Wednesday, March 16, 2011

How a Nuclear Power Plant Works

I am fairly certain that the average person knows next to nothing about how nuclear power plants generate electricity. I would also hazard to guess that the average person's research into the matter probably goes no further than the education that they have received watching Homer Simpson at work. That's okay. Until recently, that's about all the working knowledge I had of nuclear power as well. Today, I will give to you, free of charge, an elementary education on how nuclear power plants produce electricity, and shortly, why the plants in Japan are in such big trouble.

First off, nuclear power plants do not work by magic, and they are not powered by glowing goo. In reality, a nuclear power plant produces power by heating water and making steam. I know that is a little bit of a let down, but it is nevertheless true. Nuclear power plants are simply glorified coal plants. They make electricity the same way, only coal is replaced by uranium pellets.

So here is the very simplified, layman's description of how this works. Uranium pellets, which are generally no bigger than the width of a dime, are put into rods and bundled together. These uranium pellets get hot naturally by nuclear fission. By naturally, I mean that uranium naturally "fizzes" all the time. The atoms in it are unstable and fling off neurons, which strike other atoms of uranium which causes those atoms to split and fling off neurons. This process causes heat to be generated, a lot of heat. The difference between a nuclear power plant and an atom bomb is the amount of uranium in the pellets. Nuclear power plant pellets are enriched 2-3%. Bombs are enriched to 90%. In short, a nuclear power plant has less "neuron flinging" than a bomb. That's a good thing.

So theses uranium rods are inserted into water, they get really, really hot, and they make steam. The heat they make is monitored, and if they get too hot, another kind of rod can be inserted into the mix that catches flung neurons. This rod is called a "control rod" and is usually made of zirconium. (The same stuff a fake diamond is made out of.) This control rod serves to cool the uranium's heat generated by slowing the process of nuclear fission. If flung neurons are like little bullets, then the control rod is like a bullet proof vest over the uranium. If the uranium starts getting too cool, the control rod can be lifted. So these rods, combined with the liquid that the uranium rods are in, serve to cool the reactor core down. The control rods, however, do not really "cool" the uranium, they only serve to slow down the reaction. This does lead to cooling the reaction chamber, but it doesn't take away the heat already present.

Simply put, a nuclear power plant works by putting hot uranium into water, which then evaporates and pushes a turbine. The steam cools, turns back into water, and then is re-introduced to hot uranium. Neat.

So why is it that the Japanese facility keeps blowing up? The answer is that uranium gets hot. Really, really hot. After the earthquake/tsunami, all power was lost at the plant, both regular power and back-up power. Despite the fact that the control rods were inserted as a precaution during the earthquake, there was no power left to pump in cool water. The control rods do slow down the reaction, but they cannot vent the heat by themselves. This heat will eventually damage the control rods themselves, which means the nuclear fission will heat up again, and then all bets are off. To compound this problem, as the control rods fail, the water that is present gets so hot that they start popping off hydrogen atoms. This turns water into hydrogen, and hydrogen has a tendency to blow up. (Oh, the humanity!) If it blows up the reactor, then all that radioactive material goes up with it. Not good.

So the Japanese are pumping salt water into the reactors now to try and cool the uranium. Let's hope it works.


Anand said...

Thank you, I got little more details about control rod. What i am not aware or confirmed is the Uranium fizzle you say here. My question is does uranium naturally break down, i mean gets hot on it's own. I see picture of Uranium Pellet in hand. Does it require first source to start the chain and then it carries by itself ? Is there no known chemical or compound which reacts with Uranium/Plutonium to alter their radio active properties ?

Brad Williams said...

Hi Anand, thanks for the comment.

I am not an expert by any means on nuclear fission, but I will give my best shot at answering your question. Uranium does break down naturally. However, plain uranium does not break down at a rate fast enough to generate heat. In order to get the required reaction, the uranium pellet has to be "enriched" 2-3%.

This article on wikipedia should help you get a better grasp on how it works:

Anonymous said...

Why the steam was let out of the reactor? Suppose the steam is used for turbine to generate electricity.

Drew said...

Where did you learn about all this? You have a curious mind lol.

Chrissy said...

Love your post. I teach this topic in my courses for future science teachers and they are always amazed. Thanks for helping contribute to scientific literacy P.S.: it's neutrons, not neurons! Neurons are the cells in your brain!