The BGZ research mission

The strategy for the responsible and safe disposal of spent nuclear fuel and other heat-generating radioactive waste is stipulated in the German government’s National Waste Management Programme. BGZ Gesellschaft für Zwischenlagerung mbH’s mandate in the national waste management strategy is derived from the Act on the Reorganisation of Responsibility in Nuclear Waste Management.

Abbildung 1: Vereinfachte Darstellung des zeitlichen Ablaufs von der ersten Behälterbeladung bis zur Einlagerung in ein Endlager

BGZ is a company organised in private legal form that is wholly owned by the Federal Government. BGZ was founded to ensure the reliable and safe operation of interim storage facilities for low, intermediate and high-level radioactive waste. Since 1 January 2019, the interim storage facilities for high-level radioactive waste have included not only the Ahaus and Gorleben interim fuel storage facilities but also the Biblis, Brokdorf, Grafenrheinfeld, Grohnde, Gundremmingen, Isar, Krümmel, Lingen, Neckarwestheim, Philippsburg and Unterweser sites1. High-level radioactive waste is stored at interim storage facilities until it is transferred to a repository. The procedural periods for searching and selecting a site for a repository for high-level radioactive waste are established in the German Site Selection Act. The intended and approved interim storage period of up to 40 years will almost certainly not be long enough. It might take longer to commission a repository for the spent nuclear fuel (SNF) and other heat-generating radioactive waste and to clear the storage facilities. As shown in Figure 1, the time limits of the existing storage licences for interim storage facilities will expire2 between 20342 and 2047. As the operator and licence holder, BGZ is required to provide ongoing proof of where the dual-purpose casks remain as well as of compliance with the safety objectives for extended interim storage in accordance with the state of the art in science and technology.

The research programme developed here identifies the research that is required and provides an overview of the BGZ’s research strategy and activities. The research programme is constantly updated and adapted in line with the evolving state of the art in science and technology.

Figure 1 (on the left): Simplified representation of the time sequence from initial cask loading through to emplacement in a repository

Figure 1: Simplified representation of the time sequence from initial cask loading through to emplacement in a repository

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Development of the research programme

The development of the research programme follows a holistic approach to safety objectives. In this context, the safety proofs on which the storage licences and package design approvals are based were systematically and critically questioned with regard to extended interim storage.

The research programme also draws on operational experience with storage buildings, casks and inventories. International exchange and the review of current research findings that define the state of the art in science and technology also play an important role in determining the research needs that are required on extended interim storage (see Figure 2). Effects and possible countermeasures are always key factors to be taken into account when assessing the need for research. The underlying concept of dry interim storage and the associated functions of the casks, the inventory and the storage building, must also be included. The aim is to identify any need for action at an early stage in order to be able to use the remaining time until the application is submitted for corresponding research programmes and thus open up additional options for action if necessary.

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Abbildung 1: Vereinfachte Darstellung des zeitlichen Ablaufs von der ersten Behälterbeladung bis zur Einlagerung in ein Endlager

Figure 2: Procedure for the development of the research programme

National and international cooperation

BGZ’s work on the research tasks presented in this report involves extensive professional exchange at national and international level.

Within the framework of specific research projects, collaboration takes place with partners involved in the manufacture of casks and fuel assemblies as well as partners in research institutes, universities and other relevant companies. BGZ strives to transparently involve all important and relevant partners in the field of nuclear waste management in its research. Collaboration on specific projects is outlined in Chapter below for each project.

As well as collaborating on specific research projects, BGZ also cooperates with strategic partners in the field of nuclear waste management and participates in programmes, organisations and committees. These are presented separately in brief in the following.

EWN Group

EWN Entsorgungswerk für Nuklearanlagen GmbH, which is also a state-owned company that is preparing for extended interim storage, is an important strategic partner for BGZ at the national level. EWN operates the ZLN storage facility near Lubmin, which will be replaced by the ­ESTRAL storage facility by the mid/late 2020s. The Jülicher Entsorgungsgesellschaft für Nuklearanlagen mbH (JEN), which is affiliated with EWN, also operates the AVR storage facility in Jülich.

In addition, the Kerntechnische Entsorgung Karlsruhe GmbH (KTE), which is also affiliated with EWN, has stored nuclear fuel in the form of vitrified radioactive waste from the reprocessing of spent fuel assemblies in the ZLN. The shared interests and key tasks in this context provide the basis for regular mutual exchange. There are also plans for joint research projects with the EWN Group.

Cooperation in DIN standards committees

BGZ sends a permanent member to the working group NA 062-07-54 AA “Criticality safety and decay power” of the DIN Standards Committee Materials Testing (NMP). The working committee draws up and updates the relevant standards and holds important discussions on topics of criticality safety and the decay power of SNF. Cooperation in additional DIN standards committees that are relevant to interim storage is planned.

Cask manufacturers – GNS and Orano NPS

The dual-purpose casks (DPCs) used to store spent fuel assemblies and vitrified waste from reprocessing in BGZ interim storage facilities are from the German-based Gesellschaft für Nuklear-Service mbH (GNS) or from Orano Nuclear Packages and Services (Orano NPS) in France. Both companies have decades of experience in the field of nuclear waste disposal, especially in the development, approval and production of dual-purpose casks for highly radioactive waste. The CASTOR® cask type developed by GNS accounts for more than 90% of the DPCs stored by BGZ; the remaining are TN® type casks from the company Orano NPS. The cask manufacturers GNS and Orano NPS hold the package design approval for DPCs and are thus important partners of BGZ.

Partnerships with Swiss institutions

ZWILAG Zwischenlager Würenlingen AG and BGZ have been in regular contact with each other for many years. In view of the extended interim storage period, shared interests and key tasks, there are plans to extend this mutual exchange to the Swiss nuclear power plant operating companies BKW Energie AG, Kernkraftwerk Gösgen-Däniken AG, Kernkraftwerk Leibstadt AG and Axpo Power AG in the future.

The Paul Scherrer Institute (PSI) is the largest research institute for natural and engineering sciences in Switzerland and has a long tradition in energy research. The PSI is an important partner for BGZ in research on the safe disposal of spent fuel. The PSI has an extensive research infrastructure that includes the operation of hot cells in which entire fuel rods can be studied, as well as other large-scale facilities such as the Swiss Synchrotron Light Source (SLS) and the Swiss Spallation Neutron Source (SINQ).

Extended Storage Collaboration Program

The Electric Power Research Institute (EPRI) is an independent non-profit organisation in the USA that conducts research on electrical power supply. The research is mainly financed by the members and participants, who are made up of approximately 1,000 organisations from 40 countries worldwide. In 2009, the Extended Storage Collaboration Program (ESCP) was founded with the aim of expanding the technical basis for ensuring the extended interim storage of irradiated fuel assemblies and their subsequent transport. Among other things, the focus is on defining common goals, exchanging information and strengthening international cooperation. Approximately 600 participants from 19 countries take part in the ESCP programme and more than 150 participants in each of the bi-annual meetings.

BGZ chairs the modelling and benchmark working group here, which is primarily concerned with preparing and using experimental data for the validation of existing computational programmes, as well as determining and quantifying uncertainties in the forecast models (see also Chapter casks, Inventories, Thermal Modelling Benchmark).

Nuclear Energy Agency

The Nuclear Energy Agency (NEA) of the Organisation for Economic Co-operation and Development (OECD) – OECD/NEA – provides a framework in which governments can compare policy experiences, seek answers to common questions, identify best practices and work to coordinate national and international strategies. The NEA’s specific areas of competence include safety and regulation of nuclear activities, radioactive waste management, radiation protection, nuclear science, economic and technical analyses of the nuclear fuel cycle, nuclear law and liability, and public relations. The NEA Data Bank [8] provides nuclear data and computer programmes for participating countries.

BGZ is a member of the Working Party on Nuclear Criticality Safety (WPNCS) [9] and provides one member of the delegation from the Federal Republic of Germany. The WPNCS addresses technical and scientific issues relevant to criticality safety. This also includes the transport and storage of fuels.

International Atomic Energy Agency

The International Atomic Energy Agency (IAEA), which has its headquarters in Vienna, promotes the safe and peaceful use of nuclear energy. It was founded in 1957 as the “Atoms for Peace” organisation of the United Nations and has 172 member states today. The focus of its work is on nuclear safety and the safeguarding and monitoring of fissile nuclear materials. The IAEA promotes research and technology for the application of ionising radiation in medicine, food safety, agriculture and environmental monitoring.

The internationally required safety level is developed by the IAEA and defined in the Safety Standards [10]. These cover all aspects of reactor safety, radiation protection, the transport of nuclear goods and the disposal of radioactive waste. The Commission on Safety Standards manages the continuous development of technical committees that are made up of experts from the member states. BGZ is represented in the Transport Safety Standards Committee (TRANSSC, Transports of Nuclear Goods). BGZ also participates in Coordinated Research Projects (CRP) and Technical Meetings (TM) of the IAEA.

Communicating progress and outcomes

Communicating progress and outcomes

Results and progress of the research programme are communicated on several levels by different actors and with content adapted to the respective target group. This communication work is undertaken by individual scientists, by specialist departments and in the form of press and public relations work at various levels, including addressing the general public, independent experts, and the scientific community.

BGZ is already using the series of events being held as the “Interim Storage Forum” to answer and discuss essential issues relating to safe interim storage with representatives of citizens’ initiatives, public authorities, and scientific institutions as well as interested citizens. In this framework, dialogue and discussions cover various aspects of the safe storage of radioactive waste as well as current research. The internet platform of the Interim Storage Forum [6] is designed to foster the exchange of information on the interim storage of radioactive waste. The forum can be used to direct specific questions to BGZ and to provide prompt responses. All questions and the corresponding answers are published in the question forum.

The current status and results of the research programme are presented and discussed with the participants at a biennial “Expert Workshop on Interim Storage”. The majority of workshop participants are from universities, research institutions, public authorities, expert bodies, and relevant committees or are representatives of the industry in Germany and Switzerland.

Direct and ongoing exchanges take place at the scientific level with national and international project partners from research institutions and industry, as well as in the various national and international committees (see also Chapter below). Progress and the results generated by the BGZ research programme are also presented to and discussed by the broad international and national scientific community in the form of regular presentations at specialist meetings, conferences, and workshops. The research programme and selected results are also published in conference proceedings and the relevant peer-­reviewed scientific journals. Whenever possible, contents are subsequently made freely available to the general ­public (so-called open access).

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BGZ’s research programme: thinking ahead interim storage

BGZ, as a company organised in a private legal form and wholly owned by the Federal Government, ensures the safe and reliable operation of interim storage facilities for low-, intermediate- and high-level radioactive waste.

As the operator and licence holder, BGZ is obliged to provide permanent proof of the whereabouts of the transport and storage containers and compliance with the protection goals for extended interim storage in accordance with the state of the art in science and technology.

The research programme developed here shows the research required for this purpose and provides an overview of BGZ’s research strategy and activities. The research programme is continuously updated and adapted to the evolving state of science and technology.

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Ergebnisbericht zum Fachworkshop Zwischenlagerung
22. & 23. Oktober 2019 in Berlin

Am 22. und 23. Oktober 2019 fand in Berlin der erste Fachworkshop Zwischenlagerung der BGZ statt. Der Einladung zu dieser geplant zweijährlichen Veranstaltungsreihe folgten 101 Teilnehmer*innen aus Deutschland, sowie weitere Gäste aus dem Ausland. Der Teilnehmerkreis setzte sich aus Mitarbeiter*innen von Bundesministerien, Aufsichts- und Genehmigungsbehörden, Betreibern von Zwischenlagern, Energieversorgungsunternehmen, Universitäten, außeruniversitären Forschungseinrichtungen sowie Vertreter*innen weiterer Verbände und Bürgerinitiativen zusammen.

Im Rahmen von zehn Fachvorträgen wurden Forschungsthemen der verlängerten Zwischenlagerung dargestellt und fachlich diskutiert. Der Fachworkshop hatte das Ziel, offene Forschungsfragen zu identifizieren und zukünftige Aktionsfelder, die im Zusammenhang mit der Zwischenlagerung bestrahlter Brennelemente und wärmeentwickelnder Abfälle stehen, einzugrenzen und sie in die Belange der BGZ einzuordnen. Die vielschichtigen Aspekte der verlängerten Zwischenlagerung wurden im Nachgang bewertet und in die eigenen Überlegungen zum Forschungsprogramm der BGZ einbezogen.

BGZ stellt Weichen für verlängerte Zwischenlagerung radioaktiver Abfälle

Ende Oktober 2019 erörterten rund 100 Expert*innen aus Deutschland und der Schweiz auf Einladung der BGZ die wissenschaftlichen, technischen und genehmigungsrechtlichen Fragen, die mit einer verlängerten Aufbewahrung von hochradioaktiven Abfällen einhergehen. Im Nachgang zum Fachworkshop wurden die vielschichtigen Aspekte der verlängerten Zwischenlagerung bewertet und in die eigenen Überlegungen zum Forschungsprogramm der BGZ einbezogen.

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BGZ stellt Forschungsprogramm vor

Die BGZ hat im Rahmen ihres zweiten Fachworkshops ihr Forschungsprogramm zur verlängerten Zwischenlagerung vorgestellt und dieses mit zahlreichen nationalen und internationalen Expert*innen erörtert. Am 16. und 17.11. diskutierten Expert*innen in Berlin auf Einladung der BGZ die wissenschaftlichen, technischen und genehmigungsrechtlichen Aspekte, die mit der notwendigen verlängerten Zwischenlagerung hochradioaktiver Abfällen einhergehen.

Wilhelm Graf freut sich als technischer Geschäftsführer der BGZ über die gute Resonanz: „Durch unser Forschungsprogramm sind wir einen großen Schritt vorangekommen mit unseren Vorbereitungen auf die verlängerte Zwischenlagerung.“ Graf weiter: „Unser Programm zeigt den Forschungsbedarf auf und gibt einen Überblick über unsere Forschungsstrategie sowie die diesbezüglichen Aktivitäten. Der Austausch dazu mit Expert*innen aus dem In- und Ausland wie bei unserem Fachworkshop ist für uns dabei elementar.“ Er machte auch deutlich, dass die BGZ das Forschungsprogramm laufend fortschreiben und an den sich weiterentwickelnden Stand von Wissenschaft und Technik anpassen werde.

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Erster BGZ-Studierendentag: Akademischer Nachwuchs diskutiert über sichere Zwischenlagerung

Studierende aus Deutschland und der Schweiz haben sich erstmals auf Einladung der BGZ zum Austausch in Essen getroffen und erörterten dort ihre Abschlussarbeiten zur Zwischenlagerung radioaktiver Abfälle.

„Wir bieten damit ein neues, in dieser Art noch nicht vorhandenes Forum zum Austausch untereinander und fördern damit auch den Kompetenzerhalt in unserer Branche“, sagt Dr. Maik Stuke, Mitinitiator der Veranstaltung, aus der Abteilung Zentrale Fachfragen der BGZ. Das bundeseigene Unternehmen baut seine Aus- und Fortbildung kontinuierlich aus und führt diese mit ihrer eigenen Forschungsarbeit zusammen, um sicherzustellen, dass auch künftig qualifiziertes Fachpersonal und das wissenschaftliche Know-how vorhanden sind.

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BGZ stellt Forschungsprogramm in Ahaus vor

In einer Informationsveranstaltung für den Rat und die Bürger*innen der Stadt Ahaus hat die BGZ ihr Forschungsprogramm zur verlängerten Zwischenlagerung vorgestellt. Das bundeseigene Unternehmen informierte außerdem zu aktuellen Vorhaben am Standort.

„Mit unserem Forschungsprogramm stellen wir die Weichen für eine verlängerte Zwischenlagerung an den 14 Standorten der BGZ mit Zwischenlagern für hochradioaktive Abfälle“, erklärte Dr. Jörn Becker, Leiter der für Forschung zuständigen Abteilung. Mit dem Programm habe die BGZ die Grundlage dafür erarbeitet, die Sicherheit der Zwischenlagerung auch über den bisher genehmigten Zeitraum von 40 Jahren nachzuweisen.

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BGZ stellt Forschungsprogramm im Ausschuss Atomanlagen vor

Während einer öffentlichen Sitzung des Kreisausschusses Atomanlagen hat die BGZ ihr Forschungsprogramm zur verlängerten Zwischenlagerung vorgestellt. Das bundeseigene Unternehmen informierte außerdem über den Stand der vorgesehenen Nachrüstung des Zwischenlagers.

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BGZ präsentiert Forschungsprogramm in Geesthacht

Die BGZ informierte gestern Bürgerinnen und Bürger im Ratssaal der Stadt Geesthacht über die notwendige verlängerte Zwischenlagerung hochradioaktiver Abfälle. Auf diese Aufgabe bereitet sich das bundeseigene Unternehmen mit einem eigenen Forschungsprogramm vor, das an dem Abend vorgestellt wurde.

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Forum Zwischenlagerung: Reger Austausch über verlängerte Zwischenlagerung

Auf Einladung der BGZ haben Expert*innen beim „Forum Zwischenlagerung“ über die notwendige verlängerte Zwischenlagerung diskutiert und die Fragen des Publikums, das vor Ort und per Livestream teilnahm, beantwortet.

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BGZ informiert Nationales Begleitgremium über sichere Zwischenlagerung

Die BGZ hat bei einer Veranstaltung des Nationalen Begleitgremiums (NBG) in Ahaus ihre Aufgaben, die Vorbereitungen auf die notwendige verlängerte Zwischenlagerung sowie ihre Kommunikation mit der Bevölkerung erläutert. Die Mitglieder des NBG informierten sich bei einem Besuch des Zwischenlagers zudem über das Konzept der sicheren Zwischenlagerung.

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Verlängerte Zwischenlagerung: BGZ stellt Forschungsprogramm in Brokdorf vor

Die BGZ informiert interessierte Bürgerinnen und Bürger im Rahmen eines öffentlichen Dialogabends in Brokdorf über ihre Vorbereitungen für die erforderliche verlängerte Zwischenlagerung hochradioaktiver Abfälle. Dazu möchte sie als Betreiberin des Zwischenlagers Brokdorf mit der Öffentlichkeit ins Gespräch kommen.

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Zwischenlagerung radioaktiver Abfälle: BGZ stellt Mainbogen-Gemeinden Forschungsprogramm vor

Die BGZ hat Vertreter*innen der sechs Gemeinden, die zur Allianz Schweinfurter Mainbogen gehören, ihr Forschungsprogramm zur verlängerten Zwischenlagerung vorgestellt. Das bundeseigene Unternehmen informierte außerdem über umfangreiche Investitionen am Standort Grafenrheinfeld.

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Die BGZ informiert die Mitglieder des Kreistags Landshut über die sicherer Zwischenlagerung.

BGZ informiert Kreistag Landshut: Sichere Zwischenlagerung auch bei längerer Endlagersuche gewährleistet

Wie die BGZ die sichere Aufbewahrung der Behälter mit hochradioaktiven Abfällen bis zu deren Abgabe an ein Endlager gewährleistet, erläuterte das bundeseigene Unternehmen jüngst dem Landshuter Kreistag.

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BGZ informiert in Dannenberg: Sichere Zwischenlagerung auch bei längerer Endlagersuche gewährleistet

Wie die BGZ die sichere Aufbewahrung von hochradioaktiven Abfällen bis zu deren Abgabe an ein Endlager gewährleistet, erläuterte das bundeseigene Unternehmen jüngst bei einer Informationsveranstaltung der Bürgerinitiative Lüchow Dannenberg.

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Research activities

MSTOR – Long-term behaviour of metal seals

Subject matter:MSTOR – Metal seals during long-term storag
» Extension of the existing experimental basis for the temperature-dependent ageing behaviour of metal seals
» Development of a forecast model for sealing parameters
Project partners:» GNS (cask manufacturer, Germany)
» Technetics (seal manufacturer, France)
» EWN Entsorgungswerk für Nuklearanlagen GmbH (store operator)
Project period:2021 to 2031, longer if necessary

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Safe confinement of radioactive materials during storage is ensured by the double-lid sealing system with compressed metal seals.

Only Helicoflex® metal seals made by the French manufacturer Technetics are used in the sealing barrier. Helicoflex® type seals consist of a helical spring core and a stainless steel jacket covered by an outer liner made of aluminium or silver. The functioning of the seals is based on the elasticity of the helical spring (see Figure 3) that, when compressed, generates the restoring force that is necessary to maintain contact between the outer liner and the sealing surfaces. The plasticity of the outer liner also ensures that the surface of the seal is optimally adapted to the structure of the sealing surface. During compression, the liner material fills the flange imperfections in order to achieve a high degree of tightness with standard He leakage rates of less than 10-8 Pa* m3/s (see Figure 3). The stipulated standard He leakage rate of 10-8 Pa* m3/s is therefore also a system-specific quality criterion for required long-term functionality and is not a radiologically based tightness requirement, as this could also be met at higher leakage rates.

Figure 3:
Functionality of Helicoflex® metal seals

During initial compression of the metal seal, a force-deformation curve is obtained as shown in Figure 4. At deformation e0, the required tightness is achieved for the first time, but only at deformation e2 is the seal at its operating point. The seal is compressed to the operating point thanks to the depth of the groove in which the seal is inserted in the lid, as the difference between the groove depth and the torus diameter of the seal corresponds exactly to the optimum compression e2.

If the seal is decompressed as a result of external loads, i.e. if a gap emerges between the lid and the mating surface, the required tightness is maintained until e1 is reached. The permissible gap between the sealing surface and the lid corresponds to the useful elastic recovery ru (see Figure 4). In particular, this parameter and the associated force Y1 are essential for evaluating the behaviour of the metal seal under operating and accident conditions.

In the assembly state, mechanical stress and temperature exposure lead to creep processes in the metal seal. These become noticeable in the form of relaxation. As shown in Figure 4, relaxation decreases the restoring force at the operating point to Y2r with unchanged deformation. Although the minimum force required to maintain the specified tightness Y1r decreases upon decompression, the deformation e1r decreases accordingly, so that the remaining useful recovery rur for an aged seal decreases considerably compared to the assembly state.

Knowledge of the characteristic values Y1r and rur of an aged seal is therefore of great importance to reliably evaluate the long-term behaviour of the metal seals. In particular, the remaining useful recovery rur is a measure for assessing the robustness of the sealing barrier, i.e. the extent to which the seal is able to maintain the required tightness even under external mechanical influences.

Figure 4:
Change in characteristic values of metal seals due to ageing

The long-term tests carried out so far and the operating experience of more than 25 years demonstrate that the high tightness requirements are also met in the long term by the metal seals used. This can be expected to remain the case for interim storage of 40 years and longer. However, in order to validate and quantitatively predict the sealing behaviour over longer periods of time, further investigations are required to determine the change in sealing characteristics under the influence of temperature and time.

A study on the ageing behaviour of metal seals was carried out between 2013 and 2016 under the direction of GNS with RuDrift. The tests were carried out in the joint laboratory of Technetics and CEA at the Pierrelatte site (France). The stainless steel (SST) test flanges developed for the tests represent the standard sealing surface/lid combination (nickel-plated cask body/martensitic lid material). Aluminium- and silver-coated metal seals were used in the test flanges. These metal seals are of the kind used in the primary and secondary lids, the so-called main lids. The compressed metal seals used in the test flanges were stored for two years at temperatures of 100°C and 130°C and for one year at 150°C to predict sealing behaviour, particularly for the maximum design temperatures. Reference flanges were stored in parallel at room temperature. At different times, the test flanges were removed from the furnaces and the decompression curves were recorded with respect to Y1r and rur. The results describe the quantitative behaviour of the metal seals for a temperature level that corresponds to the maximum design thermal load.

In fact, seal temperatures during interim storage are significantly lower than the test temperatures at RuDrift. This means that the design temperatures are not reached even at the time of loading and continue to drop due to the decreasing heat generation. Additional tests at lower temperature levels with a correspondingly extended test duration are also planned. These will be performed to obtain valid statements on the behaviour of metal seals at the sealing temperatures expected during extended interim storage. The study will also cover seals with a smaller torus diameter. These seals are used in the so-called small lids (closure lid, protective cap and pressure switch). There are also plans to study transferability to the use of another sealing surface/lid combination. This is a combination of uncoated ductile cast iron (DCI) with stainless steel (SST), which is only relevant for main lids with aluminium-coated metal seals. The test programme (EP) under MSTOR is therefore as follows (see Figure 5):

Figure 5:
Overview of the RuDrift/MSTOR test programme

EP1/2
Continuation of the artificial ageing of aluminium- and silver-coated metal seals in main lids at 130°C (plus one year) and 100°C (plus three years), which was started with RuDrift. The aim is to improve prediction accuracy in the selected temperature range and to carry out further reference testing of seals stored at room temperature (plus eight years).

EP3/6
Expansion of the database for aluminium- and silver-coated metal seals in main lids at ageing temperatures of 60°C and 80°C, in each case over a period of at least eight years.

EP4
Creation of a database for aluminium-coated metal seals in main lids for the flange combination DCI/SST at representative ageing temperatures of 130°C (one year), 100°C (six years) and at room temperature (six years) for reference purposes.

EP5/7
Creation of a database for aluminium- and silver-coated metal seals in small lids for the flange combination SST/SST at ageing temperatures analogous to the large lid seals of 150°C (one year), 130°C (three years), 100°C (six years), 80°C (seven years) and 60°C (at least seven years) as well as at room temperature (at least seven years) for reference purposes.

Models for predicting the change in the characteristic values of the metal seals are generated based on the temperature and time-dependent measurement results. These models enable the long-term behaviour of metal seals to be evaluated, taking into account real thermal loads and decay behaviour. The results can also be used to specifically pre-age metal seals by artificial ageing, for example for further studies on aged seals. To verify the calculations, additional flanges (so-called travelling flanges) with silver- and aluminium-coated metal seals are aged at various temperatures (130°C, 100°C, 80°C and 60°C) in the EP3/6 test programmes. The respective measurement results are compared with the temperature- and time-dependent ­calculations. The aim is to demonstrate that the prediction model found is able to correctly calculate ageing under realistic operating conditions at decreasing temperatures.

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OBSERVE – Dosisleistungs- und Temperaturmessprogramm

Subject matter:OBSERVE
» Dose rate and temperature measurement programme on loaded casks
» Comparison of calculated expected values with measurement values on selected casks at different points in time during storage
Project partners:WTI
Period:Phase I (feasibility study): completed
II (measurement programme): in planning

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1observe2observe
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The distribution of dose rate and temperature on the cask surface depends on the arrangement and characteristics of the radioactive inventory and cask components (see Figure 6 and Figure 7). Measurements are routinely taken before casks are placed in storage to confirm compliance with the applicable dose rate and temperature limits for storage in the respective storage facility.

These measurements are carried out at representative measuring points specifically defined for each cask type in order to obtain reliable information about the mean value or to find maximum values.

Figure 6: Distribution of dose rates on the surface of a CASTOR® cask (left: neutron dose rate, right: gamma dose rate)

Figure 7: Comparison of calculated and measured maximum cask temperatures in an interim storage facility (left: CFD calculation, right: thermography)

Activity and thus heat generation both decrease during storage due to the radioactive decay of the inventory. Likewise, there are temperature-dependent and radiation-induced changes in the properties of the materials used (for example, thermal expansion, density changes and radiolysis of the moderator material), which in turn effect shielding and heat dissipation.

The planned dose rate and temperature measurement programme OBSERVE aims to measure the entire surface of selected casks intermittently over a long period of time. Measurements provide immediate and integral information on the ageing behaviour of the casks or the inventory. The measured dose rate and temperature curves must be compared with the predicted values for this purpose. These comparisons and the respective distribution can be used to draw conclusions about actual shielding and heat dissipation properties. If the measured values are within the expected range, this confirms that no unexpected changes have occurred and that ageing behaviour has been correctly assessed. In this way the measurement programmes can make an active contribution to ageing management.

In the first stage of the project, sensitivity studies will be carried out to check how suitable dose rate/temperature measurements are for drawing conclusions about the condition of cask components and the inventory. The results of the sensitivity analyses will be used to determine the requirements for carrying out the measurements (equipment, measurement grid, measurement location) and to develop a corresponding measurement programme.

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DPOPT – Optimierung des Druckschalters

Subject matter:DPOPT
» Optimisation of the pressure switch
» Qualification of a production-optimised component
Project partners:» GNS (cask manufacturer, Germany)
» HBM pressure switch manufacturer, Germany)
Project period:2021 to 2029 including BAM qualification

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The sealing function of the double-lid sealing system is monitored by means of a pressure switch mounted in the outer sealing barrier of the double-lid sealing system and connected to the inter-lid space. The pressure switch in turn is connected to the cask monitoring system.

If, during storage, the pressure in the inter-lid space drops below the pressure level in the reference chamber of the pressure switch due to a defect in one of the two barriers, the contact of the main switch is opened and the cask monitoring system reports “Inter-lid pressure low”.

Figure 8:
Basic design of the pressure switch

The pressure switch is a complex component (see Figure 8 and Figure 9), which is manufactured to high quality standards. However, random defects cannot be completely ruled out. The pressure switch therefore has a self-monitoring function in the form of an additional switch for the reference chamber. If pressure in the reference chamber drops below the specified switching pressure due to a pressure switch defect, the corresponding contact is opened and the cask monitoring system reports “Reference chamber pressure low”.

There have only been a very limited number of defects – so-called pressure switch events – in the more than 1,400 pressure switches that are installed worldwide. Some of these 1,400 switches have been in operation for more than 30 years, adding up to a total operating time of more than 20,000 years. The failure probability of a pressure switch is in the range of less than 10-6 per year.

Figure 9:
Basic design of the pressure switch

Figure 10 shows that pressure switch events do not correlate with the total number of operating years. It can therefore be assumed that events occur randomly, and it cannot be deduced from their distribution over time that a systematic increase in failures is associated with extended interim storage. The decrease in the frequency of failures is also related to incorporation of the lessons learned during operation or during failure evaluation into the handling of the pressure switches.

Specific evaluations of pressure switch events show that the vast majority of events were due to leakage at the feedthroughs of the contact pins. This resulted in a pressure drop in the reference chamber but not in a failure of the containment function of the lid system. However, BGZ is working with GNS to optimise the pressure switch for interim storage purposes. Preliminary studies by the manufacturers HBM and GNS show that using glass feedthroughs instead of the ceramic feedthroughs previously used is a promising solution to the problem, as glass feedthroughs do not require additional soldering material.

The DPOPT research programme will subject the new contact feedthrough design to several tests covering operating conditions. The contact pins will be subjected to different mechanical stresses, which are considered to be the cause of the previous defects. These include pure weight loads and the simulation of repeated plugging and unplugging. The new switch will also be subjected to thermal loads to demonstrate that the glass feedthroughs are suitable for the entire temperature range envisaged. Load tests will be followed by helium leak tests in which the specified maximum leak rate must not be exceeded.

Figure 10:
Comparison of years of operation and pressure switch events

 

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SCIP IV – Studsvik Cladding Integrity Project

Subject matter:SCIP IV & V – Studsvik Cladding Integrity Project IV
» Expansion of the existing experimental basis for cladding performance under conditions of extended interim storage
» Derivation of models for predicting cladding performance
Organisation:» OECD-NEA international project
» Participants from Europe, Japan, USA, China and Korea
Project period:SCIP IV: July 2019 to June 2024
SCIP V: July 2024 to June 2029

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The OECD/NEA Studsvik Cladding Integrity Project (SCIP IV) is the fourth research programme of a cooperation be­tween OECD/NEA and various organisations from 15 countries. The project was launched in July 2019 and will be implemented by Studsvik AB over a period of five years with a total budget of 14 million euros.

The project will include basic and safety research on cladding performance during loss-of-coolant accidents, overheating, operational power transients and performance under interim and final storage conditions. The content of the current research programme builds on the previous programmes and specifically addresses questions pertaining to interim storage facilities. These include the creep behaviour of cladding tubes under interim storage conditions, the hydrogen fracture mechanisms in cladding tube material and the associated hydride reorientation, the interaction between fuel pellets and the cladding tube (pellet cladding interaction, fuel swelling), the permeation of gases in the fuel, the microstructure in high burn-up fuels, the mechanical behaviour of irradiated fuel assemblies and the fragmentation behaviour of different irradiated fuel types. The current experimental programme addresses a large number of the research questions identified by BGZ, some of which are also being carried out in parameter ranges relevant to Germany. BGZ is actively participating in the research programme. This includes regular discussions about the design, procedure and sample material of the experiments, as well as the evaluation and interpretation of the results obtained.

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SpizWurZ – Stress-induced hydrogen rearrangement in fuel cladding during long-term interim storage

Subject matter:SpizWurZ – Stress-induced hydrogen rearrangement in fuel rod cladding (2020 –2024)
» Expansion of the existing experimental basis for cladding performance under conditions of extended interim storage
» Derivation of models for predicting cladding performance
Organisation:» BMWi research funding for nuclear safety as part of the 7th Energy Research Programme (funding codes RS1586A, 1501609B)
» GRS and KIT (IAM, INE) collaborative project (IAM, INE)
» BGZ has observer status
Project period:2020 to 2024

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BGZ is engaged as an observer in the collaborative proj­ect of GRS (funding code RS1568A) and KIT (funding code 1501609B) funded by the BMWi and managed by GRS.

The collaborative project focuses on experimental and theoretical studies on the behaviour of hydrogen in fuel rod cladding materials under long-term interim storage conditions. In principle, hydrogen exerts an embrittling effect on zirconium-based materials. In dissolved form, the distribution of the hydrogen stored in the cladding tube changes by diffusion under the impact of temperature, concentration and stress. There is no complete description in the literature of the hydrogen flow taking into account all relevant parameters. However, this would be necessary for a reliable assessment of fuel rod integrity. The project extends the qualitative and quantitative understanding of hydrogen diffusion on a macroscopic and microscopic level to predict the formation of hydride structures in zirconium-based cladding tube materials.

One focus of the project is the experimental determination and description of the solubility and diffusion of hydrogen in cladding tube materials under longer-term interim storage conditions. The qualitative and quantitative description of hydrogen diffusion at the macroscopic and microscopic level will be studied to improve the prediction of hydride structures forming in zirconium-based cladding tube materials and the resulting material embrittlement. The results of individual effects will then be combined to produce a consistent description for theoretical modelling purposes of real cladding tube materials under conditions of longer-term interim storage with reference to irradiation and slow cooling rates.

One of the aims is to determine the chemical potential and diffusion coefficients of hydrogen in elastically stressed zirconium alloys. The project will also produce and perform a blind benchmark for the evaluation of the existing computational code for the simulation of fuel assemblies in transport and storage casks.

Specifically, bundle experiments will be carried out in the QUENCH facility at the Karlsruhe Institute of Technology (KIT) to determine macroscopic hydrogen flow in the cladding tube. For this purpose, different cladding tube materials (Zry-4, ZIRLO®, Duplex) will be loaded with hydrogen and individual internal pressures and hydrogen concentrations set. Subsequently, the unirradiated cladding tube samples will be cooled from temperatures of around 370°C at a cooling rate of about 1 K/d in an experiment lasting several months. The hydrogen distribution in the cladding tubes will then be determined by means including neutron radiography.

The microscopic diffusion-induced hydrogen flow as a function of orientation, morphology and mechanical stress in zirconium alloys will also be investigated and quantified. For this purpose, investigations on hydrogen diffusion on cladding tube materials of a material texture previously analysed in detail will be carried out on pre-oxidised material samples. In addition, an in-situ and ex-situ investigation of the impact of stress on the concentration distribution in Zircaloy will be carried out (by means of tensile tests).

The real elastic strain in the fuel rod cladding tube after more than 30 years of storage will be determined experimentally in the hot cells of the Institute for Nuclear Waste Disposal (KIT-INE) by separating out the fuel and measuring the cladding tube diameter. A Zircaloy-4 cladding tube sample with UO2 (sample burn-up of 50.4 GWd/tHM) from the Obrigheim PWR is used as the sample. The irradiation history of the samples irradiated in the 1980s is very well known. The initial period of the project, which began in 2020, is three years.

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Thermal Modelling Benchmark

Subject matter:Thermal Modelling Benchmark
» Determining and comparing precise fuel rod cladding tube temperatures
» Uncertainty and sensitivity analysis of the different calculation and modelling approaches
Organisation:» International Thermal Modelling Benchmark of the EPRI-ESCP
» Participants from Europe, Asia, USA
» BGZ in collaboration with GNS and WTI
Project period:2019 to 2024

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PreviousNext

As almost all degradation effects of the cladding tube are temperature-dependent, it is critical to have as much precise knowledge as possible about the cladding tube temperature and its development in order to determine the integrity of the cladding tube.

After reactor deployment, cladding tubes have a temperature of the decay pool of below 50°C. Following the decay period, the fuel assemblies are packed into and dried in casks. During the drying process, the cladding tubes reach their maximum temperature (outside the reactor), which subsequently decreases according to the heat conduction in the cask and the decay heat of the fuel. Long-term temperature development during storage essentially follows a decreasing exponential function.

The time span from the drying process through to several months later is critical for the majority of the hydrogen-dependent effects that apply to longer-term interim storage. The rapid rise in temperature during the drying process and the way in which the cladding tube temperature decreases again significantly determine the behaviour of the hydrogen absorbed in the cladding tube.

An instrumented type TN® 32 standard cask was loaded with PWR fuel assemblies at the North Anna power plant in 2017 and subjected to a measurement programme. Temperature measurements were an important part of the test procedure given that knowledge and accurate prediction of the temperature distribution in the cask and the cladding tubes has a significant influence on the ageing behaviour of the components. These are used to verify the thermal calculation methods. In the current project phase, the American project coordinator “Electric Power Research Institute” (EPRI) also invited interested international institutions to participate in a sub-project of the HBU Data Project: the “International Thermal Modelling Benchmark Study”. The aim is to obtain an international overview of the methods used and their sensitivity in comparison to experimentally obtained results.

BGZ is participating in this benchmark together with GNS in order to gain further insights into safety-related verifications based on best estimate analyses for the upcoming reapplication for nuclear storage licences under Section 6 of the Atomic Energy Act.

The model of the cask was created in the first phase and the calculations carried out according to the benchmark description (see Figure 11). The results were sent to the organisers for evaluation. Evaluations are currently being undertaken and the specifications for the second phase are being defined in parallel. The second phase will involve more detailed study and quantification of the uncertainties in the modelling and their effects on the calculated cladding tube temperatures. BGZ will contribute results to the benchmark in addition to the results arrived at in association with GNS and WTI.

Figure 11: Calculated temperature distribution of the loaded cask based on the information from the benchmark description

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LEDA – Long-Term Experimental Dry Storage Analysis

Subject matter:LEDA – Long-Term Experimental Dry Storage Analysis
» Expansion of the existing experimental basis for cladding performance under conditions of extended interim storage
» Derivation of models for predicting cladding performance
Organisation:» Managed by BGZ
» Experiments will be performed in Studsvik laboratories in Sweden.
» Joint planning with partners from industry and science
» Implementation with other partners
Project period:2022 to 2029

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In the laboratories of Studsvik in Sweden, BGZ is planning an experimental campaign to answer questions about cladding performance under conditions of dry interim storage and, in particular, to investigate the significance of hydrogen.

The experimental studies will take the form of “integral effect tests”, i.e. tests and investigations on different fuel rod segments with prototypical boundary conditions representative for dry storage in Germany. Typical conditions are set and monitored. Furthermore, suitable investigations for the pre- and post-characterisation of the cladding materials are also required. The objectives of the LEDA test programme are the integral study of the behaviour of fuel rod segments representative for Germany under typical conditions of dry interim storage. The experimental data generated in LEDA will be used to extend and validate analytical models and methods for predicting fuel rod integrity for verification purposes in line with protection goals. The previous criteria for the exclusion of systematic cladding failure and their completeness will be reviewed and analysed taking account of (long-term) hydrogen behav­iour for storage times of over 40 years.

The long-term measurement campaign will be carried out with different irradiated fuel rod segments under prototypical boundary conditions. An integral approach under drying process and dry interim storage conditions is taken. The implementation of adequate pre- and post-characterisation of the fuel rod segments and the focus on hydrogen-induced effects, in combination with the use of fuel rod segments representative for Germany, in contrast to individual effect tests, allows results to be interpreted directly.

The fuel rod segments that will be studied are as similar as possible to the fuel rods used in Germany in terms of cladding tube materials, fuels and irradiation histories. This will ensure that consideration is given as far as possible to the prototypical conditions that apply to cask loads.

The tests will be carried out in the hot cells at Studsvik in Sweden in a test rig suitable for this purpose. The latter was developed and manufactured as part of the Halden Reactor Project (HRP) to investigate fuel rod behaviour under dry interim storage conditions. After the operating licence for the Halden plant expired, the Halden Board agreed to the test equipment being transport­ed to the Studsvik laboratories in Nyköping to ensure that it could continue to be used by BGZ. A sketch of the test rig is shown in Figure 12. Eight fuel rod segments can be simultaneously exposed to a typical temperature transient in the test rig, varying axially and over time. Typical axial temperature profiles are set for the tests. Over a period of several months, the temperature will be successively lowered, similar to the drop in temperature in the fuel assemblies in interim dry storage after the cask has been sealed.

The ongoing steps within the project LEDA include ­preparing the test rig at Studsvik’s laboratories and ­selecting and preparing the fuel rod samples for testing.

Figure 12:
Sketch of the test rig developed within the HRP and used further in LEDA. The test rig can accommodate
up to eight fuel rod segments. The temperature is regulated via the heating jacket and heating rod.

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DCS-Monitor II

Subject matter:DCS Monitor II – Development and testing of methods for the non-invasive analysis of the inventory condition for transport and storage casks in extended interim storage
Organisation:» BMWi research funding for nuclear safety as part of the 7th Energy Research Programme (funding codes 1501606A, 1501606B)
» Collaborative project of the Dresden University of Technology (TUD, project coordinator), Zittau/Görlitz University of Applied Science and the Helmholtz Centre Dresden-Rossendorf
» BGZ has the status of an associated project partner.
Project period:2020 to 2024

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The aim of the collaborative project, which is coordinated by Dresden Technical University, is to study approaches to radiation field-based diagnostis for gamma radiation, ­neutron flux and muons in greater depth. This will be done in the form of simulations and experiments aimed at developing a validated and applicable monitoring procedure for CASTOR® casks.

This will include field studies on real casks in interim storage facilities for the first time. The proj­ect is dedicated to the in-depth analysis and experimental evaluation of radiometric detection methods for larger, predominantly geometric changes in the cask inventory based on gamma, neutron and muon fields. The project builds on generic feasibility studies on various potential diagnostic methods for non-invasive monitoring of CASTOR® casks, which were carried out as part of a previous project (funding codes 1501513A, 1501513B). The project explicitly does not extend to research on change and damage mechanisms.

A detector based on straw tube technology provided by Forschungszentrum Jülich will be commissioned and an in-house detector concept will be developed to study the applicability of muon imaging for a monitoring procedure for dual-purpose casks (DPCs) for high-level radioactive waste. With the support of GNS and BGZ, the detectors will be used as far as possible to carry out measurements on large-scale dummies and later on DPCs in the interim storage facility. The aim is to experimentally demonstrate the feasibility of mapping fuel distribution in the DPCs using muon imaging within the framework of practical measurement times. In parallel, extensive simulation studies will be carried out to arrive at a suitable measurement concept and to generate suitable algorithms for volume reconstruction from the measurement data of the muon detectors. This work should deliver a suitable strategy for one-off and recurring cask scans using muon imaging.

The project will also investigate in more depth the use of measurements of the gamma and neutron radiation field of loaded DPCs for non-invasive monitoring. A numerical sensitivity study will be carried out to qualify the gamma and neutron measurement technology. The data generated will include information on how differences in cask loading affect the gamma and neutron field of a CASTOR®. A semi-automated radiation measurement system for measuring the gamma and neutron radiation emitted by the DPCs will also be designed and built. The radiation measurement system will be used in a specially designed experimental campaign on CASTOR® casks in the interim storage facility. The focus will be on reliably determining signature differences in the radiation field with known differences in the loading or burn-up of single fuel assemblies. Accompanying ongoing research will also be undertaken on current research results regarding potential cladding tube damage and nuclear fuel distributions during extended interim storage.

BGZ supports the preparation and implementation of the project as an associated project partner.

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Forschungsverbund Myonenradiografie

Cosmic muons are produced in large numbers by cosmic rays in the upper atmosphere. Their properties are similar to those of electrons except that they are much heavier. The material-dependent interactions of cosmic muons with structures on Earth are already used in established methods in archaeology, geology, volcanology and fission material monitoring using radio- and tomographic imaging.

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Non-invasive studies of the cask inventory using cosmic muons is still a relatively young and dynamic field of research. While initial work in 2003 focused on proliferation aspects, such as the identification of missing fuel assemblies, more recent efforts have focused on visualising individual fuel rods [34]. Some national research projects already exist in this field. These are funded from a variety of grant sources.

The BMWK, for example, is currently funding the development and testing of procedures within the DCS Monitor II research project (see Chapter above) for the non-invasive analysis of the inventory condition for transport and storage casks during extended interim storage. As well as theoretical modelling, the project also includes experimental work on muon tomography in loaded casks.

The Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Germany’s central expert organisation in the field of nuclear safety, is investigating and developing the theoretical modelling of imaging procedures using atmospheric muons as part of an in-house research project on aspects of extended interim storage. This project is funded by the BMUV.

What these projects have in common is that they deal with issues arising in the context of extended interim storage and study imaging procedures using muons on CASTOR® V/19 casks.

There are plans to establish a joint network with the participation of BGZ to undertake focused studies on the potential of the imaging methods and in particular the data evaluation algorithms as well as future implementation and application of the technology in Germany. One aim of the network is to build up broad knowledge and to further develop specific expertise. The various focuses of the work funded in each of the projects and the specific know-how of the participants means that research will cover the entire field of the application of imaging methods using atmospheric muons to the study of loaded transport and storage casks.

The planned establishment of this network will lay the groundwork for bundling national research resources and a platform for the exchange of ideas and knowledge in the field of muon tomography.

BGZ is further participating in the MuTomCa project (Muon Tomography for Shielded Casks) on fissile material monitoring. The cooperation framework includes Istituto Nazionale di Fisica Nucleare (INFN, Italy), EURATOM and the Forschungszentrum Jülich and will enable synergy effects to be exploited in the network.

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