Neutron kinetics modelling for simulations of loss of coolant accidents in the nuclear power plants

The code used by Framatome to predict the progression of a LOCA is the system scale thermal-hydraulic code CATHARE (Code Avancé de THermoHydraulique pour les Accidents deRéacteurs à Eau). It calculates the full primary and secondary circuits including the core and the fuel elements. CATHARE currently utilizes a 0D neutronic model that solves the PointKinetics Equation (PKE) to determine the evolution of instantaneous fission power.This approach is suitable for transients where the core's moderator density changes rapidly in a quasi-uniform manner, such as in large break LOCA scenarios (from the initial situation witha core full of liquid to a sudden and complete voiding leading to a full vapour environment). However, during Intermediate Break (IB) scenarios, with slower dynamics, the assumption of auniform core's moderator density is no longer valid. This assumption results in a significant underestimation of void antireactivity in the upper part of the core and a slight overestimation atthe bottom. Thus, using PKE for IB-LOCA leads to an overestimation of the fission power, and the more heterogeneous the core, the higher the conservatism of this hypothesis is expected.In fact, the specific application of IB-LOCA involves a precise neutronic calculation in a strongly diphasic fluid environment which is first due to the uncovering of the core and then to itsreflooding by the safety injections. The presented work relies on the development of a fine 3D coupling between neutronics and thermal-hydraulics at the assembly scale plunged into a fullreactor simulation. Such a development goes beyond the known limitations of current neutronics/thermal-hydraulics couplings (dealing with low void fraction situations) which are notsuitable for LOCA safety studies.