Monte Carlo Analysis of Streaming Effect through Annular Gaps around Piping Legs in Sodium Pool to Secondary Sodium Activation of the Demonstration Fast Breeder Reactor

Abstract

A top-entry loop-type reactor is one of the favorable options for Demonstration Fast Breeder Reactor (DFBR) which is now under development in Japan as a part of conceptual design study. Annular gaps around top-entry piping raise neutron streaming in upward direction in the sodium pool of reactor vessel. It enhances neutron flux level around decay heat exchangers (DHX's) in the pool and at the penetrations in primary biological shield for piping connecting to the vessels of intermediate heat exchangers (IHX's), and consequently enhance secondary sodium activations in these heat exchangers, which is one of the main issues for shielding design. In this study, three-dimensional Monte Carlo analysis method was applied to make precise evaluation of the neutron streaming effect with combination of some techniques for reduction of statistical error within reasonable CPU time. It is established that the contribution of neutron streaming to the secondary sodium activation in DHX's hardly reaches a level of 30% and that design analysis with two-dimensional discrete ordinates method gives conservative evaluation of the secondary sodium activations in DHX's and IHX's.