Resonance and Total U²³⁸ Activation in Thin Natural Uranium-Niobium Fuel Plates

Differential measurements of the epicadmium and total U²³⁸ radiative capture have been obtained in thin natural uranium-niobium plates and compared to several theoretical models. These fuel elements were located in Zircaloy-2 boxes, which in turn were placed in a light water moderated critical assembly at ambient temperatures. A Monte Carlo calculation with the assumption of a smooth 3.3 b contribution predicted the spatial epicadmium activation through a natural uranium fuel plate positioned in the center of the Zircaloy-2 box. It was observed that the integrated epicadmium U²³⁸ radiative capture in the end fuel plate of the Zircaloy-2 box was 1.27 times the capture in the center fuel plate. This sharp end-to-center dipping was due to a water channel located next to the fuel box. An analytic calculation based on the thin resonance theory of Stein gave a value of 1.16 for this end-to-center ratio. The experimental integrated values for ρ²⁸, the ratio of epicadmium to thermal captures, were 0.893 ± 0.009 in the center fuel plate, 0.765 ± 0.014 in the second from the end plate, and 0.851 ± 0.011 in the end plate. The value of ρ²⁸ obtained using a diffusion theory approximation for these plates was 4.7%, 7.5%, and 21.7% below experiment. The prediction in the end plate was improved when the Stein theory was utilized to calculate the resonance capture.