Numerical analysis of coupled thermo-hydro-mechanical behavior in single- and multi-layer repository concepts for high-level radioactive waste disposal
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.
References (60)
- et al.
Recent experiments in hard rocks to study the excavation response: Implications for the performance of a nuclear waste geological repository
Tunn. Undergr. Space Technol.
(1999) - et al.
Gas permeability in the excavation damaged zone at KURT
Eng. Geol.
(2013) Mechanical and hydraulic behavior of rock salt in the excavation disturbed zone around underground facilities
Int. J. Rock Mech. Mining Sci.
(2003)- et al.
An investigation of the excavation damaged zone at the KAERI underground research tunnel
Tunn. Undergr. Space Technol.
(2009) - et al.
Stress-dependent permeability of fractured rock masses: a numerical study
Int. J. Rock Mech. Mining Sci.
(2004) - et al.
Implications of thermally-induced fracture slip and permeability change on the long-term performance of a deep geological repository
Int. J. Rock Mech. Mining Sci.
(2013) Status of the TOUGH-FLAC simulator and recent applications related to coupled fluid flow and crustal deformations
Comput. Geosci.
(2011)- et al.
A modeling approach for analysis of coupled multiphase fluid flow, heat transfer, and deformation in fractured porous rock
Int. J. Rock Mech. Mining Sci.
(2002) - et al.
Hydrogeochemical evolution of the bentonite buffer in a KBS-3 repository for radioactive waste. Reactive transport modelling of the LOT A2 experiment
Appl. Clay Sci.
(2014) - et al.
Comparison of Klinkenberg-corrected gas permeability and water permeability in sedimentary rocks
Int. J. Rock Mech. Mining Sci.
(2009)
A constitutive model for partially saturated soils
Géotechnique
The squeezing potential of rocks around tunnels; theory and prediction
Rock Mech. Rock Eng.
Excavation damage and disturbance in crystalline rock - Results from experiments and analyses, SKB technical report TR-08-08
Fluid flow along potentially active faults in crystalline rock
Geology
Engineering Rock Mass Classifications
Analysis of Thermo-hydro-mechanical Behavior of the Engineered Barrier System of a High-level Waste Repository. KAERI Technical Report KAERI/TR-4142/2010
Korea Atomic Energy Research Institute (in Korean with English abstract)
Feasibility analysis of the multilayer and multicanister concepts for a geological spent fuel repository
Nucl. Technol.
FEBEX Project: Full-scale Engineered Barriers Experiment for a Deep Geological Repository for High Level Radioactive Waste in Crystalline Host Rock
Constitutive laws
Integral equation solution of heat extraction-induced thermal stress in enhanced geothermal reservoirs
Int. J. Numer. Anal. Met. Geomech.
FLAC3D, Fast Lagrangian Analysis of Continua in 3 Dimensions, Version 5.0
Fundamentals of Rock Mechanics
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