Study on Energy Release Mechanism during ULOF Initiating Phase of LMFBR

Liquid-metal cooled fast breeder reactor (LMFBR) cores are difficult to design to eliminate a re-criticality potential during the postulated core disruptive accidents (CDAs). This is because the cores are not designed to be optimum configuration from a reactivity point of view. Once a super-prompt criticality occurs, the core fuel will begin to melt and vaporize. The expansion of the fuel vapor drives the upper sodium slug to strike the reactor vessel head and generates a significant mechanical consequence as to threaten the vessel integrity. Hence, the amount of the energy release during CDAs is the most important issue in the licensing procedure. In this study, such a mechanical consequence generated during an Unprotected Loss-of-Flow (ULOF) initiating phase (IP) is investigated by using SAS4A for the end-of-equilibrium-cycle (EOEC) core of Japanese prototype LMFBR, MONJU. The reactor power during ULOF initiating phase is characterized by the superposition of negative and positive reactivities. The negative reactivity feedback is governed by fuel dispersion in the boiling subassemblies at the center region of the core. On the other hand, the positive reactivity insertion is introduced by fuel-coolant interactions (FCIs) in non-boiling subassemblies at the external region of the core. The mechanical consequence is evaluated based on the phenomenological event tree of ULOF IP. Each heading parameter and its value are selected keeping a conservative aspect, based on the results of CABRI experiments. Even in the case with the most conservative assumptions, the mechanical energy release is evaluated to be almost 0 MJ. (author)