A study of the evolution of the thermal field at a depth of 450 m in granite host rock induced by emplacement of ten thousand radioactive waste over-packs each of diameter 0.35m and 2.05m length has been carried out. The repository area of 4 × 106m2 included evenly distributed 504 D-shaped disposal tunnels (size: 4m × 4m) with 20 over-packs in each tunnel. The study was conducted using FLAC3D code of numerical modeling based on finite difference method. The analysis assumed that all 10000 over-packs were disposed simultaneously so that the combined effect of the repository heating could be observed and the extent of zone affected due to heating would be represented clearly. The temperature of ground surface was taken as 35°C. The upper (ground surface) and lower (450m below the disposal hole) boundaries were fixed at 35°C and at 56.6°C assuming geothermal gradient as 0.024°C/m. The heat source simulating the radioactive over-pack was assumed to decay exponentially with time. The analysis was conducted for the period of 1000 years after disposal of radioactive waste. The average heat intensity of the area after disposal of all over-packs has been found 1.25MW per square kilometer. The three dimensional numerical analysis further revealed that maximum skin temperature of the over-pack reached 104°C in 35.64 years and temperature of rock mass at 350m above the over-pack reached 40.5°C in 100 years after the burial of over-packs. The temperatures thus can be kept less than 100 either by using clay buffers with good thermal conductivity or by increasing the spacing between two overpacks. Thus numerical simulations serves as valuable tool in optimizing design parameters of deep geological repository.
Continuously increasing demand of energy is a strong call for search of an alternative source of energy. Nuclear fuel is such a source of energy that does not pollute the atmosphere like coal which is being used in thermal power plants. But, the disposal of nuclear waste becomes a challenging task, if nuclear fuel is used as an alternative source of energy. However, research and development activities carried out so far world over have demonstrated that the ultimate disposal of such waste in specifically designed underground repository at appropriate depth and rock type is feasible. Such geological disposal of radioactive waste in deep underground is a complex process and hence it requires a good understanding of response of rock masses to thermal loading imparted by the heat that emanates from the radioactive decay of radionuclide in waste disposed underground. In view of this, a generic study was carried out to assess the behaviour of rock mass on thermal loading when the high level nuclear waste is disposed in an underground repository. Study reveals that the temperature of the surface of canister containing radioactive waste reached to maximum temperature of 104.3°C in 35.6 years and therefore the granite samples were also tested up to 200°C to study change in its physico-mechanical properties at this temperature but no practical change was observed.