Laser enrichment processes have been the focus of interest for some time. They are a possible third-generation technology promising lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages. One of these processes is almost ready for commercial use. Laser processes are in two categories: atomic and molecular.
Development of the Atomic Vapour Laser Isotope Separation (AVLIS, and the French SILVA) began in the 1970s. In 1985 the US Government backed it as the new technology to replace its gaseous diffusion plants as they reached the end of their economic lives early in the 21st century. However, after some US$ 2 billion in R&D, it was abandoned in USA in favour of SILEX, a molecular process. French work on SILVA has now ceased, following a 4-year program to 2003 to prove the scientific and technical feasibility of the process. Some 200kg of 2.5% enriched uranium was produced in this.
Atomic vapour processes work on the principle of photo-ionisation, whereby a powerful laser is used to ionise particular atoms present in a vapour of uranium metal. (An electron can be ejected from an atom by light of a certain frequency. The laser techniques for uranium use frequencies which are tuned to ionise a U-235 atom but not a U-238 atom.) The positively-charged U-235 ions are then attracted to a negatively-charged plate and collected. Atomic laser techniques may also separate plutonium isotopes.
The main molecular processes which have been researched work on a principle of photo-dissociation of Uf6 to solid UF5+, using tuned laser radiation as above to break the molecular bond holding one of the six fluorine atoms to a U-235 atom. This then enables the ionized UF5 to be separated from the unaffected UF6 molecules containing U-238 atoms, hence achieving a separation of isotopes. Any process using UF6 fits more readily within the conventional fuel cycle than the atomic process.
The main molecular laser process to enrich uranium is SILEX, which utilises UF6 and is now known as Global Laser Enrichment (GLE). In 2006 GE Energy entered a partnership with Australia's Silex Systems to develop the SILEX process. It provided for GE (now GE-Hitachi) to construct in the USA an engineering-scale test loop, then a pilot plant or lead cascade, which could be operating in 2012, and expanded to a full commercial plant.
Apart from US$ 20 million upfront and subsequent payments, the license agreement will yield 7-12% royalties, the precise amount depending on how low the cost of deploying the commercial technology. In mid 2008 Cameco bought into the GLE project, paying $124 million for 24% share, alongside GE (51%) and Hitachi (25%).
Isotope separation is difficult because two isotopes of the same elements have very nearly identical chemical properties, and can only be separated gradually using small mass differences. (235U is only 1.26% lighter than 238U.) This problem is compounded by the fact that uranium is rarely separated in its atomic form, but instead as a compound (235UF6 is only 0.852% lighter than 238UF6.) A cascade of identical stages produces successively higher concentrations of 235U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.
There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion (referred to as first generation) and gas centrifuge (second generation) which consumes only 6% as much energy as gaseous diffusion. Later generation methods will become established because they will be more efficient in terms of the energy input for the same degree of enrichment and the next method of enrichment to be commercialized will be referred to as third generation. Some work is being done that would use nuclear resonance; however there is no reliable evidence that any nuclear resonance processes have been scaled up to production.
- Gaseous diffusion
- Thermal diffusion
- Gas centrifuge
- Zippe centrifuge
- Atomic vapour laser isotope separation (AVLIS)
- Molecular laser isotope separation (MLIS)
- Separation of Isotopes by Laser Excitation (SILEX)
LASER ENRICHMENT METHODS
In this method a laser is tuned to a wavelength which excites only one isotope of the material and ionizes those atoms preferentially. The resonant absorption of light for an isotope is dependent upon its mass and certain hyperfine interactions between electrons and the nucleus, allowing finely tuned lasers to interact with only one isotope. After the atom is ionized it can be removed from the sample by applying an electric field.
This method is often abbreviated as AVLIS (atomic vapour laser isotope separation). This method has only recently been developed as laser technology has improved, and is currently not used extensively. However, it is a major concern to those in the field of nuclear proliferation because it may be cheaper and more easily hidden than other methods of isotope separation. Tunable lasers used in AVLIS include the dye laser and more recently diode lasers.
A second method of laser separation is known as MLIS, Molecular Laser Isotope Separation. In this method, an infrared laser is directed at uranium hexafluoride gas, exciting molecules that contain aU-235 atom. A second laser frees a fluorine atom, leaving uranium pentafluoride which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF5 must be refluorinated (back to UF6) before being introduced into the next MLIS stage.
Alternative MLIS schemes are currently being developed (using a first laser in the near-infrared or visible region) where an enrichment of over 95% can be obtained in a single stage, but the methods have not (yet) reached industrial feasibility. This method is called OP-IRMPD (Overtone Pre-excitation - IR Multiple Photon Dissociation).
ADVANTAGES AND APPLICATION
Nuclear energy can be produced in a pilot plant. Future technologies are under detailed study. They are the future of energy source. They don’t produce green-house gases in comparison to other energy generation methods. They Requires less energy for initiating the lasers.Nuclear energy will be a clean and an affordable source of energy presently. Technology is possible for both fussion and fission.
They Needs special method to avoid radio-wastes. It Might increase terrorist activities if not handled by mature personnel's.Nuclear energy is a tool that requires a mature society to wield and properly use.The power produced by the lasers may not get properly transferred to the molecules.