Neutron Reaction Rate Measurements as Tests of Diffusion Calculations in a Fast Breeder Blanket Facility

Six capture rates, five fission rates, and one inelastic scattering rate have been measured as a function of radius in the blanket of the Fast Breeder Blanket Facility, a facility designed to simulate the transport of neutrons in fast reactor blankets. The measured reaction rates were compared with the reaction rates obtained from a typical two-dimensional calculation. The calculated reaction rates agree well with the measurements at the inside of the blanket but diverge from the measurements with increasing blanket penetration. Two effects were found to account for all of the differences between the calculated and measured reaction rates. First, a quantity approximately equal to the neutron number density decreases more rapidly across the blanket in the calculations than the measurements would indicate. Second, a self-shielding transition effect was noticeable around the converter/blanket interface. Furthermore, a mutual shielding effect between ²³⁸U cross-section resonances and detector foil resonances caused additional differences between the measurements and calculations of three capture rates for materials commonly used in neutron dosimetry experiments. The experimental techniques and the results of the reaction rate measurements are presented in detail, including a theoretical foil correction (by means of integral transport theory) that replaces the previously used experimental correction. This work completes and complements earlier experiments, comparisons, and interpretations.