Friday, November 26, 2010

What is uranium enrichment?

The great majority of civilian nuclear power reactors currently in service are thermal reactors. Thermal reactors can only use fissile nuclei as fuel (unlike fast reactors which can also use fissionable nuclei). Natural uranium consists of over 99% non-fissile U-238, with fissile U-235 being only 0.7% of the total. Uranium fuel for a thermal reactor using light water as a moderator must have at least 3% U-235. The process of producing uranium fuel with an increased portion of U-235 is known as enrichment. The primary problem with any procedure to alter the isotopic composition of a sample of any element is that chemical processes very rarely distinguish between different isotopes of the same element, so non-chemical methods are usually necessary. This has historically tended to make enrichment technology difficult, complex, expensive and energy-intensive, although the situation has improved somewhat.

Fundamental Economic Principles Of Uranium Enrichment:

There are two basic considerations governing the economics of uranium enrichment. These are the amount of separative work and the mass of natural uranium feedstock needed to produce a given quantity of enriched uranium at a given level of enrichment.

Separative Work:

The amount of separative work needed for uranium enrichment is measured in special units known as Separative Work Units (SWU). The SWU is a measure of the amount of work done in the process of seperating isotopes. They are usually expressed in units of kilogram SWUs, or tonne SWUs. An SWU will require different amounts of energy depending on the enrichment process used. A detailed description of the SWU can be found on the following site:

Natural Uranium Feedstock Mass:

In addition to the separative work units provided by an enrichment facility, the other important parameter to be considered is the mass of natural uranium required to yield a desired mass of enriched uranium. As with the number of SWUs, the amount of feedstock material required will depend on the level of enrichment desired and upon the amount of U-235 that ends up in the DU. Unlike the number of SWUs required during enrichment, which increases with decreasing levels of U-235 in the depleted stream, the amount of natural uranium needed will decrease with decreasing levels of 235U that end up in the DU.

Economic Consequences of the Interaction Between SWUs and Feedstock Mass:

Putting these two factors together in an illustrative example, in the enrichment of LEU for use in a light water reactor it is typical for the enriched stream to contain 3.6% U-235 (as compared to 0.7% in natural uranium) while the depleted stream contains 0.2% to 0.3% U-235. In order to produce one kilogram of this LEU it would require approximately 8 kilograms of natural uranium and 4.5 SWU if the DU stream was allowed to have 0.3% U-235. On the other hand, if the depleted stream had only 0.2% U-235, then it would require just 6.7 kilograms of natural uranium, but nearly 5.7 SWU of enrichment. Because the amount of natural uranium required and the number of SWUs required during enrichment change in opposite directions, if natural uranium is cheap and enrichment services are more expensive, then the operators will typically choose to allow more U-235 to be left in the DU stream whereas if natural uranium is more expensive and enrichment is less so, then they would choose the opposite.

Downblending and the Megatons To Megawatts Program:

Another factor which has influenced the economics of uranium enrichment over the past two decades is the practice of downblending. This is the production of LEU for reactor fuel by dilution of HEU sourced from nuclear warheads with depleted uranium. this has been done since 1995 under the auspices of the Megatons To Megawatts program mandated by the 1993 United States-Russia nonproliferation agreement, with 400 tonnes of Russian HEU processed into fuel for US reactors as of September 2010. This fuel has provided half the power currently generated by US civilian reactors. The program is scheduled to end in 2013, by which time a total of 500 tonnes of HEU from former Soviet warheads will have been consumed. The termination of this program will likely increase the demand for enrichment services around the world.

Enriched Uranium Grades:

Uranium can be enriched to any desired level depending on the amount of separative work performed. There are a number of recognised enrichment levels.

Depleted Uranium (DU):

DU is the uranium resulting from the tails of the enrichment process, so named because it is depleted in U-235 compared to natural uranium. DU usually has a U-235 content of 0.2-0.3%

Slightly Enriched Uranium (SEU):

SEU is a relatively new grade of enriched uranium with U-235 levels between 0.9% and 2%. SEU is sometimes used in preference to natural uranium for heavy water reactors, and has been proposed as the fuel type for the ACR-1000 reactor design, intended as an advanced variant of the CANDU series.

Low-Enriched Uranium (LEU):

LEU is uranium which has been enriched to less than 20% U-235 composition. Uranium fuel for LWRs is usually enriched to between 3% and 5%. Research reactors and medical isotope production reactors typically use higher levels of enrichment, around 12% or more. The OPAL reactor at Lucas Heights uses uranium enriched to just below 20% U-235.

Highly Enriched Uranium (HEU):

HEU is uranium enriched to above 20% U-235. It is used in some research and isotope production reactors, and is also used in nuclear power plants for naval vessels such as nuclear submarines and aircraft carriers, where the high enrichment levels allow for compact plant designs.

Weapons-Grade Uranium (WGU):

HEU enriched to 85% U-235 and above is also known as weapons-grade uranium. This is the grade of uranium used in the production of some nuclear weapons.

Enrichment processes:

The art of uranium enrichment extends back to the Second World War, when a number of approaches were rushed through development under the pressure of military demand. Some early techniques which were abandoned in favour of more effective methods were thermal diffusion, which exploited the tendency of lighter isotopes to diffuse toward warmer surfaces and heavier isotopes toward colder, and electromagnetic isotope separation, which used a powerful magnetic field to separate beams of uranium ions into different streams according to atomic mass. These techniques were largely abandoned in favour of gaseous diffusion, and more recently, the gaseous centrifuge process.

Gaseous Diffusion:

Gaseous diffusion was the first uranium enrichment process employed on a large scale in the civilian nuclear power sector. It was first employed at the Oak Ridge facility for the Manhattan Project during the Second World war, and was later used in a number of nations around the world for their civilian nuclear power programs. In this process, uranium is combined with fluorine in the form of Uranium Hexafluoride (UH6) gas and forced through semi-permeable membranes, producing a slight separation of molecules carrying U-235 and U-238. The process is based on Graham's Law, which describes the tendency of lighter molecules to escape from a container with a semi-permeable membrane wall faster than heavier molecules. By employing a cascade of many stages, any given degree of U-235 enrichment can eventually be reached.

Gaseous diffusion is an energy-intensive process, requiring on the order of 2,500 kilowatt hours to perform one kilogram SWU. Producing LEU for LWR fuel using gaseous diffusion requires approximately one fortieth of the amount of power eventually obtained from it. The gaseous diffusion process currently produces about 33% of the world's enriched uranium.

Gaseous Centrifuge Process:

The gas centrifuge process uses rotating cylinders to concentrate the heavier isotope around the rim and harvest the U-235-rich portion from near the rotation axis. This process is much more efficient than the older gaseous diffusion process, requiring as little as 2% of the energy to perform equivalent separative work. A gaseous centrifuge plant can perform one kilogram SWU for around 50-60 kilowatt hours. Gaseous centrifuge plants currently produce about 54% of the world's enriched uranium.

An improved centrifuge known as the Zippe centrifuge (named after Gernot Zippe, the German scientist credited with its invention), uses a temperature gradient along the length of the cylinder to enhance the separation of light and heavy isotopes. This type has been used by the commercial firm Urenco for uranium enrichment, and is also believed to have been used by Pakistan in its nuclear weapons program.

Laser Enrichment:

Some laser enrichment techniques have also been considered. The one currently receiving the most interest is the SILEX process, standing for Separation of Isotopes by Laser Excitation. The SILEX process is of special interest to Australia as the CSIRO played a large role in its development. SILEX is currently being developed by GE Hitachi Nuclear Energy in agreement with Silex Systems. Details of the process and performance are heavily classified, but it is said that SILEX offers an order of magnitude better performance over the gaseous centrifuge process.

Two other laser enrichment techniques have also been investigated. These are AVLIS (Atomic Vapour Laser Isotope Separation) and MLIS (Molecular Laser Isotope Separation). These techniques have encountered technical difficulties which have shut down most efforts to develop them.

Other Enrichment Techniques:

Some development work has been done on other processes, occasionally reaching the pilot plant stage, and in at least one case a full scale operational plant. The most significant of these has probably been the Helikon Vortex Separation Process developed by South Africa's UCOR corporation. UCOR operated a plant in from the 1970s through to 1991 using this technology to produce LEU for civilian nuclear fuel and WGU for South Africa's nuclear weapons program. The technique consists of forcing a stream of UH6 diluted with hydrogen tangentially into a shaped tube at near the speed of sound. The tube thus acts as a non-rotating centrifuge. The process uses large amounts of electricity, has substantial cooling requirements, and is therefore not considered economic for the production of reactor fuel. Over its operational lifetime the plant is said to have produced hundreds of kilograms of HEU. An alternative aerodynamic process has been used in a demonstration plant in Brazil which faced similar issues to the South African plant, and has also been shut down.

In addition to these aerodynamic approaches, chemical separation processes and plasma separation processes have also been tried. Most of these efforts have been shut down or scaled back, and none currently make a significant contribution to world uranium enrichment capacity.

Proliferation Issues:

Uranium enrichment is one of the most sensitive stages of the nuclear fuel cycle because of its potential to produce weapons-grade uranium. Uranium must be enriched to over 85% U-235 before it is suitable for nuclear weapons production, but it is still the same process used to create LEU for civilian nuclear fuel, although carried through to a far higher level. Enrichment capacity is therefore regarded as a potential nuclear proliferation threat, which is why Iran's uranium enrichment program has received so much sceptical attention from the international community.

Although the long-term nuclear future lies with breeder reactor technology, the main expansion in nuclear infrastructure over the next few decades will almost certainly be of the LWR type, requiring a reliable supply of LEU for reactor fuel on the global market. There are preliminary moves underway led by the International Atomic Energy Agency in cooperation with Russia, the United States and others to establish international uranium enrichment centres for the provision of LEU to the international market. This is intended to guarantee supply to the many nations now expressing interest in developing their own civilian nuclear power programs, thereby diminishing the justification for new players to develop their own national uranium enrichment capacity. Australia could potentially play a role in this system by establishing an international enrichment centre here, using either gaseous centrifuge technology or SILEX technology, enriching locally mined uranium to LEU levels and providing it to clients who have signed up to the program. In this way, Australia could play a major role in limiting the spread of nuclear weapons capability throughout the world during this particular historical phase of the use of nuclear power. Establishing such a centre would also assure Australia of local supply of LEU fuel for our own nuclear plants once we've established our domestic nuclear power industry.

Further information:

More details on the subject of uranium enrichment can be found on the following sites: