Multiscale Thermal-Hydraulic Analysis of the ATHENA Core Simulator

The Advanced Thermal Hydraulic Experiment for Nuclear Application (ATHENA) will play a fundamental role in the design, safety assessment and thermal-hydraulic (TH) characterization of the ALFRED reactor components. In ATHENA, full-scale systems and their mutual interactions will be tested in steady state and relevant transient conditions. The Core Simulator is the electrical heating system and aims to be representative of an ALFRED average fuel assembly (FA). CFD codes are gaining attention for the analysis of complex systems in pool-type reactors, since they are able to reproduce with high accuracy 3D TH phenomena. In this paper a multiscale approach based on porous media (PM) is proposed to reduce the computational cost of the Core Simulator CFD model. Firstly, the PM hydraulic behavior is characterized by means of CFD simulations on the infinite lattice domain. Then PM are applied in the system model in place of fuel assemblies (FAs). Three different approaches are investigated: (1) adopting a single porous domain for the entire FA; (2) representing the FA with two porous domains; (3) adopting the so-called hybrid medium model. The results obtained with the PM models are validated against a detailed CFD simulation of the Core Simulator to evaluate the performance of the three approaches. Then, the best solution is applied on a model of the entire ATHENA Core Simulator integrated with the feeding region. The model is tested in simple transient conditions. The numerical experiment demonstrates the effectiveness of the multiscale approach in reducing the computational cost while maintaining high accuracy in representing the quantities of interest.