Improvement the value of sodium void reactivity effect of the fast neutron reactor by the instrumentality of the Monte Carlo code

To fulfill safety of fast sodium reactors in a beyond design-basis accident (BDBA) like unprotected loss of flow accident (ULOF), the sodium void reactivity effect (SVRE) should be close to zero. Its value depends on the fuel burnup – the higher burnup the higher value of SVRE. We analyze limitation of the fuel burnup in the core of a large sodium reactor imposed by SVRE.The model of a large sodium-cooled reactor core is chosen for analysis. Two fuel types are considered – MOX and nitride uranium-plutonium. For both we follow the transition of the core from reactor startup to equilibrium reloading state, where the core passes consequently through different stages of fuel burnup. Calculations of maximal and average burnup together with corresponding value of SVRE have been done for a homogeneous model with the codes TRIGEX and MMKKENO. The latter employs transport approximation (the Monte Carlo method) and allows detailed heterogeneous representation of fuel assemblies and safety rods. The results obtained for the MOX fuel show that after the end of second core cycle (maximal burnup about 8%) the refined value of SVRE exceeds two times its maximal acceptable value (0.3% Δk/k). For the nitride fuel this exceeds is found at the end of 3rd fuel reshuffling (maximal burnup is about 8.75%); however it is considerably lower as compared to that found for MOX.