|Fig. 1: Gundremmingen Nuclear Power Plant: Unit A (left), Units B and C (right) and their cooling towers (rear) (Source: Wikimedia Commons)|
Nuclear reactors are responsible for around 6% of the worlds energy and 14% of the worlds electricity.  In 2016 alone, global nuclear power generation increased by 1.3%.  Nuclear plants, similar to plants that burn coal, oil and natural gas, generate energy by boiling water into steam. This steam then activates the turbines, which then produces electricity. Although nuclear plants are similar to plants that burn coal, oil and natural gas, they differ mainly in where their energy is produced. A fossil-fuel burning power plant gets its energy from the burning of fossil fuels, where as a nuclear power plant uses uranium fuel, which consist of solid ceramic pellets, to produce its energy through a physical process called fission. 
During this physical process, atoms of uranium are split in order to produce enough heat to create steam. The uranium fuel consists of tiny, solid ceramic pellets that are packaged into long, vertical tubes. Bundles of this fuel are inserted into the reactor. 
There are two types of uranium used for fuel in nuclear reactors, U-238 and U-235.  The half-life of U-235 is 700 million years, while U-238 has a much larger half-life of 4.5 billion years. U-238 is the most commonly used, but surprisingly U-235 fissions the easiest. In U-235 atoms, the nucleus is unstable, and as the nuclei break up, they release neutrons. When the neutrons collide with other uranium atoms, those atoms also split, releasing neutrons of their own, along with heat. This occurs over and over again creating a chain reaction. When that happens, fission becomes self-sustaining.
Rods placed inside the uranium fuel tubes control the nuclear reaction. Control rods, inserted or withdrawn to varying degrees, slow or accelerate the reaction.  Water separates fuel tubes in the reactor. The heat produced by fission turns this water into steam. The steam drives a turbine, which spins a generator to create electricity. 
Majority of todays nuclear plants were originally built to last between 30 to 40 years, but with major investments in systems, structures and components, lives can be extended. Gundremmingen Nuclear Power Plant, as seen in (Fig.1), is one of the oldest nuclear plants in the world. Due to its durability, several countries active programs have been put in place to extend operating lives In the United States more than one hundred reactors are expected to receive licence extensions from somewhere between 40 to 60 years.  This justifies significant capital expenditure in upgrading systems and components, including building in extra performance margins. 
© Trevor Speights. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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