8. Processing Chemistry 8.1 THE OXIDE CHEMISTRY OF Pa4* IN MOLTEN LiF-BeF2-ThF4
C. E. Bamberger R. G. Ross
QE= XPaO zXT h F4
C. F. Baes, Jr. D. D. Sood’
xT hO zXP aF4
where X denotes mole fractions, it was necessary to determine the composition of the solid phase. Accordingly, we decided to isolate and examine directly the few milligrams of oxlde phase present. The flanged top with the stirrer assembly was removed from the cooled equilibration vessel and replaced by another top provided with a 1-in.-OD nickel ftlter to be used for removing the molten fluorides at temperature. The system was reheated to 570°C under flowing H2and kept at temperature for 40 hr before we attempted to remove the molten fluorides. This permitted redissolution of any solid phase that might have precipitated on cooling. Only 10% of the fluoride phase was subsequently removed, probably due to plugging of the filter. The system was again cooled to room temperature and the procedure repeated with another ftlter. This time we successfully removed all but a small fraction of the fluoride phase. After cooling, the solid residue was loaded into a ‘A-in. nickel tube with a fritted bottom, and the remnant of the fluoride phase was forced out of the compartment at temperature using H2 pressure on the inside and vacuum on the outside of the compartment. A portion of the oxide residue was repeatedly washed with hot “ V e r b ~ c i t ”solution, ~ followed by treatment
A study was initiated previously2 to determine the precipitation behavior of Pa(IV) in LiF-BeF2-ThF4 (72-16-12 mole %) when oxide is introduced. This information is needed not only in connection with the development of possible MSBR fuel reprocessing methods based on oxide precipitation, but also because it is obviously relevant to the determination of the oxide tolerance of MSBR fuels. The results reported previously should correspond to the equilibrium
PaF4(d) t Th02(ss)
* Pa02(ss) + ThF4(d) ,
where d denotes a dissolved species and ss a solid solution phase. Such an exchange reaction is to be expected on the basis of the similarity in size of the ions Pa4’ and Th4’ and the fact that regular solutions of U02-Th02 are formed from fluoride melts upon oxide additions? In order to confirm the above presumed reaction (1) and to estimate its equilibrium quotient 1. Guest scientist from Bhabha Atomic Research Centre, Bombay, India. 2. C. E. Bamberger, R. G. Ross, and C. F. Baes, Jr., MSR Program Semiannu. Prop. Rep. Feb. 28, 1971, ORNL-4676, pp. 120-22. 3. C. E. Bamberger and C. F. Baes, Jr., J. Nucl. Mater. 35, 177 (1970).
4. “Verbocit” solution: (developed.by the Analytical Chemistry Division, ORNL) 50 g sodium venenate + 20 g sodium citrate in 200 ml saturated boric acid solution. Final DHI 9.