HEAT REMOVAL PERFORMANCE IN ACCORDANCE WITH MESH SIZE OF SCREEN INSTALLED AT OPENING OF THE INLETS AND OUTLETS OF CONCRETE STORAGE CASK

摘要

Spent nuclear fuel generated at nuclear power plants must be safely stored during interim storage periods. A concrete storage cask developed for this purpose should be able to adequately emit the decay heat from the spent nuclear fuel. Moreover, the concrete storage cask must ensure that the temperatures of the spent nuclear fuel assemblies are maintained within the allowable values under normal and off-normal conditions and during an accident. However, the thermal conductivity of concrete is low and the allowable temperature of concrete is lower than that of steel. Therefore, the concrete storage cask must be designed to have heat removal capabilities with appropriate reliability. Accordingly, a passive heat removal system was designed to maintain the temperatures of the fuel assembly cladding material and concrete storage cask components within the allowable limits. The passive heat removal system consists of four air inlets and four air outlets with openings that are covered by mesh screens to prevent debris or wildlife from entering the ventilation ducts. The effect of each screen on the heat removal of the concrete storage cask depends on its mesh size. Therefore, thermal tests were performed to evaluate the heat removal performance in accordance with the mesh size of the screen. The screen mesh size was estimated to have an insignificant effect on the temperature rise of the canister surface and the over-pack surface, but it had a considerable effect on the temperature rise of the components of the over-pack body. As the screen mesh size decreased, the heat removal by the natural convection cooling through the passive heat-removal system was reduced, and the temperature of the concrete storage cask rose.