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

The thermohydraulic conditions in a storage pool for spent nuclear fuel are 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 the water of the pool, the heat transfer rate into the gaseous pool atmosphere for the above scenario involves several mechanisms to a similar amount which are interacting in a complicated manner, such as heat convection, thermal radiation and heat conduction. These mechanisms are analyzed with the aid of Computational Fluid Dynamics using two complementary models that address different length scales, one for the large scales of the pool and the containment geometry, another one for the small scales of the fuel rods. The models emulate the conditions at reactor unit 4 of the former Fukushima Daiichi nuclear power plant at the time of the accident, because this is a case example for a loss of cooling scenario and sufficient material for adequate modeling is available in the literature. The pool scale model serves to analyze the developing flow patterns. It includes the fuel represented as porous medium as well as the pool and reactor building atmosphere. The results show that a characteristic flow field forms in the pool atmosphere that prevails for all studied water levels and decay heat rate distributions. The temperature level, on the other hand, 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 are extracted and applied to the second model that represents a single geometry-resolved fuel assembly with a portion of the atmosphere above it. The corresponding simulations show that the heat exchange rate between the fuel assembly and the atmosphere above is higher for the conditions near the pool wall compared to the conditions in the center of the pool. The study provides important information on how to best arrange the fuel assemblies in a spent fuel pool for optimal heat exchange in case of an accident involving loss of cooling water. Checkerboarding should be combined with a strategy to place fuel with higher decay heat rate near the pool walls.