 In this video, I will describe the science behind the fission reactors that are used in nuclear power plants around the world. At present, nuclear power is responsible for around 10% of the total electricity produced worldwide, and for a handful of countries, including France, Ukraine, Slovakia and Hungary, nuclear power produces more than 50% of their electricity. Although it is a controversial power source, primarily because of the public sphere of most things nuclear-related, it is one of the very few large-scale power production methods that is almost completely greenhouse gas-free. For this reason, it is likely that it will continue to play a major part in the world's mix of energy resources for many years to come. Most nuclear power plants around the world use the fission of uranium fuel. Natural uranium is a mixture of two different isotopes. 99.27% of natural uranium is uranium-238, with a half-life of 4.5 billion years. The other 0.72% is uranium-235, with a half-life of 704 million years. Both of these isotopes are unstable, but the half-lives are long enough that much of the uranium present in the Earth when it was created is still present today. Most reactors around the world today require uranium fuel that is enriched in uranium-235. The reasons for this were discovered during the Second World War as part of the Manhattan Project in the USA, responsible for the creation of nuclear weapons. The picture shown here is taken from the Los Alamos Primer, a book that summarizes the lectures given to incoming personnel in the Manhattan Project. It shows the probability that a neutron will induce the fission of uranium nucleus as a function of the energy of the neutron. Both axes are plotted on a log scale. The labels are in codes, since this was a top-secret project at the time. The key point is that low-energy neutrons have zero probability of inducing a fission reaction on uranium-238 nuclei. In contrast, uranium-235 has orders of magnitude higher probability for fission if low-energy neutrons are used. The isotope plutonium-239 is similar to uranium-235, but plutonium is not something that is found naturally and must instead be produced in a nuclear reactor. This higher probability for fission of uranium-235 is the reason that most nuclear reactors in the world today require uranium enriched in uranium-235.