The established commercial enrichment processes of gaseous diffusion and gas centrifuge enrichment both generate depleted tails material in the form of UF6 , with the quantity of tails material significantly greater than the quantity of product. The tails material still contains potentially valuable 235U, which is worth recovering should the price of natural uranium reach sufficiently high levels and industry preference is therefore to store tails material as UF6. The volatility and chemical reactivity of UF6 means that it is not ideal for long-term storage, however, and the quantities involved require a lot of storage capacity, so that conversion to a more compact and stable form must at some point be considered. The preferred chemical form for long-term uranium storage is U3O8, the same material that is fed into the conversion process earlier in the nuclear fuel cycle. The process of converting UF6 back into U3O8 for long-term storage is therefore known as deconversion.

A large deconversion plant has been operating successfully at Pierrelatte in France since the 1980s, with over 300 000 tonnes of UF„ of tails deconverted. The facility is owned and operated by AREVA. The process strips fluorine from the UF6 in the form of HF using steam and hydrogen to give a U3O8 product. The HF is collected as concentrated hydrofluoric acid and sold into the commercial market as a high-grade feed material for chemical processing. The chemical process is shown in Fig. 7.7.

In the first stage of the process, UF6 is mixed with superheated steam in a hydrolysis chamber, generating solid UO2F2 and HF gas via a gas phase reaction (see Eq. 7.1). The reaction takes place at 250-300 °C. The steam is added in excess, which allows the HF and excess steam to be condensed and collected as concentrated hydrofluoric acid, with the amount of excess steam driving the concentration of the acid.

In the second stage of the process, the UO2F2 is reacted with more superheated steam at a higher temperature and with some hydrogen present giving the overall reaction:

3UO2F2 + 2H2O + H2 ^ U3O8 + 6HF [7.12]

The plant at Pierrelatte uses a rotary kiln for the pyrohydrolysis with a reaction temperature of around 750 °C. The steam is again added in excess to allow condensation and collection of the HF as hydrofluoric acid. The process is designed so that the hydrofluoric acid output from the process, including the effluent from any wet scrubbing systems, is generated at a suitable concentration for onward commercial use with appropriate, standard concentrations being 40%, 60% or 70% by weight. The U3O8 is generated as a powder, which is collected and stored.

A number of new deconversion facilities have recently been, or are in the process of being, constructed. A plant has been built in Russia based upon the Pierrelatte design and another is in the course of construction in the UK. Further plants have been built at Paducah and Portsmouth in the USA to process tails material from the US military programme. These are of a slightly different design as a single fluidised bed reactor is used, rather than a hydrolysis chamber and kiln, but the chemical process of reacting the UF6 with a mixture of steam and hydrogen is essentially the same.