IMPACT OF THERMAL COUPLING ON A PEBBLE BED REACTOR EQUILIBRIUM FROM HYPER-FIDELITY DEPLETION

Abstract

Due to the complex nature of pebble bed reactors' (PBR) operation and simulations, most approaches depict the fuel evolution using large spectral zones with uniform fluxes and compositions moving with time. However, the lack of experimental data for validation and detailed distributions pushes the need for higher fidelity methods. Moreover, burnup, power, temperature distributions, and peaking factors at equilibrium are critical parameters for core design, safety analysis, and essential inputs for fuel performance, thermal hydraulics, and waste management. In this context, hyper-fidelity depletion (HF) is being developed. This approach aims to track the evolution of each pebble's location and isotopic concentrations, from insertion to discarding. This work complements previous HF developments by incorporating a fine temperature distribution to Serpent transport calculations, hence providing an additional layer of accuracy. Pebbles can, indeed, experience significantly different thermal conditions based on their location in the core and burnup. The thermal distribution is resolved using the OpenFOAM-based porous media solver GeN-Foam, which provides medium-fidelity pebbles' surface temperatures. Conduction calculations, through a newly implemented double-heterogeneous model, subsequently determine the matrix and TRISO particles' temperature profiles. This information is ultimately communicated to Serpent to account for thermal feedback on neutronics in an iterative process. This study presents a proof of concept of thermal coupling with the generic FHR model. Using an obtained equilibrium from HF with discrete motion on this core, thermal profiles and their impacts on the neutronics are determined and discussed. Although the global influence of temperatures on the power distribution is minimal, local differences impacting the outlying values are non-negligible. In particular, fuel temperature is driven by power production and fluid temperature around the pebbles.