[en] Verifications of three dimensional BWR core dynamics program STANDY have been performed by out-of-core loop data and stability test data at actual plant. Space dependent analyses of core wide mode instability and out-of phase regional mode instability have carried out, and the characteristics of the various parameters in the oscillations were evaluated. Based on these analyses, it has been shown that the flow amplitude in an unstable channel was larger in a regional oscillation than in a core wide oscillations, and that thermal margin decrease were smaller than the initial margin expected under operation conditions to cause instability. Comparison between analysis results by frequency domain program and those by three dimensional core dynamics program have also been done, and examined the analysis procedure of regional stability by the frequency domain program. Then, the long-term stability countermeasures by SRI system is described