Neutron Importance and Fission Density in Uranium-235-Enriched Uranium and Plutonium Metal Spheres

The spatial distribution of the neutron importance in bare and natural-uranium-reflected uranium (∼93.2 wt% ²³⁵U) and plutonium (∼4.7 at % ²⁴⁰Pu) metal spheres was measured using ²⁵²Cf neutron sources. The spatial distribution of the fission density from activation measurements in the bare spheres and those previously measured for the reflected spheres are presented.Comparison of these distributions with those from S₁₆ transport theory calculations showed that the measured and calculated results agreed very well for the bare spheres and in the central core of the reflected spheres. The disagreement in the natural uranium reflector increased with radius and attained values as large as ∼35% at the outer surface. The sensitivity of the calculations to the cross sections is examined.These measurements were undertaken to properly account for spatial effects in the point reactor kinetics description of Rossi-α measurements. The spatial-effects factors obtained from these measurements, which multiply the correlated amplitude of the Rossi-α measurement, were 1.123, 1.109, 1.163, and 1.214 for the bare uranium, bare plutonium, reflected uranium, and reflected plutonium spheres, respectively. The error in these values is ± 0.010.