Two-scale CFD analysis of a spent fuel pool involving partially uncovered fuel storage racks

The thermal-hydraulic conditions in a storage pool for spent nuclear fuel were studied for a loss of cooling accident leading to partially uncovered fuel racks. While under normal operating conditions, the fuel is cooled by means of single-phase natural convection in liquid water, the heat transfer rate into the gaseous pool atmosphere for the above scenario is comprised of comparably relevant portions of heat convection, thermal radiation and heat conduction. These mechanisms were analyzed with the aid of computational Fluid Dynamics using two complementary models that address different length scales. The models emulate the conditions at reactor unit 4 of the former Fukushima Daiichi nuclear power plant at the time of the accident, as it is a case example for a loss of cooling scenario and sufficient material for adequate modeling is available in the literature. A large-scale model served for the purpose of analyzing the developing flow patterns. It includes the fuel represented as porous medium as well as the pool and reactor building atmosphere. It was found that a characteristic flow field forms in the pool atmosphere that prevails for all studied water levels and decay heat rate distributions, while the temperature of the fuel can be reduced significantly by means of a checkerboard storage. However, the boundary conditions in the head region of a fuel assembly are clearly a function of its storage location in the pool. Representative conditions were extracted and applied to a second model that represents a single geometry-resolved fuel assembly with a portion of the atmosphere above it. With the corresponding simulations it was determined that, in terms of cooling efficiency, the conditions near the pool wall are favorable compared to a location close to the pool center. In conclusion, the study indicates that a beneficial storage solution combines checkerboarding with a tendency of storing fuel that exhibits a higher decay heat rate near the wall.