Within-Pin Reaction Rate Distributions: CASMO-4 and HELIOS Compared Against Tomographic Measurements at the PROTEUS Reactor

In the framework of the LWR-PROTEUS project - an extended validation program for advanced light water reactor core analysis tools conducted at the Paul Scherrer Institute - the radial, internal variations of the total fission rate (F_tot) and the capture rate in ²³⁸U (C₈) have been calculated for zero-burnup pins of a Westinghouse SVEA-96+ boiling water reactor fuel assembly using two codes, namely, CASMO-4 and HELIOS. While F_tot distributions predicted by CASMO-4 and HELIOS are in good agreement, C₈ distributions show significant inconsistencies (20 to 30%). The calculations are compared with experimental results obtained using single photon emission computerized tomography for several SVEA-96+ pins irradiated in the zero-power reactor PROTEUS. The comparisons confirm the predicted shape of the F_tot distributions within UO₂ pins and clearly indicate that HELIOS within-pin predictions for C₈ are more reliable than CASMO-4 results. This is important for the derivation of gamma-ray self-absorption corrections when pin-integrated reaction rates are to be determined using the gamma-scanning technique. Thus, the use of CASMO-4-type within-pin distributions would lead to 3 to 4% discrepancies in the absolute, self-absorption-corrected pin-integrated values deduced for C₈ and hence for C₈/F_tot. For relative C₈ distributions, the discrepancy would be much smaller, namely, up to ~1% if pins containing a burnable absorber are involved.