Dave's book review for Midnight in Chernobyl

Astonishing scholarship! Check out the bibliography and notes in the back to get a sense for the amount of research Higginbotham did for this book. Not to mention in-person interviews and even visits to the site.

I finally watched the HBO Chernobyl miniseries fairly recently and I thought it was very good, but I also wanted to know more information! It’s really interesting to see how HBO picked certain individuals to be heroes and villians to tell a story and to represent whole groups of people over a long period of time in one neat little package.

I learned a lot about RBMK reactors, life in the USSR, and a million other fascinating details surrounding the disaster.

The only thing I still didn’t completely understand from either this book nor Atoms and Ashes: A Global History of Nuclear Disasters (which I read a month prior to this one) was why RBMK reactors have a positive void coefficient.

The void coefficient of reactivity (wikipedia.org) is simply the change in reactivity as steam bubbles form in the reactor moderator or coolant.

In Chernobyl’s case, the RBMK’s reactors were graphite moderated, but water cooled. It’s really a worst-case scenario for safety. Graphite moderator burns when it gets hot enough. Coolant water doesn’t merely form steam bubbles when it gets hot enough, it can even break down into explosive hydrogen and oxygen gas!

The thing I didn’t understand: water is also a nuclear reactor moderator.

Water-moderated reactors have a negative void coefficient because steam bubbles form voids in the moderator, thereby decreasing the number of neutrons that will be reduced to thermal speeds (which are lower speeds where they’re much more likely to continue the chain reaction).

This makes water-moderated reactors safer. In simplistic terms (because I’m not a nuclear physisist), when they get too hot, the water boils away and the chain reaction can’t continue. It’s like a dead man’s switch.

I understood that an RBMK reactor used graphite as the moderator, so the loss of coolant water wouldn’t reduce the reaction. And the loss of coolant water would, obviously, be very bad for reducing the temperature of a runaway reaction.

But I was confused. If water is a moderator, why wouldn’t removing water reduce the reaction at least a little?

If you already know the answer, you’re probably already screaming at the screen, but I had to piece this together from many different sources and sleep on it for a while before I finally got the whole picture.

The answer is that an RBMK reactor, for cost reasons, used unenriched uranium. Unenriched uranium can not use "light water" as a moderator. Thus the use of graphite as the moderator.

(A water-moderated reactor needs to either use "heavy water" or enriched uranium. Both of which are more expensive.)

So when the RBMK reactor loses its coolant water, it is not losing a moderator that was boosting the reaction.

In fact, the water in the RBMK reactor was not just a coolant, but also a neutron absorber, which is why losing the coolant water wasn’t merely a neutral void coefficient, but a positive void coefficient - its loss meant even more thermal-speed neutrons were being allowed to participate in the chain reaction.

It hope that clears it up for someone else who didn’t quite understand that the first time around.