Carbon steel, which has low corrosion resistance, is mainly used as the structural material of the cooling system piping. At present, the PCV is filled with nitrogen, and the oxygen and hydrogen peroxide concentrations in the water are expected to decrease due to a decrease in the oxygen concentration in the gas phase, so the corrosion is controlled to some extent.

However, in fuel debris retrieval in the future, it is assumed that the atmosphere containing oxygen will flow inside the PCV under negative pressure control.

Up to now, the confirmation test of the cooling status of fuel debris, which temporarily stops reactor water supply, has been carried out in Unit 1 to Unit 3.

There was no significant temperature increase in the RPV and PCV during the test, and there was no effect on dust concentrations or noble gas concentrations. Judging from this, a temporary reactor water supply stoppage will not interfere with the cooled status of fuel debris.

The temperature of the RPV bottom and the PCV can be generally evaluated by the heat balance evaluation (see references for details), including the transient cooling conditions such as the water injection shutdown.

The largest temperature increase rate of about 0.2 °C/h was observed in Unit 2 during water supply stoppage. Based on this temperature increase rate, the time margin for water supply stoppage is about 10 days or more, much longer than the previous estimate of about 10 hours.