Year
2017
Abstract
Neutron multiplicity counting measurements are routinely conducted for the purpose of plutonium quantification in samples such as Mixed OXide (MOX) and Pressurized Water Reactor (PWR) fuel. These measurements use the multiplicity moments along with the sample isotopics and the 240Pu effective mass coefficients, ????238 and ????242, which respectively represent the measured coincidence rates from the even plutonium isotopes 238Pu and 242Pu relative to 240Pu in order to determine the 240Pu effective mass. Inaccurate reference coefficients used during these measurements may be an inadvertent source of systematic error. The current reference values have determined the relative neutron emissions based on tabulated nuclear data, but a more recent re-evaluation calculated slightly different coefficients using a combination of high precision experimental multiplicity measurements and corresponding MCNP simulations for several samples highly enriched in each of the even plutonium isotopes. Here a set of previously reported coefficients uncorrected for the relative doubles detection efficiency of 238Pu and 242Pu relative to 240Pu have been corrected using MCNP 6.11b simulations. These simulations used three isotopically pure plutonium sources within the Mini-ENMC (Epithermal Neutron Multiplicity Counter) and applied the prompt neutron fission spectra generated using the empirical Maxwell and Watt distributions as well as from the semi-empirical Los Alamos model. The magnitude of differences originating from the choice of model was evaluated and the resulting 240Pu effective weight fractions for commonly sampled materials were compared relative to those calculated using the current reference coefficients. As there is no clear consensus which of the prompt fission neutron energy models best matches experimental data the differences found here highlight the need for more accurate experimental plutonium fission spectra measurements.