Numerical analysis of coupled thermo-hydro-mechanical behavior in single- and multi-layer repository concepts for high-level radioactive waste disposal

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Highlights

  • Multi-layer repository design for high-level radioactive waste disposal is proposed.

  • Applicability of the multi-layer concept is analyzed by numerical simulations.

  • Multi-layer repositories meet the temperature criterion of 100 °C.

  • Stress evolutions in bentonite buffer materials are not significantly different from existing repository concept.

  • Rock mechanical behavior is different and potential for spalling failure exists.

Abstract

A multi-layer repository concept is proposed as an alternative to the single-layer repository concept to improve the disposal density of high-level radioactive waste (HLW) in Korea. Numerical simulations are performed to compare the coupled thermo-hydro-mechanical (THM) behavior in the existing Korean reference HLW disposal system (KRS) based on the single-layer repository concept to that in the new multi-layer repository concept. The applicability of the multi-layer repository concept, such as double- and triple-layer repositories, for the disposal of HLW is analyzed with respect to a maximum temperature criterion of 100 °C, evolution of saturation and pore pressure, and mechanical stability. Maximum temperatures are below 100 °C in the double- and triple-layer repository concepts, as in the single-layer reference, although two peaks in simulated temperatures are observed during the evolution of the thermal pulse, resulting from superposition of decay heat at different vertical levels in the multi-layer repository concept. There is no significant difference in the evolution of pore pressure and saturation among the three concepts. The evolutions of mean effective stress at the bentonite blocks and backfill materials are also not significantly different between the single-, double-, and triple-layer repository concepts, because the hydraulic behavior is quite similar, resulting in almost the same change in swelling pressure. Stress states in the rock mass near the tunnels and deposition holes remain below the Mohr-Coulomb failure envelopes at the repositories of 500 m depth in the single-, double-, and triple-layer repository concept simulations. However, the stress states exceed the Mohr-Coulomb failure envelopes at the repositories below 500 m depth (beneath the top layer) in the double- and triple-layer repository concepts. Also, mechanical analysis shows high potential for spalling failure around the deposition holes and tunnel roof in the double- and triple-layer repository concepts. Subsidence in the double- and triple-layer concepts due to the construction of repositories is much deeper than that in the single-layer concept, and uplift induced by heating the rock mass due to the decay heat in the double- and triple-layer concepts is much higher than in the single-layer concept. These results indicate that multi-layer repositories with larger depth intervals between layers are a feasible alternative to the KRS to safely achieve higher disposal density.

Introduction

Disposal tunnels and deposition holes are constructed at several hundred meters below the ground surface for disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW). SNF/HLW are emplaced in deposition holes with an engineered barrier system (EBS) in the deep geological disposal (DGD) concept, which is one of the candidate disposal concepts in many countries. It has been internationally agreed that the DGD concept with multi-barrier system consisted of the EBS and a natural barrier system (NBS) is the most appropriate solution for the safe long-term management of SNF/HLW. This consensus is based on scientific and technical work that has been carried out over several decades, including extensive research, development, and demonstration programs (Berg and Brennecke, 2013). In Korea, the Korean reference HLW disposal system (KRS) based on the DGD concept was developed to isolate SNF from the biosphere (Fig. 1) as long as it could be harmful to people’s health and the environment (Lee et al., 2007).

The total land area of Korea is much smaller than that of Sweden and Finland, and the population density in Korea is enormous compared to those two countries. On the other hand, a larger area is necessary to dispose of the HLW in Korea (Table 1; Fig. 2). Therefore, Korea may face more difficulties than Sweden and Finland during the site selection process. It is necessary to develop alternative concepts to alleviate the difficulty of site selection for repositories based on the KRS. Alternative concepts have been discussed, including a multi-layer repository with a two- or three-story disposal tunnel and a multi-canister repository with two or three canisters emplaced in a single deposition hole. The proposed concepts have been assessed from the viewpoint of temperature, mechanical stability, and nuclear criticality (Cho et al., 2017). In the numerical model of the previous study, however, the distance between the heat source and the lower boundary is just 200 m; therefore, the fixed temperature condition at the lower boundary affected the temperature results of the simulations.

In the DGD concept, it is expected that temperature would be increased by decay heat, which is one of the specific characteristics of HLW. Moreover, the degree of saturation and pore pressure in the EBS would be changed by groundwater flow from the surrounding rock. Not only the thermal and hydraulic behaviors but also the mechanical behavior in the EBS would be affected by thermal stress and swelling pressure induced by very expansive buffer materials. Coupled thermo-hydro-mechanical (THM) behavior is expected in the deep geological repository, as shown in Fig. 3. In this study, therefore, numerical simulations were conducted to investigate the coupled THM behavior in the KRS and alternative concepts using a large domain to eliminate the boundary effects. The simulation results were analyzed from the viewpoint of the temperature criterion of 100 °C, the evolution of saturation and pore pressure, and the mechanical stability, including investigation of the spalling failure of the near field in the repositories, to examine the applicability of the alternative concepts.

Section snippets

Repository concepts for numerical analysis

The reference concept of the KRS is based on a single-layer repository (Lee et al., 2007). Disposal tunnels of 5.0 m width and 6.15 m height are constructed 500 m below the ground surface and vertical deposition holes of 2.02 m diameter and 7.83 m length are excavated into the floor of a disposal tunnel in the case of the vertical type of the KRS. In the KRS-3 V, a disposal canister, which contains pressurized water reactor spent fuel assemblies after cooling for 40 years, has an outer diameter

Numerical simulator

An approach linking two existing codes was applied for modeling of coupled multiphase fluid flow, heat transport, and mechanical processes. The TOUGH2-MP/FLAC3D simulator was developed based on linking the finite volume multiphase flow and heat transport code TOUGH2-MP (Zhang et al., 2008), which is a parallel version of TOUGH2 (Pruess et al., 1999), and the geomechanical code FLAC3D (Itasca, 2012) based on the finite difference model. This method is similar to the one adopted for a TOUGH-FLAC

Domain size effect

Numerical simulations with different domain sizes (modeling case 1, 2, 3, and 4) were conducted. The slopes of the temperature at 10,000 years and pore pressure at 100 years near the lower boundary are somewhat different from the others, especially initial slopes before the excavation in the single-layer repository concept with 1.5 km length domain (Fig. 7(a) and (b)). This means that impacts of the thermal and hydraulic boundary condition appear in the numerical simulation with 1.5 km length

Discussions

Numerical simulations were conducted to investigate the coupled THM processes in the single-layer repository concept and to examine the differences in the coupled THM behavior between it and the multi-layer repository concept, which has been suggested to improve the HLW disposal density in Korea. In the numerical simulations, the input parameters of the buffer materials and rock mass were based on results of laboratory tests and in-situ experiments for Kyungju bentonite and KURT rock, but some

Conclusions

This study analyzed the applicability of the double- and triple-layer repository concepts from the viewpoint of temperature, pore pressure, saturation, and mechanical stability. To satisfy the temperature criterion of 100 °C, the double-layer concept with a distance of 40 m between the disposal tunnels, deposition hole spacing of 6 m, and layer interval of 200 m is applicable. The triple repository concept with a deposition hole spacing of 8 m and the same distance of 40 m between disposal

CRediT authorship contribution statement

Changsoo Lee: Conceptualization, Methodology, Software, Validation, Formal analysis, Writing - original draft, Writing - review & editing, Supervision. Jaewon Lee: Conceptualization, Software, Writing - original draft, Writing - review & editing. Seunghun Park: Software, Visualization. Sangki Kwon: Resources. Won-Jin Cho: Writing - review & editing, Supervision. Geon Young Kim: Project administration, Funding acquisition.

Declaration of Competing Interest

None.

Acknowledgment

This research was supported by the Nuclear Research and Development Program of National Research Foundation of Korea (NRF-2017M2A8A5014857) funded by Ministry of Science and ICT, Republic of Korea.

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