Nuclear energy is energy captured from the fission of heavy elements or fusion of light elements. These nuclear reactions give off heat, which can be converted into electrical, mechanical or photonic energy. Nuclear energy is most commonly harvested for human use in nuclear reactors, which can power small vessels like submarines, all the way up to cities and states. Less directly, nuclear energy from the sun can be harvested as solar energy.
Currently, most reactors in the world are fission reactors , in which heavy elements undergo fission into lighter elements with large kinetic energy, free neutrons, gamma rays. These lighter atoms lose this kinetic energy to heat through collisions with other atoms. This heat is collected by heating a liquid which then conveys it to a heat engine (such as a steam engine) that converts this heat into electricity. However, there are various different ways of conveying this heat away from the reactor core, as well as various elements that can be used for fuel, most commonly the very fissile Uranium-235.  However, uranium is not a hugely abundant material (world supplies are estimated to be 1.6 × 107 tonnes) and other fissile materials are even rarer; therefore a method of producing fuel is needed that can convert non-fissile isotopes to fissile isotopes, a process known as breeding. [3,4]
In nuclear reactors, as fuel is spent, neutrons are released. A single neutron for every fission event is necessary to sustain the reaction, i.e. to cause another fission event. However, as many fission reactions release more than one neutron, it is possible for the other neutrons to cause more fission events, as in conventional nuclear power plants and also nuclear weapons. These neutrons can also be absorbed by non-fissile isotope atoms, causing them to transmute to a higher isotope. This higher isotope atom then undergoes beta-decay to form a fissile isotope atom. The number of fissile isotope atoms generated per fission event is known as the breeding ratio . A breeding ratio of greater than unity implies that for every fission reaction, more than one fissile atom can be generated, and the fuel can be bred from non-fissile material. Nuclear reactors are being developed to make use of this to breed fissile material from relatively much more abundant materials .
U-235, the isotope of uranium with the largest nuclear cross section (a measure of the likelihood of a nuclear reaction occurring), is commonly used as a nuclear fuel, but is only found in small quantities, comprising less than one percent of naturally occurring uranium [7,8]. However, Uranium-238, which makes up over 99% of natural uranium, can be used to breed Plutonium-239, a highly fissionable isotope of plutonium that can be used in nuclear reactors.  Such breeder reactors are called fast breeders, as they breed plutonium from fast neutrons ejected from U-238 .
Thorium breeding is of great interest for nuclear power, as thorium is thought to be three to four times more abundant than uranium in the Earth's crust . Further, though Thorium-232, the isotope that constitutes most of the natural supply of thorium, needs a neutron source to initiate a chain reaction, it has a large nuclear capture cross section and breeds Uranium-233, a fissionable isotope of uranium, suitable for use in nuclear power generation.  It is of great interest to countries such as India with large reserves of thorium and low uranium reserves; India presently has the only operational Thorium nuclear reactors in Kalpakkam in the state of Tamil Nadu and Kakrapar in Gujarat, though research reactors are in operation in Canada . Finally, thorium reactors reduce the risk of proliferation as the by-products of thorium fission are generally used in-situ for power generation, and a completely new fuel cycle would need to be developed for thorium to be used in nuclear weapons .
Research has shown that breeding in nuclear reactors can generate the same amount of power using less nuclear fuel (and more abundant fuel) than conventional reactors. Further, as breeders can reduce the risk of nuclear proliferation, they present a viable technology for power generation.
© Kunal Sahasrabuddhe. 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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