Year
2011
Abstract
Active well coincidence counters (AWCCs) are widely used for nondestructive-assay for nuclearsafeguards applications. The measurement technique is based on the detection of time-correlated neutrons from fission by 3He tubes embedded in a polyethylene moderator. In the assay of uranium, an active source is required to induce fission in the material. Typically, an AmLi neutron source is used as the interrogation source. In this study, a detailed analysis of the measurement system will be presented for uranium-oxide standards ranging from 0.5 kg to 4.0 kg with a Canberra JCC-51 AWCC. MCNPX-PoliMi is an enhanced version of MCNPX version 2.6.0. The modifications made to MCNPX allow for the accurate simulation of multiple source neutrons per history while maintaining the original code structure. MCNPX-PoliMi is also capable of producing a specialized output file which characterizes all of the events occurring in the detector cells. This file contains all relevant information for each interaction, including the particle type, the interaction mode, the location and time of interaction, and the energy deposited. A specialized post-processing code has been developed to interpret this file and determine the response of a 3He multiplicity counter. The MCNPXPoliMi simulation provides the full neutron multiplicity distribution measured by the 3He tubes. This distribution is used to compute the singles, doubles, and triples rates which are the quantities used in safeguards applications for 235U mass determination. These quantities are compared with measurements performed at the Y-12 National Security Comple