Integrity evaluation for the PCV/RPV/buildings have been conducted to a certain extent, and long-term maintenance management plans considering the progress of deterioration have been formulated and are in operation. In addition, for the integrity of the Unit 1 to 3 reactor buildings, the internal investigation of the building and trend analysis using seismographs is being continuously conducted.

In particular, based on the confirmation of the exposure of the inner wall reinforcement and inner skirt inside the pedestal during the investigation inside the containment vessel of Unit 1, a safety assessment on the pedestal support function is being conducted. In this assessment, while the maximum effective dose at the site boundary in the event of loss of pedestal support function, subsidence of the RPV, and large opening of the PCV is assumed to be 0.04 mSv/event. Since it includes uncertainty due to the estimation of temperature history during the accident and the limited inspection range of the equipment, measures to mitigate the effects of dust dispersion are planned to be taken.

The survey method of integrity in the high-dose building is still under investigation.

The process of installing a large cover for Unit 1, etc. was reviewed in consideration of the application of the new seismic design policy presented by the Nuclear Regulatory Commission in FY2022 and the process related to slope countermeasures around seismic critical facilities were added to the “Decommissioning Medium- and Long-term Action Plan” in FY2024.

In addition, during the investigation inside the containment vessel of Unit 1, the inside wall reinforcement and inner skirt were found to be exposed inside the pedestal. In response to this fact, a safety assessment in the pedestal support function is being conducted. However, this safety assessment includes uncertainties due to the estimated temperature history of the accident and the limited scope of inspection of the equipment.