How does uranium become nuclear fuel?
How do you find uranium deposits?
How is uranium mined?
What happens to the ore during milling?
What is refining and conversion?
What is enrichment?
What is fuel fabrication?
How does a nuclear reactor work?
Can nuclear fuel be reused?
How are nuclear fuel wastes handled?
Uranium, as it is mined from the ground, is not directly useable for power generation. Much processing must be carried out before uranium can be used efficiently to generate electricity. Uranium's transformation from ore in the ground into nuclear fuel and ultimately the handling of waste products is described as the nuclear fuel cycle.
After a successful exploration program, uranium ore undergoes:
Steps one to four are known as the front end of the fuel cycle; steps six and seven, the back end, refers to what happens after the fuel comes out of the reactor.
Today's exploration activities are much more complex than in the past since the deposits that were close to the surface were found first because they were easier to discover. With the highest-grade deposits buried in deep rock formations, advanced technologies like satellite imagery, geophysical surveys, multi-element geochemical analysis and computer processing are required to locate and confirm the deposits.
Once geologists locate a prospective deposit, detailed geological and economic evaluation of the grade and characteristics of the orebody must be completed. Then mining engineers develop a mining plan to extract the ore. If the project looks promising, environmental impact assessments and the public consultation process begin so that applications can be made for regulatory approvals of project development. When permits and licences are in place, mine development and construction of surface facilities can begin. The timeline from discovery of an orebody to electricity production can span decades. Cameco's McArthur River mine was fast-tracked and still took 12 years to bring to commercial production.
At Cameco, uranium exploration has focused in recent years on targets in northern Saskatchewan's Athabasca Basin and in Arnhem Land in the Northern Territory of Australia.
Uranium ore is removed from the ground in one of three ways depending on the characteristics of the deposit. Uranium deposits close to the surface can be recovered using the open pit mining method, and underground mining methods are used for deep deposits. In some circumstances the ore may be mined by in situ recovery, a process that dissolves the uranium while still underground and then pumps a uranium-bearing solution to the surface.
When uranium ore is found near the surface, generally less than 100 metres deep, it is typically extracted by the open pit mining method. Open pit mining begins by removing overburden (soil) and waste rock on top of the orebody to expose the hard rock. Then a pit is excavated to access the ore. The walls of the pit are mined in a series of benches to prevent them from collapsing. To mine each bench, holes are drilled into the rock and loaded with explosives, which are detonated to break up the rock. The resulting broken rock is then hauled to the surface in large trucks that carry up to 200 tonnes of material at a time.
When an orebody is located more than 100 metres below the surface, underground mining methods are necessary since it is uneconomic to mine by open pit. For example, Cameco's McArthur River orebody is located more than 500 metres below the surface so it is mined using an underground mining method.
The first step in underground mining is to access the ore. Entry into underground mines is gained by digging vertical shafts to the depth of the orebody. Then a number of tunnels are cut around the deposit. A series of horizontal tunnels, called drifts, offer access directly to the ore and provide ventilation pathways. All underground mines are ventilated, but in uranium mines, extra care is taken with ventilation to minimize the amount of radiation exposure and dust inhalation.
In most underground mines the ore is blasted and hoisted to the surface for milling. At McArthur River, due to the potential for radiation exposure from the high-grade ore, processing systems must ensure worker safety. As a result, the ore is processed underground to the consistency of fine sand, diluted with water and pumped to the surface as slurry or mud. The slurry is trucked to the Key Lake site for milling.
In a few places geological conditions allow uranium to be dissolved directly by pumping mining solutions underground, bringing it back to the surface, and extracting the dissolved uranium. With this in situ recovery (ISR) process there is limited surface environmental disturbance. The surrounding rock remains in place while the dissolved uranium is pumped to the surface then circulated through a processing plant for extraction. ISR is suitable for certain deposits such as Cameco's US operations and is the mining method at the Inkai project in Kazakhstan.
After mining, ore is transported to a nearby mill for processing. In Saskatchewan, Cameco's mills at Key Lake and Rabbit Lake process ore from the McArthur River and Eagle Point orebodies.
Uranium ore is a mixture of valuable minerals and waste. The first step in milling is to crush the ore, unless it is in a solution already, and treat it with acid to separate the uranium metal from unwanted rock material. Then it is purified with chemicals to selectively leach out (dissolve) the uranium. The uranium-rich solution is then chemically separated from the remaining solids and precipitated (condensed) out of the solution. Finally, the uranium is dried. The resulting powder is uranium oxide concentrate, U3O8, commonly referred to as yellowcake because it is often bright yellow.
The yellowcake is packaged into special steel drums similar in size to oil barrels. When full they weigh about 400 kilograms. Approximately 43 drums per load are hauled by truck to uranium refineries, the next stage in the nuclear fuel cycle.
After milling, yellowcake requires further processing. First it is refined to remove impurities which produces high-purity uranium trioxide (UO3). Then, depending on the type of reactor for which it will be used, the UO3 is converted into powdered uranium dioxide (UO2) or uranium hexafluoride (UF6). If converted to UO2, the fuel is now ready to be fabricated into fuel pellets for Candu reactors. If converted to UF6, it must undergo two more steps, enrichment and subsequent conversion to enriched UO2, before it can be finally pressed into usable fuel pellets for light water reactors.
If the processing is completed by Cameco, refinement to UO3 is carried out at our refinery in Blind River, Ontario. Uranium trioxide is trucked 600 km to the Port Hope conversion facility, also in Ontario, for further processing into UO2 or UF6.
Naturally occurring uranium is made up of two different uranium isotopes, approximately 99.3% U-238 and 0.7% U-235. Most commercial reactors require uranium fuel to have a U-235 content of 3 - 5%. Uranium enrichment is the process that increases the U-235 concentration from 0.7% to 3 - 5%. Enrichment involves separation of the lighter U-235 atoms from the heavier and more predominant U-238 atoms in order to concentrate the U-235 portion. There are two commercial enrichment methods: gaseous diffusion and centrifuge.
In the gaseous diffusion process, U-235 and U-238 atoms are separated by feeding UF6 in gaseous form through a series of porous walls or membranes that allow more U-235 to pass through. To understand how this method of enrichment works, think of UF6 as equal sized sand particles of two different weights suspended in air. All the sand grains are blown through thousands of sieves, one after another.
Because the light U-235 particles travel faster than the heavier U-238 particles, more of them penetrate each sieve. As more sieves are passed, the concentration of U-235 increases. The process continues until the concentration of U-235 is increased to 3 - 5%. The slower U-238 particles left behind are collected as byproduct and referred to as "depleted tails" or "tails", in other words uranium with a reduced concentration of U-235. The high amount of energy required to force the UF6 through the porous membranes makes the gaseous diffusion process expensive.
In this type of enrichment process, the gaseous UF6 is placed in a centrifuge (a cylindrical container that spins the UF6 at high speeds). The rapid spinning flings the heavier U-238 atoms to the outside of the centrifuge, leaving UF6 in the centre enriched with a higher proportion of U-235 atoms. The enrichment level achieved by a single centrifuge is insufficient to obtain the desired concentration of U-235. It is therefore necessary to connect a number of centrifuges together in an arrangement known as a cascade. The U-235 concentration is gradually increased to 3 - 5% as it passes through the successive stages of centrifuge cascades. Enrichment using centrifuge technology requires little energy, giving this method a significant cost advantage. Centrifuge requires only about 2% of the energy needed for gaseous diffusion.
Enrichment services are sold in separative work units (SWU). A SWU is a unit that expresses the energy required to separate U-235 and U-238. How uranium is enriched depends on: 1) the amount of uranium feed (UF6) at the beginning of the process; 2) the amount of SWU used; and 3) the concentration of U-235 atoms left over (tails assay) at the end of the process.
A reactor operator knows the amount and concentration of uranium fuel required by each reactor. By varying the level of tails assay, a reactor operator can find the most economical combination of UF6 feed and SWU required for enrichment. To illustrate, consider the following example:
| Let's assume you are in the freshly
squeezed orange juice business. By deciding first how much
juice you are prepared to leave behind in the pulp, you can
then decide the optimum balance between the number of oranges
you require and the effort required to squeeze them. If oranges are cheap and the cost of squeezing is high you are less concerned with how many oranges you use, but you want to make your orange juice with the least amount of squeezing. If oranges are relatively expensive and the squeezing process is cheap, you will minimize costs by squeezing fewer oranges more times to get the same amount of juice. |
Now think of the oranges as uranium and the effort to squeeze them as SWU. If the price of uranium is relatively low, then you will use more uranium and less SWU to enrich the UF6. If the price of uranium is high and SWU is relatively cheaper, you will use more SWU and less uranium. Enrichment is measured both as the percentage of U-235 in the product and in the depletion. So the percentage of U-235 left behind in the tails assay is critical to the calculation of enrichment. The reactor operator always starts with the tails assay to find the best combination of UF6 feed and SWU. The following table shows two examples of how a given quantity of enrichment could be contracted. The shaded part of the table shows the relative amounts of electricity required to produce that quantity of enrichment which points to one of the key advantages of centrifuge enrichment.
1 kg of UF6 enriched to 3% U-235 could be produced by either of the following combinations:
| Gaseous Diffusion | Centrifuge | |||
| Natural UF6 Feed | Separative Work Units | Tails Assay | Approximate Kilowatt Hours Required | |
| 6.0 kg | 3.8 SWU | 0.25% | 9,500 | 190 |
| 5.1 kg | 5.0 SWU | 0.15% | 12,500 | 250 |
It takes about 100,000 SWU of enriched uranium to fuel a typical 1,000 megawatt commercial reactor for one year, which can supply the electricity needs for a city of 600,000 people.
Fuel manufacturing is the last stage of the front end of the nuclear fuel cycle before the uranium fuel is ready for use in a reactor. The process begins by pressing powdered UO2 into small cylindrical shapes and baking them at a high temperature (1600 - 1700°C) to make hard ceramic pellets.
In a light water reactor, the fuel pellets are packed in thin tubes called fuel rods. The rods are grouped together into a bundle called a fuel assembly. A typical 1,100 megawatt pressurized water reactor contains 193 fuel assemblies composed of nearly 51,000 fuel rods and approximately 18 million fuel pellets.
In a Candu reactor, fuel pellets are loaded into 28 or 37 half-metre long rods grouped into a cylindrical fuel bundle. Twelve bundles lie end to end in a fuel channel in the reactor core. A Bruce 790 megawatt Candu reactor contains 480 fuel channels composed of 5,760 fuel bundles and over 5 million fuel pellets.
See Nuclear Reactors.
After being in a nuclear reactor for several months, a portion of the nuclear fuel must be replaced with new fuel. The used (spent) fuel contains some residual U-235, plutonium (created when U-238 absorbs a neutron) and wastes from the fission process. Reprocessing is the chemical separation of spent fuel into these three components. The U-235 can again become reactor fuel. The plutonium can be blended with natural UO2 to create mixed oxide fuel (MOX), a fuel used in some reactors in Belgium, Germany, France and Switzerland. The waste is placed in secure storage.
While the costs of reprocessing outweigh its benefits at the present time, Russia and some European countries reprocess used fuel for environmental reasons or as a result of political policy. As well, countries like Japan are turning to reprocessing because they lack domestic fuel sources and wish to be energy independent.
Radioactive waste is generally divided into three categories depending on its level of radioactivity: low, intermediate and high-level waste.
Low-level waste includes slightly contaminated clothing and items that comes from places such as nuclear medicine wards in hospitals, research laboratories and nuclear plants. Low-level waste contains only small amounts of radioactivity that decays away in hours or days. After the radioactivity has decayed, low-level waste can be treated like ordinary garbage.
Intermediate-level wastes mostly come from the nuclear industry. They include used reactor components and contaminated materials from reactor decommissioning. Typically these wastes are embedded in concrete for disposal and buried.
High-level waste generally describes spent fuel from nuclear reactors. Spent nuclear fuel from nuclear power plants is initially stored in large water-filled pools. The water provides shielding from the radiation and cooling to remove the heat, which continues to be generated by the radioactive material in the spent fuel. After several years, when the radioactivity and its associated heat have diminished, the fuel is transferred to medium-term storage near the nuclear power plants.
The nuclear industry is evaluating long-term storage of high-level waste. While spent fuel is safely stored at nuclear plant sites today, the storage facilities were never intended for permanent storage. Countries operating nuclear power reactors are conducting extensive studies on how high-level wastes should be disposed. Research indicates the ideal permanent storage disposal is in deep underground caverns in stable geological formations.
At present, no country has constructed a repository, although considerable research is being done on a variety of different geologies. Belgium, for example is studying clay formation. The US is investigating the suitability of the volcanic tuffs of Yucca Mountain in Nevada. The site received government approval in 2002 and is now in the multi-year process of licensing application. Finland is the closest to implementing disposal of high-level wastes. In 2001, the Finnish parliament approved the plan to build a repository in the crystalline Precambrian rocks of southwest Finland near the nuclear power plant at Okiluoto.
At this time there is no urgency to build a permanent disposal facility for spent nuclear fuel in Canada because dry storage facilities can provide safe storage for many decades. After years of operations, the high-level waste from all the reactors in Canada would roughly fill a soccer field to the height of 1.3 metres. Nevertheless, it was deemed necessary to demonstrate the feasibility of permanent disposal.
Therefore, a research program led by Atomic Energy of Canada Limited (AECL) was launched in 1975 to develop a concept for disposing of high-level wastes deep in the Canadian Shield. Research was conducted until about 1998 at which time a panel of technical experts and non-technical representatives reviewed the disposal concept. The panel stated that the safety of the concept had been demonstrated from a technical point of view, but not from a social perspective. As the concept did not have broad public support, it was not considered acceptable as Canada's approach for disposing of nuclear waste.
After receiving input from various stakeholders and the public, the federal government introduced nuclear fuel waste legislation in April 2001, requiring nuclear utilities to establish a separate waste management organization to manage the disposal of nuclear fuel waste. The Nuclear Waste Management Organization www.nwmo.ca (NWMO) was established in 2002 to assess disposal alternatives including the option of long-term centralized storage, as well as establish a comprehensive public participation program. On November 3, 2005 the Nuclear Waste Management Organization concluded the first phase of its mandate by submitting its recommendations to the Minister of Natural Resources Canada. After a comprehensive three year study, the NWMO recommended Adaptive Phased Management for the long term care of Canada's used nuclear fuel. For more information on the study and implementation go to: www.nwmo.ca.
