Source Term Assessment and Preparation of Countermeasures in Fukushima Daiichi NPS Unit-1 in Case of Further Degradation of Support Function of Pedestal Caused by Earthquakes

Abstract

Partial loss of concrete in the inner lower part of the pedestal of Fukushima Daiichi NPS Unit-1 was confirmed during the primary containment vessel (PCV) internal investigation made from March 2022 to April 2023. In previous studies of the pedestal base seismic robustness, it was concluded that the possibility of pedestal collapse as the result of earthquakes was low, and further degradation of support function to the superstructure would be limited. It is necessary to address such an event, however, and source term assessment was carried out and sufficient countermeasures were prepared in order to reasonably mitigate potential dose exposure.

The source term was assessed with reference to the five-component linear equation in DOE HDBK-3010-94. Amounts of radioactive materials were assumed based on results from severe accident analysis codes (MAAP/ SAMPSON), muon tomography, and an actual contaminant sample collected during previous PCV internal investigations; and based on findings obtained during construction work, done using an abrasive water jet (AWJ), for the route on which an investigation machine was to approach the PCV. Airborne release of the radioactive aerosols was considered based on information from construction work using the AWJ and vibration experiments. The source term assessment concluded no significant dose exposure would occur.

Although low risk of dose exposure was confirmed, uncertainties of the assessment would exist. Thus, countermeasures were discussed. Nitrogen gas is normally injected into PCV in order to have an inert atmosphere in it. If radioactive aerosols are generated in the PCV, the radioactive aerosols might be released from the PCV via the nitrogen gas. As the result of comparing other risks, it was decided that the injection of nitrogen gas should be temporally stopped when large earthquakes occur.