Impact of aluminum on nuclear criticality safety at the Oak Ridge Y-12 Plant

The accurate prediction of k{sub eff} for nuclear criticality safety purposes using calculational methods requires the following aspects: (1) detailed representation of neutron cross-section data; (2) knowledge of the applicable neutron physics; (3) realistic geometric and material representation of a physical model; and (4) representative experiments to benchmark and validate the results against any bias. The authors address these points with respect to the determination of criticality of uranium-aluminum metal mixtures (U/Al) at the Oak Ridge Y-12 plant (Y-12). There is interest in systems with significant amounts of fissile material mixed with aluminum, as the accuracy of aluminum cross sections casts doubt on the applicability of code and cross-section usage and validations where k{sub eff} may be driven by uranium and aluminum. The accuracy of large cross-section sets in some uranium-metal mixtures has been a source of ongoing investigation for the past 6 yr. The initial interest to anomalous calculations caused by uranium-metal mixture cross-section inaccuracies can be traced to Ref. 1. Oak Ridge National Laboratory has performed studies of the cross sections and identified deficiencies in methods used for resonance processing in SCALE, with may affect criticality of intermediate spectrum systems. These deficiencies in the aluminum resonance structure (10 more » < eV < 500 keV), and in the capture cross section caused an underprediction in the Palmer k{sub {infinity}} by 14%. Thus, there is concern that the inadequate ENDF capture representation in this energy range could translate to inaccuracies in calculating k{sub eff} for practical configurations. « less