BOILING TRANSITION PHENOMENON IN BWR FUEL ASSEMBLIES EFFECT OF FUEL SPACER SHAPE ON CRITICAL POWER

A thorough understanding of the thermal-hydraulic phenomena near fuel spacer is necessary for the accurate prediction of the critical power of BWR fuel assemblies, and is thus essential for effective developmen ts of a new BWR fuel assembly. The main purpose of this study is to de velop an accurate method for predicting the effect of spacer shapes on critical power. Tests have been conducted under actual. BWR operating conditions, using an annulus flow channel consisting of a heated rod and circular-tube channel, and BWR simulated 4x4 rod bundles with heat er rods unheated just upstream of spacer. The effect of spacer shapes on critical power was predicted analytically based on the droplet depo sition rate estimation. The droplet deposition rate for different spac er shapes was calculated using a single-phase flow model. The predicti on results were compared with the test results for the annulus flow ch annel using ring-type spacers. Analytical results of critical power ag reed with measured critical power from point of the effects of changes in the rod-spacer clearance and the spacer thickness on critical powe r.