Fission meaning astronomy12/13/2023 Zheng-Tian Lu, Peter Mueller, in Advances In Atomic, Molecular, and Optical Physics, 2010 5.3 Nuclear MonitorĨ5 Kr is a product of nuclear fission, and is released into the atmosphere due to nuclear-fuel reprocessing activities. The process of thermalization, or slowing down, of fast fission neutrons is referred to as “moderation.” The material in a thermal reactor (i.e., a reactor in which fission is caused by neutrons with thermal energies) that is responsible for slowing down the fast fission neutrons is referred to as the “moderator.” It is desirable, therefore, to slow the high-energy (∼2 MeV) fission neutron (referred to as a fast neutron) to lower (thermal) energies of ∼0.025 eV at room temperature, which corresponds to a velocity of 2.2×10 5 cm s −1. During the fission process, the longer a neutron dwells in the vicinity of a nucleus of a fuel atom the greater the possibility it will be captured and therefore cause the fuel atom to fission. The γ-energy given up in the fission reaction is absorbed in the bulk of the reactor outside of the fuel, i.e., moderator, pressure vessel, and shielding. The fission neutrons give up their energy within the moderator via the process of elastic collision. The kinetic energy of the fission fragments is converted to thermal energy by successive collisions within the body of the uranium fuel mass in which the fission occurs. Nuclear fission reactors produce thermal energy from the fission of heavy isotopes, notably U 92 235(Eqn. As discussed below, the role of graphite in the fission reactor is to facilitate the nuclear chain reaction by moderation (slowing down) of the high-energy fission neutron. The fission fragments and the neutron possess kinetic energy that can be degraded to heat and harnessed to raise steam and hence drive turbine generators. Typically, an impinging neutron initiates the fission reaction, and the reaction yields an average of 2.5 neutrons per fission.
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