NEPTUNE: A New Software Platform for Advanced Nuclear Thermal Hydraulics

The NEPTUNE project constitutes the thermal-hydraulic part of the long-term Electricité de France and Commissariat à l’Energie Atomique joint research and development program for the next generation of nuclear reactor simulation tools. This program is also financially supported by the Institut de Radioprotection et Sûreté Nucléaire and AREVA NP. The project aims at developing a new software platform for advanced two-phase flow thermal hydraulics covering the whole range of modeling scales and allowing easy multiscale and multidisciplinary calculations. NEPTUNE is a fully integrated project that covers the following fields: software development, research in physical modeling and numerical methods, development of advanced instrumentation techniques, and performance of new experimental programs.The analysis of the industrial needs points out that three main simulation scales are involved. The system scale is dedicated to the overall description of the reactor. The component or subchannel scale allows three-dimensional computations of the main components of the reactors: cores, steam generators, condensers, and heat exchangers. The current generation of system and component codes has reached a very high level of maturity for industrial applications. The third scale, computational fluid dynamics (CFD) in open medium, allows one to go beyond the limits of the component scale for a finer description of the flows. This scale opens promising perspectives for industrial simulations, and the development and validation of the NEPTUNE CFD module have been a priority since the beginning of the project. It is based on advanced physical models (two-fluid or multifield model combined with interfacial area transport and two-phase turbulence) and modern numerical methods (fully unstructured finite volume solvers). For the system and component scales, prototype developments have also started, including new physical models and numerical methods.In addition to scale-specific developments, the generalized use of multiscale calculations is also expected to be a major means to meet the industrial needs. The coexistence of different simulation scales together with the fast growth of computing power multiplies the computation possibilities. In particular, thanks to the recent progress of CFD tools, one can imagine local zooms in some critical parts of the reactor components. The NEPTUNE multiscale platform will offer advanced coupling functionalities based on state-of-the-art software architecture and new numerical coupling techniques.Finally, despite the existence of a huge worldwide database of two-phase flow experiments, the validation of new physical models (more local, more complex) requires new experimental data. That is the reason why for several years we have been developing new instrumentation techniques such as four-sensor optical probes, X-ray tomography, and hot-wire anemometry. These techniques will be used for new experimental programs (currently being launched) that have been defined in connection with the high-priority industrial applications (departure from nucleate boiling, pressurized thermal shock, loss-of-coolant accident, etc.).