Maintaining and management of stable condition of fuel debris retrieval

The current conditions of Units 1-3 can be estimated to maintain the stable cold shutdown state based on the plant data of the inside of the PCV, such as the temperature, hydrogen concentration and pressure recorded since the accident.

Moreover, technical developments required for practical application of the fuel debris retrieval methods, such as the development of equipment and facilities for remote decontamination, investigation, and work are being advanced. It is, therefore, important to maintain, control and monitor the condition of plant, fuel debris and FP such as Cs in ensuring safety until fuel debris retrieval starts.

Although at this moment, it is difficult to observe the condition of fuel debris directly due to high radiation, the information is required to be understood from the perspective of maintaining and management of safety state. The status of the information obtained to date is as follows:

①Criticality control

Although it is assumed that the fuel debris is in a stable condition at present, it cannot be denied that it may become critical due to a certain cause, and so the criticality control is required.

②Cooling

Because fuel debris has decay heat and can release contained radioactive materials when it becomes a high temperature, so it needs to be cooled with water.

③Containment

Since release of fuel debris and FPs such as Cs to the outside has a significant impact on workers and general public, it is required to be prevented by the PCV, etc. so that they may not be released to the outside.

④Maintenance/reliability improvement of safety system

Maintenance and reliability improvement are required for the facilities necessary for maintaining the current safe condition of fuel debris.