Preliminary uncertainty quantification of the core degradation models in predicting the Fukushima Daiichi unit 3 severe accident

A MELCOR model of a boiling water reactor has been developed by Sapienza University of Rome in the framework of the MUSA project aiming at identifying and quantifying uncertainty sources in severe accidents analyses. To develop the model, the Fukushima Unit 3 boiling water reactor has been taken as reference, and a preliminary sensitivity analysis with RAVEN software coupled with MELCOR has been performed to quantify the influence of core degradation parameters on selected key figures-of-merit, such as pressure vessel and containment pressures, core liquid level, hydrogen generation, lower head breach time and source term. Results of base case satisfactorily predict main pressure and liquid level data measurement from TEPCO. As well as, results from the uncertainty analysis envelope the majority of thermohydraulic TEPCO reported measured data. Furthermore, median of calculated core status is in likely agreement with recent TEPCO containment inspections and muon measurements, with about 25% of fuel rods remained intact in RPV and 65% of core masses ejected to the pedestal.