Fissile material
In nuclear engineering , fissile substances are nuclides which, after absorbing a neutron , split, releasing energy and simultaneously emitting several neutrons, which in turn can trigger new nuclear fission ( chain reaction ). In the civil sector, nuclear fission is used to generate energy; in the military sector, it is used in nuclear weapons . The most important fissile materials are uranium-233 and -235 and plutonium-239 and -241 .
The smallest mass of fissile material that sets a self-sustaining chain reaction in motion is the critical mass . It is around 50 kg for uranium-235, and around 10 kg for plutonium-239. The critical mass can be reduced by technical measures.