Assessment of re-criticality in severe accident configurations using MCNP and MELCOR

The paper presents a method to evaluate the potential re-criticality issues in severe accident configurations. This method is developed by Tractebel ENGIE, using respectively MCNP and a dataset of MELCOR calculations. The work has focused on the in-vessel phase and more particularly on a TMI-2-like configuration. A model based on a corium sphere approximation associated to the detailed WIMS10 fuel characterization has allowed us to highlight some degraded configurations presenting re-criticality risks associated to the intact fuel rods that remain in the core. The resulting distributions of the multiplication factor (k_eff) in function of several parameters (core degradation fraction, boron concentration in water, core burnup, moderator and fuel temperature,...) has allowed us to identify the more sensitive parameters regarding the re-criticality risk. The core degradation fraction is the major parameter, even for a high level of degradation, which highlights the influence of the intact peripheral fuel rods. The boron concentration in the water injected after the accident also plays an important role: even with a relatively high core degradation fraction (about 60%) and a consequent boron concentration (1500 ppm) the re-criticality risk cannot be excluded. These conclusions being well dependent on the assumptions and conservatisms of the study Finally a 'surrogate model' - regression model based on artificial neural network - is presented for the prediction of the multiplication factor for TMI-2-like configurations.