Experimental and Calculated System Criticality

A continuing programme directed to the neutronics of uranium-235 system criticality has been in progress for a number of years in the United States of America. The experiments were designed to provide basic criticality data describing arrays susceptible to simple description: elementary geometry and homogeneous fuel regions. Such data fall naturally into a class which lends itself directly to analyses and, at the same time, may be applied to nuclear safety evaluations. The component variables examined included mass, shape, moderation, and the 235U content of the uranium, while the array variables studied were shape, moderation, degree of reflection and the number of units and their spacing. Data from four series of experiments and representative calculations utilizing two Monte Carlo codes are given. Series I. Unreflected two- and three-component critical assemblies of coaxial U(93) metal discs with diameters between 17.78 cm and 31.10 cm were studied. The unit mass as a function of separation was determined. Series II. Critical three-dimensional arrays of as many as 64 units of U (93.2) metal cylinders ranging from 10.5 to 26.2 kg, in five sizes, were employed to examine the effects of array moderation, reflection, shape and unit perturbation. Series III. Five-litre components of aqueous U(92.6)O2(NO3)2 solution, at a concentration of 415 g of uranium per litre, contained in right circular cylinders of methacrylate plastic were assembled in three-dimensional arrays. As many as 125 units were used to determine the number required for criticality as a function of their spacing and the degree of array reflection. Series IV. Cylinders of aqueous U(5)O2F2 solution, at a concentration of 901 g of uranium per litre, 24.1 cm diam. and up to 142.0 cm high were assembled to criticality in triangular and square patterns in planar geometry. Calculations with O5R, a General-Purpose Monte Carlo Neutron Transport Code developed at the Oak Ridge National Laboratory, yields multiplication constants in the range 0.971 to 1.028; GEM, a Monte Carlo Neutronics Code developed by the Authority Health and Safety Branch (U.K.A.E.A.), yields values in the range 1.000 to 1.032. (author)