MODELING THE THERMAL EVOLUTION DURING HELIUM LEAKAGE IN A VERTICAL STEEL ENCASED CONCRETE CYLINDER

摘要

Concrete cask storage systems used in dry storage allocate spent fuel within containers that are usually filled with helium at a certain pressure. Potential leaks from the container would result in a cooling degradation of fuel that might jeopardize fuel integrity if temperature exceeded a threshold value. According to Interim Staff Guidance, temperatures below 673 K ensure fuel integrity preservation. Therefore, the container thermal response to a loss of leaktightness is of utmost importance in terms of safety. This work develops a methodology capable of estimating peak fuel temperatures and heat up rates resulting from a postulated depressurization in a dry storage cask. The methodology is based on a linear relationship between the inner pressure and maximum temperature and on stationary Computational Fluid Dynamics (CFD) calculations. The methodology is to be verified through comparisons to actual transient CFD calculations. The period of time to achieve the temperature is a function of pressure loss rate and decay heat stored in the container; in case of a fuel canister with iOkWthe period of time to reach the thermal limit takes between half day (fast pressure loss) and one week (slow pressure loss). In case of a reduction of the decay heat (15 %), the period of time to achieve the thermal limit increase up to a few weeks. The results highlight that casks with heat loads below 20 kW would never exceed the thermal threshold (673 K).