WORKSHOP ON EVALUATION OF UNCERTAINTIES IN RELATION TO SEVERE ACCIDENTS AND LEVEL 2 PROBABILISTIC SAFETY ANALYSIS

The analysis of the main uncertain core degradation parameters impact on the consequences of a severe accident has been performed. This investigation has been performed to assess severe accident codes weaknesses and to identify the potential needs of separate effect tests to reduce uncertainties regarding the more sensitive core degradation parameters. On this purpose, a sensitivity study of the hydrogen production (total amount and flow rate), of fission product (FP) release and of the corium composition, to some degradation modelling uncertainties, has been realised with ICARE2 and ASTEC codes, taking into account a particular accidental sequence (loss of feed water) for a French 900MWe pressurized water reactor. The results of this study indicate that, depending on the different set of parameters retained in the modelling, the mass of hydrogen produced can range from 320 to 580 kg, whereas the flow rate can vary between 0,1 to 0,4 kg.s-1. Moreover, the FP release increases significantly when the amount of dissolved fuel increases. Furthermore, the uncertainty regarding the Molybdenum release (regarding also the other semi-volatile FP) induces a variation of the iodine gaseous release and of the caesium airborne from the primary circuit break. Finally, the corium composition (ratio between oxides and metal) could vary from a factor 3.