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

Abstract

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