As some of you may know, I am currently taking a class called “Myths and Misconceptions about Nuclear Science.” It’s a great class. I’ve been learning a lot: about nuclear physics and radiation and nuclear power and how stupid people can really be.
So today, on that last point, I am going to talk a bit about Chernobyl.
Chernobyl was the worst nuclear power disaster in history. You can kind of get a feel for that from the above photo. It involved a nuclear fission reactor in the Soviet Ukraine, one of four reactors on the site. (The other three kept working just fine for years after the accident.) The accident spread radiation all over Europe, killed 31 people either directly in the accident or from acute radiation sickness after it, and to this day, no one can live in a 1,000-square mile “exclusion zone” around the reactor.
So what happened? What was wrong with the reactor that caused this horrible accident?
Obviously, all kinds of things were wrong with it later on, or we wouldn’t have photographs like the one above to show you all the devastation. But initially, the reactor was working just fine, like it was supposed to.
So, we have the same question again. What happened?
Well, on April 25th, 1986, the operators of the reactor started a test to see if they could make the reactor safer.
They wanted to find out if they could keep the electricity-generating turbines (in the upper right of the diagram above) going during shutdown, so they could keep the reactor core cool without having to use a generator or the like. (It’s very important to keep the core cool, even when the reactor isn’t running. You’ve heard of nuclear meltdowns? If the fuel gets too hot, it will melt through the floor of the reactor vessel, and sometimes through the building.) At Chernobyl, as in most reactors, the coolant was water, composed of hydrogen and oxygen.
Let me take a moment to explain some other components of a nuclear reactor. The most obvious necessity is fuel; usually, as at Chernobyl, this is uranium, a mixture of two different types, one of which splits more readily when slow neutrons are shot at it. In order to slow the neutrons down so you can keep a chain reaction going, you need a “moderator,” in this case, graphite. As we’ve already discussed, you need coolant to keep the core from overheating. Last but not least, you need a control system, usually rods made of neutron-absorbing material that can be pulled in and out of the core (see diagram). This helps keep the reaction in check.
Back at Chernobyl, the first thing the operators did for the test was to turn off the emergency core-cooling system, a violation of reactor operation guidelines. They then lowered the power of the reactor, but instead of holding it at the recommended level, they let it drop too much and started pulling out control rods to try to get the power back up. They turned on two extra water pumps for the test, then realized there was too much water in the reactor and reduced the flow, causing the reaction to increase. And when, ignoring safety system warnings, they pulled out too many control rods, the reaction rate skyrocketed (by a factor of 10,000 in 5 seconds), the water in the reactor flashed to steam, the reactor exploded twice, and the graphite lit on fire for a couple weeks, ultimately spreading radiation across Europe.
Clearly, this accident was caused largely by human stupidity (don’t turn off the safety systems in a nuclear reactor!), but the reactor’s design played a role as well. The RBMK was a cheaper style of reactor, so the containment building (which in U.S. reactors is often feet-thick concrete that can withstand missile blasts) was not adequate to contain the initial boiler explosion and the hydrogen explosion that followed. Because of the design of the reactor with graphite as the moderator and water as the coolant, instead of water being both moderator and coolant as in other reactors, removing water increased the reaction rate rather than decreasing it. In addition, without graphite, there would have been less chance of a fire and thus less radiation spread.
In sum, Chernobyl, the worst nuclear power accident ever, was caused by a combination of human error and design flaws, but mostly by human error, since humans made the faulty containment and graphite-moderated design that contributed to the severity of the accident. Further, the reactor was working fine before the operators turned off and ignored various safety systems. Does this make nuclear power unsafe? The answer is complicated. Nothing is perfectly safe; humans can make grave errors with any power system. And in fact, chemical accidents have caused more deaths even than this worst nuclear accident. I think what we can learn from Chernobyl is that we need to be smart about safety: use the best designs, don’t skimp on safety systems, and never, ever turn them off.
If you’re interested in learning more about nuclear science and technology, you can check out Nuclear Choices: A Citizen’s Guide to Nuclear Technology by Richard Wolfson. Although it’s a bit dated (it was published when the USSR was still a country), it is easily readable for non-physicists and deals with many aspects of nuclear technology in an unbiased way. It is my textbook for my nuclear science class and my main source for this blog post.
What do you think of Chernobyl? Have you heard much about it before? Are you surprised at how much of a role human error played? How about design flaws? Do you have any questions? What do you think of nuclear power? Tell me in the comments!