Application of Prompt Gamma-Ray Analysis to Identify Electrorefining Salt-Bearing Plutonium Oxide at the Plutonium Finishing Plant

Abstract

A large fraction of impure plutonium oxide inventory items received in the mid-1980s from the Rocky Flats Plant (RFP) contain sodium chloride and potassium chloride salts from the electrorefining process. Sodium and potassium chloride evaporate at the ≥950°C stabilization temperature mandated for long-term storage under the U.S. Department of Energy plutonium oxide stabilization and storage criteria to plug and corrode process equipment. Because of the equipment damage and associated increased processing time and operator radiation exposure caused by these salts, items found to have these salts qualify for thermal stabilization at 750°C. The present document describes how prompt gamma-ray analysis is being implemented at the Plutonium Finishing Plant (PFP) to screen the items from the RFP for the presence of electrorefining salt. The prompt gamma ray energies characteristic of sodium, potassium, chlorine, and other low atomic weight elements arise from the interaction the light elements with alpha radiation from plutonium and americium radioactive decay. High-resolution gamma ray spectrometers designed to detect energies up to ~4.5 MeV are used to gather the high-energy prompt gamma spectra. Observation of the presence of the high-energy gamma peaks representing the natural chlorine-35, sodium-23, and potassium-39 isotopes and the sodium-to-chlorine peak area ratios in the range for plutonium oxide materials known to contain the electrorefining salts give the evidence needed to identify plutonium oxide materials at the PFP that qualify for the lowertemperature processing. Conversely, the absence of these telltale signals in the prompt gamma analysis provides evidence that the materials do not contain the electrorefining salts. Furthermore, based on calibrations using known assayed items, semiquantitative measurement of the quantity of chlorine present in materials containing electrorefining salt also can be performed by using the count rates observed for the chlorine peak, the plutonium quantity present in the measured item, and the plutonium-and chlorine-specific response of the gamma detection system. The origin and characteristics of the impure plutonium oxide, the process impacts of the electrorefining salts, and the background and technical bases of application of prompt gammaray analysis to identify electrorefining salt-bearing plutonium oxide at the PFP are described. v

A calibration to predict the concentrations of impurities in plutonium oxide by prompt gamma analysis: Revision 1

“…One correlation was developed by Fazzari to estimate chlorine content (as chloride salt) in scrap plutonium oxide from the electrorefining process. 11 The estimates of chloride concentration for the materials analyzed by this method showed good agreement with those obtained by analytical chemistry. 12 However, this correlation was detector specific and required that the concentration of 241 Am, a significant contributor to the alphaparticle activity, was the same for all materials.…”

“…Therefore, the ratios of sodium to chlorine in many of the materials from this process are consistent. 11,17 Because of the importance of chlorine detection in packaged material, this trend was used to develop a calibration line for chlorine as a function of sodium concentration. This allows the estimation of chlorine concentration in materials where it is below its 5,000 ppm lower limit of detection.…”

Complex-wide representation of material packaged in 3013 containers

“…All MIS items in this table have been characterized by PG analysis and analytical chemistry and tested in small-scale surveillance experiments. 10,14 ER scrap materials are expected to have some amount of chloride salt from the salt matrix used in the ER process (equimolar NaCl/KCl with 5% MgCl 2 ). The ER scrap materials have Na:Cl peak area ratios with a mean of 12.0 and a range of 9.5 to 15.5 and Mg:Na peak area ratios less than 0.25.…”

Chemical and Radiochemical Composition of Thermally Stabilized Plutonium Oxide from the Plutonium Finishing Plant Considered as Alternate Feedstock for the Mixed Oxide Fuel Fabrication Facility

“…Lower-temperature stabilization of plutonium oxides from Rocky Flats that contain sodium and potassium chloride from electrorefining (ER) processes has been approved (750º instead of 950ºC) based on demonstration of technical equivalency of the 3013 standard's stabilization goals (Boak et al 2003). The prompt gamma measurements used to identify the presence of ER salts also provide qualitative data on other elements that may exist as chemical impurities in the Hanford alternate feedstocks, including aluminum, beryllium, chromium, copper, fluorine, iron, magnesium, silicon, and tantalum (Fazzari 2003).…”