[논문]벤토나이트 완충재 설계 기준 온도에 따른 고효율 처분시스템 처분 간격 및 암반 조건 산정을 위한 수치해석적 연구

김광일; 이창수; 김진섭; 조동건

doi:10.7474/tus.2021.31.4.289

벤토나이트 완충재 설계 기준 온도에 따른 고효율 처분시스템 처분 간격 및 암반 조건 산정을 위한 수치해석적 연구
A Numerical Analysis to Estimate Disposal Spacing and Rock Mass Condition for High Efficiency Repository Based on Temperature Criteria of Bentonite Buffer 원문보기

터널과 지하공간: 한국암반공학회지 = Tunnel and underground space, v.31 no.4, 2021년, pp.289 - 308

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

초록
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본 연구에서는 열-수리-역학적 복합거동 수치해석을 활용하여 국내 고준위방사성폐기물 처분장의 완충재의 설계 기준 온도가 100℃ 및 125℃인 경우, 처분 간격에 따른 처분시스템의 최고 온도를 계산하고, 역학적 안정성을 확보하기 위한 암반의 조건을 도출하였다. 완충재의 설계 기준 온도를 현재와 같이 100℃로 유지할 때, 처분터널 간격이 40 m, 처분공 간격이 5.5 m인 경우와 처분터널 간격이 30 m, 처분공 간격이 6.5 m인 경우, 처분용기와 완충재가 접하는 점에서 최고 온도가 각각 99.4℃ 및 99.8℃로 계산되었다. 완충재의 설계 기준 온도를 125℃로 향상시킨 경우, 처분터널 간격을 30 m, 처분공 간격을 4.5 m까지 감소시켜 처분 면적을 KRS⁺ 기반 처분시스템 대비 55%까지 감소시킬 수 있었다. 다양한 처분 간격에 대해 암반에서의 역학적 안정성을 평가한 결과, 암반파괴가 발생하지 않기 위해서는 KRS⁺ 기반 처분시스템은 암반의 RMR 분류법의 Good rock에 해당하는 RMR 72.4 이상의 조건이어야 했다. 처분 간격이 감소할수록 암반의 RMR이 더 높아야 했으며, 처분터널 간격 30 m, 처분공 간격 4.5 m인 경우에는 RMR 87.3 이상이 되어야 암반의 파괴를 방지할 수 있었다. 그러나, 처분 이후 지하수 유입 시 벤토나이트 완충재 및 뒤채움재의 팽윤에 따른 구속압에 의한 암반 강도의 증가를 고려하면, 해석을 수행한 모든 처분 간격에 대해 암반의 RMR이 75 이상이면 역학적 안정성을 확보할 수 있었다.

Abstract ▼ AI-Helper

This study conducts coupled thermo-hydro-mechanical numerical modeling to investigate the maximum temperature and conditions for securing mechanical stability of the high-level radioactive waste repository when temperature criteria of bentonite buffer are 100℃ and 125℃, respectively. In case of temperature criterion of buffer as 100℃, the maximum temperatures at the interface between canister and buffer are calculated to be 99.4℃ and 99.8℃, respectively for a case with disposal tunnel spacing of 40 m and deposition hole spacing of 5.5 m and for the other case with disposal tunnel spacing of 30 m and deposition hole spacing of 6.5 m. In case of temperature criterion of buffer as 125℃, spacings of disposal tunnel and deposition hole could be decreased to 30 m and 4.5 m, respectively, which reduces the disposal area up to 55% compared to the disposal area of KRS+. According to analysis of mechanical stability for various disposal spacings, RMR of rock mass for KRS+ should be larger than 72.4 which belongs to good rock in RMR classification to prevent failure of rock mass. As disposal spacing is decreased, required RMR of rock mass is increased. In order to prevent failure of rock mass for a case with disposal tunnel spacing of 30 m and deposition hole spacing of 4.5 m, RMR larger than 87.3 is needed. However, mechanical stability of the repository is secured for all cases with RMR over 75 considering the enhancement of rock strength due to confining stress induced by swelling of the bentonite buffer and backfill.

주제어

표/그림 (21)

그림 Fig. 1. Schematic diagram of KBS-3V multi-barrier system (SKB, 2010)
그림 Fig. 2. Coupled thermo-hydro-mechanical (THM) processes in the high-level radioactive waste repository by decay heat and water inflow (Lee et al., 2020a)
그림 Fig. 3. Temperature criteria of bentonite with a possible layout for a deep geological repository in Opalinus Clay in Switzerland (modified from NAGRA, 2002)
표 Table 1. Temperature criteria for bentonite buffer and radioactive waste repositories in countries considering subsurface radioactive waste disposal
그림 Fig. 4. Algorithm of TOUGH2-MP/FLAC3D simulator (Lee et al., 2016)
그림 Fig. 5. Schematic diagram of the disposal tunnels and deposition holes from top-down view
표 Table 2. Properties of the numerical model (Kim et al., 2021)
그림 Fig. 6. (a) a quarter symmetric model and (b) initial and boundary conidtions of the KRS⁺ numerical model (Kim et al., 2021)
표 Table 2. Properties of the numerical model (Kim et al., 2021) (continued)
표 Table 3. Cases of numerical simulations with various disposal tunnel and deposition hole spacings
그림 Fig. 7. Evolutions of decay heat and temperature at the middle of the interface between canister and bentonite buffer (at Point A) from 0.001 year to 100 years after the disposal of the PWR R-SNF with various disposal tunnel and deposition hole spacings. Disposal tunnel spacings are (a) 40 m and (b) 30 m, respectively as deposition hole spacing varies from 4.5 m to 7.5 m
표 Table 4. Maximum temperature, time of the maximum temperature, and disposal area compared to KRS⁺ for cases 1 to 14
그림 Fig. 8. Horizontal distributions of temperature from the center of the canister to the rock mass at 0.0001 year to 100 years after the disposal of the PWR R-SNF. Disposal tunnel spacing is 40 m as deposition hole spacings vary from 4.5 m to 7.5 m
그림 Fig. 9. Horizontal distributions of temperature from the center of the canister to the rock mass at 0.0001 year to 100 years after the disposal of the PWR R-SNF. Disposal tunnel spacing is 30 m as deposition hole spacings vary from 4.5 m to 7.5 m.
표 Table 5. Empirical equations to estimate cohesion and friction angle of rock mass based on RMR and rock mass strength
그림 Fig. 10. Evolutions of the temperature and maximum effective stress at rock mass for cases (a) and (b) 1, (c) and (d) 6, (e) and (f) 11, and (g) and (h) 14. Ranges of the Mohr-Coulomb UCS based on RMR classification suggested by Bieniawski (1988); Very poor (0 < RMR < 20), poor (20 < RMR < 40), fair (40 < RMR < 60), Good (60 < RMR < 80), Very good (80 < RMR < 100)
표 Table 6. Average values of C₀ and q in terms of RMR of rock mass calculated from empirical equations in Table 5
그림 Fig. 10. Evolutions of the temperature and maximum effective stress at rock mass for cases (a) and (b) 1, (c) and (d) 6, (e) and (f) 11, and (g) and (h) 14. Ranges of the Mohr-Coulomb UCS based on RMR classification suggested by Bieniawski (1988); Very poor (0 < RMR < 20), poor (20 < RMR < 40), fair (40 < RMR < 60), Good (60 < RMR < 80), Very good (80 < RMR < 100) (continued)
그림 Fig. 11. Schematic diagram of the compressive stress around the disposal tunnel and deposition hole by the swelling pressure (Rutqvist et al., 2005)
그림 Fig. 12. Evolutions of the maximum and minimum principal stresses for cases 1, 6, 11, and 14 with Mohr-Coulomb failure criteria for RMR of 70 and 75
그림 Fig. 12. Evolutions of the maximum and minimum principal stresses for cases 1, 6, 11, and 14 with Mohr-Coulomb failure criteria for RMR of 70 and 75 (continued)

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