When a neutron is fired into the nucleus of a large atom one of two things may happen.The neutron may be absorbed by the nucleus causing the isotope to increase its atomic mass by one while retaining the same atomic number. This process is called radiative capture. The second outcome is that the large nucleus is made so unstable by the impact of the neutron that it splits into several fragments.Two of the fragments are nuclei of roughly equal mass, the rest are neutrons.
Slow neutrons
Large Hadron Collider (LCH) in CERN
The speed at which the neutrons strike the large nucleus affects the likelihood of each
outcome. Slow neutrons may cause fission whereas fast neutrons undergo radiative capture.
Chain reactions
CHAIN REACTIONS Chain reactions are reactions where at least one neutron is released during each fission reaction.
The neutrons released cause more fission when they are absorbed by another nucleus. It is a ‘self-sustaining’ reaction.
Critical mass CRITICAL MASS
Critical mass is the minimum amount of the fissile material needed in order for chain reactions to occur.
Ba-141 U-235 Ba-141 U-235 Kr-92 n U-235 Kr-92 n n Kr-92 Fig 22.21: Chain reaction involving U-235 410 INVESTIGATING PHYSICS n Ba-141 n n n n n
The critical mass of a fissionable material depends upon its nuclear properties, its density, its shape, its enrichment, its purity, its temperature and its surroundings. Having exactly the ‘critical’ mass will result in an equilibrium fission reaction; this is also referred to as a steady-state or continuous chain reaction. A subcritical mass is a mass of fissile material that does not have the ability to sustain a fission reaction, while a supercritical mass is one where there is an increasing rate of fission.
Nuclear Reactors All nuclear reactors in operation today are based on nuclear fission.They consist of the following:
• Fuel rods: a fissionable material that is the source of energy. The fuel is a heavy unstable element, such as
