Analysis of BWR core nuclear thermal hydraulic oscillation with three dimensional transient program

A three-dimensional BWR core dynamics program STANDY has been developed. STANDY takes into account parallel channel effect and evaluates fuel thermal margin. Peach Bottom 2 and Vermont Yankee stability test data have been analyzed by STANDY. Calculated decay ratios and resonance frequencies agreed well with measured data. Limit cycle oscillation at Vermont Yankee test has been also simulated. Oscillation amplitude agreed well with experiment. A hypothetical core condition has been made up to examine unstable oscillation in BWR core. Analyses of the core revealed that oscillation at conditions close to instability initiation reaches small amplitude limit cycle, and change in fuel thermal margin is very small during the limit cycle