ESTIMATION OF THE THERMAL STATUS OF THE 1F UNIT 2 CORE MATERIAL AT THE TIME OF THE RPV FAILURE WITH MELCOR

抄録

Due to the large differences in melting temperatures of the different core materials (oxides and metals), estimating the thermal status (temperature, solid / liquid state) of the core materials at the time of the Reactor Pressure Vessel (RPV) failure of Fukushima Daiichi Unit 2 (from hereinafter, Unit 2) may provide a good basis for improving estimation of the current debris distribution. In another word, the thermal status of the debris in the RPV lower plenum at the time of the RPV lower head failure determines, which components of the debris are more likely to relocate to the ex-vessel (pedestal region). One of the preceding studies with accident progression analyses of Unit 2 indicated that most of the oxidic fuel (UO₂) was unmolten when the major core relocation to the RPV lower plenum (the core slumping) took place. The purpose of this study is to estimate the subsequent thermal status of the core materials at the time of the RPV lower head failure by referring to the recorded post-core slumping pressure histories using MELCOR code.

The following present understanding / modeling limitations were considered:

●Uncertainty of the injected water reaching the RPV was considered in two cases: Water injection tuned to reproduce the estimated time of the in-vessel water depletion from the plant data (estimated March 15, 4:30) (standard case) / no injection case.

●Uncertainty in the heat transfer coefficient between the debris and the lower head wall was considered in two cases: 0.1 kW/m² (standard case) / 1 kW/m² (high heat transfer case)

●Uncertainty in estimating when the Unit 2 lower head failed from the available plant data was recognized. In the analyses, detailed stress distributions of the structures and eutectic reactions were not considered and a simple creep failure of the lower head wall was modeled.

In all analysis cases, the differences of the analyzed timing of the lower head failure were smaller than the uncertainty of the timing estimated from the recorded pressure histories. The estimated in-vessel representative debris temperature at the time of the lower head failure (2270 – 2440 K) was sufficiently low with respect to the melting point of the oxidic fuel. Thus, the melting of uranium-containing oxidic fuel at the time of the Unit 2 lower head failure was unlikely. Such estimate is consistent with the muon imaging results, which indicated significant amount of the core materials still remained in the Unit 2 RPV lower plenum after the accident.