Technology Jargon Series: Void Coefficient

Bubbles have always been interesting. Whether they are in your water or floating through the air on a summers day they always seem to capture our attention. however these darling little pockets of gas have a dark side. They are all members of Greenpeace and want to endanger nuclear plants.

Remember that nuclear reactors are built on the idea that by carefully controlling neutron absorption rates in the fuel we can control the amount of fission that happens and produce heat without producing Way too much heat. This requires careful consideration of reactor core design, moderator choice and amount, and coolant. When you mix all those features together you can determine a specific number, called the Void Coefficient.

What the Void Coefficient tells you is how much the reactivity of your reactor core changes if/when bubbles form inside it. A positive value means that your reactor starts producing more power, a negative value means that your reactor starts producing less power.

Suddenly seems like a very important number doesn’t it?

This number is one of the primary factors that lead to the event at Chernobyl. The reactor design there had a very high positive void coefficient. So when the reactor got a touch too hot and started forming bubbles, it started to get even hotter and form more bubbles and so on and so forth. The rest of that is history.

A positive void coefficient does not immediately mean that a reactor design is dangerous. CANDU reactors have slight positive void coefficients due to their use of heavy water as both a moderator and a coolant fluid. But, their core design has separated loops of water, one in the pressurized fuel pipes, and another in the low pressure calandria. This means that there are more ways to cool the reactor core in case of bubbles forming and the calandria is such a huge mass of water that it takes a long time for it to heat up. Because of these and other safety features, CANDU reactors have a sterling safety record and have been mainstays of several countries nuclear fleets for several decades.

Oh Canada…

However, I will say that a negative coefficient is better from a peace of mind standpoint. A system that is self regulating takes a lot of hassle off of the operators. This was a major focus for many Gen III reactor designs such as the AP1000, or EPR. But we have been using designs such as this for decades. Almost every nuclear seagoing vessel in the world uses a variant of the PWR design for the reasons of compact design and this self-stabilizing behaviour due to the negative void coefficient.

I’m not even going to bother talking about Gen IV reactor designs because most of them have found ways to deal with moderation and coolant that don’t even have the possibility of a void coefficient. People are always saying that we need more money to test Gen IV reactors to deal with climate change and that’s just not true. The designs we currently have are more than sufficient to power our world and help get our climate stabilized. Are Gen IVs a good idea? Yes, but they aren’t the magic bullet. They are just the second step, and we already have all we need to make our fist steps.


3 thoughts on “Technology Jargon Series: Void Coefficient

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    Not quite right, I think. The RBMK reactors at Chernobyl were graphite-moderated boiling water cores. Water is a moderator, like graphite, and increases reactivity in relation to stem or voids.

    The problem at Chernobyl is the the reactor was lowered in reactivity via many control rods so that little stem flow was exiting to the turbine stage, thus also reducing cooling uniformity across the core. That was the experiment operators were performing — see how low we can go and how low the turbine generator output will be — will it be enough to operate the plant’s electrical gear?

    The operators apparently missed a design flaw — for lubrication when moving in their sleeves, control rods were tipped with a bit of graphite.

    Because of the lowered output, the reactor was also twitchy, with steam (low reactivity) forming in some slots in the graphite, while water (higher moderation/reactivity) occupied others.

    One operator thought he’d better stop everything so he hit SCRAM to lower all remaining controls rods into place. Because the core was speckled with slots containing water or steam (void), graphite tips suddenly entering steam-filled slots suddenly increased their reactivity and supercriticality, overheating and burning graphite followed in microseconds, thus turning any remaining water to steam, blowing the core apart. Pieces of core then fell into the water pool below, turning it to steam and blowing the entire structure upward through the roof.

    I may have missed details. See also “Atomic Accidents” by Mahaffey.

    Dr. A. Cannara


    1. That would be all of it if the RBMK series used Heavy water like a CANDU, but they use light water which has a noticeable neutron absorbption cross section. So the light water was used less as a moderator and more of a sponge while the graphite moderated the remaining neutrons.
      However when the bubbles formed there was less absorption so the neutron flux suddenly shot up, but was still being moderated by the graphite. Meaning there were suddenly a large excess of thermalized neutrons to react with the fuel causing more heat and more steam which lead to less neutrons being absorbed.
      The graphite tips on the control rods would then have just exacerbated that along the lines you stated as well. Chernobyl was the result of many flaws working in concert, not any one specific problem.


  2. “Are Gen IVs a good idea? Yes, but they aren’t the magic bullet.”

    100% agree that there are no magic bullets, because electricity is responsible for 25% of emissions, agriculture is responsible for 24%, manufacturing is 21%, transportation 14%, buildings 6% and 10% from “other” sources. Source: IPCC,

    High temperature reactors like Molten-Salt Reactors are needed because current technologies are no magic bullet. Current low temperature reactors cannot produce high temperature output process heating *efficiently* for reducing manufacturing emissions which is 21%. (*efficiency: using high grade energy, electricity for heating is inefficient/expensive.)

    Liked by 1 person

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