Recovery of uranium from nuclear conversion plant waste

Abstract

The ammonium diuranate (ADU) conversion process operated in South Africa from the 1970s was based on the direct conversion of ADU obtained from different mines (Ponelis, Slabber, and Zimmer, 1986;Ponelis, 1989). This approach differed from those of other countries operating conversion processes at that time in that the feed was not of nuclear grade. Since the ADU was sourced from various South African mines, the composition of the feed to the conversion plant at the Atomic Energy Corporation (AEC) varied. In addition, tail-end distillation of the uranium hexafluoride (UF 6 ) product was used, and as a result of the variability in the ADU a considerable proportion of unwanted elements or impurities, such as sodium (Na), potassium (K), and calcium (Ca), was present during the conversion process (Ponelis, Slabber, and Zimmer, 1986;Ponelis, 1989).Basically, the ADU conversion process entailed the conversion of ADU to uranium tetrafluoride (UF 4 ), which was then converted to uranium hexafluoride (UF 6 ) in a fluorine flame reactor (Ponelis, 1989). The final product, thermally stable UF 6 gas, was then filtered to remove any foreign particles. Thereafter, the UF 6 was frozen out and distilled before being fed to the enrichment plant. Due to the incomplete conversion of UF 4 to UF 6 in the fluorine flame reactor, a significant amount of solid waste rich in uranium was formed, which accumulated at the bottom of the reactor. The incomplete conversion could have been caused by various factors, including sintering due to the low melting point of UF 4 , inhomogeneous feed compositions, and the presence of various impurities. A study to determine the effect of alkali metal impurities on the conversion of UF 4 to UF 6 showed that the presence of these impurities had a significant effect on the sintering of UF 4 (Ponelis, 1989).Owing to the high uranium content, the unreacted material is a nuclear liability and needs to be processed to recover a product that can be re-used in possible future conversion activities while reducing the amount of waste that is currently stored in drums. A search of the literature was undertaken to determine whether similar waste exists elsewhere, and how it is handled. Sasahira et al. (2007) mention an 'ash' that was formed during fluorination of UO 2 to UF 6 , while Ohashi, Recovery of uranium from nuclear conversion plant waste by M. Potgieter* † , J.C. Barry*, D.J. van der Westhuizen † , and H.M. Krieg † The ammonium diuranate (ADU) conversion process that was operated at the Nuclear Energy Corporation of South Africa (Necsa) in the past generated a significant amount of waste containing high concentrations of uranium, which can be re-used if the uranium can be recovered in a useful form. To attain this objective, the composition of the waste material and the amounts of impurities present were determined, followed by an investigation into various methods of uranium dissolution. For dissolution, water as well as different acid types and concentrations were investigated, and the efficiency of eac...

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