Qualification of a Three-Dimensional Core Dynamics Analysis Program Coupled with a Detailed Mesh Division for Commercial Boiling Water Reactors–I

In the stability licensing analysis and evaluation of boiling water reactors (BWRs), frequency-domain stability analysis programs have been used in Japan. To back up the licensing analysis and evaluation, time-domain, multiregional analysis programs have been used because more detailed analytical results can be obtained by these programs with little more computer time than that used by the frequency-domain stability analysis programs. In the backup calculation by time-domain, multiregional analysis programs, many trial-and-error experiments and much expertise on the reactor core radial regional division and on the initial disturbance input are necessary to analyze properly the stability of the BWR core, particularly its regional nuclear thermal-hydraulic stability. A three-dimensional time-domain core dynamics analysis program called SUPER-STANDY was developed with a detailed mesh division that makes various trial-and-error procedures and experience-based expertise unnecessary and that can treat the stability peculiar to the BWR core accurately. The program was applied to a plant where regional instability was observed, and the results were qualified. They showed that BWR stability can be analyzed using SUPER-STANDY by adding only the core uniform initial disturbance input without considering the reactor core radial regional division. It was determined that core regional mode instability can be properly analyzed by the multiregional analysis program (a) by dividing the core into six or more radial regions, (b) by specifying the hot fuel bundle as one region, and (c) by specifying the surrounding fuel bundles around the hot fuel bundle as one region. A visual display system was also developed for a huge number of stability data and core nuclear thermal-hydraulic characteristics, which are connected to each other in a complex way. These are obtained by the SUPER-STANDY program.