Benchmark Calculation of FHR Fuel Assembly Phase I-C Depletion Exercises

This paper presents our initial results for the Fluoride-salt High-temperature Reactor (FHR) physics benchmark calculations in Phase I-C, focusing on depletion exercises 5 and 6. In this Phase, the model is extended from the previous OECD benchmarks (Phases I-A and I-B) by transitioning from a pseudo-2D to a full 3D representation of a single FHR fuel assembly featuring TRISO fuel, graphite moderator, and FLiBe coolant. The pseudo-2D geometry is extruded axially, incorporating top and bottom reflectors (FLiBe and graphite), with radial periodic and axial vacuum boundary conditions applied. The benchmark’s challenging aspects, including the complex 2D geometry of the plate-type assembly with TRISO fuel, the double heterogeneity spectral calculation, and the use of novel materials (FLiBe coolant and europium as a burnable poison), necessitate the use of Monte Carlo methods. We used the Serpent 2 code (versions 2.1.32 and 2.2.1) with two versions of the ENDF/B library (VI.8 and VII.1). Exercises 5 and 6 subdivide the fuel assembly into 20 axial regions with distinct material temperatures, however the primary distinction between these two exercises is the inclusion of integral burnable absorbers (Eu-151 and Eu-153 oxides) in the latter. The paper presents the results for keff, recoverable energy per fission, fission density rate, and axial offset in selected burnup points up to the final burnup of 70 GWd/tU. Additionally, we assessed the impact of different cross-section libraries, xenon modeling (transient versus equilibrium), and the depletion step length on the benchmark results.