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

The nuclear community has performed two decades of research on coupling three-dimensional Computational Fluid Dynamic (CFD) models with one-dimensional thermal-hydraulic system (STH) codes. Nevertheless, the computational cost of coupled simulations is still too high for nuclear safety studies that require of a large number of different system configurations to be analyzed. As the CFD solver is accountable for the largest part of the computational time, this work proposes to replace it by a reduced order model (ROM) of a high fidelity CFD code. More precisely, the best-estimate system code RELAP5-MOD3.3 and a ROM of the finite volume CFD solver OpenFOAM are coupled by a partitioned domain decomposition coupling algorithm. An implicit coupling scheme based on a Quasi-Newton algorithm is adopted. The finite-volume based ROM is developed with a Proper Orthogonal Decomposition technique in combination with a Galerkin projection technique. The reduced bases are determined using the velocity and pressure fields obtained by performing a coupled RELAP5/CFD simulation for a certain parameter set. To demonstrate the performance of the coupled models, tests are carried out on simple open and closed pipe flow configurations. The coupling methodology is evaluated by comparing the time evolution of the mass flow rate and area-averaged pressure calculated with the RELAP5/CFD models at a coupling interface with stand-alone system thermal-hydraulics solutions. Furthermore, the performance of the coupled RELAP5/ROM models and the coupled RELAP5/CFD models are compared. The results for new parameter sets show that the coupled RELAP5/ROM models are capable of predicting the coupled RELAP5/CFD results with good accuracy. Finally, simulations performed with the RELAP5/ROM models are about 3 to 5 times faster than the coupled RELAP5/CFD simulations.