LABORATORY INVESTIGATIONS IN SUPPORT OF FLUID BED FLUORIDE VOLATILITY PROCESSES. PART I. THE FLUORINATION OF URANIUM DIOXIDE-PLUTONIUM DIOXIDE SOLID SOLUTIONS

Abstract

Work in the development of fluid-bed fluoride volatility processes is described. In these processes, uranium and plutonium in spent nuclear fuels are converted into hexafluoride compounds in a fluid-bed reactor. The uranium and plutonium hexafluorides are volatile and can be separated from fission products, cladding, and alloying materials by techniques such as vaporization and distillation. The experimental work was directed toward devising a fluorination procedure for uranium and plutonium dioxides which would result in a high degree of removal of uranium and plutonium as hexafluorides. In these experiments synthetic mixtures made up to simulate a charge for a fluidized bed reactor (100 kg U, 0.4 kg Pu, approximates 1 kg F.P., and 30 kg inert solids) were used. High-purity recrystallized alumina was found to be a suitable material for use as the fluidized inert solid. After a 10-hr fluorination period at 450 deg C, the concentrations of residual uranium and plutonium on the alumina were 0.01 and 0.03 wt%, respectively. A reaction temperature of 450 deg C was found to be optimum, since experiments at 500 and 550 deg C resulted in plutonium retentions on the alumina of 0.060 and 0.090 wt%, respectively. At all these temperatures, the residual uranium content of the residue was less than 0.01 wt%. When fission product element oxides, in quantities that would be expected in a Dresden-type fuel after 100,000 Mwd/ton burnup and 30 days of cooling, were added to the uranium dioxide-plutonium dioxide-- alumina and the mixture was fluorinated at 450 deg C for 10 hr, the concentration of plutonium on the alumina increased to a value of 0.065 U%. Additional recovery of the plutonium retained on the alumina was obtained by either pyrohydrolysis followed by refluorination at 450 deg C for 10 hr, or by refluorination alone at 550 deg C for 10 hr. These procedures reduced the residual plutonium content of the alumina to less than 0.02 wt%. Experiments were also performed to determine the feasibility of using the same batch of alumina as the inert solid for the fluorinations of five batches of the urania--plutonia solid solution. Experiments were performed in which the solid solution of plutonium dioxide in uranium dioxide was oxidized prior to fluorination. The oxidation resulted in a powdered mixture of uranosic oxide and plutonium dioxide. Fluorination of this oxide mixture in alumina resulted in the removal of essentially all of the uranium in a reaction time of 2 hr at 450 deg C when 10 vol% fluorine was used. When this fluorination was followed by a second fluorination period of 5 hr at 550 deg C with 75 vol% fluorine, the plutonium content of the alumina was 0.011 wt%. When both fluorination periods were extended to 10 hr each, the retention of plutonium was 0.007 wt%, which corresponded to a removal of 99.5% of the plutonium contained in the solid mixture. During the first part of the fluorination period, in which the major portion of the uranium is removed from the mixture of uranium dioxide, plutonium dioxide, alumina, and representative fission product element oxides, a low ternperature (450 deg C) and a low concentration of fluorine (10 vol% fluorine in nitrogen) are desirable. However, in order to remove the plutonium efficiently during the last portion of the fluorination period, it is desirable to use a higher temperature (550 deg C) and a higher fluorine content of the gas mixture (75 vol% fluorine in nitrogen). (auth)