Estimating the Mass of UO3 Powder in a Container Using MCNP-PoliMi Simulations of NMIS Measurements

摘要

The Nuclear Materials Identification System (NMIS) uses fast neutrons to conduct non-intrusive scans of both fissile and non-fissile materials. By time-tagging the source neutrons, the NMIS processor can measure the time of flight of these neutrons through the target of the scan and into detectors on the opposite side. With a monoenergetic neutron source, such as a DT generator, these time-of-flight measurements can approximately determine the number of neutrons that travel through the target uncollided. By comparing the results of a scan on an empty container and one made on a container with unknown materials inside, the NMIS processor can subtract the container and determine the configuration of materials inside. Using multiple small detectors, and many horizontal and vertical samples, the NMIS system can produce a picture of the object being scanned, known as a neutron radiograph. With this radiograph, it is thought that other characteristics of the scanned material can be inferred. In this thesis, the material being scanned consists of highly enriched uranium trioxide (UO(sub 3)) powder. Using the results of the time-of-flight measurements from two reference scans, a container with an unknown configuration was scanned and the mass of the UO(sub 3) powder inside was estimated. The MCNP-PoliMi computer code was used to simulate these scans.