Time Dependent Evolution of Heat and Stress Field in Granite-clay System Representing Geological Disposal Scheme for High Level Radioactive Wastes

A reference Geological Disposal System (GDS) for high level long lived heat emitting radioactive waste with dimension 20–10–6.25 m has been evaluated for time dependent heat field distribution, stress build up and possibility of micro-fracturing. The GDS comprises of three cylindrical heat sources (0.30 m diameter and 2 m length) mimicking radioactive waste loaded canisters emplaced within Jalore granite (JG) with barrier layers of Barmer bentonite (BB) clays, from Rajasthan. The methodology adopted in the study includes thermo- mechanical characterization of the granites and clays, followed by finite difference method (FDM) based numerical analysis for time dependent buildup of thermal and mechanical stresses, evolution of temperature field and micro fracturing phenomena within the studied GDS. The study reveals that three heat sources (waste filled SS canisters) of 500 watt intensity with spacing of 2.5 m and clay granite (CG) ratio of 60:40 and 70:30 produces maximum temperature of 92.42 and 94.69 °C and total stress (thermal and mechanical) of 24.77 and 24.97 MPa respectively within the system. 3.32 and 3.31 mm maximum displacement observed in the GDS for 60:40 and 70:30 CG ratio respectively which is within design limit. Analysis further establishes that clay admixture having bentonite clay and granite ratio in the range of 70:30 and 60:40 is capable of smooth dissipation of heat thorough them with resultant maximum temperature of < 100 °C throughout the entire duration of the time periods analysed. Based on these results it is concluded that JG and BB clay in 60:40 and 70:30 ratio with waste canister spacing of 2.5 m have appropriate characteristics suitable for hosting geological disposal system for heat emitting high level waste canisters. The study marks first analysis of multi-canister geological disposal system in Indian context.