The Determination of Plutonium in Irradiated Uranium Fuel Solutions by Controlled-Potential Coulometry

Abstract

Controlled potential coulometry has been applied to the determination of plutonium in solutions which contain plutonium within the range 0.05 to 50 g/l. The titrations are performed by direct electrode reactions at the controlled-potential electrode. No valence state adjustments or methods for detecting the end point of a titration are necessary. The electronic equipment and special cells necessary to prevent the spread of radioactivity are described in detail. The procedures are simple enough and the equipment dependable enough for routine application in control laboratories. The method has been applied to solutions containing approximately 20 g/l. of plutonium with a relative standard deviation of 0.05 per cent and with an accuracy of better than 0.1 per cent. The method has also been applied to the direct estimation of the plutonium content of solutions of irradiated natural uranium (dissolver solution) with a relative standard deviation of 0.5 per cent and an accuracy of at least 1.4 per cent, using samples containing between 100 and 200 μ g of plutonium. The titration curves follow the Nernst equation exactly for ions which undergo reversible electrode reactions. Therefore, if the concentration and half-wave potential of an interfering material are known, estimations can readily be made of the theoretical magnitude of the interference. The method is remarkably free from interferences since there are few reversibly titratable ions which have standard electrode potentials near that of the plutonium IV plutonium III couple in an acid medium. When an ion with a standard redox potential near that of the plutonium couple is known to be present, the titration can usually be carried out by the selection of a complexing medium which increases the difference between the electrode potentials of the interfering substances and that of plutonium. Several such media have been investigated and are compared. The high acid or salt content of the media used provides buffering action and prevents interference from variations in the acid content of the sample. Details concerning the procedure can be obtained from document number HW-58491. A year of experience with the method has been obtained since the publication of this document and has led to a definite preference for the titration cell using the platinum working electrode. Electrode efficiencies and reaction reproducibility have not proven to be a problem as anticipated in HW-58491. The ease of cell construction and the simplicity with which remote titrations can be performed using this cell have proved to be of great advantage.