Measurements of the Critical Parameters of Under-Moderated Uranium-Hydrogen Mixtures at Intermediate Enrichments

The critical size of intimate mixtures of 30% enriched uranium oxide and wax with H/23'5U in the range 8 to 80 and of 37½% enriched uranium metal, as used in Zebra, were estimated by extrapolation from high multiplication assemblies. The cores were constructed in rectangular geometry from small blocks to facilitate stacking changes. Reflectors included polyethylene, Perspex, concrete and wood. The effect of other reflectors was estimated by replacement of one face of an otherwise polyethylene reflected near cube by the material of interest. Evidence is given that critical masses derived by this method are correct to within 5%, even when the spectrum of neutrons returned to the core from the replacement face is markedly different from that returned from the remainder of the reflector. Density exponents were estimated from measurements with low density blocks, and one carbon dilution exponent is reported. The critical size of half-reflected cores is of interest to the criticality assessor. The parameters obtained from a near cube and a long pipe suggest that their dimensions are the arithmetic mean of those of bare and completely reflected cores of the same shape. The neutron interaction between partially reflected cores can be greater than that for the uniformly reflected case. To check the magnitude of this effect, two identical cores were reflected on all but the adjoining faces by polyethylene, and their critical separation determined. The measurement was then repeated with the cores fully reflected, or with the polyethylene filling only the space between the reflector, leaving free space between the cores. Cadmium sheet in conjunction with a moderator is commonly used as a means of absorbing neutrons in transport and storage applications. Selected assemblies were rebuilt with a layer of cadmium sheet between core and reflector to estimate its effect on critical size. (author)