Three-Dimensional Thermal Modeling of TRISO Fuel in Pebble-Bed Reactors.

摘要

The Very High Temperature Gas Reactor (VHTR) is widely accepted as one of the top candidates for the Next Generation Nuclear Power-plant (NGNP) fleet. The TRISO particle is a common element among different VHTR designs, and the performance of the TRISO particle is critical to the safety and reliability of the reactor. A TRISO particle experiences complex thermo-mechanical changes during reactor operation under high temperature and high burnup conditions. TRISO fuel performance analysis requires evaluation of these changes on micro-scale. Since most of these changes are temperature dependent, thermal analysis of TRISO fuel is a crucial part of the overall fuel performance model. A PhD dissertation work, with the goal of developing a comprehensive fuel performance model for TRISO fuel, has been completed recently at the University of Illinois, and the thermal analysis is part of the dissertation work. In this paper, 3D numerical thermal models are developed to calculate temperature distribution inside TRISO particles and pebbles under different scenarios. Given that TRISO particles are embedded inside pebbles, two thermal models are needed: one for the TRISO particle and the other for the pebble. The pebble model provides boundary conditions for the TRISO particle model. 3D modeling is required because pebbles or TRISOs are always subjected to asymmetric thermal conditions partly because they are randomly packed together. The thermal models are developed using the finite difference method, and they are bench marked against ID analytical results and also against results reported in literature. Monte-Carlo models are set up to calculate radial power density profile inside a TRISO particle and in a pebble. Complex convective boundary conditions are applied on the pebble outer surface. Three reactors are simulated using this model to calculate temperature distribution in a pebble under different power levels.