Anbindung an den Untergrund enthalten. Damit soll allen beteiligten Organisationen wie Antragstellern, Sachverständigen und BGE ein langfristig verlässlicher Leitfaden im Hinblick auf die Durchführung anforderungsgerechter Fallprüfungen unter definierten und reproduzierbaren Randbedingungen im Rahmen der Bauartprüfungsverfahren für das Endlager Konrad an die Hand gegeben werden.
Abgeleitet aus den in Teil 3 der „Systemanalyse Konrad“ [1] definierten Lastannahmen für den „untertägigen Anlagenbereich“ müssen Verpackungen, die der Abfallbehälterklasse I (ABK I) mit dem zusätzlichen Merkmal der störfallfesten (sf) Verpackung oder der ABK II zugeordnet werden, gemäß den „Endlagerungsbedingungen, Endlager Konrad“ [2], nach einem „…Fall aus 5 m Höhe auf eine unnachgiebige Unterlage…“ bestimmten Anforderungen genügen.
Eine nähere Beschreibung der Prüfeinrichtung sowie die Definition detaillierter Anforderungen an das Aufprallfundament erfolgen in der „Produktkontrolle, Endlager Konrad“ [3]: „Die Prüfeinrichtung, bestehend aus einer Hebeeinrichtung und einem Fundament ist so zu gestalten, dass der Fall des Prüfmusters in jeder beliebigen Aufprallorientierung aus 5 m Höhe, gemessen von der Unterkante des Prüfmusters zum Fundament, möglich ist. Das Fundament ist aus Beton der Mindestgüte B 35 herzustellen und muss so beschaffen und verankert sein, dass keine Zerstörung in zahlreiche kleinere Bruchstücke erfolgt und keine horizontale Verschiebung des Fundamentes oder seiner Teile auftritt. Maße und Masse des Fundaments müssen so beschaffen sein, dass der Einfluss des Baugrundes unter dem Fundament auf den Aufprallwiderstand vernachlässigbar ist.“
Im Rahmen der Bauartprüfung für Behälter zur Endlagerung radioaktiver Abfälle mit zu vernachlässigender Wärmeentwicklung im Endlager Konrad wird die Bundesanstalt für Materialforschung und -prüfung (BAM) von der Bundesgesellschaft für Endlagerung (BGE) als Sachverständige hinzugezogen und ist in der Regel auch mit der Durchführung der notwendigen Fallprüfungen auf dem Testgelände Technische Sicherheit der BAM (BAM TTS) einschließlich der Bereitstellung der benötigten Prüfeinrichtungen sowie des regelkonformen Aufprallfundamentes beauftragt. Der Prozess, beginnend von der Gewährleistung einer von allen Beteiligten als gültig bewerteten Fallprüfung bis hin zur abschließenden Bewertung und Interpretation der Versuchsergebnisse erfordert dabei u.a. eine sorgfältige Planung und Fertigung der für das Endlager Konrad repräsentativen Fundamentplatte und deren Ankopplung an einen Untergrund im Sinne der „Regulations for the Safe Transport of Radioactive Material“ [4], [5] wie er zum Beispiel auf den beiden Fallversuchsanlagen des BAM Testgelände Technische Sicherheit (BAM TTS) zur Verfügung steht.
Um diese Anforderungen zu erfüllen muss zum einen der Erhalt der Integrität der Fundamentplatte beim Fallversuch durch eine geeignete Auswahl und kraftflussgerechte Anordnung der Stabstahl-Bewehrung sowie deren Verankerung garantiert und damit eine unzulässige Energieaufnahme durch Risse sowie eine unzulässige „… Zerstörung (der Fundamentplatte) in zahlreiche kleinere Bruchstücke …“, wie in [3] gefordert, verhindert werden. Außerdem ist zu gewährleisten, dass der Beton die Anforderungen an die vorgeschriebene Mindestgüte bzw. -festigkeit zum Zeitpunkt der Fallprüfung erfüllt, die Betonfestigkeit jedoch zur Vermeidung unnötig verschärfter Prüfrandbedingungen eine zu definierende Obergrenze nicht überschreitet.
Ausgehend von dem 2009 im Rahmen der KONTEC veröffentlichten Wissensstand [6] präsentiert die BAM im vorliegenden Beitrag die wesentlichen Ergebnisse der zwischenzeitlich bei der Planung und Durchführung von zahlreichen Fallprüfungen gewonnenen Erfahrungen bzgl. Spezifikation und Fertigung der für die Fallversuche notwendigen Fundamentplatten sowie deren Anbindung an den Untergrund. Neben den immer wieder notwendigen Anstrengungen zur Gewährleistung einer im Rahmen der Definition korrekten Betondruckfestigkeit am Tag der Fallprüfung wird die Anpassung der Bewehrungsführung an verschiedene Fallpositionen diskutiert.
Zur Ergänzung des gültigen Regelwerks entsteht parallel zu diesem Dokument eine Fachnotiz, in der BGE und BAM die Vorgaben und Spezifikationen zur regelwerkskonformen Herstellung und Montage von Fundamentplatten für Fallprüfungen im Rahmen der Behälterbauartprüfungsverfahren für das Endlager Konrad zusammenfassen und veröffentlichen werden. Diese Fachnotiz soll detaillierte Vorgaben und Spezifikationen zur Planung, Beauftragung, Herstellung und abschließenden Prüfung eines solchen Fundamentes und dessen Anbindung an den Untergrund enthalten. Damit soll allen beteiligten Organisationen wie Antragstellern, Sachverständigen und BGE ein langfristig verlässlicher Leitfaden im Hinblick auf die Durchführung anforderungsgerechter Fallprüfungen unter definierten und reproduzierbaren Randbedingungen im Rahmen der Bauartprüfungsverfahren für das Endlager Konrad an die Hand gegeben werden.
Transport and storage of radioactive materials are performed in countries with policy of either closed or open nuclear fuel cycle. The related technologies have been established by accumulation of experiences and researches including demonstrative tests using full scale or scale models and analyses. Those are essential before commercialization, but are often costly and time consuming. Such demonstrative works should not be repeated meaninglessly, but can be shared through this kind of book and used by readers and the future generations to advance the technology effectively. This book systematically provides findings from lots of valuable researches on safety of transport and storage of radioactive materials under normal and accident conditions that have an impact on basis of safe regulations, designs, and operations.
The general aim of this research is the development of methods for predicting mechanical behavior and identification of limiting conditions to prevent brittle failure of high-burnup (HBU) pressure water reactor (PWR) fuel cladding alloys. A finite element (FE) model of the ring compression test (RCT) was created to analyze the failure behavior of zirconium-based alloys with radial hydrides during the RCT. An elastic-plastic material model describes the zirconium alloy. The stress-strain curve needed for the elastic-plastic material model was derived by inverse finite element analyses. Cohesive zone modeling is used to reproduce sudden load drops during RCT loading. Based on the failure mechanism in non-irradiated ZIRLO® claddings, a micro-mechanical model was developed that distinguishes between brittle failure along hydrides and ductile failure of the zirconium matrix. Two different cohesive laws representing these types of failure are present in the same cohesive interface. The key differences between these constitutive laws are the cohesive strength, the stress at which damage initiates, and the cohesive energy, which is the damage energy dissipated by the cohesive zone. Statistically generated matrix-hydride distributions were mapped onto the cohesive elements and simulations with focus on the first load drop were performed. Computational results are in good agreement with the RCT results conducted on high-burnup M5® samples. It could be shown that crack initiation and propagation strongly depend on the specific configuration of hydrides and matrix material in the fracture area.
The general aim of this research is the development of methods for predicting mechanical behavior and identification of limiting conditions to prevent brittle failure of high-burnup (HBU) pressure water reactor (PWR) fuel cladding alloys. A finite element (FE) model of the ring compression test (RCT) was created to analyze the failure behavior of zirconium-based alloys with radial hydrides during the RCT. An elastic-plastic material model describes the zirconium alloy. The stress-strain curve needed for the elastic-plastic material model was derived by inverse finite element analyses. Cohesive zone modeling is used to reproduce sudden load drops during RCT loading. Based on the failure mechanism in non-irradiated ZIRLO® claddings, a micro-mechanical model was developed that distinguishes between brittle failure along hydrides and ductile failure of the zirconium matrix. Two different cohesive laws representing these types of failure are present in the same cohesive interface. The key differences between these constitutive laws are the cohesive strength, the stress at which damage initiates, and the cohesive energy, which is the damage energy dissipated by the cohesive zone. Statistically generated matrix-hydride distributions were mapped onto the cohesive elements and simulations with focus on the first load drop were performed. Computational results are in good agreement with the RCT results conducted on high-burnup M5® samples. It could be shown that crack initiation and propagation strongly depend on the specific configuration of hydrides and matrix material in the fracture area.
Understanding the mechanical behaviour of fuel cladding is decisive for an extended dry interim storage period beyond the currently licensed period of 40 years in Germany and subsequent handling, transport, and disposal considering normal and accidental conditions. Since fuel cladding acts as the first barrier for spent fuel, its integrity should be demonstrated to enable safe and secure discharge and handling options after interim storage and subsequent transportation in preparation of final disposal. An established method for characterizing cladding material is the Ring Compression Test (RCT), where a small, cylindrical sample of the cladding tube is subjected to a compressive load. This test is a laboratory representation of a fuel rod load case and has shown a high susceptibility to failure under the occurrence of radial hydrides. Certain hydride morphologies and low temperature might even lead to brittle fracture at very small loads. The BAM research project BRUZL (Fracture mechanical analysis of spent fuel claddings during long-term dry interim storage) aims to study the results of quasi-static RCTs and to establish numerical models to gain an in-depth understanding of the stress state during the experiments. Using a fracture mechanical approach, cases of sudden failure during the RCT procedure are used to characterize the material behaviour and establish a failure criterion. As an integral part of the project BRUZL, quasi-static Ring Compression Tests have been performed to identify all experimental details, which might be helpful for numerical modelling. Unirradiated samples of the cladding material ZIRLO® have been subjected to hydrogen charging and a thermo-mechanical treatment for radial hydride reorientation. Sample preparation, testing procedures, and analysis results are presented. A numerical model has been established and an elastic-plastic material model was derived from as-received RCT samples by inverse finite element analyses. Cohesive zone modelling has been implemented to reproduce sudden load drops during RCT. The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) under contract no. 1501561.
In order to demonstrate the safety of both spent fuel and the storage system, a good understanding of the processes that might cause deterioration is required. The International Atomic Energy Agency (IAEA) continued the coordinated research on Spent Fuel Performance Assessment and Research (SPAR) into a fourth phase starting in 2015 to evaluate fuel and materials performance under wet and dry storage and to assess the impact of interim storage on associated spent fuel management activities (such as handling and transport). BAM’s understanding and experiences in the field of long-term storage of spent fuel and especially spent fuel characterization are presented to assist the preparation of the final project report.
The current research project BRUZL (Fracture mechanical analysis of spent fuel claddings during long-term dry interim storage) is presented. General aim of the project is the development of methods for risk assessment and identification of boundary conditions to prevent potential sudden brittle failure of spent fuel claddings at small deformation during long-term dry interim storage and subsequent transportation. The project is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) under contract no. 1501561.
The evaluation of cladding integrity is a major issue to be demonstrated in Germany for extended interim storage periods up to 100 years and subsequent transportation considering operational and accidental conditions with respect to reactor operation, cask drying and dry interim storage. The chemical reaction between the zirconium fuel cladding and the cooling water in water-cooled reactors produces hydrogen and zirconium oxide. Hydrogen diffuses into the cladding and precipitates as zirconium hydrides when the solubility limit is reached, preferably oriented in hoop direction. At high temperatures during vacuum drying procedures, the hydrides can dissolve. Over a succeeding period of slow cooling with existing hoop stress the hydrides precipitate again, but partly reoriented along the radial direction of the cladding. This change of microstructure in combination with a decreasing temperature (0.5...2 K/year) during (extended) interim storage and additional mechanical load by handling procedures or under accident conditions could lead to a potential cladding embrittlement and consequently increased failure probability. The current research project BRUZL (Fracture mechanical analysis of spent fuel claddings under long-term dry interim storage conditions) has been launched by BAM to investigate potential sudden brittle failure of spent fuel claddings at small deformation under long-term dry interim storage conditions and is presented.
The evaluation of cladding integrity is a major issue to be demonstrated in Germany for extended interim storage periods up to 100 years and subsequent transportation considering operational and accidental conditions with respect to reactor operation, cask drying and dry interim storage. The chemical reaction between the zirconium fuel cladding and the cooling water in water-cooled reactors produces hydrogen and zirconium oxide. Hydrogen diffuses into the cladding and precipitates as zirconium hydrides when the solubility limit is reached, preferably oriented in hoop direction. At high temperatures during vacuum drying procedures, the hydrides can dissolve. Over a succeeding period of slow cooling with existing hoop stress the hydrides precipitate again, but partly reoriented along the radial direction of the cladding. This change of microstructure in combination with a decreasing temperature (0.5...2 K/year) during (extended) interim storage and additional mechanical load by handling procedures or under accident conditions could lead to a potential cladding embrittlement and consequently increased failure probability. The current research project BRUZL (Fracture mechanical analysis of spent fuel claddings under long-term dry interim storage conditions) has been launched by BAM to investigate potential sudden brittle failure of spent fuel claddings at small deformation under long-term dry interim storage conditions and is presented.
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.
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.
Cohesive Zone Modelling Approach on Irradiated Claddings Subjected to Long-Term Dry Interim Storage
(2023)
Long-term dry interim storage may adversely affect the mechanical properties of spent fuel rods, possibly resulting in a reduced resilience during handling or transport after storage. Pre-storage drying and the early stage of interim storage can subject the cladding to higher temperatures and higher pressure induced tensile hoop stresses than those associated with in-reactor operation and pool storage. Under these conditions, radial hydrides may precipitate in zirconium-based alloys (Zircaloy) during slow cooling, which may result in embrittlement of the cladding material and eventually a sudden failure of cladding under additional mechanical loads. Especially long, continuous radial hydride structures and low temperature can cause severe embrittlement of claddings and finally failure by fracture even at small deformations. The focus of the presented research is on the development of appropriate numerical methods for predicting the mechanical behaviour and identification of limiting conditions to prevent brittle fracture of Zircaloy claddings. An iterative inverse analysis method is used for deriving the elastic-plastic material properties in the hoop direction of a ring-shaped sample. A modelling approach based on cohesive zones is explained which can reproduce the propagation of cracks initiated at radial hydrides in the zirconium matrix. The developed methods are applied to defueled samples of cladding alloy ZIRLO®, which were subjected to a thermo-mechanical treatment to reorient existing circumferential hydrides to radial hydrides. A selected sample showing sudden load drops during a quasi-static ring compression test is analysed by means of fracture mechanics for illustrative purposes.
Elastomeric seals are essential components in the infrastructure which prevent leakage of gas and proper function of technical devices and are therefore highly safety relevant. For proper function a remaining resilience and tolerance to pressure changes is required.
The ageing of 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. automotive, piping or container applications, a long lifetime or at least a timely exchange of a seal before occurrence of critical failure is desired.
Hydrogen is a key element for the energy turnaround and therefore the compatibility of seals with hydrogen is essential. In addition to the effect of hydrogen uptake and release, which might also cause destructive effects such as rapid gas decompression, ageing at high temperature under hydrogen atmosphere is of interest too. Most of the existing work is addressing the performance of new materials and comprises only very limited investigations concerning long-term use and the behaviour of aged materials in contact with hydrogen. As ageing can lead to substantial changes of material properties, it must be evaluated whether these changes are beneficial or deteriorating for the component function.
In this work we present and compare results of the characterisation of three sealing materials (EPDM, HNBR, FKM) after ageing at high temperature under hydrogen and air. Despite the common assumption that ageing under hydrogen atmosphere should be less severe for the material in comparison to air ageing, this is not the case for every material.
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.
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.
Component and material investigations on metal seals for high level radioactive waste containers
(2018)
Long-term investigations performed at BAM look to extend the state of knowledge on safety-related components of interim storage containers. Metal seals act as the primary sealing barrier in the bolted double lid closure system of the containers. The behaviour of metal seals has been investigated for ageing times up to 8.5 years and for various temperatures. The main cause for reduction in useable resilience overtime was due to creep deformation of the outer jacket of the seal.
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.
This presentation focusses on the thermal test of a typical storage container design for the German final storage repository KONRAD. The preparation of the container, the conduction of the fire test itself and results like the temperature evolution and as-sumptions for thermal material properties are shown.
As thermal test final storage containers have to be exposed to a fire of 800 °C (1472 °F) for 1 hour according to the final storage conditions provided by the federal com-pany for radioactive waste disposal (BGE). The here discussed cask contains a steel covered concrete layer of a thickness of 100/150 mm. Heating up concrete is a highly nonlinear process since the vaporization of residual water consumes high amounts of thermal energy and produce vast amounts of steam.
The constantly measured temperatures provide the basis to adjust common models of thermal properties for concrete esp. for FEAs. Finally in a later step this will allow predictions of similar designed container with different sizes.
In 2007 the license for the German Konrad repository for low and intermediate level radioactive waste was finally confirmed. Since then, the site is improved, prepared and equipped for the currently planned start of waste package emplacement in 2029. Bundesanstalt fuer Materialforschung und -pruefung is regularly contracted by the Bundesgesellschaft für Endlagerung as the responsible authority for waste product control, waste container design testing, and waste package acceptance. In this context BAM is mainly responsible for design testing of various container types including the evaluation of quality assurance measures for container manufacturing.
In addition to general requirements concerning container design, the casks have to withstand specific mechanical load scenarios. Drop tests from different heights under consideration of the most damaging drop orientation and a stacking test are two examples for necessary safety demonstrations.
If the containers are made from concrete the reinforcement has the function to absorb tensile forces the concrete is not able to withstand by its own. Because of manufacturing imperfections of cylindrical concrete containers manufactured already decades ago, the reinforcement cages are not always exactly, asymmetrically positioned when they were poured. This inspired BAM to perform various ABAQUS® finite-element (FE) simulations by using the example of a simplified cylindrical container design with generic dimensions. On the basis of the calculated tensile and compressive stress distributions suggestions are derived on how a cylindrical concrete container with an imperfectly placed reinforcement cage has to be oriented for a drop or stacking test to ensure the most damaging test scenario.
Suitable containers for receiving high-level radioactive waste are a key elements of every repository system concerning different host rocks like rock salt, claystone and crystalline rock as being considered in Germany. The disposal container design is decisive regarding boundary conditions for the transport, emplacement techniques, and the operational and long-term safety assessment. The systematic derivation of the requirements to be placed on disposal containers forms the basis for a targeted, comprehensible and transparent development of containers that meet the requirements. The presentation summarizes the outcomes of the research project KoBrA.
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.
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.
In Germany spent nuclear fuel and high level radioactive waste is stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. The used metal seals consist of three components as can be seen in the cross-sectional view in Figure 1. The innermost part is a helical spring that is surrounded by an inner jacket made of stainless steel. The outer jacket that is made of a softer material which in case of assembly in the aforementioned storage containers is silver or aluminum (i.e. Al 99.5). During application the seal is compressed and due to the restoring force of the helical spring, the outer jacket is plastically deformed and adapts to the sealing surface. Hence, leakage paths are closed and the sealing function is generated. In Germany the above-mentioned containers are licensed for up to 40 years of interim storage, which in case extended storage becomes necessary before a final repository is available will have to be extended to even longer periods. Therefore, the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads as well.
At Bundesanstalt für Materialforschung und –prüfung (BAM) long-term investigations are being conducted in which seals are assembled in test flanges and aged at temperatures ranging from room temperature to 150°C for accelerated aging. The aged seals are tested semi-annually (after the first 6 months in which the seals are tested more frequently) regarding the sealing performance, the remaining seal force, and the useable resilience upon decompression. Results of these investigations have been published over the past years (e.g. Grelle, Wolff, Probst, Jaunich, & Völzke, 2017; Völzke, Wolff, Probst, Nagelschmidt, & Schulz, 2014). It was found that the seal force and the useable resilience decrease with time and temperature, which is in agreement with the result of other studies (Sassoulas et al., 2006; Wataru et al., 2016) as well. Geometry change of the outer jacket has been identified as the main reason for this seal behavior. At the prevailing operating temperatures and stresses the aluminum is subjected to creep deformation leading to a thinning of the outer jacket. Since the seal groove depth remains unchanged the helical spring expands, which in turn leads to a decrease of the generated spring and seal force.
Although the main reason for the change of seal parameters over time and temperature is known, a detailed characterization of the seal behavior and a reliable prediction of the parameter development for aging times that exceed the experimental time frame have not been possible, yet.
For deeper understanding of the aging processes, an Investigation program, which is covered in this contribution, is conducted at Bundesanstalt für Materialforschung und –prüfung (BAM) that focusses on the behavior of the aluminum jacket and its influence on the long-term sealing performance. The program investigates properties of material samples as well as the behavior of the seal as a component.
Original sheet material of the same aluminum that is used for manufacturing of the seals is investigated in compression creep tests. For this, a DMA (dynamic mechanical analysis) machine is employed (here used for static tests) that allows for a measurement of the specimens deformation under forces of up to 500 N. The advantage of this method is that the original material can be tested in the same shape as used for the seals which is 0.5 mm thick sheet material. For investigation of tensile creep standard specimens are used, that were machined from surrogate material of the same composition and annealing condition.
Furthermore, aluminum seals that are cut into smaller segments are assembled in flanges and placed in heating chambers at temperatures ranging from 23°C to 150°C. After different periods of time from 3 days to 300 days the segments are taken out of the flanges and are investigated, thus giving information on different states of aging. Measurements of the development of the seal contact width and the aluminum jacket thickness are done with an optical microscope. Further investigations on the segments will include metallography and hardness measurements.
From the detailed material and component behavior including the results of the long-term seal force and useable resilience investigations a better understanding of the overall seal behavior can be gained. The aim is to contribute to the development of material models and analytical approaches for the prediction of the sealing behavior in dependence of time and temperature.
In Germany spent nuclear fuel (SNF) and high level radioactive waste (HLW) are stored in interim storage containers with double lid systems. Those lids are equipped with metal seals (e.g. Helicoflex®) that ensure the safe enclosure of the inventory. Being licensed for up to 40 years of interim storage the evaluation of the long-term behavior of the seals is necessary, taking into account storage conditions, decay heat and possible mechanical loads.
This publication summarizes the work carried out during an IAEA coordinated research project (CRP) on safe and reliabe management of spent nuclear fuel. This work contributes to the overall goal of demonstrating the performance of spent nuclear fuel and related storage systems components over long durations, and facilitates the transfer of this knowledge to Member States. The technical areas addressed by the CRP participants were related to potential degradation mechanisms in metal casks and concrete overpacks, such as stress corrosion cracking; long term integrity and performance of the fuel cladding; thermo-mechanical behaviour of the metal seals; and long term gamma and neutron shielding capability. The participants also considered past and ongoing demonstration programmes. For each technical topic addressed the main conclusions have been drawn, including information on the development of specific monitoring and inspection techniques as well as future opportunities for closing relevant data gaps. A major achievement is also the establishment of a worldwide network of experts working on current research projects to demonstrate the long term performance of spent fuel in dry storage systems.
In order to study the time- and temperature-dependent long-term behavior of metal seals, experimental investigations on special metal seals have been carried out at five different temperatures in a temperature range between 20 and 150°C for more than 7 years. Experimental results indicate a noticeable change of relevant sealing properties like seal force and usable resilience depending on time and temperature. In this study, the metal seals are treated as a homogeneous material block so that the identified decrease in seal force can be treated as a material relaxation effect. For the time-dependent behavior of seal force, an enhanced power-law model is introduced for the first time and is compared with the currently used power-law model. Additionally, regarding the influence of temperature, the timetemperature superposition principle is applied to metal seals for the first time with a clearly defined process. Thus, possible mistakes in the application of principle could be avoided. The introduced method is widely available for different applications regarding effects the principle with time and temperature.
Beginning in 1976 the former iron ore mine Konrad was geologically investigated concerning its suitability as a location for a German final repository for low and intermediate level waste. After a comprehensive licensing procedure it was approved and is now being prepared and equipped for the planned start of storage in 2027. At the end of the 1980s, the requirements for waste containers for storage in this repository were defined for the first time on the basis of an incident analysis of the on-site handling procedures, beginning with the unloading of the packages after the arrival at the site up to the final positioning in the underground storage galleries. A 5-meter drop onto the rock ground of the mine was identified as the covering case for high mechanical requirements (ABK II container class). In contrast to the 9-meter drop according to the IAEA Regulations for the Safe Transport of Radioactive Material, the 5-meter drop is performed onto a hard but not essentially unyielding target. The container is typically not protected by an impact limiter. The requirements for a potential test facility are described in the regulations for the Konrad repository. The mechanical strength of the target is defined as a concrete strength equivalent to the identified properties of the rock ground of the mine.
Since 1991 BAM has consistently used precast reinforced concrete slabs as target for drop tests in the framework of licensing procedures as well as in research projects. While the original design fulfilled the requirement for the integrity of the concrete slab in most cases, it failed when drop tests onto an edge of a container were performed. A redesigned concrete slab developed in a research project and suggested as a reference target in 2009 has been successfully used in Konrad licensing procedures since then.
The paper gives a brief overview of the historical development and it describes BAM’s efforts and approaches to continuously guarantee a concrete slab of defined quality and to provide a test setup for valid drop tests from the applicants and the authorities view.
Beginning in 1976 the former iron ore mine Konrad was geologically investigated concerning its suitability as a location for a German final repository for low and intermediate level waste. After a comprehensive licensing procedure it was approved and is now being prepared and equipped for the planned start of storage in 2027. At the end of the 1980s, the requirements for waste containers for storage in this repository were defined for the first time on the basis of an incident analysis of the on-site handling procedures, beginning with the unloading of the packages after the arrival at the site up to the final positioning in the underground storage galleries. A 5-meter drop onto the rock ground of the mine was identified as the covering case for high mechanical requirements (ABK II container class). In contrast to the 9-meter drop according to the IAEA Regulations for the Safe Transport of Radioactive Material, the 5-meter drop is performed onto a hard but not essentially unyielding target. The container is typically not protected by an impact limiter. The requirements for a potential test facility are described in the regulations for the Konrad repository. The mechanical strength of the target is defined as a concrete strength equivalent to the identified properties of the rock ground of the mine.
Since 1991 BAM has consistently used precast reinforced concrete slabs as target for drop tests in the framework of licensing procedures as well as in research projects. While the original design fulfilled the requirement for the integrity of the concrete slab in most cases, it failed when drop tests onto an edge of a container were performed. A redesigned concrete slab developed in a research project and suggested as a reference target in 2009 has been successfully used in Konrad licensing procedures since then.
The paper gives a brief overview of the historical development and it describes BAM’s efforts and approaches to continuously guarantee a concrete slab of defined quality and to provide a test setup for valid drop tests from the applicants and the authorities view.
Beginning in 1976 the former iron ore mine Konrad was geologically investigated concerning its suitability as a location for a German final repository for low and intermediate level waste. After a comprehensive licensing procedure it was approved and is now being prepared and equipped for the planned start of storage in 2027. At the end of the 1980s, the requirements for waste containers for storage in this repository were defined for the first time on the basis of an incident analysis of the on-site handling procedures, beginning with the unloading of the packages after the arrival at the site up to the final positioning in the underground storage galleries. A 5-meter drop onto the rock ground of the mine was identified as the covering case for high mechanical requirements (ABK II container class). In contrast to the 9-meter drop according to the IAEA Regulations for the Safe Transport of Radioactive Material, the 5-meter drop is performed onto a hard but not essentially unyielding target. The container is typically not protected by an impact limiter. The requirements for a potential test facility are described in the regulations for the Konrad repository. The mechanical strength of the target is defined as a concrete strength equivalent to the identified properties of the rock ground of the mine.
Since 1991 BAM has consistently used precast reinforced concrete slabs as target for drop tests in the framework of licensing procedures as well as in research projects. While the original design fulfilled the requirement for the integrity of the concrete slab in most cases, it failed when drop tests onto an edge of a container were performed. A redesigned concrete slab developed in a research project and suggested as a reference target in 2009 has been successfully used in Konrad licensing procedures since then.
The paper gives a brief overview of the historical development and it describes BAM’s efforts and approaches to continuously guarantee a concrete slab of defined quality and to provide a test setup for valid drop tests from the applicants and the authorities view.