IAEA CRP on HTGR uncertainties: Comparison of EX. I-2C nominal results and coupling with the PHISICS/RELAP5-3D core model for EX. II-1

Super cell models are compared to the nominal results obtained in the IAEA CRP for HTGR for single block models (Ex. I-2a and Ex. I-2b) to evaluate if such models achieve softer neutron spectra that characterizes these reactors. A comparison of a set of super cell models consisting of various moderation schemes and nuclide inventories is performed to establish the merits of this approach. The super cell are analyzed with the SCALE 6.1 deterministic code NEWT coupled with the TRITON sequence. The cross-section data generated by the TRITON/NEWT sequence is additionally utilized in the coupled neutronics/thermal-hydraulics code PHISICS/RELAP5-3D. The impact of various super cell flux-weighted cross sections in subsequent core calculations from a neutronics point of view is evaluated. The cross section uncertainties (nuclear data uncertainties) along with thermal-hydraulics uncertainties will be propagated through coupled neutronics/thermal-hydraulics calculations at a later stage. The neutron fluxes observed in super cells is shown to be softer than single block models, but depends highly on the composition and topology of the peripheral regions. The preliminary results obtained with PHISICS/RELAP5-3D indicate that such variations in neutron fluxes induce different magnitude of changes in the core's effective multiplication factor, depending whether the core is at the beginning or end of life.