07 October 2010

Moderator, Coolant & Control Rod : Part III

What is control rods???

Basically control rods are the movable pieces of neutrons absorbing material which used to control the fission reactivity. These control rod is made of chemical elements which have the ability to capture many neutrons without fissioning themselves. The most important function is to insert to shutdown or stop the nuclear fission process when required.

An automatic control system, or the operator, can initiate the shutdown. In some reactors, all rods (29 to 100+, depending on reactor size) may insert in as short a time as 2 seconds. The control rods are made of materials that quickly stop the nuclear reaction by absorbing the neutrons. The materials divert the neutrons from being absorbed or causing fissions in the fuel. Materials used include boron carbide, silver, indium, cadmium, and hafnium diboride.
Since they absorb neutrons, any movement of the rods can affects the multiplication factor of the system. When the rods are been removed, the “k” increases while when insertion the “k” decreases. Therefore, the reactor can be started up or shut down safetly. Besides, by adjusting the control rods we able to control the power output according to our needs.
They are used in nuclear reactors to control the rate of fission of uranium and plutonium. Because these elements have different capture cross sections for neutrons of varying energies, the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control. Light water reactors (BWR, PWR) and heavy water reactors (HWR) operate with "thermal" neutrons, whereas breeder reactors operate with "fast" neutrons.


  1. I do have questions.
    can the silver control rods withstand the high heat/temperature in the reactor?And how long(time period)can this rods being used before the next replacement?
    thank you.

    Kantharrajan sandaram

  2. hello..

    i got questions here...
    what is the meaning of "k" symbol?is it referring to the shutdown process in reactor?

    what i can understand after read several article in your blog,we want energetic neutron and thermal neutron.so,how neutron can become energetic?is it referring to speed of neutron or the energy carried by neutron in terms of heat?
    i hope u can clarify in this things..


    MUHAMAD danial bin dasuki

  3. to danial...

    "k" symbol is the multiplication factor which referring to the efficiency of the system. in my opinion, the heat are supply to the neutron so they can mobile and the collision will occur. after the collision, the big amount of heat energy will produced and will the use to boil the water in the power plant to generate electricity.....


  4. I'm wondering what will happen to the control rod as it absorbs neutron. any physical or chemically change to the rod?


  5. hye i got a question here. You mentioned about materials that are used to make these rods. They are boron carbide, silver, indium, cadmium, and hafnium diboride. My question is how does these elements bind together, through what process and if you could provide some videos showing how does these rods work. Maybe a simple anology.

    Zulfadhli Bin Zahudin

  6. to rajan..

    Can the silver control rods withstand the high heat/temperature in the reactor?And how long(time period)can this rods being used before the next replacement?

    For your information, a pure silver control rods unable to withstand high temperature within the reactor. But control rods made of Silver-indium-cadmium alloys which consisting 80% of silver,15% of indium and 5% of cadmium able to withstand the temperature below (~1400°C). Because Control rod silver-indium-cadmium alloy is observed to rupture the stainless steel cladding at ~1400°C, leaving
    the silver to wet the fuel cladding and form a low-melting zirconium alloy that flows downward. Relatively littlesilver escapes as a vapor or aerosol.Besides, this type of control rod can only use in pressurized water reactor (PWR).

    Well for your second question, how long time can these rods being used before next replacement?

    When the swelling of the material in the neutron flux occurs it can cause deformation of the rod, leading to its premature replacement besides the burn up of the absorbing isotopes in limiting lifetime factor. Besides, it also depending on the design of the control rods itself, vary design having its own life span.BWR control rods in the mid-1960s. The first control rod, the CR 70, was in operation in the Oskarshamn 1 BWR plant in 1970. Many original rods are still in operation after up to 35years of operation.

    Sorry, bout i unable to provide you with the exact life span of each type of the rods.But i hope the reply above may help to let you all know more bout control rods.

    Thank you.

  7. thanks park_axe for helping to answering the meaning of "K" it is the multiplication factor which are referring to the efficiency of the system.

    For the energetic neutrons it are produced from the fission or fussion reactions.The more faster is the neutrons speed the more the heat generated from the collisions.When it is not monitored or controlled, will cause the reactor to overheat and fuel to melt down.

  8. Hello,

    very nice template for your web...
    Goin to the point,can you pls explain why pure uranium ore must be enriched for it to be used as fuel in a standard PWR reactor. still abit blurr on this..........


  9. Hi,

    A little curious here. Is it the rod is the only thing that can control nuclear fusion and also control the output?

    Muhammad Damanhuri