The unusual property of uranium, neptunium, and plutonium of forming volatile hexafluorides has led to extensive work on fluoride volatility processes for separating these elements from irradiated fuel and from each other. Major programs were carried out at Brookhaven, Argonne, Oak Ridge, and European laboratories. These programs have been summarized by Jonke [J2], Barghusen et al. [Bl] and Schmets [S2],

Brookhaven made engineering-scale studies of a process in which uranium metal fuel was dissolved in a liquid interhalogen compound such as BrF3. The reaction was difficult to control; work was terminated after an explosion [В18]. Brookhaven later developed the Nitrofluor process [B19], in which fuel was converted to UF4 and PuF3 by a liquid mixture of HF and oxides of nitrogen. After dissolution, UF4 was converted to UF6 by BrF3 and distilled off. Finally, PuF3 was converted to PuF6 by fluorine and distilled off.

Gas-phase fluorination reactions were studied at Argonne [Bl] and in Europe [С13]. Fuel was first oxidized to U308 and Pu02. The crushed oxides were charged to a fluidized bed of alumina through which gases containing fluorinating agents, F2, C1F3, or BrFs, were passed. Uranium was readily separated as volatile, stable UF6. Separation of neptunium and plutonium was less satisfactory. Although these also form volatile hexafluorides, they are less stable than UF6. Stronger fluorinating conditions are needed to form them, and PuF6, in particular, is so unstable that it tended to decompose and deposit solid fluorides throughout the equipment.

Experience has shown that fluoride volatility processes are most useful when applied either to fuel containing little plutonium and neptunium or to fuel from which these elements have been largely removed by other processes. Oak Ridge National Laboratory has successfully separated and purified multikilogram amounts of irradiated, highly enriched uranium relatively free of plutonium from zirconium-235 U fuel used in submarine reactors [03], from aluminum — 235 U fuel used in research reactors [04], and from the mixture of fused salts used in the aircraft reactor experiment [С2]. More recently, 235 UF6 and 233 UF6 were recovered from the BeF2-7LiF-UF3 fuel melt used in the molten-salt reactor experiment [L3], This work led to design of a process to separate 233U and fission products from the BeF2-7LiF-UF3-ThF4 mixture proposed as fuel for the molten-salt breeder reactor nuclear power system [R9].

In the Aquafluor process [G4] developed by the General Electric Company, most of the plutonium and fission products in irradiated light-water reactor (LWR) fuel are separated from uranium by aqueous solvent extraction and anion exchange. Final uranium separation and purification is by conversion of impure uranyl nitrate to UF6, followed by removal of small amounts of PuF6, NpF6, and other volatile fluorides by adsorption on beds of NaF and MgF2 and a final fractional distillation. A plant to process 1 MT/day of irradiated low-enriched uranium fuel was built at Morris, Illinois, but was never used for irradiated fuel because of inability to maintain on-stream, continuous operation even in runs on unirradiated fuel. The difficulties at the Morris plant are considered more the fault of design details than inherent in the process. They are attributed to the attempt to carry out aqueous primary decontamination, denitration, fluorination, and distillation of intensely radioactive materials in a close-coupled, continuous process, without adequate surge capacity between the different steps and without sufficient spare, readily maintainable equipment [G5, R8].