A More Accurate and Penetrating Method to Measure the Enrichment and Mass of UF 6 Storage Containers Using Passive Neutron Self-Interrogation

This paper describes an unattended mode neutron measurement that can provide the enrichment of the uranium in UF 6 cylinders. The new passive neutron measurement provides better penetration into the uranium mass than prior gamma-ray enrichment measurement methods. The Passive Neutron Enrichment Monitor (PNEM) provides a new measurement technique that uses passive neutron totals and coincidence counting together with neutron self-interrogation to measure the enrichment in the cylinders. The measurement uses the neutron rates from two detector pods. One of the pods has a bare polyethylene surface next to the cylinder and the other polyethylene surface is covered with Cd to prevent thermal neutrons from returning to the cylinder. The primary neutron source from the enriched UF 6 is the alpha-particle decay from the 234 U that interacts with the fluorine to produce random neutrons. The singles neutron counting rate is dominated by the 234 U neutrons with a minor contribution from the induced fissions in the 235 U. However, the doubles counting rate comes primarily from the induced fissions (i.e., multiplication) in the 235 U in enriched uranium. The PNEM concept makes use of the passive neutrons that are initially produced from the 234 U reactions that track the 235 U enrichment during the enrichment process. The induced fission reactions from the thermal-neutron albedo are all from the 235 U and provide a measurement of the 235 U. The Cd ratio has the desirable feature that all of the thermal-neutron-induced fissions in 235 U are independent of the original neutron source. Thus, the ratio is independent of the uranium age, purity, and prior reactor history.