MOLTEN CLAD MOTION MODEL FOR FAST-REACTOR LOSS-OF-FLOW ACCIDENTS

The motion and location of the molten clad during an unprotected loss-of-flow accident in liquid-metal fast breeder reactors are very important because of their effect on the reactivity and the subsequent fuel motion. The present study analyzes the clad relocation problem based on a single-channel film flow model and a simple thermal transient model for fuel pins. The motion of molten clad induced by sodium vapor streaming undergoes initial rapid upward acceleration, slowing down, flow reversal, and eventual slumping down into liquid sodium at the lower end of the heated section. A possibility for freezing of the molten clad at the unheated upper plenum region as well as bottom freezing and blockage formation were also included in the analysis. A sample calculation has been made for the case of R-series 7-pin tests in the Transient Reactor Test Facility. The agreement of the overall physical behavior of the clad motion with the post-test observations is quite satisfactory.