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.

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.

Microbiology of Barrier Component Analogues of a Deep Geological Repository

Canada is currently implementing a site selection process to identify a location for a deep geological repository (DGR) for the long-term storage of Canada’s used nuclear fuel, wherein used nuclear fuel bundles will be sealed inside copper-coated carbon steel containers, encased in highly compacted bentonite clay buffer boxes and sealed deep underground in a stable geosphere. Because a DGR must remain functional for a million years, there is value to examining ancient natural systems that serve as analogues for planned DGR components. Specifically, studying the microbiology of natural analogue components of a DGR is important for developing an understanding of the types of microorganisms that may be able to grow and influence the long-term stability of a DGR. This study explored the abundance, viability, and composition of microorganisms in several ancient natural analogues using a combination of cultivation and cultivation-independent approaches. Samples were obtained from the Tsukinuno bentonite deposit (Japan) that formed ~10 mya, the Opalinus Clay formation (Switzerland) that formed ~174 mya, and Canadian shield crystalline rock from Northern Ontario that formed ~2.7 bya. Analysis of 16S rRNA gene amplicons revealed that three of the ten Tsukinuno bentonite samples analyzed were dominated by putative aerobic heterotrophs and fermenting bacteria from the Actinobacteria phylum, whereas five of the Tsukinuno bentonite samples were dominated by sequences associated with putative acidophilic chemolithoautotrophs capable of sulfur reduction.

Lifetimes of Used Nuclear Fuel Containers Affected by Sulphate-Reducing Bacteria Reactions inside the Canadian Deep Geological Repository

The present work aims at approximating the reduction of sulphate to sulphide caused by sulphate-reducing bacteria (SRB) inside the Canadian deep geological repository in order to calculate the expected lifetime of used nuclear fuel containers (UFCs). Previous studies have assumed a conservative constant concentration of sulphide at the host rock interface. The novelty of this study resides in the use of first-order kinetics to explicitly account for the SRB-induced sulphide production. This reaction term is developed following an empirical approach using published results on actual sulphate reduction by SRB and included in a coupled reaction-diffusion system. Lifetimes of UFCs are subsequently calculated following the conditions of two scenarios: having SRB active only at the region closest to the host rock and having SRB active at the host rock and throughout the bentonite clay. This study shows that the mean lifetimes of UFCs in both cases are above one million years. However, more accurate results would require the characterization of the host rock and groundwater of the prospective emplacement, as well as additional experiments on growth and sulphide production by the microbial communities from the site.

Comparison of Gamma and Neutron Dose Rate Variations with Time for Cast Iron and Metal—Concrete Casks Used for RBMK-1500 Spent Fuel Storage

The present SF management concept in Lithuania envisages that spent RBMK-1500 fuel will be stored in dry storage containers for 50 years, before being disposed of in a deep geological repository. However, the risk that a deep geological repository will not be constructed at the planned time should be taken into account, and the extension of SF storage over 50 years should be considered. This paper presents a comparison of gamma and neutron dose rate distributions and variations with planned and extended storage times for cast iron and metal–concrete containers loaded with RBMK-1500 SF. All calculations were performed using the SCALE computer codes system. The modeling results show that the overall shielding properties of the CONSTOR® RBMK-1500 container containing the same neutron and gamma sources are better than those of the CASTOR® RBMK-1500 container. During an extended storage period (from 50 to 300 years), the total dose rate would decrease considerably and the dose rate due to neutrons would become dominant for both containers.

Swelling Pressure and Permeability of Compacted Bentonite from 10th Khutor Deposit (Russia)

Bentonites from the 10th Khutor deposit (Republic of Khakassia, Russia) are considered a potential buffer material for isolation of radioactive waste in the crystalline rocks of Yeniseyskiy site (Krasnoyarskiy region). This study presents the results of a series of permeameter experiments with bentonite compacted to dry densities of 1.4, 1.6, and 1.8 g/cm3, saturated and permeated by the artificial groundwater from Yeniseyskiy Site. Permeation was conducted at hydraulic gradients of 180–80,000 m/m to simulate potential hydraulic conditions in the early post-closure phase of a deep geological repository (DGR). The respective swelling pressures of 0.8 ± 0.3, 2.2 ± 0.6, and 6.3 ± 0.3 MPa and permeabilities of (27 ± 15) × 10−20, (3.4 ± 0.8) × 10−20, and (0.96 ± 0.26) × 10−20 m2 were observed for the hydraulic gradient of 2000 m/m, which is recommended for the determination of undisturbed swelling pressures and permeabilities in permeameter experiments. Upon incremental increases in the hydraulic gradient, swelling pressures at all densities and permeability at the density of 1.8 g/cm3 remained unchanged, whereas permeabilities at 1.4 and 1.6 g/cm3 decreased overall by a factor of approximately 5 and 1.7, respectively. Seepage-induced consolidation and/or reorganisation of bentonite microstructure are considered possible reasons for these decreases.