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How exactly does a LWR/MSR start

  1. May 14, 2013 #1
    How exactly does a LWR/MSR "start"

    So I believe there were a source of neutrons in a LWR, something about a possibly decaying piece of Plutonium that would start the reaction?

    As for MSR it seems like there is dissolved fuel (thorium, uranium) in a salt bath which is just dumped into one large vat... and then graphite moderators which act as fuel rods?? I am somewhat clueless even with wikipedia at bay.
    Last edited: May 14, 2013
  2. jcsd
  3. May 14, 2013 #2
    For the first reactor startup in an LWR, spontaneous decays can start the reactor, however typically neutron sources are inserted into the core (I believe AM-241 or some Cf isotope). This boosts the number of neutrons in the core to detectable levels so that operators can see the reactor go critical on their instruments, otherwise the core could go critical before its even on scale with their instruments.

    After the first startup, provided the reactor hasn't been in a long extended shutdown, the neutrons which are emitted by the waste products are sufficient to restart the reactor.
  4. May 14, 2013 #3
    Cool, thanks for the reply. Scary to think spontaneous decays can start some reactions..

    Is this answering the MSR question?
  5. May 14, 2013 #4
    Both were the LWR question.
  6. May 14, 2013 #5
    well thank you! I hopefully someone will be able to answer the MSR question!
  7. May 15, 2013 #6


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    These statements are incorrect. Spontaneous decays do not start or stop reactors. Source neutrons do not affect criticality. Source neutrons only allow criticality to be more easily measured through subcritical multiplication. Nuclear reactors are started by adding positive reactivity, typically by withdrawing control rods (PWR's can be started by diluting the soluble boron concentration).
  8. May 15, 2013 #7
    I understand by lifting the control rods the neutrons are able to continue the reaction does this mean the control rods are put in place during the manufacturing of the bundles of fuel rods?
  9. May 15, 2013 #8


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    No. Reactor fuel is manufactured in the absence of moderating materials and in discrete quantities which are less than a critical mass. Careful manufacturing process controls are in place to prevent accidentally creating a critical configuration. Such accidents have happened in the past.
  10. May 15, 2013 #9


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    To answer your question about MSR's, the way liquid fuel reactors achieve criticality is by flowing through a channel which is surrounded by a moderator. In this case the reactor would be started up or shut down by moving control rods in between the fuel and graphite moderator.
  11. May 15, 2013 #10
    You're adding reactivity by removing control rods, but at the same time, where do the original neutrons come from? (This might be an interesting question in and of itself).

    And I believe I said that the source neutrons are used for detection.....
  12. May 15, 2013 #11


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    While your statement on its own is accurate, the OP specifically asked how a reactor starts and your explanation was somewhat misleading. It's true that you need source neutrons to be able to see how much reactivity you are adding but they are not what actually starts the reactor.
  13. May 16, 2013 #12


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    The source neutrons provide a 'signal' to the detectors. In a fresh core, or one with low burnup fuel, startup sources are required. The startup sources use an (α, n), (γ, n) or spontaneous fission. In the past, (α, n) neutron reactions from Am-Be were common. In modern systems, Cf-252 (spontaneous fission) is used as a primary source, while Sb-Be (γ, n) is used as a secondary source. The Sb-123 absorbs a neutron and becomes Sb-124, which then decays to Te-124m which emits a 1.69 MeV gamma. Once a core has some high burnup fuel, usually located near the neutron detectors, spontaneous fissions from TU isotopes may be used for 'sourceless' startup.

    The startup neutron source allows the operators to monitor the state of the core, using the core as a neutron multiplier. Positive reactivity is added, or rather negative reactivity removed, by removal of control rods, and in the case of PWRs, dilution of soluble boron in conjuction with control rod withdrawal.
  14. May 16, 2013 #13
    Well as far as my first question goes in about three days I have received three great answers and I have enough information to be content. Thanks to everyone who replied.

    For some reason my brain was not functioning while reading about MSR's and now I do understand how criticality is reached.
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