The third very big difference is that all Light Water Reactors have some sort of containment structure or containment building. The Chernobyl type reactors, of which there are 11 still operating, have no containment building. The final difference between them is basically an operational one. The BWR is controlled by both control rods and coolant jets that form a ring around the reactor—inside the reactor pressure vessel but around the core—and by basically turning up or slowing down the speed of those pressure pumps you can adjust the power. It’s a very nice way of doing it. And also the size of the BWR core is about 2.5 metres in diameter and about 3.7 metres in height. The Chernobyl reactor, in contrast, is 11.8 metres in diameter and 7 metres in height. It’s a very big reactor, and that’s what I mean by the operational differences.
The Chernobyl reactor is a ‘decoupled’ core, which means one side of the core doesn’t always ‘know’ what the other side is doing neutronically. What that means is that the reactor operators need to keep a very close eye on what’s going on in the Chernobyl type reactors. That isn’t the case for Western reactors. In those, one side of the core ‘knows’ fairly well what the other side is doing and it naturally adjusts.
This leads to the final point. In a Western Light Water Reactor, there’s something called a negative feedback principle at work. If a Light Water Reactor heats up for whatever reason, the reaction actually slows down—it’s worse for the reaction if the reactor gets too hot. In the Chernobyl reactor, it works the opposite way in certain power ranges. In other words, the hotter the reactor gets, the more you boil water. The more you boil water, the more you introduce steam voids, the more you introduce steam voids, the faster the reaction goes. This has been largely addressed since the Chernobyl accident by the Russians. Still, it doesn’t take a rocket scientist to understand that you don’t want such a situation because of the positive feedback. I’m not saying it happens all the time, but in certain power ranges it’s true. This isn’t possible in a Light Water Reactor.Enjoying this article? Click here to subscribe for full access. Just $5 a month.
So all those things together are a rough summary of how different they are in terms of structure. That leads to the second thing I mentioned, which are the causes of the accidents.
At Fukushima, the earthquake didn’t cause the accident, but caused the reactors to automatically shutdown. The systems worked—they went subcritical. The problem was that the ensuing tsunami devastated some equipment on the outside of the plant that was supposed to ensure continued core cooling. Unfortunately, that equipment got damaged and that’s why in the past week the Japanese have been pulling power lines to the reactors and restarting those things.
At Chernobyl, what caused the accident was 1) what I just mentioned before, the kind of inherent flaws in the reactor design and 2) external production pressures to conduct what was ironically a safety experiment. The accident occurred on April 26, 1986, just a few days before May Day, which in the former Soviet Union was a very big national holiday. And they were being pressured to produce as much electricity as possible so they could get their bonuses, and also this safety experiment so they could get their bonuses. In fact, the dispatcher in Kiev, about a day before the accident, ordered the reactor crew not to go down on power as they needed another nine hours of production.