Development of a new thermal-hydraulic module for FRENETIC, a code for the multiphysics analysis of liquid metal-cooled reactors

Heavy Liquid Metal Cooled Reactors (HLMCRs) are among the most promising Generation IV concepts. In view of the growing interest towards these systems, HLMCR core design activities are currently being pursued worldwide. The core design of a fission reactor is a complex task that must take into account the intrinsically multi-physics nature of the system. Computational tools that are reliable while being fast running are required both in support of the core design phase and to assess the reactor behaviour during operational and off-normal transients. The specific features of HLMCRs, namely the fast neutron spectrum and the liquid metal coolant, determine a different behaviour with respect to commercial light water reactors, thus calling for the development of specific codes. FRENETIC is a multi-physics code for the full-core simulation of liquid metal-cooled fast reactors, developed at Politecnico di Torino. The code is capable of performing steady state and transient neutronic (NE) and thermal-hydraulic (TH) coupled calculations, while maintaining a relatively low computational cost thanks to the adoption of simplified physical models. The NE module implements the nodal formulation of the multigroup neutron diffusion equations with delayed neutron precursors, whereas the TH module treats the reactor hexagonal assemblies as separate channels, which are individually modelled as 1D in the axial direction and then thermally coupled to their neighbors in the horizontal directions. In this thesis, a new TH module for FRENETIC was designed, developed and tested, in accordance with current best practices, which resulted in improved performance, portability and flexibility. Specific attention was devoted to ensure a correct memory management and a high code modularity, so to simplify future developments and/or extensions of FRENETIC. The governing equations were discretized with a finite volume treatment, taking advantage of the incompressibility of liquid metals. The performance of the new TH module were benchmarked against the previous code version. The newly developed code shows considerably improved performances in steady state and improved performance in transient calculations. Finally, as a reactor-relevant test case, the code is applied to simulate the ALFRED reactor core in steady state and in a representative transient.