DETAILED DESCRIPTION Overview of Experiment A variety of critical experiments were constructed of enriched uranium metal (oralloy a ) during the 1960s and 1970s at the Oak Ridge Critical Experiments Facility (ORCEF) in support of criticality safety operations at the Y-12 Plant. The purposes of these experiments included the evaluation of storage, casting, and handling limits for the Y-12 Plant and providing data for verification of calculation methods and cross-sections for nuclear criticality

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... afety applications. These included solid cylinders of various diameters, annuli of various inner and outer diameters, two and three interacting cylinders of various diameters, and graphite and polyethylene reflected cylinders and annuli. Of the hundreds of delayed critical experiments, two were performed that consisted of uranium metal annuli surrounding a solid beryllium metal core. The outer diameter of the annuli was approximately 13 or 15 inches with an inner diameter of 7 inches. The diameter of the beryllium was 7 inches. The critical height of the configurations was approximately 5 and 4 inches, respectively. The uranium annulus consisted of multiple stacked rings, each with radial thicknesses of 1 inch and varying heights. The 15-inch experiment was performed on June 4, 1963, and the 13-inch experiment on July 12, 1963 by J. T. Mihalczo and D. L. Bentzinger (Ref. 1) with additional information in its corresponding logbook. b Unreflected and unmoderated experiments with the same set of highly enriched uranium metal parts were performed at the Oak Ridge Critical Experiments Facility in the 1960s and are evaluated in HEU-MET-FAST-051. Thin graphite reflected (2 inches or less) experiments also using the same set of highly enriched uranium metal parts are evaluated in HEU-MET-FAST-071. Polyethylene-reflected configurations are evaluated in HEU-MET-FAST-076. A stack of highly enriched metal discs with a thick beryllium top reflector is evaluated in HEU-MET-FAST-069. Both detailed and simplified model specifications are provided in this evaluation. Both of these fast neutron spectra experiments were determined to be acceptable benchmark experiments. The calculated eigenvalues for both the detailed and the simple models are within approximately 0.6% of the benchmark values, but exceed the benchmark values by significantly greater than 3ı from the benchmark value because the uncertainty in the benchmark is very small: <±0.0004 (1ı). There is significant variability between results using different neutron cross section libraries, the greatest being a ǻk eff of ~0.67%. These benchmark experiments were compared to similar bare HEU configurations (Appendix C). There is not any reduction in critical mass obtained by replacing the core of the uranium discs with beryllium reflector. The critical mass actually increases by 13.497 kg HEU for the 13-inch-diameter configuration and 3.258 kg HEU for the 15-inch-diameter configuration because the uranium material of greatest worth in the a Oralloy stands for Oak Ridge Alloy, and consists of HEU metal with an 235 U enrichment of more than 93%. b Experimental data for these experiments can be found on page 118 of ORNL logbook 12R (East Cell -Logbook 1) and pages 19-21, 55-57, and 60 of ORNL logbook 13R (East Cell -Logbook 2). NEA/NSC/DOC/(95)03/II Volume II