Three dimensional stability analysis of BWR core

The nuclear thermal hydraulic instability in a BWR core has been analyzed by a three-dimensional BWR core dynamics program STANDY. STANDY uses space-dependent neutron kinetics and parallel channel thermal hydraulics models. Two types of instability --- core instability and channel instability --- have been analyzed using a hypothetical BWR core condition. Analyses of core instability 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. Although increase in core power causes rapid increase in power oscillation amplitude, the ratio of thermal margin change to power amplitude stays almost constant in spite of flow distribution change. It has been found from channel instability analyses that neutron flux at core top and bottom vary in opposite phase in the unstable channel. Flow and thermal margin amplitudes relative to neutron flux amplitude were larger than those in core instability. A possibility has been suggested that a limit cycle oscillation can sustain without violating thermal margin limit at operation point with a decay ratio over 1. (author)