Investigation of the SVRE influence on the safety of large size sodium fast reactor
The article provides analysis of ULOF type BDBA for different designs of BN-1200 reactor core with MOX fuel. The design feature of the reactor core is the presence of sodium plenum above the fuel area, significantly reducing SVRE, similarly to what was done earlier in the BN-800 reactor. SVRE calculations were carried out by two codes: - TRIGEX - engineering code for fast reactors neutronics calculations in the diffusion approximation and three-dimensional GEX-Z geometry; - MMKKENO - precision code for reactor characteristics calculation by Monte Carlo method. Multigroup data Library ABBN-93 and data preparation system CONSYST are used in the two codes. Studies have shown that sodium removal from the core only gives positive effect linearly dependent on the density of sodium. The results of calculation obtained by the diffusion and transport codes are in a good agreement. Sodium removal from the plenum gives a negative effect, which is nonlinear over height. The results of its calculation by the transport codes show higher (by ~0.2 Δk/k) positive value than that obtained by diffusion method. To increase inherent reactor safety in ULOF type accidents, a modified core design with axial layer of depleted uranium dioxide was also studied. Computational studies on the worst conceivable scenario of ULOF type BDBA confirmed the effectiveness of sodium plenum to ensure inherent safety of the large size reactor. Introduction of depleted uranium axial layer into the core would further increase reactor inherent safety.