![]() ![]() View of the reactor core at MNR with control rods visible (photo credit: Science Media Lab). MNR uses control rods composed of an alloy of silver, indium and cadmium because each of these three metals is able to capture neutrons of a different kinetic energy than the other two, making the alloy highly efficient at regulating the neutron population or “flux” within the reactor core, and thereby controlling the rate of fission. ![]() Materials appropriate for this application include boron carbide and the heavy metal cadmium. Schematic representation of a controlled nuclear chain reaction – note that the neutron absorbers block two of the fission pathways shown in the earlier figure.Ĭontrol rods must be fabricated from a material that readily absorbs or “captures” neutrons, but is not prone to nuclear fission as a consequence. a chain reaction is occurring), the control rods are inserted slightly further into the core in order to slow the rate of fission to the required value and maintain it at that level. To initiate a chain reaction when a reactor is being started up, the control rods are partially withdrawn so that they absorb fewer neutrons once the reactor is critical (i.e. In contrast, in a nuclear reactor, the rate at which the nuclear fuel undergoes fission is rigorously controlled using “control rods”: cylinders of neutron absorbing material that are inserted into the reactor core to absorb a portion of the neutrons released by the fission of the uranium-235 fuel. If the nuclear chain reaction is allowed to propagate unchecked, the rate of fission will increase rapidly as more and more neutrons are produced, resulting in a massive and nearly instantaneous release of energy and heat. Along with a means of instigating nuclear fission in the reactor core, it is important to be able to control the rate at which the uranium atoms are undergoing fission.
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