Thermally Constrained Conceptual Deep Geological Repository Design under Spacing and Placing Uncertainties

The temperature evolution within a deep geological repository (DGR) is a key design consideration for the safe and permanent storage of the high-level radioactive waste contained inside used nuclear fuel containers (UFCs). Due to the material limitations of engineered components with respect to high temperature tolerance, the Nuclear Waste Management Organization of Canada requires the maximum temperature within a future Canadian DGR to be less than 100 °C. Densely placing UFCs within a DGR is economically ideal, but greater UFC placement density will increase the maximum temperature reached in the repository. This paper was aimed to optimize (i) the separation between UFCs, (ii) the distance between container placement rooms, and (iii) the locations of the age-dependent UFCs in the placement rooms for a conceptual DGR constructed in crystalline rock. Surrogate-based optimization reduced the amount of computationally expensive evaluations of a COMSOL Multiphysics model used to study the temperature evolution within the conceptual DGR and determined optimal repository design points. Via yield optimization, nominal design points that considered uncertainties in the design process were observed. As more information becomes available during the design process for the Canadian DGR, the methods employed in this paper can be revisited to aid in selecting a UFC placement plan and to mitigate risks that may cause repository failure.

Development of a database for the analysis of the disposal system in the representative preliminary safety analysis

The Federal Company for Radioactive Waste Disposal (BGE) is the German waste management organisation responsible for implementing the search for a site with the best possible safety for the disposal of high-level radioactive waste for at least 1 million years, following the amendments of the Repository Site Selection Act in 2017. The selection procedure is meant to be a participatory, transparent, learning and self-questioning process based on scientific expertise. It consists of three phases with an increasing level of detail. The first step of the first phase of the site selection procedure was completed in September 2020 and resulted in the identification of 90 subareas that give reason to expect favourable geological conditions for the safe disposal. The potentially suitable subareas cover approximately 54 % of Germany and are located in three different host rocks: rock salt (halite), claystone and crystalline rock. The second step of phase one is currently in progress and includes the so-called representative preliminary safety analyses that aim to assess the extent to which the safe containment of the radioactive waste can be expected. Representative preliminary safety analyses are one of the foundations for deciding whether an area will be considered for surface-based exploration in the next phase of the site selection procedure. Within the preliminary safety analyses, the behaviour of the disposal system is analysed in its entirety, across all operational phases of the repository and under consideration of possible future evolution of the disposal system with respect to the safe containment of the radioactive waste. The development of a database is described, which aims to systematically document and provide the framework needed for the analyses of the disposal systems in the subareas regarding the safe containment of the radionuclides over the assessment period of 1 million years. This database includes the vast amount of information about the different components of the disposal system. This includes also the geological setting, the technical conception of the repository and compilations of values for the physical, geoscientific, and technical parameters characterising the various barriers of the disposal system. Furthermore, a self-contained derivation of expected and deviating future evolution of the disposal system and its geological setting is included; following the so-called features, events and processes (FEP) strategy.

Identification and evaluation of processes for joint disposal of high level radioactive waste and low to intermediate level radioactive waste

The on-going research project „Identification and evaluation of processes that can arise by disposing of both high level radioactive waste (HAW) and low to intermediate level radioactive waste (LAW/MAW) at the same site“ (GemEnd, FKZ 4719F10401), commissioned by the Federal Office for the Safety of Nuclear Waste Management (BASE), is concerned with the question which thermal, hydraulic, mechanical, chemical and biological (THMCB) processes could be of importance for the long-term safety of the geological repository for high level radioactive waste. The focus of the project is on mutual influences between the HAW and LAW/MAW repositories, which should be constructed separately according to the Safety Regulations (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, BMU, 2020). A second point of emphasis is on processes that could result from the disposal of small amounts of LAW/MAW within the HAW repository. The analyses carried out for each of the potential host rocks clay rock, rock salt and crystalline rock as well as for a combination of clay rock above crystalline rock at a generic site are divided into a qualitative and a quantitative part. As for the qualitative analyses, all potentially relevant processes are identified and evaluated as to whether they are negligible or principally relevant for the exemplary repository configurations considered and according to the current state of knowledge. With regard to the quantitative analyses, the possible extent of potentially safety-relevant processes is illustrated by means of coupled numerical simulations. Of special interest are the effects of particularly sensitive model approaches and/or parameters and notably of the distances between the HAW and LAW/MAW repositories in the different exemplary repository configurations considered. From the results of the quantitative and qualitative analyses, knowledge gaps will be identified and the possibility of their reduction by research and development activities will be discussed. Furthermore, aspects of the transferability of the results to the German site selection procedure will be illuminated. At the interdisciplinary research symposium safeND selected preliminary results of both the qualitative and quantitative analyses will be presented.

Geochemical benchmark tests to validate the conversion of thermodynamic data for TOUGHREACT

According to the ongoing site selection process for a repository for high-level radioactive waste in Germany, rock salt, clay and crystalline rock are possible host rocks. The pore water of these rocks contains saline solutions with high ionic strengths. To model the speciation and/or migration of radionuclides in long-term safety analyses for nuclear waste disposal, a geochemical code that includes thermodynamic data suitable for saline solutions is needed. Thermodynamic equilibrium in saline solutions with high ionic strengths is usually modelled using the Pitzer approach (Pitzer, 1991). Within the context of nuclear waste disposal, the THEREDA project (Moog et al., 2015) provides thermodynamic data for some widely used geochemical codes (PHREEQC, Geochemist's Workbench, ChemApp, and EQ 3/6) using the Pitzer approach; however, for modelling in long-term safety analyses for nuclear waste disposal, another geochemical code, TOUGHREACT, is used. Therefore, scripts were developed to convert thermodynamic data of the THEREDA project to be applicable in TOUGHREACT. The scripts were validated by benchmark tests and by comparing calculations using PHREEQC and TOUGHREACT (Weyand et al., 2021). In total, 50 different benchmark tests were performed considering 3 specific geochemical systems, which are relevant to long-term safety analyses: (1) oceanic salt system, polythermal: K, Mg, Ca, Cl, SO4, H2O(l), (2) actinide system, isothermal: Am(III), Cm(III), Nd(III), Na, Mg, Ca, Cl, OH, H2O(l) and (3) carbonate system, isothermal: Na, K, Mg, Ca, Cl, SO4, HCO3/CO2(g), H2O(l). Each benchmark test considered specific ion concentrations in solution and in gaseous phases in the presence of specific minerals. The benchmark tests derived the geochemical equilibria and the results of both codes were compared to each other and to experimental data. The results of the calculations using both codes showed a good correlation. Remaining deviations can be explained by technical differences of the codes.

A systematic approach for surface exploration of sites – analysis of international exploration programs

The site selection procedure for a high-level radioactive waste repository in Germany is based on the Repository Site Selection Act (StandAG, 2017), which comprises three phases. In phase 2 the Federal Company for Radioactive Waste Disposal (BGE) will conduct surface exploration. Based on the exploratory findings, the further developed preliminary safety analyses, the common requirements and criteria, and potential socioeconomic analyses will be applied feeding into proposed sites for underground exploration. Commissioned by the BGE, the Federal Institute for Geosciences and Natural Resources (BGR) contributes to this procedure with the projects GeoMePS and ZuBeMErk, which collate and assess geoscientific and geophysical methods and programs for surface exploration. Their common goal is to develop recommendations for surface exploration of siting regions. For this purpose, the BGR has developed a systematic approach that includes (1) deducing exploration targets, (2) compilation of geoscientific and geophysical exploration methods in a database structure, and (3) analysis of case studies of national and international exploration programs for high-level radioactive waste disposal. Exploration targets are based on the common criteria and requirements as defined by the StandAG. The identified exploration targets (Kneuker et al., 2020) together with a large number of geoscientific and geophysical exploration methods were integrated and linked within the BGR database “GeM-DB”. All methods were evaluated according to their suitability and applicability for (a) the three defined host rocks (crystalline rock, claystone, rock salt) and (b) the previously defined exploration targets. In step (3) the BGR reviews national and international waste disposal programs exploring for crystalline rock, claystone, and rock salt. Here, the focus is on nondestructive and minimally invasive surface exploration techniques, such as geophysical airborne and ground-based methods or investigations in drill holes and on drill cores. The aims are to identify gaps in the method catalogue of the GeM-DB and to infer exploration directives for surface exploration during phase 2. An example is the analysis of the Swedish site selection process, especially the site investigation program. There, the site investigations are, e.g. the basis for the discipline-specific site descriptive models, which were applied for design and safety assessments (SKB, 2001). The Swedish site investigation program along with programs of other countries considering crystalline host rocks, such as Finland and Canada, show a common ground, which could be adapted for surface exploration of crystalline host rock regions in Germany. The assessment and evaluation of selected programs exploring for rock salt and claystone is currently in progress. The entire systematic approach of the projects GeoMePS and ZuBeMErk aims to develop recommendations for a nondestructive and minimally invasive surface exploration program of siting regions in Germany, regarding the lithological, structural, mechanical, and hydrogeological characterization of the different host rock formations.