The Nuclear Fuel Cycle
- Mining and Milling
- Conversion
- Enrichment
- Fuel Fabrication
- Electricity Generation
- Used Fuel and Waste Management
Mining and Milling
Uranium deposits are found in rocks all over the world with the largest producers being Canada, Australia, and Kazakhstan. The ore is recovered by either mining the hard rock, or by a process called "Situ Leaching. " Hard rock mining is just like it sounds - the ore is mined in either open pit mines (when it is close to the surface), or in underground mines (120 meters or more below the surface). The ore is then sent to a mill where the rock is crushed into a fine slurry. The uranium is leached from the ore with sulfuric acid and then recovered from the solution in the form of concentrated U3O8, commonly known as "Yellow Cake".
The yellow cake contains 80% uranium with the original ore containing from .1% to 20% uranium. The remaining waste rock contains small concentrations of low level radioactive material and small amounts of heavy metals. The radioactive elements are far less than the original ore and are much shorter lived. The material is stored in tailing ponds which are usually mined out pits and are isolated from the environment. The second method of Uranium mining, Situ leaching, is the process by which oxygenated ground water is circulated through the deposit to dissolve the uranium. It is then pumped to the surface where the ore is again milled to precipitate the uranium from the water. This method is becoming more widely used as it has a far less impact on the environment.
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Conversion
The U3O8, Uranium Ore Concentrate or Yellow Cake needs to be converted and enriched to be used as a fuel in a Nuclear Power Plant. First the U3O8 needs to be converted to UO3 (Blind River), then the UO3 is further converted to UF6. The UF6 is produced by mixing UO3 with fluorine gas, and remains a gas at low temperatures. The gaseous UF6 is then pumped in to Cylinders where it solidifies and is transported to an enrichment facility.
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Enrichment
The UF6 that arrives at the enrichment facility has about .7% U-235. Most reactors need the Isotope U-235 enriched to 3-5% as a fuel. This is accomplished by one of 2 methods:
The first method is by Gaseous Diffusion where the UF6 gas is forced through a porous membrane, where the lighter U-235 has a better chance of passing through than the heavier U-238. This is done some 1400 times to reach the required 3-5% enrichment. Each pass is called a "Cascade." At the end of the cycle, the enriched UF6 gas is extracted from one end and the depleted UF6 gas is removed from the other and is stored on sight.
The second method is similar to Gaseous Diffusion whereby the weight difference of the two isotopes is utilized, but in this process the UF6 gas is pumped in to a series of vacuum tubes each containing a rotor spun at 50-70,000 times per minute, generating 1 million times the acceleration of gravity. The centrifugal force causes the heavier U-238 molecules to concentrate on the outer edge while the lighter U-235 molecules stay in the center. The slightly enriched UF6 gas travels on to another Cascade or Centrifuge and the slightly depleted UF6 is pumped back in to the previous cascade. This process is repeated several times to reach the required 3-5% enrichment. This method is much more cost effective and energy efficient.
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Fuel Fabrication
At the fabricators, the enriched UF6 gas is converted to a UO2 powder which is compressed in to pellets and then baked at 1400 degrees C. These pellets are then encased in tubes of zirconium alloy or stainless steel to form fuel rods. The fuel rods are then grouped together to form fuel assemblies which are used in the nuclear reactor core. A 7 gram pellet of UO2 will release as much energy as 1 ton of coal. Each fuel assembly contains 200-500KG of fissile material and the reactor core will house anywhere from 157-241 assemblies that last 3-4 years each.
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Electricity Generation
Nuclear Power stations and conventional fossil fuel stations generate power in the same manner; that is they produce heat to generate steam which drives turbines to create electricity. The difference is that instead of burning coal or gas to generate the heat, nuclear power stations use the fission chain reaction. The chain reaction takes place in the core of the nuclear reactor and is controlled by rods, which absorb neutrons. These rods can be inserted or removed from the core and control the reactor power level. The fuel elements are surrounded by a moderator that slows down the emitted neutrons and enables the reaction to continue. Water, graphite, and heavy water (11% heavier than regular water and contains one or more hydrogen atoms than regular water) are used as a moderator in different reactors.
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Used Fuel and Waste Management
The used uranium fuel can be either reprocessed into a new uranium fuel or into MOX (Mixed Oxide Fuel) where the plutonium and uranium are combined. MOX is used in many modern reactors.
All waste that cannot be reprocessed is stored underground in specially designed repositories.
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