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Negative or 0 void coeficient graphite moderated reactor

  1. Nov 1, 2013 #1
    Is it possible to create a graphite moderated light water cooled reactor w/ at least 3 GW thermal power output and a non-positive void coefficient that runs on slightly enriched uranium (0.9-2%) w/ at least 60 GWd/tHM burn-up? If not, is it possible to create a graphite moderated light water cooled reactor w/ at least 3 GW thermal power using the same fuel as current commercial light water moderated reactors w/ at least 70 GWd/tHM burn-up, similar construction costs (to current western commercial reactors w/ similar thermal power), and a negative void coefficient? I read the Soviet-built RBMK reactors ran on 2% enriched uranium before the Chernobyl disaster (w/ a very positive void coefficient and the use of graphite-tipped control rods being the main causes of the disaster), but that these reactors were fairly cheap to build and operate.
  2. jcsd
  3. Nov 4, 2013 #2


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    The Soviet reactors were cheap to build and operate because they lack many of the engineered safety systems of western reactors and did not apply the same scrutiny to design and construction as we do (i.e. "nuclear-grade"). The characteristic of having a positive void coefficient is not related to the cost of the plant. CANDU reactors also have a positive void coefficient.

    Having a positive void coefficient does not make a reactor inherently less safe, it is just an additional failure mode that must be accounted for in the design and operation of the reactor.
  4. Nov 4, 2013 #3
    I am aware the soviet reactors were cheap to build in part because they lacked the safety features of western reactors (I think the RBMKs laccked a reactor conainment vessel) and that candu reactors, which are prbly as safe as other commercial western reactors, have a slight positive void coefficient. The Soviet pre-Chernobyl RBMKs, however, had a very high positive void coefficient that made the reactor difficult to control at low power output when the trubines are to be kept operating at normal speed, a condition created in the experiment preceding the disaster; I think the difficulty in controlling the reactor in the conditions described is because coolant flow rate was slower in those conditions than at full thermal and electric power. The control rods used (to shutdown the reactor in an emergency, which was attempted when the reactor's power was spiking as a result of voiding) were also graphite tipped, so the bottoms of the rods moderated the reactor and displaced neutron absorbing light water; the rods fractured as a result of the ensuing power spike, which caused more coolant voids and prevented the insertion of more rods, more core damage and the disaster followed. Since the common isotopes of carbon are more massive than hydrogen-1 or hydrogen-2, a greater moderator to fuel volume ratio is generally needed in graphite moderated reactors than in light or heavy water moderated reactors, a bigger reactor vessel requires a bigger reactor containment vessel. Graphite moderator needed can be reduced by using more enriched uranium; Could lowering moderator to fuel volume ratio also decrease void coefficient? After the Chernobyl disaster, the Soviets reduced the void coefficient by adding more absorbers to the RBMK reactor core, and compensated for this by increasing loaded fuel enrichment from 2% to 2.4%. They also started using emergency control rods that were tipped at the bottom w/ materials that had less of a moderating effect or absorbed more neutrons.
    Last edited: Nov 4, 2013
  5. Nov 4, 2013 #4
    I will add a little:

    the Soviet Union could not produce reactor vessels in large numbers.
    Do not have the equipment and experience to make such a big and fortified "bowlers."

    However, there was much experience in the production of reactors for the production of plutonium.
    RBMK is remade ( modernized ) military reactor.
    Also was a highly trained staff who has been trained in military reactors , and later taught the beginners.

    RBMK also has the wrong raschitana graphite lattice in consequence of that steam ( void coefficient ) was very large.
    before the accident to 4 - 5 beta .
    For the first time, he was working on a fuel with an enrichment of 1.8 % .
    And a lot of additional neutron absorbers .
    The longer the the fuel campaign was , the more unstable it behaved .
    It was a huge steam positive coefficient, and destroyed Chernobyl.

    Now in fuel composition also includes erbium.

    But the RBMK reactor is short-lived.
    Wigner energy distorts the graphite.
    * in England since the reactor burned when they tried to anneal the graphite.
    Currently at the Leningrad nuclear power plant also struggling with this effect.
    They are sawing graphite blocks to reduce their curvature.
    Very soon we will see the result.
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