Uncertainty analysis of radionuclide release into containment during severe accidents: Application of MELCOR 2.2 to the PHEBUS FPT-1 test

A reliable prediction on the behavior of source terms is essential for devising effective accident response measures to cope with the severe accident of a nuclear power plant. However, modeling the release, transport, and deposition of radioactive materials under severe accident conditions inherently involves significant uncertainties. This study was intended to evaluate epistemic uncertainties in predicting source term behavior during the severe accident on the release of fission products into the containment in the PHEBUS FPT-1 experiment using the MELCOR 2.2 code. The release fractions of cesium, iodine, and all fission products into the containment were selected as the figure of merit (FOM) in the uncertainty quantification. This study categorized the uncertain parameters into two groups: variables used to model fuel degradation phenomena and those involved in describing radionuclides behavior from the fuel. A total of 140 simulations were performed for each set of calculations, followed by regression analysis and correlation analysis to quantify the influence of selected model parameters. The possible distributions of selected FOMs were quantified, and critical parameters were identified for analysis packages of the MELCOR code in predicting the release fraction of major radionuclides.