PROMPT NEUTRON DECAY FOR DELAYED CRITICAL METAL SPHERES OF PU, AND NATURAL-URANIUM-REFLECTED PU AND HEU

摘要

Prompt neutron decay constants at delayed critical have been measured by Oak Ridge NationalrnLaboratory for unmoderated natural-uranium-reflected uranium and plutonium metal spheresrn(FLATTOP) and an unreflected and unmoderated plutonium metal (4.5% 240Pu) spherern(JEZEBEL) at the Los Alamos National Laboratory (LANL) critical experiments facility. Thernaverage prompt neutron decay constants obtained from hundreds of Rossi-α and randomlyrnpulsed neutron measurements with 252Cf at delayed criticality are 3.868 ± 0.007 x105 sec –1,rn2.219 ± 0.002 x105 sec –1, and 6.450 ± 0.010 x105 sec –1, respectively. These values werernmeasured with a variety of source detector-locations in the diametral holes of the JEZEBEL andrnFLATTOP assemblies. These values are in agreement with previous prompt neutron decayrnconstant measurements at delayed criticality by LANL for FLATTOP and JEZEBEL (3.8 ± 0.1;rn2.14 ± 0.05 and 2.29 (uncertainty not reported); and 6.4 ± 0.1 × 105 sec-1, respectively) but havernsmaller uncertainty because of the larger number of measurements. For the FLATTOP andrnJEZEBEL assemblies there is agreement between measurement and calculations. Traditionally,rnthe calculated decay constants for bare uranium metal sphere GODIVA I and the Oak Ridgernsphere have been higher than the experimental by ~10% whereas other measured quantities forrnthe bare uranium sphere that depend on energy distributions have calculated to experimentalrnratios with ~1% of unity.