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 (Ftot) 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 Ftot 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 Ftot 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 C 8 and hence for C8/Ftot. For relative C 8 distributions, the discrepancy would be much smaller, namely, up to 1% if pins containing a burnable absorber are involved.