Study of a Slightly Enriched UO₂ Lattice with H:U=0.42 — Measurement and Analysis

Parameter measurements in a 1.3% enriched UO₂ lattice with H:U = 0.42 have been performed. These measurements are an extension of an experimental program in the TRX critical facility of the Bettis Atomic Power Laboratory. Earlier measurements were made for a wide range of water-to-uranium (H₂O:U) volume ratios (1:1 to 8:1) using 4-ft (1.2-m)-high slightly enriched, 0.387-in. (0.98-cm)-diam uranium metal or oxide fuel rods clad with aluminum. The new data have been compared with current analytic techniques, using both P-1 and P-3 multigroup analysis in the epithermal neutron energy range and Monte Carlo multigroup methods for thermal neutrons. This extremely undermoderated lattice provides a very stringent test for both the computational methods and the neutron cross sections used. The quantities measured were: the ratio of epithermal-to-thermal radiative captures in U²³⁸ (ρ²⁸); the ratio of captures in U²³⁸ to fissions in U²³⁵ (the modified conversion ratio, CR*); the ratio of U²³⁸ fisions to U²³⁵ fissions (δ²⁸); and the ratio of epithermal-to-thermal U²³⁵ fissions (δ²⁵). In addition, activations were obtained with thermal-neutron detectors of widely different spectral response. The results indicate that the calculational methods predict the parameters very well, except for δ²⁸. The discrepancy in δ²⁸ may be due to inadequate U²³⁸ inelastic scattering cross sections, but this conclusion requires additional study. Monte Carlo calculations of thermal-neutron detector activations show that use of either the Nelkin or Koppel kernel gives results that agree with the data.