Improved Solvent Extraction Separation of Molybdenum from Nuclear Fuels for Isotope Dilution Mass Spectrometric Analysis S. Fredric Marsh' Idaho Nuclear Corporation, Idaho Falls, Idaho
MOLYBDENUM, the end member of fission product mass chains 95, 97,98, and 100, is produced by nearly 2 5 x of the fissions ~ in uranium and plutonium fuels. Radioactive 9 g M (66.5hour half life) is often used as a fission monitor for shortcooled fission products. Recently, the stable isotopes of molybdenum determined by isotope dilution mass spectrometry have been proposed as burnup monitors in aged nuclear fuels ( I , 2). To evaluate these proposals, as well as to measure accurately the molybdenum fission yields, a carrier-free separation procedure for molybdenmn was needed. The main requirements were freedom from natural molybdenum Contamination, simplicity for remde handling application, and compatibility with isotope diiution mass spectrometry. One of the most specific extractable complexes of molybdenum is formed with a-benzoinoxime; however, the complete separation of molybdenum from the reagent is difficult and traces of the reagent may cause mass interference, Molybdenum is partially extracted into various solvents from various halogen acids, but complete extraction requires multiple contacts. Previous work ( 3 ) in this laboratory indicated that molybdenum distributes into amyl acetate from a mixture of sulfuric and hydrochloric acids better than from either acid alone. In addition to the high distribution coefficient, this system offered near specificity, with only tellurium (which causes no mass interference) following molybdenum. The resulting analytical procedure for molybdenum is simple, rapid, quantitative, and well suited to precede mass spectrometric measurement. EXPERIMENTAL Apparatus. Common laboratory apparatus was used throughout the procedure. To minimize natural molybdenum contaminants, all glassware was boiled in A.R. hydrochloric acid, then quartz-distilled hydrochloric acid, and finally was rinsed with quartz-distilled water. Reagents. Hydrochloric acid, sulfuric acid, and water were distilled in quartz apparatus. Amyl acetate was redistilled in glass apparatus. Highly enriched g'Mo, obtained from the Isotopes Division of Oak Ridge National Laboratory, was used as the isotopic spike. A natural molybdenum solution at the mg/gram level was standardized gravimetrically both GS molybdenum trioxide and lead molybdate. The standard then was diluted to the pglgram level and was used ~ solution. for mass spectrometric calibration of the 9 4 Mspike
* Present
address, The Babcock and Wilcox Co., Box 1260, Lynchburg, Va.
(1) A. J. Fudge, A. J. Wood, and M. F., Banham, if.S. AI. Energy Cornrn. Rept. TID-7629,p. 152 (1961). (2) R. S.Forsyth, D. G. Guthrie, and A. E. Ross, [bid.,p. 166. (3) Marsh, S. F. and L. A. Weinrich, Idaho Nuclear Corporation, Idaho Falls, Idaho, unpublished data (1966).
696
ANALYTICAL CHEMISTRY
Procedure. Cautiously add 1 ml of concentrated sulfuric acid to the combined sample and spike. Heat to dense fumes of sulfur trioxide. When cool, transfer the fumed solution to a 25- or 30-ml separatory funnel. Complete the transfer by rinsing with 5 ml of 4.8M hydrochloric acid-3.6M sulfuric acid. Extract the molybdenum into 5 ml of amyl acetate for 1 minute. Drain and discard the aqueous phase. Scrub the organic phase with 5 ml. of 6 M sulfuric acid-4M hydrochloric acid for 30 seconds. Drain and discard the aqueous phase. Back-extract the molybdenum into 2 ml of 1M hydrochloric acid for 1 minute. Drain this strip solution into a 5-ml conical centrifuge tube and evaporate to dryness under a heat lamp. (An increase in the heat is necessary to remove the last of the sulfuric acid.) Dissolve the residue in 1 drop of water for loading the filament of the mass spectrometer. DISCUSSION
The proposed procedure efficiently separates both molybdenum and tellurium from fission product mixtures. No attempt is made to eliminate tellurium as it neither interferes in the mass analysis of molybdenum nor poses a radiological hazard in aged fuel. The presence of tellurium in the separated fraction does, however, make this procedure unsuitable for the radiochemical analysis of molybdenum in short-cooled fission products. The first step in the procedure, sulfuric acid fuming, serves to concentrate the sample, to oxidize molybdenum to the hexavalent state, and to ensure chemical identity between sample and spike molybdenum. This fuming should be performed in a well-ventilated hood as radioactive ruthenium will distill from nitrate-containing samples. Aluminum sulfate may salt out, from aluminum-containing samples, but does not interfere. As is the case with all isotope dilution measurements, quantitative recovery is unnecessary; however, a high yield is desirable to facilitate the mass analysis. In the particular triple filament, tandem, double dispersion mass spectrometer in use at Idaho Nuclear Corporation, sample aliquots were chosen to contain 2 to 5 pg of molybdenum. Under these conditions, the natural molybdenum contamination is typically less than 1 %. The recommended procedure has been used successfully to measure molybdenum fission yields from * 3 3 U , *35U,and 239Pu nuclear fuels. ACKNOWLEDGMENT
The assistance of Maxine E. Kussy in chemistry, and of R. M. Abernathey, R. E. McAtee, and G. D. Workman in mass analysis is gratefully acknowledged. RECEIVED for review January 9, 1967. Accepted March 13, 1967. Division of Nuclear Chemistry and Technology, 153rd Meeting ACS, Miami Beach, Fla., April 1967. Work performed under Contract AT(10-1)-1230 for the U. S. Atomic Energy Commission.
