1. To prevent PCV failure due to hydrogen explosion as well as to control and prevent leakage of radioactive materials from the PCV.

Current status

  1. Prevention of leakage of radioactive materials from the PCV gas phase part
  2. The radioactive materials released to the environment are reduced by extracting and filtering the gas in the PCV of Units 1-3 using the PCV gas control systems and by monitoring the concentration and amount of radioactive materials using radiation monitoring and control equipment. Also, since the pressures in the PCV of Units 1-3 is slightly positive, no serious damage is considered to have been caused in the gas phase of the PCV.

  3. Prevention of a leakage of the contaminated water (liquid phase) from the R/B
  4. The contaminated water leaking from the PCV of each Unit is accumulated such as in the R/B. The water level gauges are installed to monitor the conditions of contaminated water accumulated in the building and other facilities so that the contaminated water does not leak and the level of the accumulated water is controlled so as to be kept lower than the underground water level. In addition, the underground water level is being checked by the water level gauges installed in appropriate sub- drains in the vicinity of the building.

  5. Hydrogen explosion prevention
    1. The nitrogen filling to RPV/PCV is continued. The amount of injecting nitrogen gas is controlled when injecting nitrogen so that the hydrogen concentration in the PCV of each Unit does not exceed the burning limit concentration (4%) and the hydrogen concentration are monitored.

    2. In addition to these efforts, for Unit 1 where intermittent increase in hydrogen concentration in the PCV was observed, hydrogen released from the water remains in the suppression chamber (hereinafter referred to as "S/C") to the upper part of the S/C was replaced with nitrogen, and the stable condition has been achieved. Because a small amount of hydrogen continues to be released from the accumulated water in the S/C, nitrogen gas injection is carried out in order to maintain the stable condition, reducing the risks due to hydrogen. For Unit 2 where increase in the hydrogen concentration in the PCV due to by pressure fluctuation was observed, nitrogen gas injection to the S/C was carried out and replacement by nitrogen gas was completed. The changes made in the parameters are being checked. For Unit 3, because the increase in hydrogen concentration has not been observed and it is considered that the condition of the closed space in the S/C is stable, the change of the parameters is being checked.

    3. Hydrogen concentration in the PCV indicates that a certain value as previously described and its concentration is being controlled sufficient low to the inflammability limiting concentration (4%).