Numerical study of the cooling stability of spent nuclear fuels in storage and deactivation pools : analysis of the natural convection in the pool

Details

Supervisors

Papalexandris, Miltiadis

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

Since the Fukushima disaster in 2011, which caused the partial evaporation of the water contained in the spent fuel pools due to pump failure, the scientific community has set itself the goal of developing new safety systems to increase the safety of these pools. To this end, it is very important to develop numerical models in order to accurately model all the phenomena of convection, boiling and all the chemical reactions that can take place near the immersed assemblies. In this master thesis, we established a simplified model of such a pool in order to quantitatively and qualitatively analyze the water flow in the assemblies. The goal is to analyze the flow when the cooling system pumps are deactivated in order to simulate the worst case scenario. Several assembly arrangements were implemented and analyzed in order to know which one allows to limit the local temperature increase. It regularly happens that a local backflow of water in the channels causes a heating which could induce a local boiling of the water and the premature degradation of the assembly sheaths. In this study, a two-dimensional numerical model was developed using Code Saturne, a French software developed by EDF. As the assemblies can’t be resolved in detail, a porous medium approximation was used. We found that in order to minimize the local temperature rise during the first 4 minutes of the outage, the most powerful nuclear waste should be placed in the middle of the rack. Furthermore, in extreme cases, hot spots are sometimes 25 degrees hotter than the surrounding water, which can cause local boiling to occur faster than expected.