Long-term storage of nuclear fuel in spent fuel casks

SKODA JS has designed a new final disposal cask for long-term storage of nuclear fuel from a VVER-440 reactor. The aim of the research is to investigate the influence of enrichment, burn-up of spent fuel and water introduced during an accident on neutron production in spent fuel. Also, the aim is to investigate the influence of the fuel assemblies with spent fuel on each other. The research is based on the implementation of a new methodology of radiation safety analysis. A radiation field model was created in the initial research stage. A simplified model of 60 fuel assemblies in a square lattice was created. The FMM (Fission Matrix Method) is used for the calculation. Monaco and KENO-6 Monte Carlo neutron transport codes were used for the simulation. The codes are part of the SCALE code package developed at Oak Ridge National Laboratory. The most difficult issue is to meet the safety limit in the criticality safety analysis during the condition of optimum moderation, when moderation is maximum. The most important parameter is the distance between the fuel assemblies. Also, during an accident underground water can be introduced between the fuel assemblies inside the spent fuel cask, and this is why the influence of the water density was investigated, and was found to be very high.