[논문]수치해석을 활용한 향상된 한국형 기준 고준위방사성폐기물 처분시스템의 열-수리-역학적 복합거동 성능평가

김광일; 이창수; 김진섭

doi:10.7474/tus.2021.31.4.221

수치해석을 활용한 향상된 한국형 기준 고준위방사성폐기물 처분시스템의 열-수리-역학적 복합거동 성능평가
A Numerical Study of the Performance Assessment of Coupled Thermo-Hydro-Mechanical (THM) Processes in Improved Korean Reference Disposal System (KRS+) for High-Level Radioactive Waste 원문보기

터널과 지하공간: 한국암반공학회지 = Tunnel and underground space, v.31 no.4, 2021년, pp.221 - 242

김광일 (한국원자력연구원) , 이창수 (한국원자력연구원) , 김진섭 (한국원자력연구원)

초록
AI-Helper

기존의 한국형 기준 처분시스템의 처분 효율을 높인 향상된 한국형 기준 처분시스템(Improved Korean Reference Disposal System, KRS⁺)의 열-수리-역학적 복합거동 성능평가를 위해 TOUGH2-MP/FLAC3D를 이용한 수치모델링 연구가 수행되었다. 사용후핵연료 처분 이후 방사성 붕괴열에 의해 처분시스템의 온도가 상승하고, 방사성 붕괴열이 빠르게 감소함에 따라 온도가 감소하여 최대 온도가 설계기준 온도인 100℃를 넘지 않는 것으로 나타났다. 완충재의 초기 포화도는 온도 상승으로 인한 공극수의 증발로 인해 감소하였다가 주변 암반으로부터 지하수가 유입되어 처분 약 250년 후 포화 상태에 이르렀다. 암반에서는 완충재와 암반의 흡입력의 차이로 인해 암반에서 완충재로 지하수가 유입되어 처분 직후 포화도가 감소하다가 이후 원계 암반으로부터 지하수가 유입되어 포화 상태에 도달했다. 처분시스템 내 열응력과 팽윤압 발생에 의한 주변 암반의 파괴 가능성을 평가하고자 모어-쿨롱 파괴기준식과 스폴링 강도를 사용하였다. KRS⁺ 처분시스템의 처분공의 간격을 감소시키면서 처분시스템의 열적 거동 변화를 확인하였는데, 처분공 간격이 5.5 m 이하에서는 완충재의 설계 기준 온도를 초과하게 된다. 다만, 벤토나이트 완충재 부피의 56.1%의 온도는 90℃ 이하로 유지되었다. 본 연구에서 사용한 수치해석 기법은 향후 응력 모델, 지온 경사 및 입력 물성을 변화시킨 다양한 조건에서의 처분시스템의 THM 복합거동 성능평가에 활용할 수 있을 것으로 판단된다.

Abstract ▼ AI-Helper

A numerical study of the performance assesment of coupled thermo-hydro-mechanical (THM) processes in improved Korean reference disposal system (KRS+) for high-level radioactive waste is conducted using TOUGH2-MP/FLAC3D simulator. Decay heat from high-level radioactive waste increases the temperature of the repository, and it decreases as decay heat is reduced. The maximum temperature of the repository is below a maximum temperature criterion of 100℃. Saturation of bentonite buffer adjacent to the canister is initially reduced due to pore water evaporation induced by temperature increase. Bentonite buffer is saturated 250 years after the disposal of high-level radioactive waste by inflow of groundwater from the surrounding rock mass. Initial saturation of rock mass decreases as groundwater in rock mass is moved to bentnonite buffer by suction, but rock mass is saturated after inflow of groundwater from the far-field area. Stress changes at rock mass are compared to the Mohr-Coulomb failure criterion and the spalling strength in order to investigate the potential rock failure by thermal stress and swelling pressure. Additional simulations are conducted with the reduced spacing of deposition holes. The maximum temperature of bentonite buffer exceeds 100℃ as deposition hole spacing is smaller than 5.5 m. However, temperature of about 56.1% volume of bentonite buffer is below 90℃. The methodology of numerical modeling used in this study can be applied to the performance assessment of coupled THM processes for high-level radioactive waste repositories with various input parameters and geological conditions such as site-specific stress models and geothermal gradients.

주제어

표/그림 (22)

그림 Fig. 2. Schematic diagram of the thermo-hydro-mechanical (THM) coupled processes in the multi-barrier system (Lee et al., 2020c)
그림 Fig. 1. (a) Concept of an engineered barrier and a disposal canister (Lee et al., 2007). (b) Size of the disposal tunnel, deposition hole and a disposal canister for KRS (Choi et al., 2008), KRS-HB (Kim et al., 2013), KRS+ PWR (pressurized water reactor) spent nuclear fuel (Lee et al., 2020d)
그림 Fig. 3. Schematic diagram of the coupling algorithm of TOUGH2-MP/FLAC3D simulator (Lee et al., 2016)
그림 Fig. 4. A quarter symmetric model of KRS⁺ with data monitoring points for engineered and natural barrier systems
그림 Fig. 5. Initial and boundary conditions of the KRS⁺ numerical model (not to scale)
그림 Fig. 6. Decay heat of high-level radioactive waste in a canister after disposal of the PWR R-SNF (modified from Lee et al., 2020e)
표 Table 1. Parameters for the decay heat model of KRS⁺ PWR R-SNF after release from the nuclear power plant (modified from Lee et al., 2020e)
표 Table 2. Properties of the KRS⁺ numerical model (Lee et al., 2020c)
그림 Fig. 7. Variations of decay heat and temperature at (a) bentonite buffer (b) rock mass and (c) backfill from 0.001 year to 100,000 years after the disposal of the PWR R-SNF
그림 Fig. 8. Temperature profiles along the vertical line passing through the center of the canister at 0, 1, 10, 100, 1,000, 10,000 and 100,000 years after the disposal of the PWR R-SNF
그림 Fig. 9. Variations of pressure at (a) bentonite buffer (b) rock mass and (c) backfill from 0.001 year to 100,000 years after the disposal of the PWR R-SNF
그림 Fig. 10. Variations of saturation at (a) bentonite buffer (b) rock mass and (c) backfill from 0.001 year to 100,000 years after the disposal of the PWR R-SNF
그림 Fig. 11. Variations of mean effective stress at (a) bentonite buffer (b) rock mass and (c) backfill from 0.001 year to 100,000 years after the disposal of the PWR R-SNF
표 Table 3. Equations for estimating cohesion and friction angle of rock mass
그림 Fig. 12. Variations of maximum effective stress at rock mass from 0.001 year to 100,000 years after the disposal of the PWR R-SNF. Mohr-Coulomb UCS values are calculated in case that RMR is asssumed to be 70 and 75. Spalling failure criterion is set as 50% of the UCS of the KURT granite
그림 Fig. 13. Stress paths of the rock mass near (a) the deposition hole (Rock1, Rock2, and Rock3) and (b) the disposal tunnel (Rock4 and Rock5) with Mohr-Coulomb failure criteria
표 Table 4. C₀ and q in Eq. (12) in case that RMR is assumed to be 70 or 75
그림 Fig. 14. Variations of decay heat and temperature at the center of the interface between bentonite buffer and canister (Buffer2) from 0.001 year to 100 years as deposition hole spacing varies from 5.5 m to 7.5 m
표 Table 5. Deposition hole spacing, maximum temperature, time of the maximum temperature and duration of the temperature over 90°C
그림 Fig. 15. Temperature distributions along the distance from the center of the canister at 0, 0.001, 0.01, 0.1, 1, 5, 10, 15, 20, and 25 years after the disposal of the PWR R-SNF. Deposition hole spacing of (a) 7.5 m (b) 7.0 m (c) 6.5 m (d) 6.0 m (e) 5.5 m
표 Table 6. Deposition hole spacing, volumetric ratio of bentonite buffer with the temperature over 90°C, thickness of bentonite buffer for performance preservation, and enhanced disposal efficiency compared to KRS⁺ repository system
그림 Fig. 15. Temperature distributions along the distance from the center of the canister at 0, 0.001, 0.01, 0.1, 1, 5, 10, 15, 20, and 25 years after the disposal of the PWR R-SNF. Deposition hole spacing of (a) 7.5 m (b) 7.0 m (c) 6.5 m (d) 6.0 m (e) 5.5 m (continued)

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