Controlled-Potential Coulometric Determination of Plutonium-238 in Plutonium-238 Dioxide Fuels C. S. MacDougall and G. R. Waterbury University of California, 10s Alamos Scientific Laboratory, Los Alamos, N. M . 87544
T h e importance of P u metals and oxides, 80 to 95% enriched in the 238Pu isotope, as radioisotopic heat sources for powering heart pacers, artificial hearts, and radiotransmitters has grown rapidly during the few years since their inception. Concurrent with this growth in importance, t h e need has increased for more reliable chemical characterization, especially measurement of the P u in these fuels. At the time this work was started, many methods existed for the assay of P u materials, as summarized by several authors (I,2). These included potentiometric (3, 4, volumetric, amperometric ( 5 ) , and coulometric titrations (3, 6, 7). T h e volumetric and potentiometric 'titrations required too large a sample for use with 238Pu because of radiolytic effects, and the amperometric titration did not seem t o offer any advantages over a coulometric method. Favorable experience with t h e coulometric titration of 239Pu suggested that a modification of this method might yield a quick, precise, and accurate determination of milligram quantities of 238Pu. T h e radiolytically induced self oxidation and reduction of the sample by the 238Pu isotope are major problems that were not overcome in previous methods. T h e relative standard deviation for titrating 238Pu in sulfuric acid media was large, *0.5%. The intense a-particle bombardment by 238Pu of the aqueous solutions undoubtedly results in formation of hydrogen and hydroxyl radicals (Equation 1).
H,O -% H. -I-*OH From these radicals are formed reducing and oxidizing agents (Equations 2-3). 2H. 2*OH
--
H,
(2)
H,O,
(3)
Either the radicals or their by-products can further react with other ions present to increase the complexity of this ever-changing solution. This behavior has been observed in 239Pu solutions to a very limited extent (8-II),but only becomes very important in the analysis of the intensely a-active 238Pu, which can produce significant quantities of reducing and oxidizing agents during the short time period of the titration. These contaminants not only react a t the electrode to give erroneous results, but also hasten t h e "Selected Measurement Methods for Plutonium and Uranium in the Nuclear Fuel Cycle," R. J. Jones, Ed., USAEC Rep., TID-7029 (1963), pp 307-367. C. F. Metz and G . R. Waterbury, "The Transuranium Actinide Elements," Treatise on Analytical Chemistry, Part I I , I . M. Kolthoff and G. J. Elvin, Ed., John Wiley and Sons, New York. N.Y., 1959, Chap. 52. G. R. Waterbury, G . B. Nelson, K. S. Bergstresser, and C. F. Metz, "Controlled-Potential Coulometric and Potentiometric Titrations of Uranium and Plutonium in Ceramic-Type Materials," Los Alamos Scientific Laboratory Report, LA 4537 (1971). J. T. Byrne. M. K. Larsen. and J. L. Pflug, Anal. Chem., 31, 942 (1959). K. S. Bergstresser and G . R. Waterbury, "An Evaluation of the Amperoinetric Titration of Plutonium," Los Alamos Scientific Laboratory Report, LA 3091 (1964). W. D. Shults, Talanfa, 10, 633 (1963).
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disproportionation of Pu(1V) to Pu(II1) and Pu(VI), which cannot be quantitatively reduced in HC1 and leads to low results. I t is therefore necessary to either limit the a - p a r ticle-induced formation of reductants and oxidants, or eliminate these interferences as they form. Several possible solutions to the radiolytic interference were considered. If the 238Pu concentration in the coulometric cell were to be kept very small (
