High-level Nuclear Waste Research Articles

Stainless steel corrosion under anoxic, highly saline and elevated temperature conditions

Abstract. Several countries consider hosting a deep geological repository for high-level nuclear waste (HLW) in salt rock. In the unexpected case of solution access to the emplacement caverns during the long-term evolution of such a repository, metallic containers will be exposed to highly saline brines. In this study, corrosion experiments under conditions expected to be representative of HLW disposal in salt rock have been performed with stainless steel, a material typically used to construct containers containing vitrified HLW. Experiments were performed in closed vessels under anoxic and highly saline conditions at 90 ∘C. Polished stainless steel coupons were suspended in 5 mol L−1 NaCl or 3.4 mol L−1 MgCl2 at their natural pH. At the end of the experiments (up to 294 d), vessels were cooled down to room temperature, pH and Eh were measured in situ, the ultra-centrifuged contacting brines were analyzed by (HR)ICP-MS (high-resolution inductively coupled plasma mass spectrometry) and the formed corrosion products were identified. In all corrosion experiments, pHm and Eh hardly changed with exposure time, and the nature of the salt had no significant effect. The overall corrosion rates were low, and the quantities of dissolved metal ions revealed the formation of only sparingly soluble corrosion products. The formation of a passivation layer mostly made of Cr(III) (hydr)oxide was evidenced, but no localized surface attack could be detected. The coupon corroded in 5 mol L−1 NaCl was embedded in resin and cross-cut for further analyses, using synchrotron-based techniques with a small beam footprint (X-Ray fluorescence (μXRF), X-Ray diffraction (μXRD), and X-ray absorption near-edge structure (μXANES)). Element distribution maps revealed the presence of a very thin (<1 µm) layer of corrosion products, with regions enriched either in Cr or in Fe and Ni. X-ray diffractograms identified the presence of spinel-type compounds (Fe3O4 and NiFe2O4) and Cr2O3, which was corroborated by recording μXANES at the Cr, Fe and Ni K-absorption edges. The additional presence of layered double hydroxides in addition to NiO and Ni(OH)2 was also evidenced. Overall, results point to a corrosion layer having a duplex structure, with an inner layer mostly of chromium (hydr)oxides and an outer layer made of Fe- and Ni-based spinel compounds admixed with nickel (hydr)oxides.

Climate scenarios, groundwater models, and uncertainties in long-term safety

Abstract. Assessing future climate scenarios plays a fundamental role in evaluating the long-term safety of a repository for high-level radioactive waste (HAW). Over the assessment period of 1 million years, the repository is affected by various processes and developments that must be considered in the design of the repository. Predictions are complicated by the dynamics of the climate and are subject to uncertainties. The aim of this study is to evaluate uncertainties for various climate-induced processes that are relevant for the safety assessment of a repository for HAW. This research addresses fundamental questions concerning the impact of future cold stages on different regions of Germany, the differences between past and future cold stages, the transfer of future climate developments to stylized climate models, and the uncertainties resulting from these models. The study identifies the primary climate processes that could potentially affect the evolution of a HAW repository, including the formation of glaciers or ice sheets, the development of permafrost conditions, fluctuations in sea level, erosion, and isostatic adjustment. These processes can be represented in numerical groundwater models through parameter variations. In this project, these parameter variations and the impact of climatic states on different processes will be modeled for a sensitivity study. Finally, the results will be analyzed with respect to uncertainties, and possible improvements to dealing with uncertainties will be discussed. The focus of the presentation is on the climate impacts and the implementation in the model.

How do uncertain parameters affect the analysis of host rock integrity? Methods and preliminary results for a generic clay rock site

Abstract. The thermal, hydraulic, and mechanical effects of excavating the repository mine and the thermal loading of nuclear waste on the safety of the geological barrier system are addressed in the regulations as integrity analyses. For repositories in clay rock, a modelling and assessment methodology has been developed in the ANSICHT (Methodik und Anwendungsbezug eines Sicherheits- und Nachweiskonzeptes für ein HAW-Endlager im Tonstein) project based on thermo-hydro-mechanically (THM) coupled simulations using the open-source software project OpenGeoSys and has been illustrated by its application to a generic clay rock site in northern Germany. However, the assessment can be significantly influenced by uncertainties in the model inputs, which result, on the one hand, from the limited ability to characterize the geological barrier and, on the other hand, from fundamental difficulties and inaccuracies in the measurements. Hence, the results of these established integrity analyses have to be enriched by statistical information. The developed workflow comprises different steps beginning with a realistic characterization of parameter uncertainties. A surrogate model based on mathematically sound techniques is built to capture the propagation of these uncertainties through the THM simulation and the integrity criteria evaluation. The model can then be evaluated to obtain probabilistic results for rock integrity assessment and a sensitivity analysis to identify the inputs that primarily impact the results. In this contribution, we present preliminary results based on the generic repository system from the ANSICHT project and the developed tools for stochastic analyses. This work is done as part of the BGE-funded research cluster URS (Uncertainties and Robustness with regard to the Safety of a repository for high-level radioactive waste).

Pressure evaluation of a gap space in the engineered barrier system in a high-level radioactive waste repository

Disposal canisters and buffer materials are main components in an engineered barrier system designed for high-level radioactive waste disposal. The design temperature of the buffer material is below 100 °C in most countries, and many studies have been conducted to increase the design temperature of the buffer material to increase the disposal density of the repository. A 1-cm air gap exists between the canister and buffer material that can cause several variations in the thermal–hydraulic–mechanical performance of the compacted bentonite buffer material. Therefore, considering the air-gap effect under high-temperature conditions is necessary because high temperatures can cause pressure variations in the air gap and compacted bentonites. Therefore, an experimental system was developed to measure the pressure variations in a confined cell, which contained air and compacted bentonites from room temperature to 150 °C. The pressure of the confined cell increased with increasing water content of the compacted bentonite, and the pressure changes were similar up to 100 °C irrespective of the presence of the 1-cm air gaps. Moreover, the pressure values at 150 °C were 10–15% higher in samples without gaps.

Optimising the safety case through transdisciplinary research?

Abstract. The primary goal of the management of radioactive waste is safety. On the disposal path leading to a repository for radioactive waste, safety cases for disposal facilities have an important function as a basis for management-related, licensing, and political decisions. In the German site selection process, the preliminary safety analyses play a central role: they must inform siting decisions at three important steps of the process. Many stakeholders therefore have high expectations of these safety assessments. Past experiences suggest that their expectations are not always met by the current safety cases, which have grown historically, therefore follow internationally established conventions, and are extensive, complex, and difficult to comprehend. Transdisciplinary research has the potential to provide the implementer, BGE (Federal Company for Radioactive Waste Disposal), the regulator, BASE (Federal Office for the Safety of Nuclear Waste Management), and other actors in waste management with valuable information for the design and review of safety analyses that better meet the expectations of different stakeholders and actors. It provides new impulses, opens up other perspectives on questions concerning the safety case, and introduces novel proposals for solutions. In this way, it contributes to robust safety assessments and a resilient safety of repositories for radioactive waste. Based on tangible results from the research project TRANSENS (Transdisciplinary research on the management of high-level radioactive waste in Germany), we want to discuss how the safety case can be optimised in a transdisciplinary way. Topics that will be addressed in the workshop are the following. The design of a digital safety case. A digital safety case opens possibilities not only for improving the assessments of safety-relevant information, but also new perspectives on the cooperation of all those involved in the development of a safety case and their exchange with the interested public. Extensions of the features, events, and processes (FEP) catalogue. The FEP catalogue often gives rise to discussions with experts and the interested public. How can it be ensured that, as far as possible, all relevant information is covered and finds appropriate input into further activities (scenario development or modelling)? Here, digital and transdisciplinary methods offer new approaches to solutions. The benefits of extended peer communities. Extended peer communities can contribute to the quality, legitimacy, and acceptance of science-based documents such as safety cases. This finding from the social sciences can be applied well to the participatory, science-based, transparent, self-questioning, and learning site selection process. However, what about feasibility in practice? At the same time, we will introduce and try out transdisciplinary methods and tools like Research Marketplace, Soft Systems Methodology or Rich Picture. Transdisciplinary research differs from other forms of social science research in its higher degree of openness to inputs from participants. Toolboxes offer a wide range of instruments that can be used to suit the research question at hand. The workshop therefore not only offers the opportunity to exchange ideas on topics concerning the safety case, but also to get experiences with transdisciplinary methods and tools first hand.

Extended peer reviews for resilient safety

Abstract. In reviews, the content of a scientific or science-based document is assessed by invited experts. Classical peer reviews are carried out by experts who belong to the same specialist community as the experts whose work is being reviewed. Performing peer reviews is an essential element of the publishing process for articles in scientific journals. This established practice aims at ensuring scientific quality and at fostering scientific discourse. Reviews can also be carried out on a science-based safety case. International experience already exists with peer reviews of safety cases. Such peer reviews can be initiated by the implementer, the regulator, or political bodies that issue a mandate to invited experts. The aim of a peer review is to examine and, if necessary, improve the quality of a safety case, which can also strengthen the credibility of the safety case in the eyes of various actors, including practitioners in waste management and scientists, but also the interested public. Classical peer reviews have long been viewed critically in academic science. In view of complex issues, pluralistic values, and unpredictable developments, it is argued that it is appropriate to involve non-scientific peers in the review of documents on socially relevant issues. This gave rise to the idea of “extended peer-reviews” (Funtowicz and Ravetz, 1993), or, more precisely, reviews in an extended peer community, which can also be applied to reviews of safety cases. In the research project TRANSENS (Transdisciplinary research on the management of high-level radioactive waste in Germany), such extended peer reviews are placed in the context of transdisciplinary methods and tools. Our research indicates that extending the concept of peer reviews by involving various types of actors and using transdisciplinary methods and tools, so-called Td reviews, has the potential to improve the quality, legitimacy, and acceptance of a safety case and is suitable for contributing to a participatory, science-based, transparent, self-questioning, and learning site-selection procedure in the sense of the Site Selection Act (Eckhardt et al., 2023). Resilient safety is based on the ability to adapt to and recover from damaging impacts. The socio-technical system of radioactive waste management should be able to quickly return to a safe state in the event of a disruption. With Td reviews, further bodies of knowledge are introduced into the safety case, different perceptions and values, and an external perspective, which is not biased in the same way as that of peers in the classical and narrower senses. Thus, the quality, legitimacy, and acceptance of the safety case can be strengthened, and Td reviews are expected to have a positive impact on the resilience of the safety case and on the repository planning and optimization associated with the safety case (Eckhardt et al., 2023). So far, the systematic implementation of peer reviews has not been legally anchored in Germany, and experience with such processes is limited (Röhlig and Sträter, 2022). Therefore, there is a good chance at present of involving – initially on an experimental basis – not only peers, but also an extended peer community in the review of upcoming safety cases.

Acceleration potential of a Swiss-inspired approach to the site selection procedure

Abstract. The Federal Company for Radioactive Waste Disposal (BGE), the implementer of Germany's site selection procedure to identify a site suitable to safely dispose of its high-level nuclear waste, recently proposed a timeline suggesting the period between 2046 and 2068 as potential time slot for the final site decision (BGE, 2022). Thus, the decision is expected to be made substantially later than envisaged by the Site Selection Act, which states that one should aim for 2031 (StandAG, 2017). The BGE document not only details the timeline resulting in a site decision by mid-century but also outlines an alternative exploration approach inspired by the Swiss site selection procedure, which – if pursued – would enable substantial time gains. The core of this approach is a paradigm shift from exploration programs designed to acquire all data legally required for the safety analyses to data acquisition explicitly aiming at a site comparison during the respective exploration phase. Consequently, full-scale exploration would only be performed for those siting regions having passed the comparison. This contribution aims to detail and discuss this approach more thoroughly, based on the BGR's experience regarding the exploration of the subsurface. It focuses on the geoscientific aspects of this approach, while neglecting others such as the legal and administrative challenges. Owing to the basic idea of this alternative exploration approach, first a set of parameters suitable to differentiate potentially “better-suited” siting regions from “worse-suited” ones has to be identified. There are two main requirements for this set of parameters, namely the (i) general acceptance by the scientific community and (ii) capability to account for the specific characteristics of the individual host rock types while (still) ensuring the comparability of different rock types.

Historical contextualization of the site selection process for the German high-level nuclear waste (HLW) repository as a tool for information and awareness preservation

Abstract. It is always the same old story – after a couple of years most people cannot remember how decisions in nuclear waste disposal were made because many important documents are not available to the public as they have not been officially archived or are not accessible. This is why site selection Act § 6 requires nuclear waste organizations not only to store, but also to publish those documents. So, the Bundesgesellschaft für Endlagerung (BGE) provides information about all the relevant documents on the BGE website (https://www.bge.de/de/endlagersuche/wesentliche-unterlagen/, last access: 25 July 2023) and gives them to the Federal Office for the Safety of Nuclear Waste Management (BASE), which publishes them on the following website: https://www.endlagersuche-infoplattform.de/webs/Endlagersuche/DE/_home/home_node.html (last access: 25 July 2023). For the siting process, the BGE knowledge management works on methods to keep the relevant information at least for the next generation – as a first step in the process of long-term preservation, which is the task of the BASE. When we set up the historical contextualization project, our idea was not only to create an archive platform of the relevant BGE-internal and BGE-external documents on the siting process such as papers, letters, minutes, files, logs, and geological data, but also to show how the documents relate to each other. In order to do this, we had to take the historical background into account. At first, we began exclusively with the files and the eyewitness accounts of our site selection department, but we became increasingly aware that we also have to consider the interactions between all stakeholders involved in the site selection procedure. The presentation will show techniques and examples of how all information from documents, eye witnesses, and other sources can be saved and related to each other in order to understand how and why decisions have developed in a certain direction. A set of cohesive storylines, central documents, and oral history methods taken from contemporary historical research forms the background for this historical contextualization. A storyline consists of chronological actions in a certain topic, e.g. the writing of the sub-areas' interim report. The most important actions that are part of the storylines are related to the most important documents. Oral history methods are used for interviews with eyewitnesses to elicit their tacit knowledge and to reflect on the historical background and their emotions. The aim is not to create documentation from the BGE's perspective only, but also to collect a variety of perspectives as reservoirs of information for future historians, who can create a broader historical image from them. Another goal is to keep an open mind with regard to the multidisciplinary and transdisciplinary approaches of future social scientists.

Basis for a research project on marking strategies for a deep geological high-level radioactive waste (HLW) repository in Germany

Abstract. Marking has been considered as an approach for the preservation of information and awareness after the closure of a deep geological repository (DGR) for high-level radioactive waste (HLW). The discussion to use markers to warn people off from – in oversimplified terms – digging into a DGR dates back to the 1980s. Based on the recent recommendations by the Nuclear Energy Agency of the Organization for Economic Co-operation and Development (OECD NEA) regarding the preservation of records, knowledge and memory (RK&M) of DGRs in a systemic approach (NEA, 2019), the Federal Office for the Safety of Nuclear Waste Management (BASE) has identified the need for research regarding relevant aspects of potential future marking strategies in Germany. The first elaborate and sophisticated concept has been developed for the Waste Isolation Pilot Plant (WIPP) in the USA (DOE, 2004). Far from being “one marker”, it is a monumental multi-component system. Internationally, the WIPP concept provided abundant food for thought and inspired a new wave of discussions – not as a design to be copied to other DGRs (the WIPP concept has been developed for this specific DGR at its unique desert location), but as a model for site-specific ambitions to design a marker system that aims in earnest to preserve information and awareness on-site by non-mediated means. In NEA (2019) marking is described as one approach – out of nine – for the preservation of RK&M of DGRs. Marking was found to be focused, in particular, on preserving awareness. However, for a robust system, individual preservation mechanisms should not be planned in isolation, but rather “the components of a systemic RK&M preservation strategy should work together, complement and strengthen each other, act as indexes to each other and thus provide diversity and redundancy with a view to maximising information accessibility, understandability and survivability over the various timescales considered” (NEA, 2019, p. 95). In Germany, there are no regulatory requirements with regards to marking. The Site Selection Act (StandAG, 2017/2023), which sets the regulatory framework for the targeted DGR for HLW, does not mention marking. It mentions, however, two points with specific relevance to RK&M preservation: (1) important data and documents must be permanently stored, and (2) the DGR concept must ensure recoverability of the waste for 500 years after closure, thereby setting a post-closure time frame of specific interest. Against this background, a research project is planned to explore the relevant aspects of potential future marking strategies in Germany. Different types of markers may play different roles. And these roles may again change over time: what are the specifics imposed by the 500-year recoverability requirement? How could surface markers be combined with, e.g., concepts for the re-use of the site or possible added-value projects for the local community? Could off-site markers/monuments play a role? Who would be the stakeholders in a participatory process? With respect to deep geological markers, different questions arise, like the following: could markers within the DGR potentially play a role in recoverability concepts without negatively impacting long-term safety? What dependencies exist on the type of host rock yet to be selected? Could the same markers help warn an inadvertent intruder in the long term? The potential role of markers in a systemic RK&M preservation strategy needs to be investigated by taking other preservation approaches into consideration, too. Which mechanisms could strengthen the efficacy of markers? Which could be strengthened by markers? This presentation will deliver insight into the preliminary studies preceding the planned research project.

The anaerobic corrosion of the carbon steel overpack under anoxic alkaline conditions representing the Belgian supercontainer concept

Abstract. The Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) is responsible for the management of all radioactive materials (currently existing and future waste) generated in Belgium. Disposal in deep stable geological formations is, at present, accepted at an international level to be the most promising option for the long-term management of high-level and/or long-lived radioactive waste. ONDRAF/NIRAS currently recommends underground disposal in a geological stable and poorly indurated clay formation as the reference management option to steer their RD&D research programmes. The supercontainer concept is currently being considered as the reference design for the final disposal of vitrified high-level radioactive waste (VHLW) and spent fuel (SF) in Belgium. It comprises a prefabricated Portland cement-based buffer that completely surrounds a carbon steel overpack. In this highly alkaline environment (pH ∼ 13.6), and under normal conditions (i.e. without the ingress of aggressive species), the carbon steel overpack will be protected by a passive oxide film, which is expected to result in very low uniform corrosion rates. The main goal of the ongoing R&D corrosion studies performed at ONDRAF/NIRAS is to provide confidence that the integrity of the carbon steel overpack will not be jeopardized at least for the duration of the thermal phase. Considering the long timescales applicable to waste disposal, determining accurate and reliable estimates of the uniform corrosion rate under anoxic conditions forms a very important part of the R&D methodology of ONDRAF/NIRAS' corrosion programme. On the other hand, the occurrence of localized forms of corrosion cannot be neglected, and therefore it is also crucial to demonstrate that no other form of corrosion apart from uniform corrosion can take place. This paper gives an overview of the status of the research performed at ONDRAF/NIRAS with respect to the uniform corrosion, pitting corrosion and stress corrosion cracking behaviour of the carbon steel overpack of the supercontainer.

Requirements for inventory models of radioactive waste

Abstract. The aim of the final disposal of radioactive waste is to ensure the best possible safety for the long-term protection (i.e. 1 million years) of humans and the environment from ionizing radiation and other harmful effects of this waste. In Germany, the Site Selection Act (StandAG) regulates the procedure for determining the site with the best possible safety for a repository with high-level radioactive waste. One of the mandatory aspects of the StandAG is to perform various preliminary safety assessments in order to assess the expected extent of the safe confinement of the radioactive waste. For this purpose, all information (in particular on the quantity, type, composition and activity) of the waste (i.e. the inventory) is required to conduct the respective safety assessment. On behalf of the Federal Office for the Safety of Nuclear Waste Management (BASE) and within the framework of the research project “Significance of the radioactive waste inventory in the site selection for a repository (InvEnd)”, Brenk Systemplanung GmbH identified the characteristics of the inventory required for the preliminary safety assessments. To determine these inventory characteristics, two main data sources were analysed, namely the generic but host-rock-specific concepts for a German high-level radioactive waste repository and the inventory data and approaches of other countries with technologically developed repository programmes. The identified characteristics can be divided into different groups (e.g. geometric, chemical, radiological and physical) and categories such as primary inventory data that are essential for carrying out the preliminary safety assessments or secondary inventory data which validate the primary inventory data. The evaluation of the impact of the identified characteristics shows that especially data on component-specific chemical composition and component-specific radionuclide inventory have a high impact on the results of safety assessments. These characteristics are, in turn, dependent on other characteristics, such as the irradiation history and the original chemical composition of the fuel in addition to the structural parts of the fuel assemblies. Furthermore, the study assesses uncertainties of the inventory characteristics, such as data uncertainty and conceptual uncertainty. The extent of data uncertainty depends on the inventory characteristic. However, in most cases, only qualitative assessments are possible. Conceptual uncertainties depend on the degree of simplification used to define reference wastes but cannot be quantified yet, as no reference wastes have been defined. The results of this study enable authorities and waste management organizations to identify the key data of high-level radioactive waste inventories that are essential for conducting safety assessments of repositories, even during early stages of site selection and planning.

Using a multi-criteria approach for a regional differentiation of the likelihood of future volcanic activity in Germany

Abstract. Future volcanic activity, which might influence the long-term stability of radioactive waste repositories, must be evaluated critically. In Germany, according to the Repository Site Selection Act (StandAG), areas in which Quaternary volcanism is either present or future volcanic activity is expected within the next 1 Ma must be excluded from the site selection process for a high-level radioactive waste repository. However, while regions with Quaternary volcanism are reasonably well known in Germany, forecasting volcanic activity in intraplate fields is difficult, as many processes and their interaction control the spatial distribution of the volcanic centres. To develop an appropriate method to forecast possible regions of volcanic activity, a semi-quantitative, multi-criteria approach is used that combines a variety of indicators including, amongst others, seismic anomalies in the mantle, gravity data, tectonic activity, sutures, ground motion, earthquakes, mantle degassing centres and the geochronological data of volcanic rocks. The used indicators provide possible information about the source of melting in the mantle, the ascent of melt through the lithosphere and the eruption process at the surface. In total, 30 indicators were defined and ranked according to their presumed relevance for the long-term prediction of volcanism, following the results of two expert surveys. Quantifiable parameters were selected for the individual indicators, and threshold values were assigned and related to levels of significance. The parameters were uniformly applied for Germany in order to enable a transparent and equal risk assessment of future volcanic activity in all of the federal states. To consider the uncertainties regarding the distribution and manifestation of parameters in addition to their mostly unknown underlying large-scale processes, buffer zones were defined in which the values of relevance decrease with increasing distance from the initial parameter extent. By combining all parameters into an overall index, the presented results allow the regional differentiation of the likelihood of future volcanic eruptions in Germany. The robustness of the results is shown by varying the weights of the used parameters in the Germany-wide risk map. Different categories for the probability of future volcanic activity can be defined and used, depending on the needs and assessment of the volcanic hazard potential.

Biosphere modelling in the calculation basis for dose assessment for a final disposal of high-level radioactive waste

Abstract. The government of Germany decided to isolate high-level radioactive waste (HLW) in deep geological formations. Such a site should guarantee the highest possible level of safety over a period of 1×106 years. The Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) asked the Federal Office for the Safety of Nuclear Waste Management (BASE) and the Federal Office for Radiation Protection (BfS) to develop a directive for dose assessment for a final disposal of HLW, which has to be used for the site selection process by the Federal Company for Radioactive Waste Disposal (BGE mbH), the operator of the future HLW repository. This regulation contains detailed provisions for the gas-bound and water-bound radionuclide transport in the geosphere and biosphere and for the dose estimation of the public. It is built upon established concepts in radioecology and dosimetry. The comprehensive set of instructions is a toolbox for the dose estimation of a fictive representative person, one of the indicators of the safety of the repository. Modelling of the biosphere over 1×106 years is impossible because natural processes and human activities affect the evolution of the biosphere in an unpredictable way. In addition, human habits like the way of life and agricultural practices cannot be predicted even for only hundreds of years. Therefore, it was laid down in another German regulation that the current human habits have to be assumed over the period under consideration. Existing models and parameters of current German regulations for public dose calculations have been adjusted to the specific conditions of a final repository and complemented where necessary. The challenge of how to take into account the future climate and its implications on the biosphere was solved by using reference biospheres covering all possible evolutions of the biosphere including climate transitions. The application of reference biospheres to cover climate changes is a common approach in comparable safety analyses. Germany favours the enveloping approach described above that implicitly comprises all possible climatic conditions and the corresponding evolutions of the biosphere. The decisions on how to deal with the unknown future were the results of discussions between BMUV, BASE, and BfS. To ensure transparency and participation in the site selection of the HLW repository, the draft regulation was presented to the public, stakeholders, and the scientific community, providing the opportunity to comment on every part of the content. This consultation process and an expert workshop were the basis of the revision of the draft regulation. The final regulation was published at the end of 2022. The participation process helped to improve the document and to achieve a broader public acceptance. BASE and BfS are currently working on a comprehensive explanatory text to address all of the several hundreds of comments in detail.

Demonstrating the possibility of safe operation in the first phase of the site selection procedure in Germany

Abstract. The Federal Company for radioactive waste disposal (BGE) is responsible for the search for a site with the best possible safety for the disposal of high-level radioactive waste in Germany. The site selection procedure is regulated in a law that was adopted by the German Federal Parliament (Repository Site Selection Act – StandAG, 2017, last updated 2020 (StandAG)) and aims to be a participatory, transparent, learning, and self-questioning process based on scientific expertise. The first step of the first phase of the site selection procedure was completed in September 2020 and resulted in the identification of sub-areas that give reason for expecting favourable geological conditions for the long-term storage of nuclear waste in the sub-surface. These sub-areas cover approximately 54 % of Germany and are located in three different host rocks: rock salt–halite, claystone, and crystalline rock. The challenge for the next step is to find suitable siting regions within the previously determined sub-areas that are then considered further in the next phase of the site selection procedure (Hoyer et al., 2021). The preliminary representative safety analyses in Phase I, Step 2 of the site selection procedure require according to Section 7 (6) No. 4 EndlSiUntV that “the basic possibility of safe operation shall be demonstrated, but a complete operational safety analysis does not need to be performed”. The method is described in BGE (2022a, b) and can be summarized as follows. The demonstration of the basic possibility of safe operation is carried out within the framework of the representative safety analyses for the individual aspects of operational safety in different degrees of detail. The non-radiological aspects of operational safety, which are, for example, derived from occupational health and safety, mining law and fire protection, a site-independent consideration of the regulatory basis and the derivation of corresponding actions, are considered requirements for and are taken into account by the preliminary repository design according to Sect. 6 (4) EndlSiUntV. To demonstrate the basic possibility of safe operation for the operational safety aspects “radiation protection” and “release of radioactive substances” in the investigation areas to be considered, a general catalogue is compiled that summarizes possible events and actions to fulfill the requirements for operational safety. This general catalogue is independent of location and coverage of all host rocks and repository concepts. From this general catalogue, possible specific events are identified for each individual investigation area, and the necessary actions are derived. In addition to the demonstration of the basic possibility of safe operation, the robustness of operational safety within the investigation area is evaluated in a second step.

The collection, digitisation, interpretation and publication of geological data in the German site selection procedure – status and challenges

Abstract. The German site selection procedure for a repository site for high-level nuclear waste is subdivided into three major phases. During the first phase, the subsurface of Germany is evaluated based on existing data, with the aim of defining siting regions for surface-based underground exploration. Although Germany is covered by large amounts of geoscientific data – including ca. 50 000 boreholes deeper than 300 m and more than 340 000 km of reflection seismic lines – the legal obligation to make these data publicly available on a national scale is fairly new and dates back to the Geological Data Act of 2020 (GeolDG: Geologiedatengesetz vom 19. Juni 2020; BGBl. I S. 1387). However, significant amounts of data are available in analogue form only, posing a significant challenge to the timeline of the site selection procedure. As the German nuclear waste management organisation, the Federal Company for Radioactive Waste Disposal (BGE) is in charge of collecting, interpreting and publishing geoscientific data that are relevant to the decisions made within the site selection procedure. In this contribution, we outline the path of data within the first phase of the site selection procedure. We start with an overview of the BGE's current and past activities related to data collection and digitisation, focussing on a major campaign through which more than 16 000 drilling reports are being digitised in cooperation with the State Office for Mining, Energy and Geology in Lower Saxony (LBEG). We then report on the different approaches of data quality control, processing and supply, highlighting challenges related to the federal organisation of geodata storage in Germany. We present selected examples of how data are used for evaluating the subsurface in terms of suitability as a potential repository site. Finally, we give an outlook of how data could be integrated into a web application that is currently being developed as an interactive tool for the public presentation of results and underlying decisions.

Hydride reorientation in fuel cladding under interim storage conditions with low hoop stress

Abstract. The restart of the search for a final repository for high-level nuclear waste in Germany and the resulting so-called white map lead to a reassessment of the safety of interim storage as well. Due to the resulting increase in interim storage periods with a prospected timescale until 2068, among other things, renewed consideration of cladding tube integrity is required for the handling of the fuel assemblies during repackaging into the final storage containers. Besides oxide formation during operation, hydrides can have a negative influence on stability during interim storage and handling later on. The hydrides, which are initially precipitated tangentially in the tube material during operation, can reorient during interim storage as a result of the temperature and load history and precipitate again in a radial direction during cooling under load. In recent years, a number of investigations have already been carried out with respect to this topic, with cladding conditions that are present in high burn-up fuel assemblies with corresponding high hydrogen contents. In these cases, reorientation effects did occur with stresses that can be encountered in interim storage, but the proportions of radial hydrides were very low and were classified as not being harmful. In addition to the tests with cladding tubes containing high hydrogen contents, sporadic tests with low hydrogen contents were also carried out, for example, by Hardie and Shanahan (1975) and later by Chu et al. (2008). These indicated that strong reorientation effects could occur because of the complete dissolution of hydrogen under high temperature and tensile stress. In recent years, Kaufholz et al. (2018) also formulated a hypothesis that describes this effect as being the consequence of free nucleation without the presence of precipitates. This results in the possible formation of extremely pronounced radial hydrides, which can even extend through the tube wall. This is possible because residual hydrides that can hinder free precipitation or represent preferential points for nucleation are absent. The results of the present investigations confirm this behaviour. They show that massive reorientations can already occur at stresses at the lower end of the literature values and that, until now, these reorientations were interpreted as being uncritical. Considering the hysteresis in solubility between terminal solid solubility for dissolution (TSSD) and precipitation (TSSP), it becomes apparent that precipitation at hydrogen contents of 100 ppmw (parts per million by weight) already starts at approx. 538 K and that the hydrides precipitate radially under a stress of 52 MPa and form extremely pronounced hydrides. This results in a need for further research, since the thermal and mechanical loads under which the tests were carried out are well below the permissible limit loads in interim storage and below the values expected so far. This applies in particular to materials and cladding tubes that either absorb less hydrogen during operation or have only low burn-ups. The study provides an overview of the status quo, presents part of the investigations carried out during the project MUDZ (Modellierung und Untersuchung der Degradation von Hüllrohrmaterialien aus Zr-Legierungen durch Hydridbildungs- und Hydridverteilungsprozesse im Hinblick auf die Langzeitzwischenlagerung) in the KEK (Kompetenzerhalt in der Kerntechnik) initiative and the results obtained, and discusses the ramifications of the results.

SpannEnD – a 3D geomechanical model of Germany for the prediction of the recent crustal stress state

Abstract. The recent crustal stress state is a crucial parameter in the search for a high-level nuclear waste repository. It is relevant not only for site selection and short-term stability during operation but also for predicting long-term safety. However, the level of knowledge in this regard is still limited in Germany. It is based on stress orientations and stress regime information from the World Stress Map (WSM) project and pointwise stress magnitude records. We present the results of a 3D geomechanical numerical model that improves the state of knowledge by providing a continuum-mechanics-based prediction of the recent crustal stress field in Germany. The model extends over an area of 1000×1250 km2, covering Germany and nearby regions. It contains 22 geological units, each parameterized with individual properties (density, Young's modulus and Poisson's ratio). We assume linear elasticity, and the finite element method is used to solve the equilibrium of forces. The model is calibrated with horizontal stress magnitude records and validated with additional data, for example, vertical stress magnitudes or data of the WSM project. Our model results are in a good agreement with a mean orientation of the maximum horizontal stress (SHmax) derived from the WSM, as indicated by a mean of the absolute differences of 12∘. Furthermore, the model results lie entirely within the standard deviation of the derived orientation of SHmax. Mean values of the absolute stress differences between calibration data and model results of 4.6 MPa for the minimum horizontal stresses (SHmin) and 6.4 MPa for the SHmax also show a good agreement. The model results can be used, for example, for the calculation of fracture potential, for slip tendency analyses or as boundary conditions for smaller local models. For smaller model volumes, where little or no stress information is available, it provides synthetic stress magnitude data that can be used for calibration. As such, the model provides a valuable starting point for the detailed assessment of stability at yet-to-be-determined siting regions for a high-level nuclear waste repository.

Communicating scientific uncertainties

Abstract. Communicating a message of scientific uncertainty to a general audience is difficult because laypeople usually lack the conceptual tools to understand uncertainty (e.g., quantification methods). In addition, uncertainties can lead to mistrust and can be misused to prevent or delay radioactive waste management. On the proposed poster, we report the results of an experimental study (N=177) that investigated preferences for and trust in different visualizations of simulation results and tested different modes of communication (verbal, numerical, and visual/graphical). The participants processed three different parts, namely (1) the variation in the form of presentation of a scientific result, (2) the variation in the sender/source of a scientific result in numerical format, and (3) a direct comparison of two representations of simulation results (deterministic and probabilistic). Besides socio-demographics, we used items to check for the potential impact of numeracy and verbal skills, the need for a cognition concept, and questions on science in general, as well as the acceptance of a potential deep-ground repository. The dependent variable in each part was the degree of trust in the scientific validity of the information. For an example result, see Fig. 1. Clear influences of the non-dependent variables are only evident to a limited extent. Women rate their numerical abilities less highly than men do. There is no difference in verbal skills. The latter variable is also not explanatory, while numerical ability is relevant for the choice of the probabilistic variant and the corresponding trust rating in Experiment 3. The most striking result is that the attitude towards a deep geological repository for high-level nuclear waste (HLW; degree of acceptance) is the decisive parameter in all experimental conditions. The oral presentation will focus on another challenge of uncertainty communication, namely unquantifiable model uncertainties. In a repository, these may arise from the heterogeneity of the host rock. One of these potential uncertainties may be an undetected fault parallel in the drift axis in clayey host rocks. At the Konrad site, an anchor failure in an infrastructure drift resulted from such a fault. To deal with this kind of uncertainty, a certain kind of flexibility is needed to react to such events. The talk should initiate a discussion on the importance of communicating these potential uncertainties and on the impact of these uncertainties on the repository program.

Compaction of crushed salt for safe containment – a summary of the KOMPASS projects

Abstract. For the underground disposal of high-level nuclear waste in rock salt formations, the safety concept includes the backfilling of open cavities with crushed salt. For the prognosis of the sealing function of the backfill for the safe containment of the radioactive waste, it is crucial to have a comprehensive process understanding of the crushed-salt compaction behavior. The crushed-salt compaction process is influenced by internal properties (e.g., grain size, mineralogy, and moisture content) and boundary conditions (e.g., temperature, stress state, and compaction rate) and, therefore, involves several coupled thermal–hydro–mechanical (THM) processes (Hansen et al., 2014; Kröhn et al., 2017). With the paradigm shift from the limited release of radionuclides to safe containment due to the German Repository Site Selection Act passed in 2017, the importance of crushed salt as geotechnical barrier has increased, with a focus on the evolution of its hydraulic properties. Based on the knowledge gaps in the current process understanding, the “Compaction of crushed salt for safe containment” (KOMPASS) projects were initiated to improve the scientific basis behind using crushed salt for the long-term isolation of high-level nuclear waste within rock salt repositories. The efforts to improve the prediction of crushed-salt compaction begun during the first phase of the KOMPASS projects (Czaikowski et al., 2020) and were followed up in a second phase ending in June 2023. The primary achievements of the projects are as follows (Czaikowski et al., 2020; Friedenberg et al., 2022): specification of the KOMPASS reference material, an easily available and reproducible synthetic crushed-salt material, for generic investigations; development of pre-compaction methods and successful production of samples in the short term and under in situ loading conditions; formulation of an extended laboratory program addressing the isolated investigation of known relevant factors influencing the compaction behavior of crushed salt (Düsterloh et al., 2022); execution of long-term compaction tests addressing isotropic and deviatoric load changes, temperature, and compaction state; construction of a backfill body using the KOMPASS reference material in the Sondershausen mine through collaboration with the SAVER (Entwicklung eines salzgrusbasierten Versatzkonzepts unter der Option Rückholbarkeit) project (Schaarschmidt and Friedenberg, 2022); advancement of the tools for microstructure investigation methods (Svensson and Laurich, 2022); generation (first stages) of a microphysical process list combining literature research with our own findings; benchmarking of long-term compaction test for model development and optimization of various existing models as well as the development of new models; application of a virtual demonstrator (2D model representing a backfilled drift in rock salt) for the visualization of developments and the quantification of the models (Rabbel, 2022). In summary, the KOMPASS projects contributed to the reduction of uncertainties and the strengthening of the safety case for using crushed salt within rock salt repositories.

Does the Fukushima nuclear accident still matter? Analysis of its mediated effects on five dimensions of nuclear power acceptance by using the parallel multiple mediator model

Although 10 years have passed since the Fukushima nuclear accident, it continues to greatly impact global energy policy and the acceptance of nuclear power. This study aims to empirically analyze the Fukushima accident's effects on five dimensions of nuclear power acceptance and the interactions of these effects with various determinants of nuclear power acceptance. We analyze survey data from 1020 Korean respondents and find that they remain highly worried about the Fukushima accident 10 years later. The Fukushima accident has negative impacts on the general acceptance of nuclear power, acceptance of nuclear power's contributions to addressing climate change, acceptance of high-level radioactive waste disposal sites, and acceptance of extensions of old nuclear power plants. However, the accident has a positive impact on the acceptance of denuclearization policies. Our parallel multiple mediator model shows that the Fukushima accident has only an indirect effect on general acceptance; this effect is negative and significant. The accident has a positive direct and indirect effect on acceptance of denuclearization policies but negative direct and indirect effects on the acceptability of high-level nuclear waste disposal and extensions of old nuclear power plants. Finally, we find a positive direct effect and a negative indirect effect of the accident on acceptance of nuclear power's contributions to addressing climate change. The latter is greater than the former, meaning that acceptance of nuclear power in the context of addressing climate change fell in response to the accident. In short, the Fukushima accident has different direct and indirect effects across different dimensions of nuclear power acceptance.