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I have been trying to understand a particular aspect of the Chernobyl accident - the role of the graphite displacers on the end of the control rods. The basic story is that the graphite displacers caused the reactivity of the core to increase when the rods moved down, a non-intuitive effect that the operators were not expecting/sufficiently aware of.

All literature I can find tells basically the same story, but I shall refer here to this PDF.

On page 17 of that PDF it describes the graphite displacers and shows a schematic of a control rod in the raised position and in the "initial insertion" position.

The text indicates that:

On moving down of absorbers into the core, their displacers displace water columns from the lower part of the core. Thus, inserting of absorbers from their extreme top position introduces a positive reactivity into the core because graphite absorbs neutrons much less than water

The graphic shows a control rod "extreme top" and "initial insertion" positions. In the second case the graphite has moved in the channel, displacing water there and increasing reactivity in the lower part of the core. The image has a "+" symbol showing this.

Also, there is reduced reactivity above the displacer as a result of water replacing the graphite in that position, and reduced reactivity at the very top of the core due to the presence of the boron control rod. There are "-" symbols showing these effects.

My question is, if the graphite displacer was already in the water column, how could moving it downwards change the overall reactivity of the core?

Obviously the reactivity distribution changes, but it seems to me looking at the diagram that the (+) and (-) from the graphite movement would cancel out overall, leaving the (-) from the boron rod: an overall reduction in reactivity.

Clearly I'm misunderstanding something.

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This diagram should explain the basic problem. Prior to the accident, the control rods were in position II - that is to say, in the normal fully withdrawn position. The actual neutron absorber was sandwiched between two graphite displacer sections - so when the SCRAM button was pushed, the graphite displacer descended into the active region of the core and caused a localised spike in reactivity in the bottom of the core - the region which was most unstable.

Edit for clarity: the design of the control rods at the time left a 1.25m column of water above and below the graphite displacer itself when in position III. I'm unsure of the exact state of the control rods immediately prior to the accident, but, in any case: it was very likely the lower 1.25m of the reactor which suffered the power excursion when the rods were inserted.

This caused water in the lower part of the reactor to flash-boil to steam, damaging the reactor and causing the rods to become stuck, and in turn fed into a positive void coefficient, causing more water to boil, and so on and so on until a steam explosion blew the reactor apart. The RBMK is what's called a "loosely coupled" reactor, in part due to its size. Different parts of the core can have very different power levels, and the core was particularly dangerous at low power.

Diagram of RBMK control rods

In any case, the control rod design was the final trigger on a sequence of events which destabilised the reactor - the design of the rods themselves was deeply unsafe and caused an increase in reactor power on insertion, and then a decrease. In Chernobyl-4's case, the rods never moved far enough into the core to stem the chain reaction.

The rest, as they say, is history.

Graphic source: http://consumedland.com/page_06_en.html

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    $\begingroup$ You say that the insertion of the rods caused a spike of reactivity at the bottom of the core. However, looking at this diagram one would think it would happen at the top, since in the transition from position II to position III, the purple graphite displacer is in line with the top parts of the fuel rods. Can you clarify? $\endgroup$ – dain Dec 6 '18 at 17:36
  • $\begingroup$ @dain from II to III the graphite is moved to be fully aligned with the fuel rods. I think the point is that, for other bad design reasons, the bottom of the fuel rods was already the least stable region of the reactor. (The big purple blobs are more graphite blocks under the fuel rods) $\endgroup$ – Carl Witthoft Dec 6 '18 at 18:41
  • $\begingroup$ @CarlWitthoft so is it correct to say that there was a spike in reactivity starting at the top of the core that travelled down with the rod, but caused the most issues when the rod neared the button of travel? $\endgroup$ – jfowkes Dec 6 '18 at 19:41
  • $\begingroup$ Edited for clarity. Even when the rods are in position III there's a column of water above and below the displacer due to horrible design. I am unsure if the rods were in position II or position III immediately prior to the accident (I suspect position II or possibly a mix) - but either way, the mode of failure was the same. $\endgroup$ – Lindsay B Dec 7 '18 at 10:25
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    $\begingroup$ @LindsayB why did the technicians move the rods into the position II initially? Wasn't Position III the optimal one for maximizing power output? $\endgroup$ – Eugene Pankov Jul 6 '19 at 21:43
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I was looking at the same thing. I think this answers it better: http://accidont.ru/ENG/rodes.html

Looking at the diagram from the accepted answer: enter image description here I think that they did not pull the control roads all the way up as in position II as that would make the system slower in getting more power.

given the following:

  • the up and down parts of the fuel rods are made of graphite (marked with purple in the diagram)
  • there is a space between of 1.25m from the bottom of the graphite part of the control rod and the bottom of the graphite part of the fuel rod
  • the water in that space acts as an inhibitor slowing down the reaction in the bottom part of the core and absorbing neutrons.

when:

  • the control rod is lowered to the point where the previous gap that was filled with water is now displaced by graphite

then:

  • you have a zone of only graphite at the bottom of the core, a hotspot of graphite
  • graphite from the control rod and from the core itself will accelerate the fission
  • as you continue lowering the control rod, the bottom of the core you will continue to accelerate the fission at the bottom of the core as there will always be graphite at the bottom until the control rods get in position IV
  • it takes 18 seconds for the control rods to get in the IV position

So i think that is why there is a power surge at the bottom of the core, but as i have no taring on this subject, and this only a theory i could be wrong.

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I read Chernousenko's book in the 90s (it's one of the references in the paper). If I remember correctly, he was one of the leads of the cleanup effort. It's a damning indictment of Gorbachov, the communist party, the people who designed the experiments while ignoring warnings of consequences, laziness on the part of the designers and deliberate inattention to safety in order to get the reactors operational as soon as possible.

In the late 80s, I attended a presentation in Chicago given to the Ukrainian community by a nuclear engineer with extensive background in the area. His assessment was that this type of reactor was originally designed for plutonium production with power generation being an add-on.

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    $\begingroup$ Posted answers are supposed to make an attempt at answering the question. I don't see where you touch on the effect of control rod position on reactivity. $\endgroup$ – hobbs Dec 6 '18 at 20:57

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