In Japan, Risk-informed activity using PRA is becoming increasingly active and Reactor Oversight Process (ROP) will be introduced in 2020. Then, PRA has attracted more attention, and it is necessary to immediately develop its infrastructures.

Therefore, as the first step, TEPSYS/TEPCO conduct to upgrade the internal event level 1 PRA that is the basis of all events targeting unit 7 (ABWR) of TEPCO Kashiwazaki-Kariwa Nuclear Power Plant. In particular, LOCA inside PCV for all piping connecting RPV including instrumentation and control piping reclassified based on its break size, break location, success or failure of the vapor suppression function, the accident progression during mission time, and PCV control procedures.

Especially, the finding presented in this paper address the newly grouping that break location is classified into not 2 (steam phase/liquid phase) groups [1] but 3 groups as below.

✓ RPV nozzles are located above L8 reactor water level (steam phase)

✓ RPV nozzles are located between TAF and L8 reactor water level (steam phase / liquid phase)

✓ RPV nozzles are located below TAF reactor water level (liquid phase)

In the Results, completeness of the accident sequence for LOCA inside PCV was improved to be applied to risk-informed activity and the proactive safety review.

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