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Ionizing radiation often enters the body through ingestion of contaminated materials. For more information, see: For a lesson about this topic, see: |
What Happens to the Ore During Milling?
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. They weigh about 400 kilograms when full. Approximately 43 drums per load are hauled by truck to uranium refineries, the next stage in the nuclear fuel cycle.
Cameco's Blind River refinery (left) and Port Hope conversion plants (right) process uranium from Cameco's mines as well as uranium from other mines around the world.
What is Refining and Conversion?
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 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 its 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.
What is Enrichment?
–5%. Uranium enrichment is the process that increases the U-235 concentration from 0.711% to 2–5%. 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.
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United States Enrichment Corporation, in the US, uses gaseous diffusion technology to enrich uranium. Source: NAC Worldwide Consulting |
Gaseous Diffusion
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 2–5%. 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.
Centrifuge
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 that is 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 2–5% 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.
Separative Work Units
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 pointing to one of the key advantages of centrifuge enrichment.
Gaseous Diffusion |
Centrifuge |
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Natural UF6 Feed |
Separative Work Units |
Tails Assay |
Approximate Kilowatt Hours Required |
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 in turn supply the electricity needs for a city of 600,000 people.
What is Fuel Manufacturing?
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 (1,600–1,700°C) to make hard ceramic pellets.
In CANDU reactors, fuel pellets are inserted into fuel rods and grouped together in bundles which are loaded into fuel channels in the reactor core.
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 MW 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. The Bruce A 795 megawatt CANDU reactor contains 480 fuel channels composed of 5,760 fuel bundles and over five million fuel pellets.
Reprocessing
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) which is 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.Source: Cameco Corporation: Uranium 101: www.cameco.com/common/pdfs/uranium_101/U101.pdf