Two-temperature homogenized model for steady-state and transient thermal analyses of a pebble with distributed fuel particles

In a pebble-bed type very high temperature gas-cooled reactor (VHTGR), a typical fuel pebble consists of over ten thousand five-layer TRISO particles in a graphite-matrix. The high heterogeneity in composition leads to difficulty in explicit thermal calculation of pebble fuels. Thus, a homogenization model becomes essential. Currently, a simple volumetric-average thermal conductivity approach is used. However, this approach is non-conservative and underestimates the fuel temperature. In this paper, we describe a homogenization model that is not only easy to implement but also gives a more realistic temperature distribution in a fuel pebble, providing the fuel-kernel and graphite-matrix temperatures separately. Steady-state and transient thermal analyses are performed using the homogenization model, and the point kinetics model is then coupled with the homogenization model to incorporate fuel-kernel temperature feedback.