Experimental Study on Fluoride Volatility Process for Thorium Fuels

Abstract

Selective removal of uranium from (Th/U)O2 by fluorination with fluorine was studied experimentally. The fluorination was performed both in a small boat and in a 2 inch inner diameter fluid-bed reactor. Fuel particles tend to agglomerate in the reactor due to the large amount of reaction heat and the comparatively low melting point of ThF4. The fluorinated fuels produced in the fluid-bed reactor were found to be partially agglomerated. Fractional retention of uranium was smaller in the agglomerated parts than in the un-agglomerated, and smaller in the outer layers of the cakes than in the core. On the other hand, it was also established beyond doubt from the results of the small boat fluorination experiment that heavy agglomeration inhibits the volatilization of uranium in the form of UF6. Inhibition of the violet exothermic reaction by lowering the fluorine pressure in the early stage of fluorination was found to be a very effective method of obtaining high uranium recovery. It was demonstrated that more than 99% of the uranium could be volatilized within 4 to 5 hr at a temperature of 580°C. The experimental results on the effects of temperature, particle size and fluorine concentration are presented. The variations of reaction rate observed in the course of fluorination are also discussed.