Worldwide there has been an enormous interest to fully understand the Fukushima accident unfolding, as a way to gain key insights for prevention of these accidents and mitigation of their consequences. This paper focuses on the CIEMAT’s forensic analysis of the Fukushima Unit 1 (1F1) accident sequence. By identifying the major challenges faced for a consistent interpretation of the data available, a description of the MELCOR 2.2 model built to capture the main accident signatures is presented, with particular emphasis on fission product release and transport. Even though this study should be seen as a work in progress, the results here presented are based on a defendable set of hypotheses and approximations and highlight some interesting observations that might have affected fission product release and transport. Among them are worth mentioning the deposit remobilization during the transient (Cesium), the moderate retention in the suppression pool due to the WW bypass meant by direct leaks between RPV and DW and the potential reentrainment as a result of pool saturation. This being said, it’ll be hard to confirm any of these insights concerning fission products. This work is framed under the BSAF Phase 2 project of the OECD and it has been supported by the Spanish Nuclear Regulatory Body through the CSN-CIEMAT collaboration agreement on Severe Accidents (ACAS).