Chemistry · Ch 13 — Nuclear Chemistry and Radioactivity
Nuclear Fission
Nuclear Fission
Nuclear fission is the splitting of a heavy nucleus into two nearly equal, lighter fragments, accompanied by the release of a large amount of energy. The classic trigger is neutron capture: when a heavy nucleus such as uranium-235 absorbs an incoming neutron, it becomes unstable and breaks apart, releasing heat, additional neutrons, and other radiation.
A representative fission reaction: ----------. Crucially, there is no single fixed way for a 235-92-U nucleus to split -- roughly 400 different fission pathways are known, together producing around 800 distinct fission products, many of which are themselves radioactive and go on to undergo further spontaneous decay of their own. Each individual fission event releases roughly 200 MeV of energy, and on average emits about 2.5 neutrons (typically 2 to 3 per event). …
Worked out. One representative fission reaction: 235-92-U + 1-0-n → 142-56-Ba + 91-36-Kr + 3(1-0-n) + Energy. When a 235-92-U nucleus absorbs a neutron it becomes unstable and splits into two lighter fragments, releasing heat, additional neutrons, and other radiation. There is no single, unique way for 235-92-U to split -- the source states that around 400 different fission pathways are known, together producing about 800 different fission products, many of which are themselves radioactive and undergo further spontaneous decay. Each fission event releases about 200 MeV of energy and emits on average about 2.5 neutrons (typically 2 to 3). Since those emitted neutrons can go on to trigger fission in further 235-92-U nuclei, which release still more neutrons, the process can become a self-sustaining CHAIN REACTION that continues even after any external source of bombarding neutrons is removed. If the mass of fissile material exceeds a threshold called the critical mass, the chain reaction proceeds so rapidly that it results in a nuclear explosion (as in an atomic bomb); below the critical mass, or when moderated and controlled (as inside a nuclear reactor), the same chain rea …