Prompt Neutron Decay for Delayed Critical Bare and Natural-Uranium-Reflected Metal Spheres of Plutonium and Highly Enriched Uranium

Abstract

Prompt neutron decay at delayed criticality was measured by Oak Ridge National Laboratory for uranium-reflected highly enriched uranium (HEU) and Pu metal spheres (FLATTOP), for an unreflected Pu metal (4.5% 240Pu) sphere (JEZEBEL) at Los Alamos National Laboratory (LANL) and for an unreflected HEU metal sphere at Oak Ridge Critical Experiments Facility. The average prompt neutron decay constants from hundreds of Rossi-α and randomly pulsed neutron measurements with 252Cf at delayed criticality are as follows: 3.8458 ± 0.0016 × 105 s−1, 2.2139 ± 0.0022 × 105 s−1, 6.3126 ± 0.0100 × 105 s−1, and 1.1061 ± 0.0009 × 106 s−1, respectively. These values agree with previous measurements by LANL for FLATTOP, JEZEBEL, and GODIVA I as follows: 3.82 ± 0.02 × 105 s−1 for a uranium core; 2.14 ± 0.05 × 105 s−1 and 2.29 × 105 s−1 (uncertainty not reported) for a plutonium core; 6.4 ± 0.1 × 105 s−1, and 1.1 ± 0.1 × 106 s−1, respectively, but have smaller uncertainties because of the larger number of measurements. For the FLATTOP and JEZEBEL assemblies, the measurements agree with calculations. Traditionally, the calculated decay constants for the bare uranium metal sphere GODIVA I and the Oak Ridge Uranium Metal Sphere were higher than experimental by ~10%. Other energy-dependent quantities for the bare uranium sphere agree within 1%.