Three weeks analysis of the Fukushima Daiichi Unit 3 NPP by the SAMPSON code: Contribution to the BSAF-2 project

As a participant in the Benchmark Study of the Accident at Fukushima Daiichi phase 2 project, which aims to improve our understanding of the Fukushima Daiichi (1F) nuclear power plant conditions, the Institute of Applied Energy has performed a comprehensive evaluation and analyses of the 1F units using the SAMPSON code. The objective of this study is to clarify the accident progression and the distribution of debris/fission products in the plant as well as evaluate points of uncertainty in severe accident codes. The paper presents the results obtained for 1F Unit 3. Firstly, a likely accident scenario is evaluated taking into account all of the available measurements and recent information obtained from plant investigations. Subsequently, the SAMPSON code is applied to confirm the validity of the estimated phenomena and explain points of uncertainty in the the accident. The calculation is performed over the course of three weeks following the scram, which is representative of the time it takes to achieve stable plant conditions according to theories of thermal-hydraulics and fission product settling. The analysis confirms that a large portion of the core melted and relocated in the lower plenum of Unit 3. Thereafter, due to lack of cooling, the reactor pressure vessel failed, which caused the debris to be gradually transferred to the pedestal, where it interacted with concrete, generating flammable gases. It is estimated that a small amount of debris may have remained in the lower plenum in the form of a solidified crust, while a considerable portion relocated to the pedestal with limited spread to the dry well due to prolonged top-flooded conditions.