Multiscale analysis of nuclear reactors thermal-hydraulics - the NEPTUNE project

The NEPTUNE project aims at building a new two-phase thermal-hydraulics platform for nuclear reactors simulation. It is being developed jointly by CEA-DEN and EDF-R&D and also supported by IRSN and FRAMATOME-ANP. NEPTUNE is a new generation multi-scale platform. The "system" scale models the whole reactor circuit with 0D, 1D and 3D modules and is generally applied with a coarse meshing including about a thousand meshes. The component scale models components like the reactor Core or Steam Generators with a finer nodalization and is generally applied using 104 to 105 meshes. Since these components contain rod or tube bundles the physical modelling is based on a homogenization technique using the porosity concept. For some specific applications it was found necessary to add a two-phase CFD tool able to zoom on a portion of the circuit where small scale phenomena are of importance for design purpose or safety issues. Here the basic equations are still averaged like in RANS approach for single phase, but the space resolution is finer than in component codes and typical applications may require from 105 to 107 meshes. These three scales have to be coupled in order to simulate many reactor transients where both local effects and system effects play a role. In addition, two-phase Direct Numerical Simulation tools with Interface Tracking Techniques can be used for even smaller scale investigations leading to a better understanding of basic physical processes and allowing the development of new closure relations for averaged models. The main challenges of this project are here presented and some first results are presented.