Coupled Neutronic/Thermal-Fluid Benchmark of the MHTGR-350 MW Core Design: Results for the Lattice Physics Exercises

NEA/NSC/R(2019)6
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The purpose of this prismatic High Temperature Reactor (HTR) benchmark is to provide a solution set that can be used in future code benchmarking and to better understand the effects induced by the presence of burnable poisons and different neighbours in a super-cell arrangement. This configuration is particularly representative of the fuel in the MHTGR-350 reactor core. Solutions from two Monte Carlo and two deterministic codes have been analysed. Means and standard deviations are provided at beginning, middle, and end of life for a variety of parameters. The report includes multiplication factors, detailed block-power distributions, spectral parameters, two-group cross-sections, as well as actinide and fission-product inventories. The analysis is focused on two spatial locations with very different neutron-energy spectra. The results from the benchmark indicate that there is very good agreement between participants in the integral parameters, but that significant differences appear in the concentrations of various isotopes. Some are due to the use of the reactivity physical transformation methodology, which captures reactivity effects very well, but does not produce the correct self-shielding in 238U. This leads to poor predictions of the 239Pu and other higher actinides.