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Components of Nuclear Power Plant – Control Rods

written by: Ricky • edited by: Lamar Stonecypher • updated: 9/13/2008

What do control rods in a nuclear power plant do? It has been often said that power corrupts and absolute power corrupts absolutely. This maxim is usually associated with politics but applies equally to a nuclear reactor as well. Unless there is something to control the immense power that a nuclear reaction wields, the reactor would simply go haywire.

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    Introduction

    Nuclear fission is a source of tremendous energy which could be either used for destructive purposes such as nuclear weapons or constructive purposes such as a nuclear reactor for producing electrical energy. Even though a nuclear reactor in a power plant has got peaceful intentions, the tremendous power, heat and energy which is associated with nuclear fission cannot be left on its own but needs to be controlled in a predictable manner. It is here that controls rods come in the picture and form an important part of the components of nuclear power plant.

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    Why the Need to Control?

    It does not require much reflection to imagine why proper control is necessary with in nuclear reactor. Some of the basic reasons are as follows.

     

    • A nuclear chain reaction should be started when a reactor fires from the cold condition. In the absence of such a reaction the process would soon die out.
    • It is not only necessary and sufficient to start the chain reaction but it is equally necessary to ensure that the reaction is sustained in the long run as long as the power requirements are present.
    • In case of emergency situations such as a sudden mechanical or structural damage, the reactor needs to be shut down quickly in order to prevent any major disaster like say Chernobyl which could be very costly in terms of loss to life and environment.
    • Fuel rods inside the reactor should be prevented from melting or getting disintegrated and therefore a control mechanism is absolutely necessary.

     

    We have seen the reasons for controlling and taming the wild nuclear power and the best method to achieve this is through the use of control rods which can be inserted or withdrawn from the core and help to control the nuclear reactions taking place inside the reactor.

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    What do Control Rods Do?

    One property which is a must for control rod material is the heavy absorption capacity for neutrons so that they can carry out the control function effectively. The commonly used materials which satisfy these criteria include cadmium, boron, iridium, silver and hafnium. Another property of control rods is that the material should not start a fission reaction despite the heavy absorption of neutrons. Infact you can imagine the function of a control rod just like a blotting paper which sucks the extra ink that has spilled somewhere but doesn't let it spread in a wider region.

    The mechanism of control consists of arranging control rods in assembles which are usually mounted vertically within the reactor core and are inserted into the guide tubes with the fuel elements. For purposes of safety of a reactor in case the lifting mechanism also suffers a failure, the control are arranged in such a way that they will get into the stop position and shut down the reactor completely in such a case.

    Hence we see that control rods tend to provide a mechanism wherein the immense nuclear energy can be tamed within reasonable limits and ensure safety and security of the reactor as well the outside environment.