Thermo-mechanical analysis for multi-level HLW repository concept

This work aims to investigate the influence of design parameters for the underground high-level nuclear waste repository with multi-level concept. B. Necessity o In order to construct an HLW repository in deep underground, it is required to select a site, which is far from major discontinuities. To dispose the whole spent fuels generated from the Korean nuclear power plants in a repository, the underground area of about 4km2 is required. This would be a constraints for selecting an adequate repository site. It is recommended to dispose the two different spent fuels, PWR and CANDU, in different areas at the operation efficiency point of view. It is necessary to investigate the influence of parameters, which can affect the stability of multi-level repository. It is also needed to consider the influence of heat generated from the HLW and the high in situ stress in deep location. Therefore, thermo-mechanical coupling analysis should be carried out and the results should be compared with the results from single-level repository concept. Three-dimensional analysis is required to model the disposal tunnel and deposition hole. It is recommended to use the Korean geological condition and actually measured rock properties in Korea in order to achieve reliable modeling results. A FISH routine developed for effective modeling of Thermal-Mechanical coupling was implemented in the modeling using FLAC3D, which is a commercial three-dimensional FDM code. The thermal and mechanical properties of rock and rock mass achieved from Yusung drilling site, were used for the computer modeling. Different parameters such as level distance, waste type disposed on different levels, and time interval between the operation on different levels, were considered in the three-dimensional analysis. From the analysis, it was possible to derive adequate multi-level repository concept. Results and recommendations for application From the thermal-mechanical analysis for the multi-level repository concept with consideration of different design parameters, which might influence on the stability of near-field rock as well as engineered barriers, the following conclusions could be made: In the case of single-level repository on 500m level, the temperature at the canister and buffer contact reaches the highest point around 20 years after the emplacement of canister. The time delays significantly in multi-level repository concept. For instance, when the two levels are located on 500m and 550m levels, the buffer temperature on 500m level reaches peak temperature of about 93 .deg. C at around 180 years after emplacement. Multi-level repository shows higher temperature and stress, and more displacement around the disposal tunnel. However, the thermal criteria of 100 .deg. C at buffer could be satisfied, when the rock properties from Yusung drilling site were used. With multi-level concept, a little more stress concentration could be observed, but not enough to generate plastic zone in near field rock. The difference between the temperature in single- and multi-level cases increases with time. When the level interval is 100m, the buffer temperature increase compared to single-level is almost 10 .deg. C in 200 years after emplacement. It is about 20 .deg. C, when the level interval is 50m. The most possible multi-level repository concept is that PWR is disposed on upper level and CANDU is disposed on lower level, which is located about 100m below the upper level. The thermal criteria of 100 .deg. C in buffer can be satisfied even with no time delay between the operation at the levels. When CANDU spent fuels are completed on lower level first and the PWR disposal is followed with 10 or 20 years time interval, the peak buffer temperature can be decreased. Even though the multi-level concept with 50m level interval can satisfy the thermal criteria, it should be carefully decided whether the multi-level design with 50m level interval is acceptable or not. From the modeling results, it was possible to derive a reasonable multi-level repository concept, which can satisfy the thermal criteria, as followings: - Level interval and location: 100m interval (Upper level 500m and lower level 600m) - Waste type : CANDU on lower level and PWR on upper level - Disposal sequence : CANDU disposal first and then PWR disposal - Disposal time interval : several tens of year