(558d) Modeling of a System of Equations to Determine the Concentrations of U235 and Pu239 in a Sample

Environmental Pu239 arises from the development, production and testing of nuclear weapons. Fall out of this weapon debris has produced an average worldwide Pu239 level of about 1 ppb. Traditional chemical separation methods are available to measure Pu239 at fCi levels, but these methods requires significant works on chemical separation process and are unable to detect Pu239 at environmental levels (~100aCi). Fissile samples are exposed to distinct neutron spectra to achieve quantitative analysis of each fissile isotope using track registration method with minimum chemical processing. U235 and Pu239 have unique fission cross-section for different neutron energy ranges. It is possible to differentiate between the isotopes based on the number of fission tracks produced. The fission tracks can be generated using a neutron source such as a nuclear reactor, high-energy gamma source or a neutron generator. In this research, a Monte Carlo method (MCNP 5) used to generate theoretical number of fissions and from this data a system of equations was developed. MCNP 5 will generate 2 or 3 distinct neutron spectra that can be obtained from a nuclear reactor. The 3 distinct energy spectra are cold, thermal and epithermal. In the real world, choosing adequate neutron filtering system can generate these 3 different neutron spectra. The proposed method can measure or predict the amount of Th232, U235, and Pu239 in the sample simultaneously. Theoretically, the detection limit for these isotopes was ~100 aCi/gram. This calculation must be compared with an experimental measurement. The uncertainties associated with this theoretical approach stem from the differences in theoretical and actual neutron flux in the irradiators. By providing more realistic neutron spectra and neutron filtering system, these uncertainties can be reduced.