The HEMERA (Highly Evolutionary Methods for Extensive Reactor Analyses) 3D software package is used to perform 3D simulations of nuclear reactor accidents involving interactions between thermal-hydraulic and neutronic behaviours (reactivity accidents). The tool, developed by the IRSN in close collaboration with the CEA, links the CATHARE2 and FLICA4 thermal-hydraulic simulators and the 3D neutronic simulator CRONOS2. The CATHARE2 code performs 1D simulations of the primary and secondary circuits of a reactor, excluding the core. FLICA4 simulates the thermal-hydraulic behaviour of the core in 3D, and CRONOS2 simulates the 3D neutronic behaviour of the core.
The software package has been developed using an explicit linking technique, solving the neutronic and thermal-hydraulic equations independently. Data is then transferred between the individual simulators using an iterative process until the results converge. A dedicated software tool (ISAS) is used to control the individual simulators and manage the exchange of data.
Concerning the reactor core, the neutronic and thermal-hydraulic simulations are therefore performed in 3D by the CRONOS2 and FLICA4 simulators linked via ISAS.
CRONOS2 solves the diffusion equation for a number of energy groups (two in the generic case for pressurised water reactors) using the mixed dual finite element method (solver MINOS). This gives the 3D distribution of power in the core over a number of homogeneous micromeshes (usually one or four per fuel assembly).
FLICA-4 is dedicated to the simulation of two-phase flows (liquid water and steam) under steady state or transient conditions. The 3D thermal-hydraulic simulation code uses a finite volume method. The two-phase flow containing both liquid and vapour phases is modelled by four conservation equations (conservation of the mass of the vapour phase and conservation of the mass, energy and quantity of movement of the liquid-vapour mixture), together with a set of closure laws.
The remainder of the reactor, the primary coolant circuit and the secondary circuit, is simulated in 1D by the CATHARE2 simulator developed jointly by the CEA, EDF, AREVA-NP and the IRSN. This simulator is based on a two-fluid model (water-steam) with six equations (mass, quantity of movement and energy for each phase), together with a set of closure laws.
The IRSN uses the HEMERA software package during reactor safety assessments, to simulate reactivity accidents, especially core cooling accidents resulting from a steam line break (SLB) and rod ejection accident (REA). The first type of accident is simulated by linking CATHARE2, CRONOS2 and FLICA4, the second using CRONOS2 and FLICA4.