When uranium or other suitable nucleus splits, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits very quickly a number of fast neutrons, the same type of particle that initiated the fission of the uranium nucleus. This makes the neutrons possible to achieve a self-sustaining series of nuclear fissions and produce a chain reaction - continuous release of energy. The minimum amount of fissile material required to maintain the chain reaction is known as the critical mass. The successive generation of fission produces a supercritical assembly, which leads to a possible explosion. A heavy material, called a tamper, surrounds the fissile mass and prevents itspremature disruption.