Thermal dimensioning of Olkiluoto repository for spent fuel

This report contains the updated temperature dimensioning of the KBS-3V type spent nuclear fuel repository in Olkiluoto for the fuel canisters, which are disposed at vertical position in the horizontal tunnels in a rectangular geometry according to the preliminary Posiva plan. This report concerns only the temperature dimensioning of the repository and does not take into account the possible restrictions caused by the rock mechanical restrictions of the rock. The dimensioning criterion is the maximum temperature on the canister/buffer interface, which due to very good conductivity of copper is practically constant all around the canister surface. The postulated dry conditions for thermal dimensioning are very unlikely to exist, especially in long-term. The normal expected condition is that the buffer around canisters is water-saturated within shorter time. For this reason it is justified to apply lower safety margin for the maximum temperature in dry conditions than in saturated condition when dimensioning the canister spacings in the repository. The maximum temperature on the canister-bentonite interface is limited to the design temperature of +100°C. For the reasons presented above and further, due to uncertainties in thermal analysis parameters (like scattering in rock conductivity or in predicted decay power) the nominal calculated maximum canister temperature is set to 95°C having a safety margin of 5°C. Correspondingly in saturated condition, which is more probable, maximum nominal temperature is set to 90°C. The nominal temperature is controlled by adjusting the space between adjacent canisters, adjacent tunnels and the precooling time of the spent fuel affecting on the decay power of the canisters. For the Olkiluoto repository, the dimensioning was made assuming the canisters to be in rectangular panels of 900 canisters of BWR, VVER or EPR spent fuel. The analyses were performed for each fuel types with an initial canister power of 1700 W, 1370 W and 1830 W, respectively. These decay powers correspond to the average decay heat of the spent fuel with pre-cooling time of 32.9, 29.6 or 50.3 years for fuels with average burn-up of 40, 40 or 50 MWd/kgU, respectively. The analyses gave the resulting canister spacing (7.5-10.5 m), when the tunnel spacing was 25 m.