THE SCIENCE BEHIND DISASTER: UNDERSTANDING CHERNOBYL

In April 1986, reactor No. 4 of the Vladimir I. Lenin nuclear power plant had a bit of a hiccup. A few things went wrong, a few people made a few mistakes. All in all, a minor hiccup. 

At least that is how the Soviet Union would want you to remember what we now call the Chernobyl disaster. 

The reality of the situation is that it was a money, political human and scientific problem. So what happened? Not as far as politics and humans are concerned, but regarding the science? 

The first thing that went wrong was the way the reactor was built. Its design was called the RBMK-1000. It was a water-cooled, graphite- moderated reactor. That means, it was cooled by water, so think of the water as brakes. But also, moderated by graphite, although “moderate” does not have the same meaning when discussing reactors. Moderator means gas pedal in reactor-speak. This gave the reactor something called positive void coefficient, or positive feedback loop. So as the reaction gets more and more violent, the reaction speeds up.

This can cause things to get ugly very quickly if something were to go wrong. But there was a second brake: the control rods. They were made of boron, which can slow down or slam the brakes on a reaction. But they were tipped with graphite, which hits the gas pedal.

This is often used as a way to rail on the Soviets, saying, “Why would you make part of your brakes your accelerator!?”

BEST INTENTIONS

On the surface, it seems insane, but those tips actually were part of how the reactor works. It made the reaction more controllable because if it they weren’t there and the boron rods were pulled out of the reactor, there would just be water. Which can act like a gas pedal, but a very bad one. The reaction would stop if the control rods were pulled out, which also is a bit like hitting the gas. This is okay; it is not as safe as most reactors, but it works and it is powerful – but most important, it is cheap.

The one thing you can never, ever do is pull all of the control rods out and then put them back in, all at once. But that is silly! That would never happen. Right?

Wrong. The second thing that went wrong began with, ironically, a safety test. The reactor had to be powered down for the day. Not a problem until you understand a little bit about nuclear chemistry. In a nuclear fission reaction such as this, uranium-235 is hit with neutrons, which causes it to split. The output can become a few different things, but the most important of them in this case was xenon. Xenon, like boron, absorbs neutrons and slows a reaction. The only way to get rid of it is to burn it off. But when it builds up too much, the reactor is poisoned. If that happens, then you need to run the reactor at low power for a few days and wait for it to burn off.

Well, needless to say, the power plant officials didn’t. Which brings us to problem No. 3, the night of the Chernobyl disaster. The engineers would continue with the safety test needed to power the reactor down even more. Since there was xenon present, the reactor started to power down too much, until it was almost in cold shutdown.

The reactor technicians, reasonably, were scared by this, because while they wanted to operate at low power, the reactor was uncontrollably plummeting each second.

So they began pulling control rods, the reactor continued to drop in power, they pulled more and more, until only a tiny fraction of the rods were even in the reactor. Only at this point was the reactor beginning to increase in power. The workers thought the crisis was averted, when in reality they had simply burned off the xenon. Now, all of the control rods were out, and once the xenon burned off, power began to increase again, uncontrollably.

DEADLY SURGE

The positive void coefficient began to kick in and steam started to form in the reactor, forcing the release of radioactive steam. As the operators realized how dangerous the situation had become, they hit the emergency scram button, which throws all rods into the reactor at once. They thought it would be safe, but as the graphite entered the core, Chernobyl’s output, designed for 3,200 megawatts of power, soared past 33,000. 

The worst environmental disaster in modern history was about to unfold. The first explosion came as steam blew the top off the reactor. Chernobyl was now a nuclear bomb. The second came as oxygen in the air reacted with radioactive uranium, and blew the top off the building. What followed (aside from many deaths) was a cleanup that lasts to this day. Chernobyl and the surrounding areas are not safe to live in and will not be for centuries. 

But, there’s a lesson to be learned. Chernobyl was built cheaply, with few resources, and was dangerous from the start. Reactors built here in the United States are miles safer. We build reactors with a negative void coefficient, containment buildings and we go the whole nine yards in building safe nuclear facilities.

Chernobyl was a lesson in reactor safety, and Chernobyl taught us what not to do to keep the power of the atom at bay.