METHOD FOR ANALYZING LOW-ENRICHMENT LIGHT-WATER CORES. SUPPLEMENTARY STUDY RELATED TO BONUS AND NUCLEAR SUPERHEAT PROGRAMS

A three-group method is developed for analyzing lightwater UO/sub 2/ cores of low enrichment. The method is composed of a series of subroutines that are consolidated to produce a set of self-consistent multigroup parameters. The thermal cross sections are determined by averaging the cross section over a "hardened" or shifted Maxwellian spectrum, the amount of hardening being a function of the macroscopic absomption-to-slowing-down ratio. The disadvantage factor is calculated using diffusion theory for the moderator and a transport condition at the surface of the rod, the accuracy of the calculation being comparable to a g a shielding effects even for very close-packed lattices. The fast-fission factor is calculated relative to the experimental measurements for UO/sub 2/. The diffusion coefficients are found by averaging the measured cross sections over the appropriate neutron flux in each neutron energy group. The slowing-down cross section is defined as the diffusion coefficient divided by the neutron age, the age being determined by calculating equivalent transport and slowing-down factors relative to light-water measurements. Utilizing the two- dimensional IBM-704 PDQ digital computer code, a comparison is made with the available experimental information using oxide fuel. These experiments, which include rod-type fuel elements, use U/sup 235/ enrichments from 1.3 to 4.0 wt.% and water-to-uranium-metal ratios from 2.2 to 4.9. The reactivity and power- distribution predictions are very good: The reactivity calculations check to within 2% delta k for all cases, most cases being with 1% delta k. (auth