A novel Ansys CFX – RELAP5 coupling tool for the transient thermal-hydraulic analysis of liquid metal systems

Thermal-hydraulic analysis of three-dimensional systems and phenomena is essential for the design and safety assessment of new Generation IV reactor concepts. While System Thermal-Hydraulic (STH) codes serve as established tools for studying Light Water Reactors, their intrinsic limitations cannot allow to accurately model pool-type systems and fluids with high thermal conductivity, where three-dimensional phenomena cannot be accurately replicated. On the other hand, Computational Fluid Dynamics (CFD) codes have demonstrated significant potential in simulating components at a detailed scale and in predicting their thermal-hydraulic behaviour. To exploit the benefits of both code families, coupled tools are gaining attention as a viable solution to enhance the representation of system thermal-hydraulics, while maintaining reasonable computational times. This paper presents a novel coupling tool between Ansys CFX and RELAP5/Mod3.3 codes, developed for different numerical time advancing schemes and domain discretization approaches, and achieved through the development of in-house scripts for memory management, code synchronization, and information exchange. The tool is then applied to three different test cases with increasing modelling complexity, examining the performance and consistency of the coupling method with respect to the RELAP5/Mod3.3 standalone code. As expected, the adoption of different numerical time advancing schemes and domain discretization approaches lead to consistent results, highlighting the efficacy and limitation of each proposed solution. Furthermore, in cases where the approximations of the STH codes are valid (test cases 1 and 2), the coupled tool and the RELAP5/Mod3.3 standalone simulations give similar results. Instead, an application of the tool where RELAP5 is expected to be not reliable is proposed in test case 3, i.e., 3D flow mixing in a large volume, highlighting the potentiality of the tool and the conditions where a TH analysis can benefit from a STH/CFD coupled calculation.