Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How do control rods get in there?

  1. Dec 25, 2013 #1
    How do control rods get in there!?

    This is a bit annoying for me, hopefully someone can help.

    You have fuel pellets in rods. They are en capsuled by cladding to make sure fission fragments cannot escape into the moderator. Do the fission neutrons pass through this cladding into the open space between the rods (where the moderator slows them down) and then pass the cladding of the next adjacent fuel rod to enter and fission with that fuel??

    ..if not, then how does it work? Please and THANK YOU.

    (and happy holidays to all)
  2. jcsd
  3. Dec 25, 2013 #2


    User Avatar
    Gold Member

    The neutrons do need to bounce around a bit to get down to the right speed before they can fission another atom, so your understanding is correct.
    The control rods manage the overall neutron economy of the reactor, removing neutrons to slow or stop the fission rate.
  4. Dec 25, 2013 #3
    Thank you for the reply I think my main misunderstanding is this: does the fission neutron escape one fuel cladding and enter into the cladding of another fuel assembly?
  5. Dec 25, 2013 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    Forgot which sockpuppet you are using again?
  6. Dec 25, 2013 #5
    lol apologies I replied via email. You know, originally I wanted middle physics as a way to rid myself of all my stupid question history. Looks like I still have very stupid questions. I think the neutrons exit the cladding of one rod pass through the moderator and enter the cladding of another rod to continue the reaction.

    I have no qualms with the moderators deleting kidphysics. There is no motive or troll here. I just wanted to rid myself of my stupid question history and have a fresh slate.
  7. Dec 26, 2013 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Most commercial reactors are light-water reactors (LWRs), of which most are pressurized-water reactors (PWRs and VVERs), others are boiling-water reactors (BWRs), and fewer are heavy-water reactors (PHWRs or CANDUs) and graphite moderates (RBMK, AGRs).

    In PWRs, the control rods reside above the core, with the tips engaged in the upper part of the assembly, just above the top of the fueled region. They are suspended by a magnetic coupling that will de-energize when tripped. The control rods fall into the core under gravity. Neutrons are also absorbed by burnable poisons in the fuel, e.g., gadolinium, erbium or boron (in the form of ZrB2) for the purpose of reactivity control, or otherwise soluble boron (in the form of boric acid, H3BO3, which must be buffered with LiOH, or in the case of VVERs, KOH) in the coolant.

    In BWRs, the control rods are used in the core for reactivity control, and they are 'swapped' in groups periodically to balance the power peaking and burnup distribution in the core. BWR control rods are hydraulically operated with water. The moderator/coolant boils in the core.

    In LWRs, fission neutrons are produced in the fuel. Some fast neutrons will produce fissions (about 8-10% of fissions come from fast neutrons). Otherwise, fast neutrons slow to low (thermal) energies in the moderator, and they must pass out of the fuel, through the cladding, and into the moderator, then through cladding again into the fuel. It is possible that a neutron will return to the fuel rod from which it originated, but the probability is low.
  8. Dec 26, 2013 #7
    this is exactly what I was looking for, thanks Astronuc!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for control rods
Proton-boron fusion
Fuel rod leak (failure) frequency