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RBMK neutron moderation question

  1. Jul 7, 2017 #1
    This is more or less a question to which I think I know the answer to but want to make sure i'm right.

    So in light water reactors one of the safety features is that the light water coolant also serves as the neutron moderator slowing them down to the "thermal" range so that they can cause fission more efficiently in U 235, so if somehow the water level drops too low or there is too much steam neutron moderation is lost there are fewer thermal neutrons and more fast ones which can't fission U 235 effectively and hence the reaction decreases and so does the power,

    Now in Chernobyl's RBMK-1000 due to a sudden increase in power water was boiling off fast and turning into steam while still in the pipes in the active zone of the core, normally this would or should lead to a decreased reactivity but in the RBMK it lead to an even greater increase in reactivity, what is essentially know as the positive void coefficient, now was that because the specific design of the RBMK unit used essentially two moderators, the main one being graphite and also water so even when the water was lost to steam the neutrons still were moderated in the graphite and so there was no decrease in reactivity?
  2. jcsd
  3. Jul 7, 2017 #2
    The graphite is the moderator in the RBMK. While you have a positive void coefficient, if sufficient control rods are inserted then boiling water also exposes more control rod material and can allow the overall reactivity response to be stable and controllable.

    During the Chernobyl accident many of these rods were removed, such that a rapid voiding event had the potential to runaway like it did.
  4. Jul 8, 2017 #3
    And they had those rods removed beyond the minimum allowed limit of rods that must be in the core at all times so violating the safety handbook instructions, if I'm correct they did this because the reactor had built up considerable amounts of xenon 135 which is an extremely powerful neutron absorber/reaction killer.
    Now a bit less clear to me is the reason this xenon built up, I assume it is because the reactor was running at low power levels, but the reactor manual says it can run on minimum 700MW thermal power of the core, so technically if it was run on this power level the xenon shouldn't have built up so did they manage to run it on an even lower level and why?

    Oh and also the xenon135 is an isotope of xenon gas right?
  5. Jul 8, 2017 #4


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    Xe-135 is a fission product, and also a decay product of another fission product I-135. During steady-state or constant power operation, the I-135 and Xe-135 concentrations are in equilibrium. When the reactor reduces power or shuts down, the I-135 decays to Xe-135, which also decays, but at a slower rate, so the concentration of Xe-135 increases rapidly before decreasing.
  6. Jul 8, 2017 #5
    The thing with xenon is that you need to establish sufficient power to burn xenon out before it passes a threshold. They hung out at a reduced power level for too long before they started their test, causing a much higher xenon inventory than expected. The they lowered lower enough that their xenon inventory crossed the threshold of what the reactor was capable of overriding.

    Not all reactors have total xenon override capability. Bwrs have it virtually all the time during their operating cycle. PWRs have it for all but the end of the fuel cycle. But CANDU reactors have less than one hour following a major transient to establish sufficient reactor power levels to override xenon following a reactor runback, otherwise xenon will poison the core and cause it to shut down for a couple days.

    The RBMK reactor is capable of operating at a reduced power level, however they need to establish that power level before xenon exceeds certain limits otherwise the core will have a tendency to shut itself down. When they operated at reduced power for several hours at the grid dispatched request they had more xenon than planned and when they later reduced power further to set up the test, xenon kept building in and caused the core to shut down.

    While I can't speak for an RBMK specifically, I can tell you that low power operation is very difficult to manage, especially in boiling type reactors like the RBMK and BWR. The boiling boundary is much higher and may be out of your power peak. You have reduced void response in the core. And you can drop power to 5-10% and try to hold steady only to have xenon keep pushing the reactor power level down. In pwr plants there have been a lot of inadvertent transitions between subcritical (or critical below the point of adding heat) and critical as operators try to manage low power operations. Bottom line is it is heavily discouraged for operators to keep the reactor in low power conditions because the core is less stable and xenon will force the core power level to continue to drop causing the operators to have to fight it.
  7. Jul 10, 2017 #6
    If I remember correctly they had to run the reactor at minimum allowed power which was 700MW thermal to wait for the okay for the test but by mistake one of the operators inserted the control rods too low or something which is when they dropped to barely 200MW and the whole situation became unstable so they now pulled the rods out and more than allowed by the manual to compensate for the xenon buildup but what they probably did not anticipate or expected was that the reactor indeed managed to burn away the xenon and all of a sudden a major and rapid power excursion started happening which was then made even worse by the reactors specific properties like the positive void/feedback loop and other aspects,

    tell me is this the correct chain of events I here just described? I kind of think this was the way it happened after many years of hearing bits about what happened.
  8. Jul 10, 2017 #7
    They were supposed to lower power to an acceptable test level and perform the test. While they were lowering power another power station tripped off and the power grid operator told them to stop lowering power so they hung out at 50% for several hours and built up a ton of xenon. By the time they were allowed to continue the test they were too deep into the xenon pit. They dropped to 700 MW but that was too low power for the xenon inventory they were building up and the reactor's power level continued to drop and they operators removed control rods beyond the allowable safety limit setting up the conditions for the accident to occur.
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