Estimation of Long-Term Ex-Vessel Debris Cooling by Water in Fukushima Daiichi Nuclear Power Plant Unit-3

Interpretation for the two-week long Unit 3 ex-vessel debris cooling behavior was conducted based on 1F plant and site data such as pressure, temperature, gamma ray level and live camera pictures. It was estimated that the debris relocated to the pedestal was in partial contact with liquid water for about initial two days. With the reduction of the sea water injection flowrate, the debris, both in-vessel and ex-vessel ones, became “dry”, in which the debris was only weakly cooled by vapor and this condition lasted for about four days until the increase of the sea water injection. During this dry period, in-vessel and ex-vessel debris were heat up and it took another four days to re-flood the heated up debris.