Criticality of Graphite- and Polyethylene-Reflected Uranium (93.2%)-Metal Cylinders and Annuli

Cylinders and cylindrical annuli of uranium metal (93.15 wt% ²³⁵U), with diameters varying from 7 to 15 in., were assembled to delayed criticality with graphite reflectors varying in thickness up to 18 in. or with an effectively infinite thickness polyethylene on all outer surfaces. In one series of measurements a polyethylene reflector was also placed adjacent to only one flat surface of the cylinders. The multiplication factor calculated by both S_n transport and Monte Carlo methods, with various sets of cross sections agreed very well with the experimental values. As a result of the high order of S_n and the large number of spatial intervals required, the computing time for the transport theory calculations was a factor of 10 larger than that required for the Monte Carlo calculations with standard deviations of 1%.