The Infinite Neutron Multiplication Constant of Homogeneous Hydrogen-Moderated 2.0 wt%-U₂₃₅-Enriched Uranium

The infinite medium neutron multiplication factor, k_∞, of a mixture of 92.1 wt% UF⁴ and 7.9 wt% paraffin has been measured both in the Physical Constants Testing Reactor at the Hanford Atomic Products Operation and in critical experiments at the Oak Ridge National Laboratory. The density of the mixture is 4.5 gm/cc and the U²³⁵ enrichment of the uranium is 2.0 wt%, resulting in an H:U²³⁵ atomic ratio of 195. The values of k_∞ 0.013 and 1.197 ± 0.013 and 1.197 ± 0.015, respectively. In the analysis of the critical experiments a two group model was assumed for the nonleakage probability. The neutron age to thermal was determined from buckling perturbation measurements as 43.1 ± 3.4 cm². The critical buckling was measured to be (4344 ± 65) × 10⁻⁶ cm⁻², the bare extrapolation distance 2.7 ± 0.3 cm, and the fast fission factor 1.039 ± 0.004. Within the experimental error, the values of k_∞ from critical experiments at ORNL and from the PCTR at HAPO agree.