Chemistry of inorganic compounds – Radioactive – Uranium compound
Patent
1996-03-07
1998-03-03
Jordan, Charles T.
Chemistry of inorganic compounds
Radioactive
Uranium compound
252643, 264 05, C01G 43025
Patent
active
057231001
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the production of oxides of uranium, in particular the production of uranium dioxide (UO.sub.2).
BACKGROUND OF THE INVENTION
The production of ceramic grade UO.sub.2 powder is an important part of the nuclear fuel cycle. Ceramic grade UO.sub.2 has been produced for many years by a variety of methods.
A number of known methods for the production of ceramic grade UO.sub.2 utilise fluidised bed reactors and other reaction vessels connected in series to convert uranium hexafluoride (UF.sub.6) sequentially into uranyl fluoride (UO.sub.2 F.sub.2) and subsequently UO.sub.2 F.sub.2 into UO.sub.2. In these methods the reactants and products are sequentially passed from vessel to vessel on a once through basis. Such methods are described in Patent Specifications GB 1280865, GB 1281508, GB 1536239 and EP 0230087A.
In one particularly successful method for the production of ceramic grade UO.sub.2, UF.sub.6 is converted directly into UO.sub.2 by reaction with a gaseous mixture of steam and hydrogen in a single stage kiln process. In carrying out the process, an inclined rotating barrel type kiln is employed. An inlet chamber comprising a filter hopper is connected to the upper end (gas outlet end) of the kiln barrel and this inlet chamber forms the first region of the single process vessel. The barrel of the kiln forms the second region of the vessel. The kiln is operated with counter-current gas solids flow.
UF.sub.6 is vaporised and metered into the inlet chamber where it reacts with steam at a jet in a zone at the base of the filter hopper at the gas outlet end of the kiln. The reaction of UF.sub.6 and steam produces a solid intermediate product of UO.sub.2 F.sub.2 which passes into the kiln by means of a scroll feeder. In the kiln the UO.sub.2 F.sub.2 is pyrohydrolysed and reduced by a countercurrent stream of steam and hydrogen, fed into the lower end (powder discharge end) of the kiln barrel, to form ceramic grade UO.sub.2 powder which is discharged from the lower end of the kiln.
The required product quality is achieved by controlling the kiln temperature in three temperature zones, to an appropriate profile in the range 500.degree.-800.degree. C. The UO.sub.2 powder produced by the process typically has an oxygen to uranium (O/U) ratio of about 2.05, a mean specific surface area (SSA) of approximately 2.7 m.sup.2 /g, and a fluorine impurity level of around 25 ppm on a uranium weight basis.
However, even in a successful process such as that described above, on rare occasions a small quantity of product UO.sub.2 may be out of the desired specification range. We have found that the out of specification UO.sub.2 can often have a higher than acceptable fluoride content and a higher than acceptable O/U ratio. This impurity is usually in the form of uranyl fluoride (UO.sub.2 F.sub.2), such a product being referred to as "high fluoride UO.sub.2 ".
For a given process, any out of specification product can often represent a significant cost with no benefit and it is therefore desirable that this cost be reduced.
A known process for recycling out of specification UO.sub.2 to a UF.sub.6 conversion process is described in Patent Specification EP 0148707A. In this process out of specification UO.sub.2 is re-fed into a kiln type UF.sub.6 conversion apparatus at the same time as UF.sub.6 is being fed to the apparatus to produce `fresh` UO.sub.2. Such a process necessitates close control of operating parameters and involves careful adjustment of the input quantities of reagents in order to obtain a constant output from the reaction vessel. One disadvantage of operating in this way is that the recycling of the out of specification material could interfere with the main UF.sub.6 converion reaction and thereby produce further out of specification material. An additional disadvantage is the use of screw feeders outside the reaction vessel to convey the material to be recycled into the reaction vessel as this can lead to operational problems including the need for increased mainte
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Carter Robert
Semeraz John Terry
British Nuclear Fuels PLC
Hardee John R.
Jordan Charles T.
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