Potential Backfill Materials for Deposition Tunnels in a High-Level Waste Repository

Jae Owan Lee,Yohan Cha,Jin-Seop Kim 한국방사성폐기물학회 2022 한국방사성폐기물학회 학술논문요약집 Vol.20 No.2

Backfill is one of the main components of engineered barrier in a high-level waste repository. The material selection of the backfill determines the barrier performance of the backfill. Overseas, its related research has been carried out mainly in Sweden, Finland, Canada, and Japan. However, Korea has recently started backfill research, and it is urgent to select a potential material for establishing the concept of backfill material and conducting backfill research. This study reviews NEA report, potential materials for overseas backfill research, advantages and disadvantages of single and mixed backfill materials, cases of license applications in Finland and Sweden for the selection of potential materials for backfill in Korea’s high-level waste repository. The review results indicated that it is reasonable to carry out backfill research according to the following plan: Both single and mixed materials are considered as potential materials for backfill research; experiments and performance studies are conducted with these materials; and, based on the results, a potential material or candidate material for the backfill suitable for the HLW repository in Korea is determined. For this plan, the single material is tentatively selected, as in Sweden, as bentonite with a montmorillonite content of about 40-50%. Then, if the selection criteria for montmorillonite content are determined through experiments and performance studies, we determine the final potential backfill material. As for the mixed backfill material, the bentonite/crushed rock mixture seems to be more advantageous than the bentonite/sand mixture considering the disposing problem of crushed rock generated from tunnel excavation and economic feasibility through its recycling. It is thought that the bentonite used in the bentonite/crushed rock mixture should have a higher montmorillonite content than bentonite used as a single backfill material since the crushed rock acts as an inert material in the mixture. The results of this study can be used as basic data for selecting the backfill material to be applied to the high-level waste repository in Korea, and can be used as a guideline for selecting the potential material required for backfill experiments and performance studies to be carried out in the future.

Suggestions on the Performance Demonstration Test of Backfill Materials in the Deep Geological Repository for High-Level Radioactive Waste in Korea

Dong-Jin Kim,Seong-Su Jeon,Sukhoon Kim 한국방사성폐기물학회 2023 한국방사성폐기물학회 학술논문요약집 Vol.21 No.2

Properties of bentonite, mainly used as buffer and/or backfill materials, will evolve with time due to thermo-hydro-mechanical-chemical (THMC) processes, which could deteriorate the long-term integrity of the engineered barrier system. In particular, degradation of the backfill in the evolution processes makes it impossible to sufficiently perform the safety functions assigned to prevent groundwater infiltration and retard radionuclide transport. To phenomenologically understand the performance degradation to be caused by evolution, it is essential to conduct the demonstration test for backfill material under the deep geological disposal environment. Accordingly, in this paper, we suggest types of tests and items to be measured for identifying the performance evolution of backfill for the Deep Geological Repository (DGR) in Korea, based on the review results on the performance assessment methodology conducted for the operating license application in Finland. Some of insights derived from reviewing the Finnish case are as follows: 1) The THMC evolution characteristics of backfill material are mainly originated from hydro-mechanical and/or hydrochemical processes driven by the groundwater behavior. 2) These evolutions could occur immediately upon installation of backfill materials and vary depending on characteristics of backfill and groundwater. 3) Through the demonstration experiments with various scales, the hydro-mechanical evolution (e.g. advection and mechanical erosion) of the backfill due to changes in hydraulic behavior could be identified. 4) The hydro-chemical evolution (e.g. alteration and microbial activity) could be identified by analyzing the fully-saturated backfill after completing the experiment. Given the findings, it is judged that the following studies should be first conducted for the candidate backfill materials of the domestic DGR. a) Lab-scale experiment: Measurement for dry density and swelling pressure due to saturation of various backfill materials, time required to reach full saturation, and change in hydraulic conductivity with injection pressure. b) Pilot-scale experiment: Measurement for the mass loss due to erosion; Investigation on the fracture (piping channel) forming and resealing in the saturation process; Identification of the hydro-mechanical evolution with the test scale. c) Post-experiment dismantling analysis for saturated backfill: Measurement of dry density, and contents of organic and harmful substances; Investigation of water content distribution and homogenization of density differences; Identification of the hydro-chemical evolution with groundwater conditions. The results of this study could be directly used to establishing the experimental plan for verifying performance of backfill materials of DGR in Korea, provided that the domestic data such as facility design and site characteristics (including information on groundwater) are acquired.

Study on the Swelling Pressure Characteristics of Bentonil WRK-silica Sand Mixed Backfill Material

Yohan Cha,Minhyeong Lee,Jin-Seop Kim 한국방사성폐기물학회 2023 한국방사성폐기물학회 학술논문요약집 Vol.21 No.1

Backfill is one of the key elements of deep geological disposal. The backfill material is used to fill disposal tunnels and is mainly composed of swellable clay, preventing the migration of nuclide and structurally supporting the tunnel. The selection and application of backfill material are critical for the stable and efficient disposal of spent fuel. Therefore, it is essential to secure various candidate materials for backfill and to comprehensively understand the properties and behavior of these materials. Recently, the Korea Atomic Energy Research Institute has selected a candidate material called Bentonil-WRK and is evaluating its applicability. To utilize this material as backfill, the safety function of a mixed backfill concept, consisting of sand and Bentonil-WRK, was assessed. The swelling pressure was measured as a function of dry density for a bentonite/silica sand mix ratio of 3/7. The results showed that the swelling pressure ranged from 0.15 to 0.273 MPa, depending on the dry density, with higher dry densities resulting in higher swelling pressures. The measured swelling pressure met the target performance criteria suggested by SKB and Posiva (i. e., 0.1 MPa), but did not meet the design requirement for swelling pressure (i. e., 1 MPa). This indicate the need for further research after increasing the mass fraction of bentonite (e. g., mix ratio 4/6 or more). The results of this study are expected to be used in the selection of candidate backfill materials and the establishment of design guidelines for engineered barrier backfill.

Backfill-simulated moving bed operation for improving the separation performance of simulated moving bed chromatography

Kim, K.M.,Lee, C.H. Elsevier 2013 Journal of chromatography A Vol.1311 No.-

The Backfill-SMB (BF-SMB) strategy was designed to improve the separation performance of simulated moving bed (SMB) chromatography. In the BF-SMB operation, a limited amount of products from raffinate and/or extract was re-fed to the SMB circuit as a backfill-feed. Two additional operating variables, backfill ratio (BR) and backfill length (BL), were suggested to determine the amount and injection length of backfill-feed. This strategy was applied to a four-zone SMB with one column (1-1-1-1) and two columns (2-2-2-2) per zone using a binary mixture with a nonlinear isotherm. Various BF-SMB operational methods were designed to supply backfill-feed to the feed node and/or intermediate node. The separation performances of conventional SMB and BF-SMB were compared in terms of purity, recovery, and eluent consumption. The BF-SMB successfully improved the separation performance of the conventional SMB because backfill-feed led to the rich condition of the main component at each product withdrawal node. Due to the 'TMB effect' caused by backfill-feed, the BF-SMB operation was more efficient in the 1-1-1-1 configuration than in the 2-2-2-2 configuration, showing maximum improvement of 5-7% purity and recovery from the performance of conventional SMB. In addition, partial recycling of eluent by backfill-feed resulted in a decrease in eluent consumption up to 10% even with improved purity and recovery in BF-SMB.

뒷채움재 특성변화에 따른 교대구조물 해석

정범석,김광수 동의대학교 산업기술개발연구소 2005 産業技術硏究誌 Vol.19 No.-

The compaction-induced earth pressures are much greater than the at-rest values near the surface of a compacted backfill. At large depth, the overburden pressure induced by the weight of the overlying backfill is significantly larger than the vertical stresses applied during compaction. Therefore, in short walls, the magnitude of the total horizontal force on the wall may be controlled by compaction-induced earth pressures. In higher wall, the total horizontal force on the wall may be controlled by the at-rest pressures for normally consolidated soil backfill. From a series of bridge abutment analyses, various sets of design parameters versus backfill height relationships for different densities and different internal friction angles of the backfill are developed. The influence of changing the backfill material on the design factors for the type of bridge abutment increases with increasing wall heights. Depending on the properties of the backfill material, a certain load combination can give harmful effect to the wall structure specially in the poorly designed drainage systems. The structural design must be adjusted by several methods until the analysis results matched the target values of the design parameters from each code. Therefore, use of new backfill material for the type of bridge abutment is recommended for a close examination of its properties and economical advantages.

A Study on the Thermal Conductivity of Bentonite-Sand Mixed Backfill According to Size and Ratio of Sand for Deep Geological Disposal

Yohan Cha,Jae Owan Lee,Mihye Kong,Jin-Seop Kim 한국방사성폐기물학회 2022 한국방사성폐기물학회 학술논문요약집 Vol.20 No.2

The backfill refills the deep geological disposal system after the installation of buffer in the disposal hole. SKB and Posiva have established the safety function for the backfill such as hydraulic conductivity of 10-10 m/s and swelling pressure of 0.2 MPa. The study on the thermal properties is required for the evaluation of performance design and long-term stability of backfill, since the thermal condition affects the hydraulic and mechanical behavior of backfill. Thermal conductivity is a key characteristic of thermal properties due to heat dissipation from spent fuel. In this study, thermal conductivities of bentonite-sand mixed blocks were measured. The silica sands were used instead of the crushed rock with bentonil-WRK, one of the candidate bentonite of the Korean repository system. The effects of size distribution and mass ratio of sand were evaluated. Four different size of silica sand (i.e., 0.18-0.25, 0.7-1.12, 1.6-2.5, 2.5-5.0 mm) and five mixing ratio (i.e., 1:9, 2:8, 3:7, 4:6, 5:5 of bentonite and sand) were used for characterization of thermal conductivity. As a result, the thermal conductivities were measured ranging from 1.6 to 3.1 W/m?K depending on the size and mass ratio of the sand. The smaller the size or higher the mixing ratio of sand or the higher the water contents, the higher the thermal conductivity on the surface of backfill block. The higher compressing pressure induce higher thermal conductivity. Meanwhile, the feasibility study of backfill block productivity was reviewed according to the variables of this study. The excessive sand ratio and water contents lead to poor quality that results in the failure of the block. In Korea, the research of backfill is only now in fundamental steps, thus the results of this study are expected to use for setup the experimental conditions of hydraulic and mechanical performance, and can be used for the design of safety function and evaluation of long-term stability for deep geological disposal system.

바톰애쉬를 이용한 지하매설관용 유동성뒤채움재의 FEM 해석

이관호(Lee, Kwan-Ho),이경중(Lee, Kyung-Jung) 한국산학기술학회 2012 한국산학기술학회논문지 Vol.13 No.5

본 연구에서는 유동성뒤채움재와 일반모래뒤채움재를 이용한 지하매설관 시공시 발생하는 관의 변형 및 지 표면변위를 유한요소해석을 이용하여 평가하였다. 해석에 사용된 조건은 2개의 관종(연성관인 PE관, 강성관인 콘크리 트 흄관), 2개의 매설관 직경(30cm 및 60cm), 2개의 매설관 부설깊이(60cm, 150cm), 2개의 굴착폭(1.5D 및 2D), 5종 의 뒤채움재(일반모래 및 4종의 유동성뒤채움재) 등을 이용하여, 다양한 조합의 해석을 수행하였다. 연성관인 PE관의 경우 직경 60cm 매설관의 수직변위가 직경 30cm 매설관의 수직변위 보다 평균적으로 3배이상 크게 나타났다. 또한 일반모래 뒤채움시 0.320mm로 나타났고, 이에 비해 유동성뒤채움재를 이용한 Case B, C, D, 및 E에서의 수직변위는 0.135-0.155mm 로 일반모래 뒤채움에 비해 약 40% 수준의 변위가 발생하였다. 강성관인 콘크리트 흄관의 경우 직경 30cm인 경우 수직변위는 뒤채움재 종류에 상관없이 0.004mm 정도이다. 직경 60cm 인 경우 일반모래 뒤채움재의 경 우 0.636mm, 유동성 뒤채움재의 경우 0.081-0.121mm 범위로 나타났다. 부설깊이에 따른 유동성뒤채움의 효과는 연성 관인 PE관에서 더 크게 나타났다. 강성관인 콘크리트흄관의 경우 부설깊이에 따른 일반모래뒤채움과 유동성뒤채움재 에 따른 차이는 거의 없는 것으로 나타났다. In this research, finite element method was carried out to evaluate the defomation of pipe and surface displacement for backfill of underground ficility. Various conditions for analysis were employer, including two different pipes(PE and concrete pipe), two different excavation depth(60cm and 150cm) and width(1.5D and 2D), a regular sand backfill, and four different flowable backfills. The vertical deformation of 60 cm diameter for PE was measured three times more than that of 30 cm diameter. The measured deformations for regular backfill and four flowable backfills were 0.320mm, and 0.135mm to 0.155mm, respectively. It ratio was around 40%. In case of 30cm diameter of concrete pipe, the measured vertical defomation was around 0.004mm for all the backfill materials. In case of installation depth, the effect of flowable backfill for flexible pipe is better than for rigid pipe. There is little effect on the deformation of concrete pipe with regular sand backfill and flowable backfill.

Active Earth Pressure of Narrow Backfill against Inverted T-Type Retaining Walls Rotating about the Heel

Yan-Bin Zhang,Fu-Quan Chen,Yu-Jian Lin,Hao-Biao Chen 대한토목학회 2022 KSCE JOURNAL OF CIVIL ENGINEERING Vol.26 No.4

Inverted T-type retaining walls are commonly used in subgrade or slope support engineering, which inevitably satisfies a narrow backfill. Using the classical earth pressure calculation method in a narrow-backfill case causes an inevitable error. The current narrow-backfill earth pressure theory does not apply to inverted T-type retaining walls. In this study, the failure mechanism in a narrow backfill when the inverted T-type retaining wall rotates about the heel is investigated using adaptive finite element analysis method. Numerical analysis reveals multiple sliding surfaces. A theoretical model for calculating earth pressure using difference and limit equilibrium methods is proposed. The proposed model is suitable for more complex conditions, including narrow backfill, irregular ground, and non-uniform overload, than previous models. Parameter analysis reveals that the cross-sectional area of the plastic zone and active earth pressure have a positive correlation. Further, the interface friction influences the decrease in active earth pressure. Fitting formulas for assessing the cases of long and short heel and the critical size of backfill width are presented to facilitate practitioners to evaluate the backfill.

기초형식 및 뒤채움재 종류별 강성관용 하수관거의 안전율

이관호(Kwan-Ho Lee),김성겸(Seong-Kyum Kim) 한국산학기술학회 2019 한국산학기술학회논문지 Vol.20 No.4

도심지 도로하부에서 발생하는 지반침하 및 싱크홀의 주요 원인은 하수관로 기초 및 관로뒤채움재의 부적절한 다짐 등이다. 이로 인해 하수관거의 이음부 파손 및 접합 불량, 관의 파손 및 균열 등 많은 문제점이 발생하고 있다. 이를 해결하기 위해 하수관거 기초와 관련된 받침계수 및 굴착 깊이에 따른 안전율을 평가하였다. 강성관용기초로는 쇄석기초, 버림 콘크리트 기초, 그리고 최근 새로 개발된 현장조립식 경량플라스틱 기초를 이용하였고, 뒤채움재는 현장발생토사(사질토 및 점성토), 현장발생토사를 재활용한 유동성뒤채움 등을 적용하였다. 굴착 깊이 및 하수관 기초별 안전율을 평가하기 위하여 하중계수 및 받침계수 등을 고려한 설계하중을 평가하였다. 받침계수는 쇄석기초 0.377, 버림 콘크리트 기초(180° 및 120°) 0.243 및 0.220, 경량 플라스틱 기초와 유동성 뒤채움재는 0.231로 적용하였다. 전체적으로 쇄석기초 사용 시 안전율이 작게 나왔고, 받침각 180° 버림 콘크리트 기초 사용 시 안전율이 가장 크게 나타났다. 또한, 경량 플라스틱 기초와 유동성 뒤채움재의 조합을 이용할 경우 받침각 120° 버림 콘크리트 기초보다 안전율이 크게 나타났다. 이는 새로 개발된 재활용 경량 플라스틱 기초가 강성관의 또 다른 대안 기초로 활용이 가능함을 의미한다. The main causes of subsidence and sinkholes in the lower part of urban roads are sewage line foundation and inadequate compaction of backfill material. This leads to many problems, such as the breakage of joints in sewer pipes, poor connection, pipe breakage, and cracks. To solve this problem, the support factor related to the sewer foundation and the safety factor according to the excavation depth were evaluated. For the foundation of rigidity tolerance, crushed stone foundation, and abandoned concrete foundation, a recently newly developed site assembly-type lightweight plastic foundation were used. Backfill materials were applied on site (sandy soil and clayey soil) and fluid backfill was recycled onsite. To evaluate the depth of excavation and the safety factor of each sewer pipe foundation, the design load considering the load factor and the support factor was evaluated. The support coefficients were 0.377 for a crushed stone foundation, 0.243 and 0.220 for an abandoned concrete foundation (180° and 120°), and 0.231 for a lightweight plastic foundation and fluid backfill. Overall, the safety factor was low when using the crushed stone foundation, and the safety rate was the highest when the foreclosed concrete foundation (180°) was used. In addition, when the combination of lightweight plastic and fluid backfill materials was used, the safety factor was higher than that of abandoned concrete foundation (120°), which means that the newly developed lightweight plastic foundation can be used as another alternative base of a steel pipe.

구조물 배면에 사질토 되메움시 유발되는 수평토압에 관한연구

이상덕(Lee Sang-Duk),강세구(Kang Se-Gu) 한국토목섬유학회 2011 한국지반신소재학회 논문집 Vol.10 No.4

본 연구에서는 모형실험을 통하여 구조물 배면의 좁은 공간을 사질토로 되메움 할 때, 되메움 지반의 공간크기와 형태 및 지오그리드의 포실 유/무에 따른 수평토압의 크기와 분포형태를 측정하였다. 8개의 층으로 구성된 벽체를 설치하고 좁은 되메움 지반을 벽체 하단으로부터 이격거리 10cm, 20cm, 30cm, 절취면의 각도는 90°, 75°, 60°로 모사하고, 되메움 지반내 지오그리드의 설치 유/무의 조건으로 수평토압을 측정했다. 측정결과 좁은 지반에서 되메움에 의한 수평토압은 아칭형태를 보였고 사면 파괴체 인에 모형지오그리드를 포설할 경우라도 수평토압이 감소된 결과가 나타났다. 본 연구와 유사한 현장의 벽체에 미치는 토압을 산정할 때 되메움 공간의 형상과 크기, 지오그리드 설치 등을 고려한 토압산정식이 적용되여야 할 것으로 확인되었다. In this study, the reinforcing effect of geogrids in the narrow backfill by sand was experimentally studied. In the model tests, the size of space and the slope of the cut off slope were varied out. The resultant and the distribution of lateral earth pressure were measured. Width of backfill space varied 10 cm, 20 cm, 30 cm at the lower wall level and angle of the cut off slope varied 90°, 75°, 60°. Geogrids were installed in the backfill. Measured results showed that the distribution of the lateral earth pressure due to the narrow backfill developed in a arching shape. And the earth pressure was reduced due to the reinforcement of the backfill by geogrid. geogrid helps reduction of lateral earth pressure.