Monte Carlo modeling and simulations of AHTR fuel assembly to support V&V of FHR core physics methods

The Fluoride Salt Cooled High Temperature Reactor (FHR) is a liquid salt-cooled reactor utilizing TRISO-based fuel. Its low operating pressure and large thermal margin offer attractive safety. However, its complex core design, in particular the double heterogeneity introduced by TRISO particles, as well as the presence of several moderating materials, require careful verification and validation (V&V) of the core physics modeling and simulation (M&S) methods. Due to their ability to accurately simulate complex geometries and utilize continuous-energy nuclear data, Monte Carlo methods are used in this study to evaluate core physics phenomena and provide reference solutions for verification of lower fidelity methods. This work is focused on the FHR design with hexagonal fuel elements consisting of fuel plates (“planks”), i.e., the Advanced High-Temperature Reactor (AHTR) developed by ORNL. The pebble-bed FHR design, PB-FHR, is not directly considered in this work, although the results of this study are relevant for PB-FHR as well. The SERPENT Monte Carlo code is selected and utilized to perform core physics analyses of an FHR fuel assembly. The issues investigated include detailed fission distribution, quantification of certain approximations, study of neutron spectra in different material regions, and impact of graphite density, temperature, and scattering matrix data. This study contributes to V&V of modeling and simulation methods for FHR.