Static and Pulsed Reactivity Measurements on Large Uranium-235 Fuel Forms in Water

Subcritical multiplication constants have been derived from static and pulsed measurements for arrays of large, hollow cylinders of uranium-aluminum alloy. The cylinders were of two types: 11.21-cm-i.d., 20.07-cm-o.d. bare ²³⁵U-Al alloy castings and 10.07-cm-i.d., 12.27-cm-o.d. Alclad “logs” extruded from the castings. The alloy was 9.985 wt% enriched uranium (92.2% ²³⁵U) in aluminum. These measurements extended previous benchmark experiments with small diameter rods of lower enrichment into the region of larger diameters and higher enrichments. The transport diffusion theory codes MGBS-TGAN overestimated the values of k_eff for the arrays tested by 7 to 12%. The more sophisticated Monte Carlo code KENO was more accurate, with errors less than 5%.