When a rapid groundwater inflow is introduced from the adjacent rock mass in the early stage of disposal, hydraulic pressure build-up occurs, which may cause piping erosion at the buffer material itself and the interface of the gap-filling material. Such piping erosion in compacted bentonite buffer via interaction between the buffer and the adjacent rock mass may deteriorate the performance of the buffer material. Therefore, it is necessary to understand the conditions and scenarios in which the piping phenomenon around the buffer material occurs for the long-term health of the repository. In this study, laboratory-scale experimental tests of piping erosion in buffer and interfacial rock was introduced. ø 100 mm × 200 mm height compacted bentonite specimens were placed in a cylindrical acetal cell, and the distilled water was continuously injected at a flow rate of 0.068 L/min using a dual syringe pump. The inflow of water was generated from the bottom and side cell of buffer material. During water injection, injected water pressure and amount were measured with visual observation. The results showed that the external saturation of buffer firstly occurs followed by piping crack generation along the wetting front. The additional piping channels were generated and merged with others. As the injection stopped, the swelling and self-sealing behavior of buffer material were observed. Moreover, X-ray CT scanning of the cell was conducted after the piping simulation to analyze the piping channels and saturation depth. The results highlight the piping erosion phenomenon mainly occurs due to the presence of a gap outside the buffer material. Further experimental cases is need to comprehensively understand piping phenomena in buffer material for assessing the long-term stability of underground radioactive waste disposal systems.

It is expected that around 576,000 bundles of CANDU spent nuclear fuels (SNF) will be generated from the four CANDU reactors located at the Wolsong site, according to the 2nd National Plan for the management of High-Level radioactive Waste (HLW). The CANDU SNFs are currently stored at the dry storage facilities at the Wolsong site. The authors proposed KRS+ geological disposal system consisting of two different concepts, Swedish KBS-3V type and Canadian NWMO type, for the final management of CANDU SNF. Both the concepts were designed based on the geological data obtained from the KURT (KAERI Underground Research Tunnel). The NWMO type is an in-room horizontal placement method. In this study, we try to determine the reference concept among the two proposed concepts at 500 meters below the ground surface. Assuming 10,000 tU of CANDU SNF and the KURT site, we design two engineered barrier systems, that is disposal canisters and buffers. The copper disposal canister is designed with a copper thickness of 10 mm based on a cold spray coating technique for both the disposal concepts. The domestic Ca-bentonite is used for the compact bentonite buffer with dry density of 1.6 g/cm3. Two concepts are compared in terms of safety, economics of the engineered barriers, and environment-friendliness. Because the same amounts of CANDU SNF are disposed of at the same depth, the differences in the disposal area are neglected. For the comparison in terms of safety, the corrosion lifetimes of the disposal canisters of two disposal systems are quantitatively calculated, and the capacities for retarding radionuclide releases of the compacted bentonite buffers are assessed. A computer tool developed by the authors is used in order to assess the lifetime of a disposal canister. In this study, the case that corrosion of a copper canister by sulfide from groundwater through intact buffer is analyzed. The sulfide concentration in groundwater is assumed to be 3 ppm. The most important safety function of buffer is to retard the radionuclide release. Twelve long-lived radionuclides are selected to compare the capacities for retarding the radionuclide transport through the buffer using an analytical solution. The retention time by an engineered barrier consisting of a disposal canister and a buffer is compared with twenty times the half-life of each radionuclide for both the disposal systems. The selected reference concept will be compared with the alternative geological concepts through a further study.

The structural integrity of concrete silos is important from the perspective of long-term operation of radioactive waste repository. Recently, the application of acoustic emission (AE) is considered as a promising technology for the systematic real-time health monitoring of concrete-like brittle material. In this study, the characteristics of AE wave propagation through concrete silo of Gyeongju radioactive waste repository were evaluated under the effects of groundwater and temperature for the quantitative damage assessment. The attenuation coefficients and absolute energies of AE waves were measured for the temperature cases of 15, 45, 75°C under dry and saturated concrete specimens, which were manufactured based on the concrete mix same as that of Gyeongju concrete silo. The geometric spreading and material loss were taken into account with regard to the wave attenuation coefficient. The attenuation coefficient shows a decreasing pattern with temperature rise for both dry and saturated specimens. The AE waves in saturated condition attenuate faster than those in dry condition. It is found that the effect of water content has a greater impact on the wave attenuation than the temperature. The results from this study will be used as valuable information for estimating the quantitative damage at the location micro-cracks are generated rather than the AE sensor location.

For safety assessment of a high-level radioactive waste disposal system, it is important to predict and analyze the coupled thermo-hydro-mechanical (THM) behaviors of bentonite, which is a buffer candidate material in the engineered barrier system. The Barcelona Basic Model (BBM) is a constitutive model to describe the geomechanical behaviors of partially saturated soils. Complicated tests are required to directly measure BBM parameters of bentonite. In this study, we demonstrate that probable BBM parameters can be sought by calibrating the BBM parameters to match simulation results to observed ones for two kinds of simple tests (swelling pressure test and free swelling test) instead of the complicated direct tests. In the swelling pressure test and free swelling test that were conducted by Japan Atomic Energy Agency (JAEA), water was injected into constrained and unconstrained bentonite core samples, and then swelling pressure and displacements were measured, respectively. We find optimal BBM parameters using a quasi-Newton optimization method that reproduce the observed swelling pressures and displacements in hydro-mechanical simulations. The optimal BBM parameters that are sought in the inversion process can be used to predict the THM behaviors of bentonite barriers in a high-level radioactive waste disposal system.

For the transport of spent nuclear fuel, it is necessary to evaluate the amount of radioactivity for each assembly and the total amount of radioactivity for each cask. Currently, KHNP is evaluating the radioactivity using the Express mode of the OrigenArp program in the SCALE6.1 code. Express mode is a method to evaluate the radioactivity assuming that it has been burned with the same power per cycle, and Detail mode is a method to evaluate the actual combustion history such as power and cooling time for each cycle. For a total of 3,795 assemblies, including 1,391 assembliess for Kori Unit 1, 1,427 assemblies for Hanbit Unit 2, and 977 assemblies for Hanul Unit 3, the radioactivity was evaluated in Express mode and Detail mode, respectively, and the results were compared. As a result of the evaluation, it was confirmed that the results of the Express mode were evaluated more conservatively by 2.5~12.9% than that of the Detail mode. Accordingly, KHNP established a plan to change the evaluation method from Express mode to Detail mode in order to improve the accuracy of the radioactivity assessment results and eliminate conservatism.