Synthesis of spent fuel pool accident assessments using severe accident codes

•Assessment of Spent Fuel Pool accidents using Severe Accident codes is presented.•Identification of some limits and needs for improvements of SA codes.•Knowledge on natural convection phenomena in SFP building are to be increased.•Simulation of air ingress in Fuel Assemblies needs improvement.•Nitrogen effect knowledge on Zircaloy cladding behavior needs to be improved. The accident at the Fukushima Dai-ichi Nuclear Power Plant has highlighted the vulnerability of nuclear fuels that are stored in spent fuel pools (SFP) before their evacuation and final disposal or possible reprocessing. This vulnerability is due to the potential loss of sufficient fuel cooling in case of internal events or of extreme external events such as earthquake or flooding. Further, the limited number of barriers containing the radioactive products, the fuel cladding is typically the only barrier in an SFP whereas there are three physical barriers when the fuel is in the reactor (the fuel cladding, the envelope of the primary circuit and the containment building), induces higher risks of radioactive releases to the environment if sufficient fuel cooling cannot be recovered. This paper presents the results of various assessments of SFP accidents performed with different severe accident codes for different SFP geometries. In these studies, both loss of cooling and loss of water transients were analyzed. These calculations have been performed in the framework of the SARNET2 project. The analysis of these different SFP scenarios was conducted to identify some limits and needs for improvement of the SA code developed and usually used for reactor applications. More specifically, some questions about the SA code capabilities to evaluate the impact of air flow on coolability of the fuel assemblies (FAs) and the rate and path of air/steam flow in these FAs, have arisen. This work has also enabled us to identify the need to improve the knowledge and the models related to the effect of mixed air/steam atmospheres, especially the role of nitrogen on the acceleration of the corrosion rate of Zircaloy claddings and on the degradation of their mechanical properties.