Analysis of isolation condenser behavior at Unit 1 of Fukushima Daiichi NPS

This report described analysis of isolation condenser behavior at Unit 1 of Fukushima Daiichi NPS, which was done by JNES. The accident initial progression of Unit 1 (before meltdown occurred) was analyzed by plant dynamics analysis code RELAP5/MOD3.3 and compared with actual data of transients' recorder and chart. Reactor pressure and reactor water level for about one hour after the earthquake were well reproduced by the analysis with isolation condenser (IC)'s heat transfer capacity as designed and so the initial reactor pressure drop was considered to be a normal response as designed. The reactor water level had been maintained almost at the normal level for about 50 minutes until the tsunami attacked. From this analysis, it was considered a large leakage might not occur immediately after the earthquake. In order to contribute to reconsider future operation procedures and design improvement, sensitivity analysis of accident progression after tsunami attacked was performed with assuming IC continued or restarted operation so as to show the reactor behavior at IC operation. If IC had been operated before core uncovered and valve opening were maintained so as to balance decay heat reduction with possible cooling water supply to IC tank, long time operation of main steam safety relief valve was avoided and reactor water level would be maintained. Decay heat could be removed by IC operation and it was possible to maintain core soundness with no problem of IC heat transfer capacity existed. (T. Tanaka)