This nucleus is relatively unstable, and it is likely to break into two fragments of around half the mass. Using U-235 in a thermal reactor as an example, when a neutron* is captured the total energy is distributed amongst the 236 nucleons (protons & neutrons) now present in the compound nucleus. It is nonetheless possible to use this so-called fast fission in a fast neutron reactor whose design minimises the moderation of the high-energy neutrons produced in the fission process. We therefore say that the fission cross-section of those nuclei is much reduced at high neutron energies relative to its value at thermal energies (for slow neutrons). In nuclei with an odd number of neutrons, such as U-235, the fission cross-section becomes very large at the thermal energies of slow neutrons.ДЄs implied previously, high-energy (> 0.1 MeV) neutrons are travelling too quickly to have much interaction with the nuclei in the fuel. The fission and other cross-sections increase greatly as the neutron velocity reduces from around 20,000 km/s to 2 km/s, making the likelihood of some interaction greater. This may be imagined as an area surrounding the target nucleus and within which the incoming neutron must pass if the reaction is to take place. The probability that fission or any another neutron-induced reaction will occur is described by the neutron cross-section for that reaction. U-238 and Th-232 are the main naturally-occurring fertile isotopes. U-235 is the only naturally occurring isotope which is thermally fissile, and it is present in natural uranium at a concentration of 0.7%. Each of these is produced artificially in a nuclear reactor, from the fertile nuclei Th-232 (in certain reactors), U-238 and Pu-240 respectively. Other heavy nuclei that are fissile (implying thermal fission) are U-233, Pu-239 and Pu-241. For more information see page on Nuclear Power Reactors. * There are two main varieties, pressurized water reactors and boiling water reactors. The most common examples of this are light water reactors*. Hence the main application of uranium fission today is in thermal reactors fuelled by U-235 and incorporating a moderator such as water to slow the neutrons down. Neutron cross-sections for fission of uranium and plutoniumДЄ neutron is said to have thermal energy when it has slowed down to be in thermal equilibrium with the surroundings (when the kinetic energy of the neutrons is similar to that possessed by the surrounding atoms due to their random thermal motion). (Newly-created fission neutrons are in this category and move at about 7% of the speed of light, while moderated neutrons move a lot slower, at about eight times the speed of sound). For nuclei containing an even number of neutrons, fission can only occur if the incident neutrons have energy above about one million electron volts (MeV). Thermal fission may also occur in some other transuranic elements whose nuclei contain odd numbers of neutrons. However, low-energy (slow, or thermal) neutrons are able to cause fission only in those isotopes of uranium and plutonium whose nuclei contain odd numbers of neutrons ( e.g. Nuclear fissionДЇission may take place in any of the heavy nuclei after capture of a neutron. Whether fission takes place, and indeed whether capture occurs at all, depends on the velocity of the passing neutron and on the particular heavy nucleus involved. But in certain cases the initial capture is rapidly followed by the fission of the new nucleus. In this example, U-239 becomes Np-239 after emission of a beta particle (electron). The new nucleus may decay into a different nuclide. A simple example is U-238 + n => U-239, which represents formation of the nucleus U-239. Capture involves the addition of the neutron to the uranium nucleus to form a new compound nucleus. When a neutron passes near to a heavy nucleus, for example uranium-235 (U-235), the neutron may be captured by the nucleus and this may or may not be followed by fission. For more information on how a nuclear power plant works, see information page Nuclear Power Reactors. Neutrons in motion are the starting point for everything that happens in a nuclear reactor.

Isotope separation to achieve uranium enrichment is by physical processes.

For reactors using light water as moderator, enriched uranium is required.

Fission reactions may be moderated to increase fission, or unmoderated to breed further fuel.Radioactive decay of both fission products and transuranic elements formed in a reactor yield heat even after fission has ceased.Nuclear reactors work by containing and controlling the physical process of nuclear fission.

Nuclear reactor meltdown yield