Monte Carlo analysis of sneak-12a core disruption in liquid-metal fast breeder reactors – the path for innovative severe accident studies in ZPR

Severe core accident (SCA) in Liquid Metal-cooled Fast Breeder Reactors (LMFBRs) could occur either due to prompt/super-prompt recriticality or serious loss of heat sink . The progression of a SCA in fast breeder reactors (FBRs) is significantly influenced by the core neutron physics, which is the primary focus of this work. The examination of a SCA due to loss of heat sink is not considered in this work. Since a FBR core loaded with MOX fuel is not in its most reactive configuration, redistribution processes of the core materials (fuel, sodium, absorbers and structural materials) have the potential of leading to positive reactivity insertion and unexpected high power excursions. Therefore, the study of SCA progression requires an adequate description of the core's neutronic behavior during the different stages of the SCA. Reactivity changes in the core could result from many different reasons, whereas the main contributors are: Loss of coolant – this type of accident leads to neutron spectrum hardening and increased neutron leakage. Change in material arrangement – could either increase or decrease neutron streaming. Relocation of fuel and structural materials – could lead to changes in the neutron direct and adjoint fluxes. Accumulation of relocated fuel in certain regions of the core – could lead to an increase in the core multiplication factor. In this paper we present a new benchmark problem based on the SNEAK-12A critical assembly experiments. During the program validation of several neutronic codes such as SNOW, TP2 , and the neutronic solver of SIMMER-II were validated against the experimental program results. The benchmark problem is introduced by the Commissariat a l'Energie Atomique et aux Energies Alternatives (CEA) and Ben-Gurion University of the Negev (BGU), and is calculated using TRIPOLI 4.9 and Serpent neutron transport Monte Carlo codes, as part of a future experimental program to be implemented in a zero power experimental reactor program, aimed to study recriticality possibilities in both light water reactors (LWR) and fast reactors (FR)