Nuclear fuel is the material used for the generation of nuclear energy. It is a material that can be fissioned or fused depending on whether its use is nuclear fission or nuclear fusion.
We refer to nuclear fuel both to the material (uranium, plutonium, etc.) and to the set made with nuclear material (fuel rods, the make-up of nuclear material, and the moderator or any other combination.
The most widely used nuclear fuel is uranium because it is the most suitable in nuclear fission reactors. Currently, all nuclear reactors in production for the generation of electrical energy are fission. At another level, plutonium is also used as a nuclear fuel.
Tritium and deuterium are light hydrogen isotopes that are used in the nuclear fusion process. Nuclear fusion, at the moment, is not sufficiently developed to be applied in nuclear power plants.
For what is nuclear fuel used?
A nuclear power plant uses nuclear fuel to power the reactor.
When used in a reactor, the fuels used can have various forms: a metal, an alloy, or some sort of oxide. Most nuclear reactors use a compound made up of uranium dioxide.
Atoms in nuclear fuel are bit by bit separated by the process of nuclear fission. In each of these reactions, the material is transformed into other elements releasing thermal energy.
This heat energy is used to obtain steam and drive a turbine coupled to an alternator. In this way, the nuclear power plant generates electricity.
For the reactor to function, the nuclear fuel mass present in the reactor reaches the so-called critical mass. Critical mass is the amount needed to start a chain reaction that is stably self-sufficient.
Setting fuel rods in a nuclear reactor
The nuclear fuel is placed in rods in the reactor. Laying on bars provides the following advantages:
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It makes it easier to transport.
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Allows you to alternate the fuel with the neutron moderator and control rods.
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It simplifies the extraction of fuel at the end of the cycle.
The fissile material should be placed in a geometric array that maximizes the efficiency of the knock-on effect. This arrangement must take into account the need to leave enough space to insert the neutron moderator.
During the design phase of a nuclear reactor, it is also necessary to allow space for control rods and diagnostic devices.
In theory, the ideal shape would be spherical; however, a cylindrical shape is used, obtained by combining a large number of bars. Typically, a reactor core will have between 150 and 250 fuel assemblies.
Nuclear fuel cycle
The nuclear fuel cycle is the set of operations necessary to manufacture fuel for nuclear power plants. Cycle operations also include the treatment of spent fuel.
In the case of uranium, the closed cycle includes:
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The mining to extract natural uranium. Currently, almost all the uranium used in the United States commercial reactors is imported.
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Production of uranium concentrates.
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Obtaining enriched uranium.
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Manufacture of fuel elements.
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The use of fuel in the reactor.
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The reprocessing of the irradiated fuel elements to recover the remaining uranium and the plutonium produced.
The enriched uranium is converted to uranium dioxide powder in a fuel fabrication plant. This powder is then pressed to form small fuel pellets. Finally, the pellets are inserted into thin metallic tubes (fuel rods). The uranium fuel rods are grouped together to form fuel assemblies.
Nuclear fuel depletion and replacement
Unlike traditional fuel (e.g., fossil fuels), fuel used in a nuclear reactor is very slow. Once loaded into the reactor, it generally lasts for years.
On the other hand, refueling operations are considerably more complicated.
Unlike what happens with other types of fuels, the product of the reaction (the so-called slag) is not dispersed. These products remain mainly within the immediately adjacent bars or elements.
Over time, fuel rods become increasingly poor in fissile material. When the rods reach the point where it is no longer efficient to explode them, they must be replaced.
Depending on the reactor’s geometry, a part of the fuel may run out faster than other regions: generally, the central part runs out more quickly than the outer part. The bar configuration is useful because it allows the replacement of only the most depleted parts.
The spent rods, as well as the material in the immediate vicinity, have become highly radioactive. It is because of:
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The presence of fission products generated by the reactions
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The exitance of other material that can become activated during the neutron capture process
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As a result of other similar processes.
Disposal of spent rods is, therefore, the most complex part of nuclear reactor slag shut down.