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A nuclear power station contains machines that are used to create a controlled nuclear fission reaction that generates electricity.
The centerpiece of a nuclear power station is a nuclear reactor, which creates controlled nuclear fission using radioactive elements such as uranium. Nuclear fuel for a reactor comes in units called pellets that are capable of generating about the same quantity of energy as about 150 gallons of oil. To be used in a reactor, pellets are stacked inside 12-foot metal rods. A collection of fuel rods, often numbering in the hundreds, is known as a fuel assembly, and a reactor core often contains many assemblies.
Fuel assemblies are kept in storage bins until needed. At this point, fuel assemblies are considered mildly radioactive, with basically all radiation held inside the metal tubes.
A cylindrical structure of fuel bundles about 12 feet across and 14 feet tall, the reactor core is where nuclear fission occurs. A steel pressure chamber surrounds the core with walls that are many inches thick.
The reactor core does not have any moving parts beyond a few control rods that are positioned to manage the fission reaction. Putting the fuel assemblies alongside each other and adding water triggers the nuclear reaction. Reactor operators normally switch out 40 to 90 fuel assemblies, around one-third of the reactor core, every one or two years.
In the reactor, uranium atoms break apart and, as they divide, they release small particles known as fission products. These particles cause other uranium atoms to divide, creating a chain reaction. The cumulative energy released by all of these reactions generates a substantial amount of heat.
The reaction heat warms water or another "cooling agent" such as molten metal or salt. The heated cooling medium generates steam, which is then used to drive turbines. The turbines then drive generators, producing electricity.
Operators use rods of material known as 'nuclear poison' to manage an on-going chain reaction. Typically made of the element xenon, these control rods absorb fission products and the more they are inserted into the reactor core, the slower and more managed the reaction. Taking out the rods leads to a more robust chain reaction and higher rate of electricity production.
After being used to drive the turbines and generators, steam from the reactor is cooled down inside tall structures referred to as cooling towers, the same towers that give a nuclear power plant its signature silhouette. Much of the steam is cooled to the point it reverts to water to be used again in the reactor. Some excess steam is harmlessly released into the atmosphere.
After being spent in the reactor, fuel assemblies are highly radioactive. They are removed and stored underwater on-location in a 'spent fuel' tank for many years. Although the fission reactions from this material have stopped, the spent fuel carries on releasing heat due to the radioactive decay of uranium by-products produced in the fission reactions. The spent fuel tank cools the material and blocks the discharge of radiation.
There are currently more than 200,000 spent fuel assemblies being stored at nearly 120 nuclear reactors in the United States alone.
After a few years, the spent fuel cools to the point it can be moved to an on-site dry storage container. A growing number of reactors now store this older spent fuel in outdoor concrete or steel containers.
The last step in the life cycle of nuclear fuel is the gathering of spent assemblies from dry storage for sequestration in a permanent underground location.
References and Further Reading
https://www.nationalgeographic.org/encyclopedia/nuclear-energy/
https://www.eia.gov/energyexplained/index.php?page=nuclear_power_plants#tab1
https://www.eia.gov/energyexplained/index.php?page=nuclear_fuel_cycle
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