Due to the earthquake that occurred off the coast of Fukushima Prefecture on February 13, 2021, a drop in the PCV water level was confirmed in Unit 1 and 3. It is assumed that this drop in water level may have been caused by an increase in the amount of leakage from inside the PCV due to changes in the status of the PCV damaged areas that have been confirmed so far and the occurrence of new damaged areas. The necessary technology development is required to strengthen the monitoring system to grasp changes in plant conditions, to evaluate the impact of such changes on the maintenance and management of facilities and buildings with a medium- to long-term perspective, and to understand the status of the plant.

Since the suppression chamber (hereafter S/C) legs are submerged by stagnant water, it is assumed that corrosion thinning will progress. Therefore, in order to suppress corrosion thinning in Government-led R&D Program on Decommissioning, Contaminated and Treated Water Management, an electrolytic corrosion prevention method considering the actual equipment environment is being investigated.

A survey conducted in 2023 in the PCV of Unit 1 revealed the loss of concrete in the pedestal section. Based on this result, it was determined that the following should be done: (1) evaluating the impact of dust dispersion at the site boundary, including the case when the RPV sinks to cause an opening equivalent to a main steam pipe in the PCV, based on the assumption that the support function of the pedestal cannot be expected, (2) implementing possible countermeasures regardless of the evaluation results, and (3) conducting evaluation regarding the structural impact on the RPV and PCV in the event that the support function is lost and the RPV sinks.

Regarding the evaluation of the loss of support function, it is evaluated that this function will not lead to large-scale damage as a result of the strength evaluation assuming earthquake motion.

The PCV, which forms a confinement boundary for use in the atmosphere, is made of carbon steel with low corrosion resistance. After the accident, the 1F is forming a special environment containing suspended matter and deposit with high radiation under high room temperature and knowledge about corrosion phenomena in this environment is insufficient. Water is supplied into the PCV to cool fuel debris, and carbon steel is immersed in water. It is also known that oxidizing chemical species such as hydrogen peroxide and various radical species are generated by radiolysis of water.

If the PCV is maintained under negative pressure during fuel debris retrieval, the oxygen concentration will increase due to an air inflow, and there is a concern over a progress of corrosion in structural materials of the RPV and the PCV and necessary piping. Based on the past investigations, however, it is expected that the seismic safety margin against standard earthquake motions can be secured even if reduced steel thickness due to corrosion for 40 years is taken into account.

For further corrosion inhibition measures, on the other hand, Government-led R&D Program on Decommissioning and Contaminated Water Management is investigating the applicability of corrosion inhibition measures to prevent corrosion of structural materials of the RPV and the PCV and necessary piping over the long-term decommissioning period and maintain the current status in actual equipment.

For the PCV, the RPV, and the piping, the effectiveness of corrosion inhibitors for steel materials has been tested in consideration of the effects of radiation environment and seawater input. Candidates for effective corrosion inhibitors have been identified for general or localized corrosion.

In order to control corrosion thinning of the S/C pedestal, which is submerged by stagnant water, an electrolytic corrosion prevention method considering the actual equipment environment is being investigated in Government-led R&D Program on Decommissioning and Contaminated Water Management.

The survey in the PCV of Unit 1 conducted in 2022 and 2023 confirmed that a part of the concrete in the pedestal was missing.

Regarding integrity assessment using inspection and evaluation technologies, the investigation on evaluation methods for seismic resistance was conducted in the past.