Enrichment is the process by which the proportion of 235U in natural uranium is increased above natural levels in preparation for nuclear fuel manufacture. The level of enrichment is typically expressed in terms of the proportion of 235U in the uranium, so that uranium compounds containing 4% 235U in relation to the total uranium is referred to as enriched to 4% or 4% enriched. This is also referred to as the assay or isotopic abundance. If enriched material is generated then some material with a level of 235U below natural must inevitably be generated as well. This material is known as depleted uranium (DU), with UF6 that has been depleted in 235U known as ‘tails’. The enrichment of uranium used for commercial power reactors is typically in the range of 3-5%.

The unit of measurement used to express the degree of enrichment applied through an enrichment process is the Separative Work Unit (SWU). It is related to the mass and enrichment level of the feed material, the product and the tails. The number of SWU in an enrichment process may be calculated using the equation:

W = P Vp + T Vt — F Vf [7.10]

where

W = Separative Work P = Mass of Product T = Mass of Tails F = Mass of Feed

Vp, Vt, Vf = Value function for the product, tails and feed respectively.

The value function is in turn given by the equation:

V = (1-2x).ln((1-x)/x) [7.11]

where

x = concentration of 2 35U in the material (product, tails or feed) expressed as a proportion, i. e. 0.0071, rather than 0.71%.

Despite being referred to as ‘work’, the SWU is not a true unit of energy (the dimension of the SWU is actually mass), although, for a given technology, there is a proportionality between the calculated SWU and the amount of energy used to achieve the desired separation. It requires relatively little energy to generate a small deviation from the feed concentration hence few SWU, while

large deviations require a great deal of energy and therefore a much larger number of SWU. Table 7.1 provides some examples of the relationship between SWU and product and tails enrichment.

The product enrichment is set by the requirements of the reactor, so that the amount of SWU required to generate that product is effectively determined by the extent to which the tails are depleted. The price of natural uranium therefore has a significant impact on the target tails depletion. The situation may be likened to squeezing juice from an orange, where the 235U may be viewed as the juice. It is relatively easy to get a small amount of juice from an orange, but to get nearly all of the juice requires a great deal more effort and it may well be easier to use more oranges! The concentration of 235U in tails material is typically in the range 0.2-0.3%. Table 7.2 provides some examples of the relationship between SWU, tails enrichment and the mass of feed material required.

The four main providers of commercial uranium enrichment services are:

1 Rosatom/JSC TVEL (Russia)

2 URENCO Enrichment Company Limited (UEC, UK/Germany/Netherlands/ USA)

3 AREVANC (France)

4 US Enrichment Corporation (USEC, USA)

Some capacity exists elsewhere in the world, notably in Japan and China, with the latter expected to become more prominent in the future particularly in its domestic market.

gas through it. The greater speed of the 235UF6 molecules means that they impact with the membrane more frequently and are therefore more likely to pass through one of the pores. The low pressure (product) side will therefore become slightly enriched in the lighter isotope while the high pressure (tails) side becomes slightly depleted. A continuous bleed on both product and tails side captures the enrichment. A diagram of a single diffuser unit is shown in Fig. 7.3.

The mass difference between the 235UF6 and 235UF6 is very small so that a single diffuser is capable of only a very modest enrichment, the theoretical maximum separation being 1.0043; in practice, the actual separation factor may be little more than half of this. This means that a great many diffusion stages are required to operate in series to give a product suitable for nuclear fuel manufacture. Furthermore, the tails material from many of the diffusers still contains commercially viable concentrations of 235U, so that many more stages are required to recycle the tails in a complex sequence of feed and re-feed so that only tails depleted to concentrations well below natural are discarded from the process. This interlinked series of stages is called a cascade and is illustrated in Fig. 7.4 . The diagram shows the principles of a cascade linked in both series and parallel, as is the case in centrifuge plants; diffusion plant cascades are simpler than this as they are linked in series only, with the throughput of each individual stage controlled by the size of the unit and the pressure applied. A diffusion plant contains thousands of stages, each of which contains a compressor, associated drive motor and a cooling unit in addition to the diffuser (the Oak Ridge Gaseous Diffusion Plant contained 5098 stages, although far fewer stages are required to enrich to commercial levels). The net result is a very large industrial facility; the Oak Ridge Gaseous Diffusion Plant was housed in the world’s largest industrial building when it was established in the 1940s, while the much smaller capacity Capenhurst plant was the largest industrial building in Europe under a single roof when built in the 1950s. Both plants occupied an area of close to 200 000 m2.

Although the principle of the technology is relatively simple, the design and manufacture of a membrane is no easy task. The membrane must be thin and must have a very small pore size, likely to be 20 nm or less in diameter. The number of pores must be maximised to keep the differential pressures as low as possible, yet inconsistencies in pore size must be minimised or performance will be compromised. The membrane must also be chemically resistant to UF6 and be of sufficient quality to perform reliably over many years. Materials that have been used or proposed for the membrane include nickel, aluminium oxide and fluorinated polymers.

A significant drawback of the gaseous diffusion process is its very high power consumption. The pressure drop across the membrane in each diffuser requires recompression of the gas before it is fed into the next unit. This continuous recompression requires a great deal of energy, with a commercial scale gaseous diffusion plant consuming as much electricity as a large city or small country; the French Eurodif plant consumes electricity at a rate of over 2000 MW when operating at full capacity, greater than the average 1800 MW electricity consumption rate for Wales in 2009. The recompression of the UF6 also generates a lot of excess heat, which must be removed from the system. This requires highly efficient cooling systems, which in turn require the use of refrigerant gases. Chlorofluorocarbon (CFC) gases such as Freon are highly efficient refrigerant gases and were a natural choice when gaseous diffusion plants were built. The
ozone depleting properties of CFCs are now known and their manufacture is banned under the Montreal Protocol, making operation of the old diffusion technology increasingly difficult.

It is a credit to the developers of gaseous diffusion technology that it has operated successfully for over 60 years and that during that time it has been responsible for producing the nuclear fuel required to power hundreds of power stations that, in turn, have provided electricity to many millions of homes and industrial premises. The technology has been under severe challenge from gas centrifuge enrichment for many years, however, and has now, finally, become economically obsolete.