My question:

What type of material is used in the control rod of a nuclear reactor? (Nuclear power plant)

  • $\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of the site, visit the help center or take a tour. By the way, you could do a little research. e.g.: Your answer could be hidden in this wikipedia page about the control rod in nuclear power plants. $\endgroup$ – M.A.R. ಠ_ಠ Dec 31 '14 at 18:04
  • $\begingroup$ The answer to your question is easily obtained by using Google. See this answer $\endgroup$ – ron Dec 31 '14 at 18:14

Control Rods: They have the ability to capture the slow neutrons and can control the chain reaction at any stage. Boron and Cadmium are the best absorbers of neutrons.

To start fusion in $\ce{^{238}U}$ we need very fast electrons having kinetic energies more than about 1 $MeV$.In a nuclear reactor, the probability that of a neutron will cause $\ce{^{235}U}$ to fission is more for a slow moving neutrons.Thus there is less probability that the fast neitrons produced in fission will cause other fission to occur and chain reaction will start. To slow down these neutrons (to initiate more fission events) in a nuclear reactor moderator is used which may be ordinary water or heavy water($\ce{D2O}$).

  • 2
    $\begingroup$ 1) Usually if an answer is one or two sentences it might be best to post it as a comment. 2) Control rods can also be designed to capture fast neutrons. 3) Many other factors go into control rod material selection in addition to neutron capture cross-section, so pure boron and cadmium won't make for the best control rods. $\endgroup$ – ron Dec 31 '14 at 19:07
  • $\begingroup$ 1)Try hovering your mouse over "add comment" to know what it specifically does, If not many users post short answers on this site, this should not be discouraged, [as another point :I wish not go my answer go waste as a comment ] 2)So What? To start fusion in $^{238}U$ we need very fast electrons having kinetic energies more than about 1 $MeV$.In a nuclear reactor, the probability that of a neutron will cause $^{235}U$ to fission is more for a slow moving neutrons.3) I didn't said pure elements, in either ways that is what you learn if you study chemistry $\endgroup$ – RE60K Dec 31 '14 at 19:19
  • $\begingroup$ See here for a discussion on short answers. For items 2) and 3), I'm just telling you how I read what you wrote. $\endgroup$ – ron Dec 31 '14 at 20:00
  • $\begingroup$ But one question, why slow down the chain reaction. Isn't it better if the chain reaction were faster then the amount of energy in a period grow $\endgroup$ – tor Jan 1 '15 at 9:56
  • $\begingroup$ @tor the probability that of a neutron will cause 235U to fission is more for a slow moving neutrons $\endgroup$ – RE60K Jan 1 '15 at 9:56

There are various designs and materials for control rods, which are more technically correctly called control assemblies. The choice depends on the type and purpose of the reactor, the size of the core, the availability of other means for power control (e.g. boric acid in the reactor coolant, burnable poison rods, $\ce{UO2-Gd2O3}$ fuels, recirculation pumps, or thermal-hydraulic feedback), patent rights, and tradition.

The control assemblies of typical boiling water reactors (BWRs) are inserted from the bottom because, in a BWR, the neutron flux is larger in the lower part of the core and the space above the core is occupied by the water separator and the steam dryer. Therefore, the design should consider saving weight. BWRs usually use cross-shaped control assemblies. The four blades of one cruciform control assembly are inserted between four fuel assemblies. The blades are made of stainless steel and contain tubes that are filled with boron carbide ($\ce{B4C}$) as neutron-absorbing material. In addition, hafnium ($\ce{Hf}$) absorber may be placed in selected locations of selected control assemblies.

The control assemblies of typical pressurized water reactors (PWRs) are inserted from above. Each control assembly has multiple rod fingers, which enter openings at the upper end of a fuel assembly. These control assemblies are very heavy because, when the reactor is tripped, they have to quickly drop into the core against the pressure of the reactor coolant. The neutron absorbing material mainly is an $\ce{Ag-In-Cd}$ alloy (usually 80 % $\ce{Ag}$, 15 % $\ce{In}$, and 5 % $\ce{Cd}$) encapsulated in an austenitic steel cladding. These three neutron absorbers together effectively block the entire energy spectrum of fast, epithermal, and thermal neutrons. In other PWR core designs, the control rods are divided into two groups: so-called black assemblies made up of rods consisting of an $\ce{Ag-In-Cd}$ alloy in the lower section and of $\ce{B4C}$ in the upper part, and so-called grey assemblies containing absorber rods made of $\ce{Ag-In-Cd}$ alloy and steel.


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