Parametric study of effective thermal conductivity for VHTR fuel pebbles based on a neutronic and thermal coupling method

Pebble bed very high-temperature gas-cooled reactor (VHTR) has many economic and safety advantages, such as high safety and high outlet temperature. It uses fuel pebbles with Tristructural-isotropic (TRISO) fuel particles. The effective thermal conductivity of fuel pebbles is a key parameter to evaluate the peak temperature and average temperature of the fuel pebbles. The currently available effective thermal conductivity models do not consider the internal heat sources and the complex neutronic and thermal (N/T) coupling effects in the fuel pebbles. In this study, a three-dimensional fuel pebble heat transfer and a Monte Carlo neutronic coupled model were developed. The model has a random distribution of TRISO fuel particles and an accurate geometric model. The results show that traditional multi-scale heat conduction methods based on the existing effective thermal conductivity model, such as those based on the Chiew and Glandt models, are conservative in predicting the temperatures of VHTR fuel pebbles. The degree of conservatism is affected by fuel pebble power and packing fraction. Under the N/T coupling condition, the effective thermal conductivity is not much affected by the power of the fuel pebble but decreases with the increase of the fuel pebble boundary temperature. Geometric param-eters affect the internal temperature distribution and peak temperature by changing the uniformity of the heat source distribution.