The small difference in the size of the 235U and 238U nuclei results in them having slightly different ionisation potentials, slightly shifted absorption lines and forming chemical compounds with slightly different bond energies. Lasers produce beams of light at a single wavelength and therefore a very specific energy, making it possible to tune a laser so that it will preferentially interact with 235U, rather than 238U. This selective activation offers a means of differentiating between the two isotopes so that they may then be separated. Very high separation factors can be achieved, which offer the possibility of a single-stage enrichment process. Energy consumption is low, comparable with or potentially less than for centrifuge enrichment.

The technology required to manufacture and tune laser systems to the very precise wavelength necessary to interact preferentially with 235U is sophisticated but within current capabilities. Perhaps a greater challenge for deployment within a commercial facility is to ensure that the systems operate reliably and without any wavelength drift for prolonged periods, as even a fractional drift will prevent them from performing their function. The greater path length that the laser light is required to travel at large scale will also increase inefficiencies caused by factors such as absorption, reflection, refraction and diffusion as the light passes through the gas.

The activated 235U species are produced as an intimate mixture within a bulk 238U matrix, requiring that they be separated and collected efficiently if high selectivity is to be retained. This must be done quickly to avoid the activated species recombining, exchanging with bulk 2 38U isotopes or interacting with construction materials before separation can be effected. The larger the unit, the longer the residence time is likely to be and the more difficult separation is likely to become.

Laser technologies are capable of producing uranium at commercial enrichment levels in a single stage; however, a commercial plant must also have a high throughput and make efficient use of feed material. Whilst a full-scale, commercial laser plant may be more compact than an equivalent centrifuge plant, it will still be of significant size and contain a great many laser systems and optical components.

A number of different laser based enrichment technologies have been explored, notably: [11]

• Chemical Reaction Isotope Selective Laser Activation (CRISLA)

• Condensation Repression Isotope Selective Laser Activation (also CRISLA)

• Separation of Isotopes by Laser Excitation (SILEX).

In the AVLIS process, uranium metal is melted by means of an electron beam that generates a stream of uranium atoms in gaseous form. A dye laser, powered by a copper laser, is used to preferentially ionise the 235U atoms in the vapour to 235U+ as it passes an ion extractor with an applied electromagnetic field. The field draws the charged ions towards the collector where they are separated and collected as a liquid metal. Some neutral 238U atoms will coincidentally deposit as they pass the collector as will 2 38U+ ions that have become charged by exchange with 2 35U+, reducing the overall efficiency of the enrichment process. The uncharged bulk passes to a second, tails collector where it is again recovered as a liquid metal. The technology has been investigated in a number of countries, most notably the USA, where the Lawrence Livermore National Laboratory developed the process to a stage where a demonstration in 1992 produced uranium enriched to commercial levels from tonne quantities of uranium feed. USEC sought to commercialise the process later in the 1990s but abandoned it at the end of the decade as not cost effective.

The MLIS process uses UF6 in a cooled carrier gas as the feed material, which sits more comfortably within the existing nuclear fuel cycle than the uranium metal used in AVLIS. The 235UF6 is preferentially energised and then stripped of a fluorine atom to form uranium pentafluoride (UF5) using a one — or two-laser system (ultraviolet and infra-red or infra-red alone). The UF5 is not volatile and solidifies more readily within the gas stream than the UF6 so that it can be preferentially filtered from the carrier gas. The feed gas also contains a scavenger gas, such as methane, that will capture the free fluorine atoms generated during laser excitation. The technology was pursued by a number of organisations in the 1980s and 1990s but was abandoned towards the end of this period, with the notable caveat that the limited information available on the SILEX process suggests that it is related to MLIS.

I n the chemical reaction CRISLA process UF6 is mixed with a proprietary chemical reagent known as RX. A laser is used to preferentially excite 235UF6 so that its reaction rate with the RX compound is significantly increased compared to the non-excited 238UF6. The reaction product, which is enriched in 235U as a result of the increased reaction rate, may then be separated from the UF6 using techniques appropriate to the physical and chemical nature of the product. This CRISLA process was developed and patented by Dr Jeff Eerkens in the late 1970s with the technology later transferred to Isotope Technologies Inc and Cameco. Changes in market conditions led to the process being dropped in the early 1990s.

Dr Eerkens has also been involved in the development of the condensation repression CRISLA process. For this process, the feed gas of UF6 in a xenon carrier gas is cooled adiabatically down to less than 60 K through expansion from a nozzle as a supersonic jet stream. UF6 dimers are formed in the gas stream as a result. A suitably tuned laser is used to provide the 235UF6 molecules in the gas stream with enough energy to prevent them from forming dimers, thus creating a substantial difference between the mass of the non-excited, dimerised 238UF, and the laser-maintained 235UF6 monomer. This results in different radial escape rates for the two isotopes in the jet stream and allows separation using appropriately positioned skimmers.

Neither CRISLA process has been used for commercial production.

The most promising laser based enrichment process at present appears to be the SILEX process, which was originally developed by Silex Systems Ltd in Australia. Global Laser Enrichment, a subsidiary of GE Hitachi Nuclear Energy and also supported by Cameco (24% ownership), has stated that it intends to use this process as the basis for a commercial enrichment facility in Wilmington, North Carolina. The process was also the subject of a significant development programme led by USEC from around 1996 to 2002 but was not pursued to the full scale commercialisation now proposed. For reasons of both commercial and nuclear proliferation security, very little technical information on the process has been published. It is known that the feed stream is a cooled mixture of UF6 in a carrier gas with the 235UF6 preferentially excited at the 16 pm wavelength (similar to the MLIS setup). The process is based on UF6 in all process streams. Further details have not been published in the open literature at the time of writing.