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.
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.