Fissiilejä

Fissiiliä refers to materials that can sustain a nuclear fission chain reaction. These are typically heavy atomic nuclei that become unstable when they absorb a neutron. Upon absorption, the nucleus splits into two or more lighter nuclei, releasing a significant amount of energy, gamma rays, and more neutrons. These released neutrons can then go on to be absorbed by other fissile nuclei, continuing the chain reaction.

The most common fissile material used in nuclear reactors and weapons is Uranium-235 (²³⁵U). Natural uranium consists of about 0.7% ²³⁵U and 99.3% Uranium-238 (²³⁸U). ²³⁸U is not fissile by thermal neutrons, though it can undergo fission with fast neutrons. For most nuclear applications, uranium must be enriched to increase the concentration of ²³⁵U.

Another important fissile material is Plutonium-239 (²³⁹Pu). Plutonium is not found in significant quantities in nature; it is produced in nuclear reactors by the absorption of a neutron by ²³⁸U, followed by beta decays. ²³⁹Pu is highly fissile and plays a crucial role in both power generation and nuclear weapons.

Other fissile isotopes include Uranium-233 (²³³U), which can be produced from Thorium-232, and a few other transuranic elements. The property of being fissile is crucial for the controlled release of nuclear energy in reactors for electricity production and for the uncontrolled explosive release of energy in nuclear weapons. The critical mass, the minimum amount of fissile material needed to sustain a chain reaction, is a key concept in the design of nuclear devices.