Measurements of Reaction Rates in Zone-Type Cores of Fast Critical Assembly Simulating High Conversion Light Water Reactor

Abstract

Measurements of reaction rates have been performed in three uranium-fueled zone-type cores of the FCA constructed for a series of experiments on a high conversion light water reactor (HCLWR). These cores possess central test zones of different fuel enrichments and moderator to fuel volume ratios. Radial and axial fission rates of 236U, 239Pu, 238U and 23,Np were measured in each test zone by means of the micro-fission counter traverse. A region where the fundamental mode spectrum is established in the test zone were determined by utilizing these fission rate distributions. Central reaction rate ratios relative to the 235U fission rate were obtained from the measurements by the micro-fission counters and metallic uranium foils to examine changes in the reaction rate ratios among the three cores. The measured data were analyzed by the SRAC code system on the basis of the nuclear data file JENDL-2. The calculated fission rate distributions agree well with the experimental results for the all cases. The results of reaction rate ratios show that the calculations over- predict the experimental values of the 238U capture/235U fission and 238U fission/235U fission rate ratios in the three cores.