The cooling states in the PCV are not clear after the TEPCO’s Fukushima Daiichi NPS accident. But from the standpoint of the long-term stability cooling, the understanding of the PCV cooling condition is important.

Therefore, the simple method was developed in which the PCV water and gas leak diameters and those locations, etc. could be estimated based on the time transient data. By applying this method to Fukushima Daiichi NPS accident after the nitrogen injection for prevention of the hydrogen explosion, the PCV leak locations, diameters and partial pressures, etc. were estimated.

It was found that the partial pressure of hydrogen and oxygen was kept low and the nitrogen injection to PCV was effective as a prevention of hydrogen explosion. The decay heat from the fuel was mainly removed by the sub-cooling water injected and steam condensation on the PCV wall. The heat release by leakage was estimated to be small.

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