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

As participant in the BSAF phase 2 project aiming at improving the understanding of Fukushima Daiichi (1F) nuclear power plant conditions, IAE has performed a comprehensive evaluation and analyses of the 1F units with the SAMPSON code. The objective of such activity is to inform about the accident progression and debris/fission products distribution in the plant as well as evaluate uncertainty points in the SA codes. The present paper presents the results obtained for the 1F Unit 3. At the beginning, a likely accident scenario has been evaluated deeply considering all the available measurements and recent information obtained from investigation at the plant. Subsequently, the SAMPSON code has been applied to confirm the estimated phenomena and explain uncertain points of the accident. The calculation has been performed from scram until three weeks of the accident, which is considered to be representative of the achievement of plant stable conditions from the point of view of thermal-hydraulics and FP settling. The analysis has confirmed that in Unit 3 a large portion of the core melted and relocated in the lower plenum. Thereafter, because of lack of cooling, RPV failed and subsequently debris was gradually transferred to the pedestal interacting with concrete and generating flammable gases. It is estimated that a little amount of debris might have remained in the lower plenum under the form of solidified crust, while a considerable portion relocated to the pedestal with limited spread to the Dry Well (DW) due to prolonged top-flooded conditions.