A comparative study of neutron attenuation in magnesium oxide and stainless steel as core catcher in FBR 1 & 2

FBR 1 & 2 is a homogeneous core 500 MWe fast breeder reactor. Magnesium oxide (MgO) has been used as a thermal insulator at the top of core catcher of FBR 1 & 2. To prevent high temperature corium from interacting with the main vessel, insulating layer of the core catcher plays a crucial role during the event of a Core Disruptive Accident (CDA). The core catcher positioned between reactor core and the detectors mounted at the bottom of the main vessel. The monitoring of the reactor core at full power will be done using these detectors. Neutron transport has therefore been carried out to investigate the impact of the magnesium oxide layer on detector flux and to compare it to the equivalent thickness of stainless steel (SS) used as a heat shield above the core catcher in PFBR. It has been observed that leakage neutron through MgO improves more than twice with respect to same thickness of SS in the thermal energy range whereas it is almost unchanged in the fast energy range. At core catcher region neutrons populated mainly in the range of 0.1 eV to 0.1 MeV thus use of MgO plates as core catcher top layer in place of SS will improve the neutron economy at the exit of the core catcher. (author)