Abstract: The highly multi-scale heterogeneous configuration in a pebble bed type reactor makes a significant challenge and complexity for theoretical analysis. In a typical pebble fuel, the fueled region that consists of a graphite matrix and numerous dispersed fuel particles is surrounded by a thin non-fueled graphite shell. Moreover, an individual fuel particle named tristructural-isotropic(TRISO) particle typically consists of five distinct regions. These high heterogeneities lead to difficulty in explicit thermal calculation of a pebble fuel. Currently, a simple volumetric-average thermal conductivity approach and a harmonic-average thermal conductivity approach are used. However, the volumetric-average approach is non-conservative as well as underestimates both the fuel temperature and the graphite matrix temperature, and the harmonic-average approach is excessively conservative that overvalues temperatures too much. In this paper, we propose a two-temperature method (TTM) to do the temperature distribution calculation in a pebble fuel. The method is not only convenient to perform but also gives more realistic results due to particles and graphite matrix are considered separately.

Keyword: two-temperature method; pebble fuel; heat transfer