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

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.