We have developed a best-estimate transient code Toshiba TRAC version, a Toshiba advanced version of the TRAC code for BWR. This code is expected to be capable of simulating thermal hydraulic and neutron kinetics phenomena three-dimensionally with high accuracy for BWR transients. A licensing topical report for this code’s validation analyses will be submitted to the Japanese regulatory body for application to future licensing safety analysis. As one of the validation analyses, analysis for BWR stability is also important, and this code’s validation analyses for BWR stability based on the international OECD/NEA Oskarshamn-2 BWR benchmark have been implemented. The Oskarshamn-2 BWR experienced a core instability event, which culminated in diverging power oscillations with a decay ratio greater than 1.3 on February 25, 1999. This paper shows this code’s validation analyses of the core instability event. In conclusion, the initial conditions and the results of the instability event are also in good agreement with the measured data. This code is capable of simulating instability events well.

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