A Coarse-mesh Methodology for the Analysis of One and Two-phase Nuclear Reactor Thermal-hydraulics in a Multi-physics Context

The analysis of nuclear reactors for performance and safety assessment benefits from the use of computational tools. In this context, this work aims at the development and application of a thermal-hydraulics methodology and related software that respond to emerging needs in the computational field: 1) greater geometric and physics modelling flexibility; 2) streamlined coupling with other single-physics to enable multi-physics capabilities; 3) parallel scalability on High Performance Computing clusters; 4) adoption of modern programming practices. A coarse-mesh approach is proposed to offer a reasonable balance between computational accuracy, comparable to that of sub-channel codes, and computational burdens. The developed approach can make use of general 3-D geometries with unstructured meshes, which are beneficial to the aforementioned geometric flexibility needs. Additionally, in a multi-physics context, field transfer operations between the different physics are simplified both by the adoption of a coarse-mesh approach and by the use of standardized mesh formats. The employed programming framework consists of the Finite Volume Method-based OpenFOAM library, which offers the desired features of massive parallel scalability and of a modern object-oriented programming paradigm. The coarse-mesh methodology is presented alongside a thorough theoretical derivation of the governing equations for a generic multi-phase system. Based on this, a computer code is developed for the modelling of one-phase and two-phase flows, with a focus on the simulation of Sodium-cooled Fast Reactors (SFRs), which represent the nearest-term deployable fast reactor technology. These were also chosen as the simulation of phase change in sodium represents a challenging case for the numerical stability of two-phase solution algorithms. The main achievements of this development effort consist of: 1) a novel solution algorithm for two-phase pressure-velocity coupling that enhances stability and performances compared to existing algorithms; 2) implementation of the code based on object-oriented programming practices, which allow for a seamless implementation of different working fluids and structure models; 3) code verification via an ad-hoc implementation of the Method of Manufactured Solutions; 4) demonstrated good parallel scaling of the code up to thousands of computer cores. In terms of applications: 1) preliminary validation based on sodium boiling experiments; 2) detailed investigation of existing and novel features for SFR fuel elements. Furthermore, the multi-physics capabilities of the developed methodology are demonstrated by integrating it within the GeN-Foam multi-physics environment. As a test case, the resulting software is applied to the simulation of a Loss Of Flow Without SCRAM test performed at the Fast Flux Test Facility. This benchmark re-analysis takes place within the framework of a coordinated research project by the International Atomic Energy Agency and is set to provide valuable feedback in terms of code-to-code comparison data.