Development and verification of a coupling methodology between a system thermal-hydraulic code and a reduced order CFD model

The System codes have been successfully used for many years in the nuclear safety field for PWRs and BWRs plants. Nevertheless, since the next generation of nuclear reactors relies on passive safety systems, basing on a natural circulation/convection and facing more complex geometries the use of a lumped parameter 1D approach is no more sucient. As a consequence,
the use of new computational tools able to deal with the 3D phenomena such as ComputationalFluid-Dynamics is currently being considered, in particular when adopted in coupled applications with System Thermal-Hydraulics codes. The coupling techniques are expected to enhance the capabilities to simulate and predict the behavior of nuclear reactors during normal and incidental conditions. Nevertheless, Computational Fluid-Dynamics often requires large computational eorts and computational times. Therefore, the use of Reduced Order Computational Fluid-Dynamics Model is here proposed and used in the coupling procedure in order to reduce the global computational cost.
In the present work, a computational tool has been developed coupling the System Thermal-Hydraulic code RELAP5/mode3.3 and the ITHACA-FV code as Reduced Order Computational Fluid-Dynamic Model. ITHACA-FV code (In real Time Highly Advanced Computational Applications for Finite Volumes), is a C++ library based on the nite volume solver OpenFOAM currently developed and maintained at SISSA (International School for Advanced Studies, Trieste,
Italy). This coupling procedure is thought to deal with the 3D phenomena that appears in open pool reactor like MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications), currently in the design phase at SCKCEN.
To demonstrate the capabilities of the new computational tool, a series of case has been set up and the obtained results were veried against the predictions of the coupled tool RELAP5- OpenFOAM 6.0. Firstly, the simple pipe
ow has been tested and, subsequently, the flow reversal and to the closed loop system were also investigated. Afterwards, the heat transport equation for temperature has been introduced in the Reduced Order Model, applying the coupling procedure to the non-isothermal test case. The analysis conducted have demonstrated the reliability of the Reduced Order Model: it was successfully used in the frame of the coupled calculations and provided results in accordance with the ones obtained adopting Full Order Computational Fluid-Dynamic codes.