Transdisciplinary research on the topic of long-term near-field monitoring of a geological repository with a view to building trust

Within the framework of disposal of radioactive waste in Germany, the question arises how trust in the safety of a future deep geological repository and therefore the acceptability can be increased. One aspect that could contribute to this is the option of long-term monitoring of a deep geological repository by participation of the civil society. Whether and exactly how long-term monitoring of a deep geological repository leads to more trust, is being researched in the transdisciplinary work package TRUST within the research project TRANSENS in cooperation with members of the civil society. For the transdisciplinary processing of specific repository topics, a group of 16 persons from the civil society were recruited, none of which were stakeholders with respect to the topic of repositories. This group is designated as the Working Group Civil Society (AGBe). With the help of 12 members of the AGBe a first workshop on the topic “Monitoring and trust” was carried out on 13 March 2021, supported by partners of the LUH-IRS, the TUBS-IGG and the ETH-TdLab. This article is concerned with the preparation work, the course itself and the knowledge gained from the workshop. It deals with the preparation work in the form of a website and a report on information of the AGBe suitable for those who have been addressed, which has meant a challenge in view of the complexity of the topic of monitoring of a deep geological repository and prior knowledge of the AGBe. Furthermore, the course of the workshop, which was carried out online due to the coronavirus pandemic, is discussed, in which the 12 members of the AGBe and 10 scientists came together. The workshop began with a brainstorming on the topic of monitoring. This was followed by two specialist lectures, in which information on deep geological disposal and monitoring as well as the possibilities and limits of monitoring and monitoring conceptions was presented. The members of the AGBe were then divided into three groups, in which the central research questions of the workshop were discussed: Does a long-term near-field monitoring contribute to trust in the safety of deep geological disposal of radioactive waste? Which aspects of monitoring conceptions could increase trust in the implementation of near-field monitoring and which do not? After the presentation of the results from the group discussion a common discussion was first carried out in the plenum, in which a picture of the sentiments within the AGBe was finally formulated. Furthermore, the knowledge from the three discussion formats of the workshop is presented in this article. This includes the characterization of information and communication with the civil society as a central aspect in relationship with monitoring and trust. Furthermore, it showed that the AGBe views the long-term monitoring of a repository as a suitable measure for gaining trust. However, which aspects of monitoring conceptions contribute significantly to trust in the safe storage of radioactive waste is less clear after this first workshop and could not be conclusively answered. The results of this workshop with the AGBe reflect a first impression in the discussion on monitoring and trust. As monitoring is a complex topic with many interfaces on repository storage and sealing concepts, repository processes and multiple physical simulations as well as on societal topics, the discussion with the AGBe on the topic of monitoring should be continued during the course of the project. Thereby, it must be taken into consideration if the first positive assesments as well as the first AGBe specific requirements regarding long-term near-field monitoring activities will change with increasing knowledge.

Knowledge Management in EURAD: the Roadmap

Knowledge management (KM) is a core activity of the European Joint Programme on Radioactive Waste Management (EURAD), which has driven the development of the EURAD Roadmap, a roadmap for the implementation of radioactive waste management (RWM) leading to geological disposal, using the experiences of advanced national and EU programmes. Because these programmes have been developed over more than 40 years and have adopted different technical and strategic approaches to deep geological repository (DGR) development, the roadmap has been structured to capture largely generic best practice in its presentation of programme Phasing and Thematic goals breakdown structure: 5 Phases of Implementation: Initiation, Site Selection, Site Characterization, Construction, and Operations and Closure. For each phase the roadmap explains how activities and existing knowledge are used to fulfil generic safety and implementation goals common across RWM programmes. 7 themes: Programme Management, Predisposal, Engineered Barrier Systems, Geoscience, Design and Optimisation, Siting and Licensing, and Safety Case. Each theme is further elaborated into sub-themes and domains. The intention of the EURAD roadmap is to provide information and guidance to three primary user groups: Organisations that are developing or updating their national RWM programmes with the objective of moving towards deep geological disposal of some of their wastes and requiring information on the steps involved; Organisations with advanced RWM and DGR programmes that require an informative training tool for new staff and a means of propagating knowledge across the groups involved in diverse activities; All organisations concerned with identifying potential future gaps in capabilities that could hinder implementation of their DGR programmes in decades to come. We recognize that there is not a unique route through the roadmap – like any roadmap, it can be used to stimulate consideration of alternative options and can be adapted by each Member State to suit national priorities. Because of its generic nature, the EURAD roadmap will be populated with content that is common to all programmes, creating a basis and strategy for a long-term project of Knowledge Management and Networking in RWM. The current focus is on signposting to existing content and the generation of new content to fill identified gaps for State of Knowledge, State of the Art, Guidance, Training and active Networks or Communities of Practice.

Lessons from archaeology and heritage studies for the long-term preservation of records, knowledge and memory concerning deep geological disposal sites for nuclear waste

Safe disposal of nuclear waste in deep geological repositories requires secure knowledge transfer or knowledge recovery in time spans of many tens of thousands of years. Never before has any detailed record, knowledge or memory been reliably preserved or recovered over comparable time periods. This challenge has been extensively addressed since the late 1980s, initially during the SANDIA workshops in the USA and more recently in the Nuclear Energy Agency/Organisation for Economic Co-operation and Development (NEA/OECD) project on Preservation of Records, Knowledge and Memory Across Generations (Schröder, 2019). Experts from many disciplines including engineering, the natural sciences, information technology, social studies of science and technology, semiotics, public management, and design as well as artists have contributed to these discussions. Some scholars from the humanities have been involved in working on these issues, especially in recent decades. At the same time, much of the existing work has drawn on assumptions about human history, archaeological monuments and cultural heritage that have been scrutinized and deemed deeply flawed by Joyce (2020). The authors of the present paper are archaeologists and cultural heritage experts. For the past decade, they have been working with the challenge of preserving records, knowledge and memory concerning deep geological disposal sites for nuclear waste (Holtorf and Högberg, 2021). From the perspective of the human sciences, in particular archaeology and heritage studies, the unique task at hand involves not only the previously recognized challenges that require consideration of long-term material durability, linguistic intelligibility, and appropriate sense-making of any communicated information but also two challenges not previously addressed: Human action as informed by cultural and social processes. In designing of various long-term mechanisms, we risk overlooking that what people will do is not going to be governed by mechanics. How human beings learn, reason, value, decide, and act is informed by specific cultural and social processes creating context and meaning. We must avoid ignoring these complexities governing human thinking and agency. This challenge requires more work on understanding how sentient and intelligent beings like humans act in variable contexts across time and space. Our anticipatory assumptions. A proverb states that “nothing ages faster than the future”. In making assumptions about future generations' understandings, meanings, and significances of our nuclear waste we risk “colonizing” the future, fail to embrace variability over time, and miss realizing multiple futures and emerging conditions. We must therefore not foreclose uncertain futures but instead create circumstances favorable for change and transformation of relevant knowledge and memory. This challenge requires more work with processes of translation between generations. The challenges of assessing our anticipatory assumptions and understanding how humans act will also need to be addressed in transmitting records, knowledge and memory for the benefit of future generations.

The European Joint Programme on Radioactive Waste Management EURAD: update of its strategic research agenda

The European Joint Programme on Radioactive Waste Management EURAD brings together various research actors, namely waste management organisations (WMO), technical support organisations (TSO) and research entities (RE), to work on a joint strategic research agenda (SRA) focusing on deep geological disposal of radioactive waste. In total, 116 project partners from 23 countries have worked jointly since 2019 in collaborative RD&D work packages, strategic studies and various knowledge management activities. EURAD research is driven by the need for implementation of a deep geological repository and its safety, while aiming for scientific excellence. EURAD has developed a roadmap which is seen as a representation of a generic radioactive waste management (RWM) programme. The content is focused on what knowledge and competencies (including infrastructures) are considered most critical for RWM and implementation of deep geological disposal, in alignment with the EURAD vision. Here, the current SRA update process will be outlined from the perspective of Europe's research entities contributing to EURAD. In this context, the international network of research entities EURADSCIENCE plays a key role. EURADSCIENCE addresses – and will address during decades to come – scientific excellence in (the full lifecycle of) radioactive waste management from cradle to grave. As an independent, cross-disciplinary and inclusive organization, its overarching aim is to ensure scientific excellence and credibility in decision-making on RWM, regardless of national implementation status, waste type or national inventory. To this end, EURADSCIENCE will define and update its own SRA. The approach here is to maintain a holistic view of scientific disciplines and provide scientific excellence to advance progress of national radioactive waste management programmes, and to ensure scientific credibility of waste management concepts as well as addressing fundamental requirements related to knowledge management. More generally speaking, EURADSCIENCE aims to bring forward a vision that assures that scientific excellence and ever-developing scientific advances are integrated at any given time into the multigenerational implementation process of geological disposal. Similarly, the respective WMO and TSO networks, IGD-TP and SITEX, have developed their SRAs based on their specific roles and perspectives. Ultimately, the overlap between these SRAs will define the envelope for future European RD&D activities in the context of RWM. The update process has recently been consolidated after consultations between the three actor groups. Ultimately, the EURAD general assembly will have to approve the SRA update process regarding its alignment with the EURAD roadmap, the development of the seven existing SRA themes, the development of future RD&D activities via an EURAD exchange forum and the focus of RD&D planning for the next 10 years.

Twenty years of research on spent nuclear fuel in the context of final disposal at KIT-INE in multinational collaborative projects

Disposal of spent nuclear fuel (SNF) in deep geological repositories is considered a preferential option for the management of such wastes in many countries with nuclear power plants. With the aim to permanently and safely isolate the radionuclide inventory from the biosphere for a sufficient time, a multibarrier system consisting of technical, geotechnical and geological barriers is interposed between the emplaced waste and the environment. In safety assessments for deep underground repositories, access of water, followed by failure of canisters and finally loss of the cladding integrity is considered in the long-term. Hence, evaluating the performance of SNF in deep geological disposal systems requires process understanding of SNF dissolution and rates as well as quantification of radionuclides release from SNF under reducing conditions of a breached container. In order to derive a radionuclide source term, the SNF dissolution and alteration processes can be assigned to two steps: (i) instantaneous release of radionuclides upon cladding failure from gap and grain boundaries and (ii) a long-term release that results from dissolution of the fuel grains itself (Ewing, 2015). In this context, research at KIT-INE has focused for more than 20 years on the behavior of SNF (irradiated UO2 and MOX fuels) under geochemical conditions (pH, redox and ionic strength) representative of various repository concepts, including the interaction of SNF with backfill material, such as bentonite as well as the influence of iron corrosion products, e.g. magnetite and radiolytic reactions on SNF dissolution mechanisms. Since 2001, KIT-INE has contributed with experimental and theoretical studies on the behavior of SNF under repository relevant conditions to six Euratom projects viz SFS (2001–2004), NF-PRO (2004–2006), MICADO (2006–2009), RECOSY (2007–2011), FIRST-Nuclides (2012–2014) and DISCO (2016–2021). Moreover, since 2007, overall 4 consecutive projects for the Belgian waste management organization, ONDRAF-NIRAS, were performed on the behavior of SNF under conditions representative of the Belgian “Supercontainer” concept. In this contribution, we summarize major achievements of theses research projects to understand and quantify the radionuclide release from dissolving SNF under repository conditions. In particular, the dependence of radionuclide release on the chemical composition of the aqueous and gaseous phase in the proximity of repositories in different types of host rock is discussed.

Evaluation of the corrosion behavior of modern spent nuclear fuels under repository conditions

In Germany it is planned to directly dispose spent nuclear fuel (SNF) from nuclear power plants together with other high-level radioactive wastes (HLW) from former SNF reprocessing (e.g., vitrified waste), in a deep geological repository for heat-generating wastes – the siting process for this repository was started in 2017 and is ongoing. Based on several decades of research, development, and demonstration (RD&D) it is generally accepted at the technical and scientific level that direct disposal of HLW and SNF in deep mined geological repositories is the safest and most sustainable option (CEC, 2011; IAEA, 2004). The current efforts to improve the performance and accident tolerance of fuels in nuclear power generation resulted in an increased utilization of a variety of new types of light-water reactor (LWR) fuels such as fuels doped with Cr, Al, and Si. This doping leads to a significant change of the microstructure of the fuel matrix. The corrosion behavior of these types of fuels under conditions relevant to deep geological disposal has hardly been studied so far; however, this is of crucial importance as the development of a robust safety case for deep geological disposal of SNF requires a solid understanding of its dissolution behavior over very long time scales (up to 1 million years). To fill this knowledge gap, additional systematic studies on modern doped UO2 fuels were needed. Corrosion experiments with SNF cannot entirely unravel all of the various concurring effects of the dissolution mechanism due to the chemical and structural complexity of SNF and its high beta and gamma radiation field during the first 1000 years; moreover, technical restrictions only allow a very limited number of experiments. Therefore, within the EU-DisCo project (https://www.disco-h2020.eu, last access: 11 October 2021), a very ambitious programme of corrosion studies on irradiated Cr and Al/Cr doped fuels was carried out, which was complemented by systematic single-effect dissolution studies (e.g., with respect to doping level, grain size and thermodynamic aspects) performed on carefully prepared and characterized, simplified UO2-based model materials. Here, we present recent results on the dissolution behavior of tailor-made UO2 model materials in accelerated static batch experiments using H2O2 as simulant for radiolytic oxidants, present in long-term disposal scenarios for SNF in failed container conditions due to the alpha irradiation of water. In these dissolution experiments pure UO2 reference pellets exhibiting different densities and grain sizes, as well as Cr-doped UO2 pellets with various Cr-doping levels, produced using different doping methods having different grain sizes, were used. In addition, Nd-doped and industrially produced Cr- and Cr/Nd-doped UO2 pellets were used to determine the influence of these parameters on the dissolution rates. The dissolution experiments were performed under strictly controlled conditions with respect to exclusion of oxygen, temperature control, and exclusion of light. This bottom-up approach was followed to understand how the addition of Cr-oxide into the fuel matrix affects SNF dissolution behavior under repository relevant conditions. The results of the dissolution experiments performed with real SNF and the model materials obtained by the DisCo partners build the basis for numerical simulations on the dissolution behavior of modern SNF. First results of the data evaluation indicate that the addition of dopants and the consequential modification of the fuel matrix does not lead to a significant change of the dissolution behavior of these fuels under repository relevant conditions compared to standard SNF (i.e. dissolution rates agree within an order of magnitude).

Michigan International Copper Analogue (MICA) project – current status

One of the key requirements for the deep geological disposal of high-level nuclear waste is the assessment of its long-term performance and safety (up to 1 Ma). Regarding engineered barrier system materials, such as copper, much of the data available comes from short-term investigations, such as laboratory experiments at different scales. Copper is an important part of many waste packaging and disposal concepts, e.g. KBS-3 developed in Sweden and Finland and Mark II developed in Canada. Natural analogues provide another important way of obtaining understanding on potential repository system behavior. Observations made from the geological systems can be utilized in the safety case, providing information on the assessment time scale. Copper analogue studies (both natural analogues and archaeological analogues) have been reported in the literature and they have been extensively reviewed by various authors (e.g. Miller et al., 2000) and by safety case projects (e.g. Reijonen et al., 2015) within waste management organizations. So far, only a few studies have focussed on the general stability of native copper within its natural media (e.g. Milodowski et al., 2000; Marcos, 2002). Keweenaw native copper occurrences (Lake Superior, USA) have been mentioned as a qualitative source of information (e.g. in Miller et al., 2000); however, data to be used in process-based safety assessments for geological disposal are lacking. These deposits have been mined for a long time and there is a great deal of knowledge related to them as well as samples collected, but no formal review has been made from the geological disposal point of view. The native copper at the Keweenaw area reflects various geological environments from bedrock to sediment and even anthropogenic mine site remnants and geochemical environments (e.g., anoxic vs. oxic, sulphur-free vs. sulphur-bearing). It thus provides a unique complementary data source that will be useful for estimating processes governing behavior of metallic copper. The MICA project phase I systematically collects and reviews the existing literature and data on the Michigan copper analogue sites and available sampling potential. Here, we present the current status of the project.

Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels

Cr-doped UO2 as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO2 nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels. Here, we present new wet chemical approaches providing the basis for such model systems, namely co-precipitation and wet coating. Both were successfully tested and optimized, based on detailed analyses of all synthesis steps and parameters: Cr-doping method, thermal treatment, reduction of U3O8 to UO2, green body production, and pellet sintering. Both methods enable the production of suitable model systems with a similar microstructure and density as a reference sample from AREVA. In comparison with results from the classical powder route, similar trends upon grain size and lattice parameter were determined. The results of this investigation highlight the significance of subtly different synthesis routes on the properties of Cr-doped UO2 ceramics. They enable a reproducible tailor-made well-defined microstructure, a homogeneous doping, for example, with lanthanides or alpha sources, the introduction of metallic particles, and a dust-free preparation.

Numerical Study on the Effect of Enhanced Buffer Materials in a High-Level Radioactive Waste Repository

In deep geological disposal system designs, it is important to minimize the installation area for cost effectiveness while satisfying the thermal requirements of the systems. An effective method to reduce the installation area for the systems is to employ an enhanced buffer material, as this can decrease the spacing between the disposal tunnels and deposition holes. Therefore, this study aims to evaluate the effect of an enhanced buffer material on the thermal behavior of the systems and their spacing. First, the discrete element method (DEM) was adopted to validate the thermal conductivity of the enhanced buffer material used, which was a mixture of bentonite and graphite. Then, a 3D finite element method (FEM) was conducted to analyze the proper disposal tunnel and hole spacing considering three cases with thermal conductivities values of the buffer as 0.8 W/(m K), 1.0 W/(m K), and 1.2 W/(m K). The results showed that the disposal tunnel and hole spacing could be reduced to 30 m and 6 m, respectively, when the temperature of the buffer surface facing the canister was 100 °C with a thermal conductivity value of approximately 1.2 W/(m K) or if more than 3% of graphite is added.

Ex and In Situ Reactivity and Sorption of Selenium in Opalinus Clay in the Presence of a Selenium Reducing Microbial Community

79Se is a critical radionuclide concerning the safety of deep geological disposal of certain radioactive wastes in clay-rich formations. To study the fate of selenium oxyanions in clayey rocks in the presence of a selenium reducing microbial community, in situ tests were performed in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland). Furthermore, biotic and abiotic batch tests were performed to assess Se(VI) and Se(IV) reactivity in the presence of Opalinus Clay and/or stainless steel, in order to support the interpretation of the in situ tests. Geochemical modeling was applied to simulate Se(VI) reduction, Se(IV) sorption and solubility, and diffusion processes. This study shows that microbial activity is required to transform Se(VI) into more reduced and sorbing Se species in the Opalinus Clay, while in abiotic conditions, Se(VI) remains unreactive. On the other hand, Se(IV) can be reduced by microorganisms but can also sorb in the presence of clay without microorganisms. In situ microbial reduction of Se oxyanions can occur with electron donors provided by the clay itself. If microorganisms would be active in the clay surrounding a disposal facility, microbial reduction of leached Se could thus contribute to the overall retention of Se in clayey host rocks.