Impact of the Uncertainties Associated with Core Degradation in H₂ Production During the In-Vessel Phase

In this paper, the impact of the uncertainties associated with the modeling of core degradation in the production of hydrogen in the vessel (total mass and mass production rate) is evaluated for a generic PWR1300MWe reactor design. Different studies involving propagation of physical and modeling uncertainties were carried out with the ASTEC V3 severe accident code, following a dedicated step-by-step approach to allow a better analysis of the results. With a focus on the hydrogen generation during core degradation, the selection of a relevant scenario is done using adapted boundary conditions starting from the core dewatering. Several studies are performed, each focusing on a different physical phenomenon or a group of related phenomena, and allow detailed understanding of the code response. In the end, the propagation of all uncertain parameters previously identified is done to evaluate the global hydrogen variation. A total of 37 uncertain parameters are defined, and 600 calculations performed (400 for the preliminary studies and 200 for the final one combining all physical uncertainties). For the selected scenario (small break scenario of 2 in.), a variation of less than ±20% is obtained for a total mass of H₂ of 700 kg calculated with the reference modeling. The analysis carried out demonstrated that the uncertainties associated with the loss of geometry of the rods are responsible for the most significant variations and for the maximum values in terms of total H₂ produced. In addition, compensating effects between claddings and corium oxidation are identified during the transient evolution and discussed.