Fast Neutron Imaging for Materials Accountancy of Uranium Hexafluoride

One of the most important problems in nonproliferation is monitoring the degree of enrichment from uranium enrichment plants. Enrichment facilities are necessary to produce fuel for commercial power plants but are also capable of producing highly enriched uranium, which can be configured into a nuclear weapon. Existing technologies for measuring 235 U enrichment in a UF6 container (for example, a 30B cylinder) require controlled conditions for accurate measurements, making them susceptible to systematic uncertainties and possibly diversion scenarios. Low-energy neutrons and gammas used by current technologies have short penetration through dense UF6, and since the UF6 thickness near the measurement location is not known a priori, the current measurement techniques are sensitive to the geometry of UF6 in the cylinder. Sandia is investigating the use of fast neutron spectrometry and imaging to ascertain the UF6 enrichment inside the cylinder. This submission considers the imaging of deeply-penetrating fast neutrons to ascertain the UF6 distribution inside a 30B cylinder. Imaging the UF6 material distribution within the cylinder allows compensation of geometry-dependent measurements when the UF6 mass is unevenly distributed. MCNP5 and MCNPX-PoliMi modeling tools were used in the imaging with two distinct 30B cylinder filling profiles. Results from the modeling demonstrate that imaging can help in the determination of the filling profile, but may have some limitations that are dependent on detector response. Accurate imaging of the UF6 material distribution within the cylinder allows compensation of geometrydependent measurements when the UF6 mass is unevenly distributed.