Evaluation of core material energy change during the in-vessel phase of Fukushima Daiichi Unit 3 based on observed pressure data utilizing GOTHIC code analysis

Abstract

The vapor formation within the reactor pressure vessel (RPV) is regarded to represent heat removal from core materials to the coolant, while the hydrogen generation within the RPV is regarded to represent heat generation by metal oxidation. Based on this understanding, the history of the vapor/hydrogen generation in the in-vessel phase of Fukushima Daiichi Nuclear Power Station Unit 3 was evaluated based on the comparison of the observed pressure data and the GOTHIC code analysis results. The resultant vapor/hydrogen generation histories were then converted to heat removal by coolant and heat generation by oxidation. The effects of the decay power and the heat transfer to the structures on the core material energy were also evaluated. The core materials are suggested to be significantly cooled by water within the RPV, especially when the core materials are relocated to the lower plenum. The results of this study combined with the existing severe accident (SA) code analyses suggested that the debris relocated to the lower plenum was effectively cooled down near the liquid water temperature within a few hours after the relocation.

Estimation of long-term ex-vessel debris cooling behavior in Fukushima Daiichi Nuclear Power Plant unit 3

“…This observation shows that there are flow paths at the lower head of the RPV on one hand. On the other hand, the former study (Sato, et al, 2021a) suggested that some of the SRVs would have had significant leakage to the D/W during the accident progression phase. These two flow paths are believed to have been effective during the debris cooling phase studied here.…”

“…It was finally followed by the temperature decrease down to the saturation and the debris was inundated by liquid water as can be observed presently. It is estimated (Sato, et al, 2021a) that the "vent-clear condition", i.e., D/W pressurization resulting in total clearance of liquid water in the S/C vent pipe, was established just after 9:00 of March 13th when the operation of the Automatic Reactor Depressurization System (ADS) took place. This vent-clear condition lasted until 20:40 of March 13th.…”

“…The in-vessel phase of the Unit 3 debris cooling behavior was presented in another paper (Sato, et al, 2021a) and this study concentrates on the ex-vessel phase. The ex-vessel debris cooling phase lasted for about two weeks and its overall characteristics is presented in Chapter 2.…”