The chemical and physical behaviour of different isotopes of the same element are virtually identical; however, there are very small differences that can be exploited to allow artificial concentration of a particular isotope. The binding forces between atoms or molecules are stronger in the solid and liquid phases than in the gaseous phase, so that the gas phase often offers the greatest potential for separation, based on the physical properties of the individual molecules.

Uranium metal and most uranium compounds have very high boiling points, making any form of gas phase processing very difficult. The exception to this is UF6 , which is a volatile solid at room temperature, having a vapour pressure of 10.6 kPa at 20 °C and a sublimation point of 56 °C. The critical point occurs at 64 °C and 151.7 kPa, meaning that liquid UF6 is not formed at atmospheric pressure. Figure 7.1 shows a phase diagram for UF6. The high vapour pressure of the material means that it can be handled as a gas at low pressure and room temperature, or at slightly elevated temperatures. Furthermore, fluorine has only one naturally occurring isotope, with a relative atomic mass of 19.00, so that any difference in the behaviour of 235UF6 and 238UF6 is due solely to the difference in the uranium isotope.

While UF6 exhibits a number of properties that make it well suited for use in a physical separation process, there are also properties that make it difficult to

handle. It reacts rapidly with water to form uranyl fluoride (UO2F2) and hydrogen fluoride (HF) via the reaction:

UF6 + 2H2O ^ UO2F2 + 4HF [7.1]

If the water is present as a vapour then the HF will tend to form as a gas whereas in bulk water the HF will form in solution, as hydrofluoric acid. UF6 may also react with organic materials, including hydrocarbon oils, to release HF. Both HF gas and hydrofluoric acid are toxic and corrosive.

UF6 can also react with metals directly although the rate of reaction with non­reactive metals tends to be slow and some metals, such as nickel and high nickel alloys are effectively resistant. Other metals, such as stainless steel, carbon steel and aluminium may be used for construction under appropriate conditions and with an appropriate allowance made for corrosion. Hydrofluoric acid will be formed in the presence of water, which tends to be far more corrosive than dry UF6.

It is important to take appropriate safety precautions when working with UF6. It reacts rapidly with water in the atmosphere to generate HF, presenting a significant and immediate chemical hazard. The main hazard from the uranium in UF6 arises if it is absorbed into the body via internal exposure pathways (i. e. ingestion, inhalation or injury) with the chemotoxic hazard considered to outweigh the radiological hazard at low to moderate enrichment levels. Its volatility means that it poses a greater threat for internal exposure than most uranium compounds and also makes the spread of contamination more likely. Once enriched, the potential for accidental criticality must be considered, for the UF6 itself and for any reaction products and residues. The key factor in ensuring safety is the prevention of any means of uncontrolled release.

Despite the challenges noted above, UF6 is used in most enrichment processes and in particular those that have been commercially exploited. The process of manufacture is known as conversion.