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We can follow up exercise 3-5 by using its results to consider problems in nuclear physics. For example, a nuclear reactor generates energy by initiating a controlled chain reaction in which fast neutrons from radioactive decay strike nuclei of uranium, thereby causing the nuclei to break up or fission. During the break up, the nuclear fragments produced are accelerated to high speeds by the energy of the fission. These fragments strike many other atoms and release their kinetic energy in the form of heat that can then be used to turn water to steam for electrical power generation. Some of the nuclear fragments produced during fission are themselves fast neutrons which can then go on to break up other uranium nuclei (hence the chain reaction). It is necessary to slow down the fast neutrons from fission in order to allow them to effectively participate in the chain reaction process. Typically, the neutrons are allowed to collide with a moderator material (e.g. paraffin, a waxy substance whose molecules contain lots of hydrogen atoms) before they encounter the nuclear fuel. Control rods which both absorb neutrons and slow them down further are used to keep the chain reaction under control and prevent a core meltdown.
Do exercise
3-11 to
examine the case of neutron control via moderators.