Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Patent
1996-08-16
1998-03-03
Gorgos, Kathryn L.
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
20415741, 423253, 423258, 422159, 422186, B01D 5300, C01G 4300, G21C 100, B01J 1908
Patent
active
057238375
DESCRIPTION:
BRIEF SUMMARY
This is a national stage application of PCT/GB94/02670 filed Dec. 6, 1994.
BACKGROUND OF THE INVENTION
This invention relates to the purification of gaseous uranium hexafluoride (UF.sub.6) and, more particularly, to the removal of fluoride impurities from a UF.sub.6 gas stream.
In the reprocessing of irradiated nuclear reactor fuel to produce a recycled uranium feedstock which is fluorinated to yield UF.sub.6, a number of impurities are usually present which, upon fluorination of the feedstock, can form fluorides with volatilities similar to that of UF.sub.6. For example, a feedstock deriving from irradiated commercial nuclear reactor fuel contains the transuranic elements Np, Pu and Am as well as the transition elements Tc, Rh and Ru. The transuranic elements typically constitute about 0.95 weight percent on a metals basis and the aforementioned transition elements constitute about 0.38 weight percent. Uranium is usually present at a level of about 96 weight percent.
While the fluorides of the aforementioned transition elements generally have volatilities sufficiently different from that of UF.sub.6 to permit separation by fractional distillation, the fluorides neptunium hexafluoride (NpF.sub.6) and plutonium hexafluoride (PuF.sub.6) do not, and consequently remain in the UF.sub.6 gas stream following processing by fractional distillation.
Previously known methods for the removal of fluoride impurities from a uranium feedstock arising from irradiated nuclear reactor fuel have involved a number of techniques. In some instances the feedstock has been contacted with an aqueous solution prior to the formation of UF.sub.6. This has the disadvantage of resulting in an increase in the volume of radioactive waste. In other instances, alkali metal fluorides have been employed to separate the impurities by selective sorption. Such a process again leads to an increase in radioactive waste products.
Other known processes include that described in U.S. Pat. No. 4,364,906 in which calcium carbonate has been used as a trapping agent to purify a UF.sub.6 gas stream. However, such a method tends to produce large volumes of waste products. U.S. Pat. No. 3,806,579 describes the distillation of impurities of MoF.sub.6 and WF.sub.6 from UF.sub.6. U.S. Pat. No. 4,311,678 describes the use of a brominating agent to remove deposits of UF.sub.6 hydrolysis products from apparatus in which UF.sub.6 is handled. Such a process would result in the loss of some UF.sub.6 as well as the impurities. A somewhat more successful technique is disclosed in U.S. Pat. No. 4,555,318 in which a UF.sub.6 gas stream is contacted with a bed of solid UF.sub.5. The technique relies on the reduction of gaseous NpF.sub.6 by UF.sub.5 to remove the impurity, thereby creating solid NpF.sub.5 in the process. It is indicated that this process is not an efficient one and it is likely that the required reduction and removal of the impurity is not the only process which occurs. A similar approach is described in European Patent numbers 0 087 358 and 0 088 006 in which PbF.sub.2 and CoF.sub.2 are employed in a manner analogous to that described for UF.sub.5. The use of a solid fluoride bed to remove impurities will necessarily create large volumes of solid wastes which must subsequently be treated. It is also known that the use of CoF.sub.2 is not a particularly efficient method for the removal of NpF.sub.6. Both of these methods suffer from the disadvantages of poor efficiency and high waste production.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for the purification of a UF.sub.6 gas stream without a concomitant increase in the volume of waste products.
According to a first aspect of the present invention there is provided a method of purifying a UF.sub.6 gas stream, the method comprising irradiating the UF.sub.6 gas stream with laser radiation in a vessel in order to selectively convert fluoride impurities in the gas stream to involatile products, removing the purified UF.sub.6 gas stream from the vessel and sepa
REFERENCES:
patent: 4555318 (1985-11-01), Beitz
Beitz et al., "Photochemical Removal of Neptunium Hexafluoride and Plutonium Hexafluoride from Uranium Hexafluoride Gas Streams", Transuranium Elem. Symp., Meeting Date 1990, 168-180, ?, 1992.
Fields Mark
Jeapes Andrew Philip
McGrady Gerard Sean
British Nuclear Fuels PLC
Gorgos Kathryn L.
Wong Edna
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