Characterization of Chemical Impurities in Plutonium Oxide Inventories at the Plutonium Finishing Plant by Prompt Gamma-Ray Analysis

Prompt gamma-ray analysis is based on identification and intensity measurements of gamma rays produced by the interaction of plutonium and americium alpha particles with the nuclei of other elements. Alpha particles (a) emitted by the radioactive decay of plutonium and americium transmute elements, particularly low atomic number elements, present in plutonium oxide materials by (a,n?) and (a,p?) nuclear reactions, releasing a neutron (n) or proton (p), respectively, and associated gamma ray (?). Inelastic scattering alpha recoil reactions (a,a'?), without nuclear transmutation but emitting a characteristic gamma ray, may also occur. These reactions leave the product nuclei in excited states that decay to produce gamma rays with energies characteristic of the excited product isotope. Prompt gamma-ray analysis was implemented at the Plutonium Finishing Plant (PFP) on the Hanford Site to screen impure plutonium oxide inventory items for the presence of sodium and potassium chloride salts. Approximately 65% by mass of the PFP plutonium oxide inventory required screening. Destructive analysis was performed on 18 samples from a cross section of the PFP inventory items, and the results were compared with the prompt gamma-ray analysis. The nondestructive prompt gamma spectral measurements performed at PFP were found to provide a semi-quantitative analysis of the Cl- and, to a lesser extent, Na+ concentrations in the bulk material. The analytical data show that F- also could have been quantified by prompt gamma analysis. Sodium and potassium chloride salt impurities present in electrorefining salts have been implicated in processing and moisture measurement difficulties associated with high-temperature (950ºC) thermal stabilization of plutonium oxide. Evaporation of the chloride salts may cause plugged offgas filters, localized corrosion, and high loss-on-ignition values that can be falsely attributed to water. Processing these oxides at lower temperatures (750ºC) mitigated these problems and resulted in an acceptable product. The presence of high-energy prompt gamma peaks representing the natural 35Cl, 23Na, and 39K isotopes and sodium-to-chlorine peak area ratios in the range measured for plutonium oxide materials known to contain electrorefining salts provided the necessary evidence to identify the plutonium oxide materials that qualified for lower-temperature processing.