Comparison of Measured and Calculated Uranium Reaction Rates in a Steam-Cooled Fast Assembly in Sneak

An absolute determination of uranium reaction rates by means of foil irradiation was carried out in the Fast Zero Power Reactor SNEAK, assembly 3A-2 This requires the measurement of reactionrate traverses as well as their fine structure. These rate distributions were compared with transport and diffusion calculations. The traverse measurements indicate a strong dependence of the axial “buckling” (relative second derivative of the rates) on energy. The calculations with the Karlsruhe SNEAK cross-section set and the Russian ABN set yield these bucklings for ²³⁸U capture and ²³⁵U fission in diffusion approximation on the average 5% higher than measured. Only the fast-fission rate distribution of ²³⁸U is almost correctly calculated using the cross-section sets. The ratio of total plutonium production to ²³⁵u fission along the z axis was measured and compared with theory. It agrees within the error limits of the experiment. Beyond the error limits are the discrepancies found for the spectral index σ_ƒ²⁸/σ_ƒ²⁵ The fine structure of reaction rates within the fuel platelets is well described by an integral transport code, but again, there are considerable discrepancies between calculated and measured fission-rate ratios.