Investigation of velocity gradient as driving force of flow pulsation in fuel assemblies

Abstract

The presence of quasi-periodic flow pulsations in fuel assemblies has been observed since the 1960's but is still not fully understood. Current design and licensing practices for nuclear reactor fuel mostly rely on 1-dimensional subchannel simulation tools, which might not accurately predict the increased subchannel mixing caused by flow pulsations. The present work develops a quantitative relationship between subchannel mixing and the inter-subchannel velocity gradient, shown to be the driving force of flow pulsation. A sensitivity study on rod-bundle geometry, based on an experiment by Bardet and Balaras at George Washington University, was conducted with a URANS method in transient simulations using the commercial software Star-CCM+. A linear relationship was observed between crossflow mixing and [delta]vbulk, defined as the difference in bulk velocities of adjacent subchannels. A threshold value of [delta]vbulk was seen close to 0.4 m/s, below which very little crossflow mixing was observed. Using these results, an analytical relationship between inter-subchannel velocity gradient and crossflow mixing could be developed and implemented into subchannel codes for more accurate modeling of flow in a fuel assembly.