Regularities in the Radiation Decomposition and Synthesis of Plutonium Hexafluoride

Abstract

Different methods of stabilizing plutonium hexafluoride using trifluoromethylperfluorocyclohexane and high-valence inorganic fluorides whose physical and chemical properties differ very little from those of plutonium hexafluoride are examined. The interaction of plutonium tetrafluoride with solid krypton difluoride is studied, and the kinetic regularities of this process are investigated.One problem of nuclear technology is stabilization of gaseous plutonium hexafluoride, which decomposes under α self-irradiation. Stabilization can be accomplished by storage in vessels with packing and dilution with inert gases as well as chemically. The first two methods have been studied quite well [1,2]. Attempts have been made to use fluorine and krypton difluoride as chemical stabilizers. Much better stabilization is attained by using krypton difluoride, which is a donor of atomic fluorine. However, technological problems require using stabilizers, which differ very little from plutonium hexafluoride (physical and chemical properties, molecular mass, viscosity, vapor pressure). For this reason it is of interest to investigate the radiolysis of plutonium hexafluoride in the presence of completely fluorinated hydrocarbons.Trifluoromethylperfluorocyclohexane (Freon-350) and standard quality plutonium were used in the present work. The saturated vapor pressure of Freon-350 differs very little from that of plutonium hexafluoride. The molecular mass of Freon-350 is 350 amu. The hexafluoride was obtained by fluorinating plutonium dioxide in a glow discharge using the procedure described in [3] followed by purification to remove volatile impurities by repeated vacuum distillation. To investigate radiolysis, a 3.4 liters nickel cylinder was filled with plutonium hexafluoride to pressure 2.9 kPa. Next, Freon gas was added to total pressure 5.6 kPa. Radiolysis was monitored radiometrically according to the activity of the gas phase in the top part of the cylinder. The maximum error of measurement did not exceed 5%. The corresponding curve of the partial pressure versus time is displayed in Fig. 1. The degree of composition of plutonium hexafluoride in time (Fig. 2) was calculated on the basis of the experimental data. The rate of radiolysis of pure plutonium hexafluoride during the first few days is 3.2% and the average value 1.5%. The average rate of decomposition under α self-irradiation equals 0.2%/day. Thus, Freon-350 accelerates radiolysis of plutonium hexafluoride.The experiments showed that dilution by trifluoromethylperfluorocyclohexane does not increase the stability of gaseous plutonium hexafluoride; quite the contrary, the effect is sharply negative. Apparently, the destabilization effect can be explained by the formation of radicals under α-irradiation and the occurrence with their participation of reactions leading to the decomposition of plutonium hexafluoride molecules. It can be supposed that under α self-irradiation pure plutonium hexafluoride decomposes via the reaction PuF 6 → PuF 4 + 2F • with the formation of atomi...