Nuclear-thermal analysis of a unit fuel element in HTGRs based on two-temperature homogenized model

Abstract

In either pebble-bed or block type design of high temperature gas-cooled reactors (HTGRs), heterogeneous composition and structure leads to difficulty in thermal analysis of the fuel elements (fuel pebbles and fuel compacts, respectively). Thus, a homogenization model becomes essential. Currently, a simple volumetric-average thermal conductivity approach is used. However, this approach lacks basis and is non-conservative in that it underestimates the fuel temperature. In this thesis, we propose a homogenization model that is not only easy to implement but also gives a more realistic temperature distribution in a fuel pebble or a fuel compact, providing the fuel-kernel and graphite-mixture temperatures separately. For a given problem, homogenized parameters are obtained at steady-state through matching the analytic solution for the homogenized fuel element to the reference solution of the heterogeneous fuel element. The reference solutions are provided by the Monte Carlo method. Transient thermal analyses are performed using the homogenization model, in which finite difference method (FDM) in space discretization and explicit scheme in time scale is applied. By providing the power history, temporal changes in temperatures can be computed. A reactor point kinetics model is then coupled with the two-temperature thermal transient model for more accurate neutronics evaluation with Doppler temperature feedback. Several test scenarios are studied, including external reactivity insertion and coolant cooldown events.