The Effective Thermal Conductivity Model of Dispersion Fuel Elements Based on Effective Matrix Approximation

Abstract The modular high temperature gas-cooled reactors use fuel elements with huge number of Tri-structural isotropic (TRISO) fuel particles dispersed in graphite matrix, which contributes to the inherent safety and enables high temperature process heat applications. Featured by complex geometry and material combinations, a lumped model of effective thermal conductivity (ETC) is practically used to describe the heat transfer performance and temperature distribution of the fuel element the in thermal hydraulic design and safety analysis of HTGRs. Since the fuel particles release fission or decay heat, classical analytic ETC models fail in predicting the temperature distribution inside the fuel elements, without denoting the inner heat sources. Previous researches have explained the definition of ETC model with inner heat sources. Some new ETC models that indicate the function of distributed inner heat sources in the dispersion fuel element were also proposed. In this paper, an ETC model to predict the volumetric average temperature of dispersion nuclear fuel elements is proposed. The proposed model considered the influence of distributed inner heat sources in the composite media by introducing effective matrix approximation. The ideal dispersion fuel pebble, which is simplified from the element currently used in pebble bed HTGRs, is taken as an example to show the application of the proposed ETC model. And the validity and conservativeness of the proposed ETC model are preliminarily revealed by finite element numerical calculation results.