Optical Spectroscopy for Process Monitoring of 238Pu Production

Abstract

238Pu is a highly energetic isotope of plutonium recognized for its particularly high energy alpha particle emissions that can be easily attenuated to produce heat. The produced heat is then used for power production through radioisotope thermoelectric generators or radioisotope heater units which are commonly employed on spacecraft and probes. Manufacture of 238Pu has been sporadic in the United States, with isotope reserves decreasing rapidly. Oak Ridge National Laboratory’s High Flux Isotope Reactor has once again begun producing 238Pu for spacecraft and research applications with plans to scale to kilogram quantities per year. Fabrication of 238Pu involves processing of highly radioactive material that poses both proliferation, safety, and cost concerns. To ensure proper nuclear materials accountancy of 238Pu in its production, in situ species monitoring techniques must be developed. The efficacy of optical spectroscopic techniques have been studied to improve process monitoring capabilities of plutonium production. Raman spectroscopy and laser-induced breakdown spectroscopy calibration models have begun development for active determination of species concentrations in aqueous and organic streams. Raman spectroscopy is being investigated for use as an on-line, in situ process monitoring technique in 238Pu production due to its ability to probe samples rapidly and accurately. Deconvolution of process information using a nondestructive technique free of sample preparation will allow efficient processing of neptunium and plutonium and lowering of resource consumption. Design of experiment and multivariate analysis will be used to optimize calibration procedures for the Raman instrument in the system. While both organic and aqueous phases are of interest, focus has been placed on the aqueous modified direct denitrification (MDD) feed stream for this study. Species will primarily include nitrate, neptunyl, and plutonyl molecules. Measurements of single-component systems including nitric acid, sodium nitrate, and nitric acid containing concentrated cerium (as an initial neptunium surrogate) have been conducted allowing principal component analysis to elucidate important information regarding the nitrate species. Linear regression models were then compared to determine the repeatability of measurements on the basic systems. A fiber optic Raman system that can be used in a glovebox or hot cell for remote analysis has been designed and assessed, as well. Determination of phosphorus levels in the neptunium feed and plutonium product in the production of 238Pu is also crucial for optimizing process conditions and minimizing contamination of plutonium product. Spectroscopic and chromatographic techniques have been developed to produce models that can be used to quantify phosphorus levels and the degree of degradation of tributyl phosphate, an important extractant in the neptunium/ plutonium separation scheme. Laser-induced breakdown spectroscopy can be used to provide quantitative analysis of phosphorus contamination in unknown samples of material taken from the 238Pu production streams. Additionally, the degree of radiolysis of the tributyl phosphate extractant into its degradation products of dibutyl phosphate, monobutyl phosphate, and phosphoric acid can be determined by thin layer chromatography followed by laser-induced breakdown spectroscopy analysis of the separated species. Phosphorus in tributyl phosphate has been detected by laser-induced breakdown spectroscopy on samples of tributyl phosphate spotted onto alumina thin layer chromatography plates. Calibration sets that employ an internal standard and are representative of the expected system will be developed and analyzed in a manner that minimizes variance among the system’s constituents. Thus, an accurate prediction model for quantification of the amount of contamination contributed by phosphorus containing species can be determined in a rapid and repeatable manner.