Effect of double heterogeneity treatment on neutronics modeling of HTGR unit cell - 18524

The High Temperature Gas-cooled Reactor (HTGR) utilizes unique coated fuel particle design, which is embedded within a graphite matrix and shaped into spherical or hexagonal fuel element. The feature of the inherent double heterogeneity in the HTGR fuel element therefore raises difficulties in modeling and requires special simulation strategies to address its impact on the self-shielding calculation. In this study, neutronics calculations with various levels of heterogeneity processing were performed for both spherical and hexagonal HTGR fuel elements. The results indicate that ignoring either level of double heterogeneity can lead to significant underestimation of self-shielding effect and consequently yield as much as 8% deviation in the prediction of the multiplication factor. Fission and capture rates were compared for the spherical HTGR fuel model and over 30% deviation was found in the resonance energy range if homogenized model used. Further investigation on the neutron spectrum demonstrated similar tendency for both types of fuel elements when applying the simplified self-shielding models, that is, #approx#20% underestimation of neutron flux in 6.5-6.75 eV energy interval was observed due to the incomplete evaluation of the cross section resonance self-shielding effect. (authors)