Burn-leach process for recovery of uranium from Rover fuel: terminal report of laboratory development

Abstract

Declassified 30 Aug 1973. Several aqueous methods for recovery of uranium from irradiated NbC-lined graphite-base Rover (nuclear rocket) fuels were evaluated on a laboratory scale. The burn-leach process, which is described in this report, is considered to be the simplest and best of the aqueous methods. ln the first step of this process, the fuel is burned at 700 to 750 deg C, preferably in a fluidized-bed of inert alumina. Volatilization of all fission products except ruthenium is expected to be low during combusion. The fluidized- bed product is transferred to a leacher and is leached first with hot 6 to 10 M HNO/sub 3/ to recover 95 to 99% of the uranium; then, the residue is leached at 115 deg C with 10 M HNO/sub 3/-5 M HF (F/Nb mole ratio of about 10) to recover the remaining 1 to 5% of the uranium and practically all of the niobium. After leaching, the system is washed with water and all solutions are combined, along with the appropriate amount of aluminum nitrate solution, to produce a final solution having the composition 3 M HNO/sub 3/-- 1 M total F--0.06 M UO/sub 2/(NO/ sub 3/)/sub 2/--0.1 M Hsub 2/NbOF/sub 5/-0.2 M Al(NO/sub 3/)/sub 3/ . The uranium can be recovered from this solution and decontaminated by conventional tributyl phosphate solvent extraction procedures. Because the burnups of the Rover fuels are quite low (<0.04 at.%), plastic or plastic-lined vessels can be used for containing the highly corrosive HF--HNO/sub 3/ solutions in the leaching and feed adjustment steps. With aluminum nitrate present in the feed solution, rates of corrosion of types 309SCb and 347 stainless steels were <0.4 mil/month at room temperature, indicating that these alloys could be used as materials of construction for the solvent extraction system. Combustion of Rover fuel in a fixed-bed bunner at 700 to 800 deg C has also been studied. The U/sub 3/O/sub 8/- Nb/sub 2/O/sub 5/ combustion ash (even that produced at 1200 to 1300 deg C) can be dissolved in HF-HNO/sub 3/ (F/Nb mole ratio of about 10), using the two-stage leaching process. However, fixed-bed burning may not be as satisfactory as fluidized-bed burning because of engineering problems. Direct leaching of the ash (from either a fluidized-bed or fixed-bed burner) with 10 M HNO/sub 3/--5 M HF is an alternative to the two-stage leaching procedure but is not as practicable. Uranium and niobium recoveries are slightly lower in the direct leaching method; furthermore, uranyl fluoride will precipitate from the leach solution unless it is diluted with water while still hot. (auth)