Updated analysis of Fukushima unit 3 with MELCOR 2.1. Part 1: Thermal-hydraulic analysis

Highlights: • The accident sequence of Fukushima Unit 3 was analyzed with MELCOR 2.1. • Three plausible scenarios were selected as the most representatives. • The calculations predicted remarkably well the main signatures. • The study was complemented with plant observations. • A plausible state of the core 350 h after SCRAM was estimated. - Abstract: The accident sequence of Fukushima Unit 3 was analysed with MELCOR 2.1. Hundreds of calculations have been performed and three plausible scenarios which predicted remarkably well the main signatures (i.e. pressure in RPV and containment and containment water level) have been selected and studied in the present analysis. The main signatures, namely pressure in the RPV and drywell and water levels were satisfactorily reproduced by the three proposed plausible scenarios. Major uncertainties concerning the time of RPV failure, possibility of MCCI as well as the transport of hydrogen responsible for the explosion of unit 3 and unit 4 were addressed in this study. The study was complemented with plant observations such as containment inspections, muon measurements and photographs taken during the accident. The results point out that the plausible sequence lie between case 1 and case 2 presented in this study. The state of the core after 350 h seems to be in agreement with our case 2, where ca. 64 tons of debris and stainless steel structures remained in the lower head and ca. 70 tons of debris were ejected to the pedestal. Taking into account the limitation in the modelling of the failure modes of a BWR, this estimation is very uncertain. Further discussion will be provided in the second part of the paper where the fission product release and transport will be studied and compared against available source term calculations.