Nuclear Reactors, the China Syndrome, and Waste Storage

© 2001 Richard A. Muller

 

Nuclear Reactors

A nuclear reactor is a device in which a "sustained" chain reaction takes place. It doesn't involve doubling; instead, from each fission, only one of the emitted neutrons (on average) hits another nucleus to cause another fission. It is as if every man and woman had, on the average, two children. Then the population would not grow. The power output is constant.

The power comes out in the form of heat, just as it does when burning coal or gasoline. Frequently the heat is used to boil water into steam. This steam is then used to run a turbine. (A turbine is really just a fan; as the steam expands through it, it makes the fan turn.)

Imagine this: a super high-tech nuclear submarine really just uses uranium to boil water!

For their fuel, reactors use primarily U-235, just as in a nuclear bomb. But the uranium is not enriched to bomb quality. Recall that natural uranium has only 0.7% U-235; the rest is U-238. For use in a bomb, the U-235 has to be enriched to about 80%. But for a nuclear reactor, it only has to be enriched to about 3%. (An exception is the Canadian reactor, called "Candu". We'll discuss this in a moment.)

Why can a reactor use less enriched fuel? There are two reasons: the first is that they don't require that both neutrons hit U-235; only one. So if one of the two neutrons is absorbed, that's ok -- for the reactor, not the bomb. Having a lot of U-238 around isn't so bad.

But there is a more important reason: a nuclear reactor uses a "moderator." A moderator is a chemical mixed in with the fuel that tends to slow down the neutrons without absorbing them. The most popular moderators are ordinary water, H20, heavy water: D2O (the D stands for deuterium, which you recall is a hydrogen atom that has both a neutron and a proton in the nucleus), and graphite (which is nearly pure carbon). The moderators consist of nuclei which are light and don't absorb neutrons. The neutrons hit the moderator, and bounce off, but in the process they lose a little energy. After enough such bounces, the neutrons are no faster than expected from their temperature. They are called "thermal neutrons" to reflect the fact that they have slowed down to thermal velocities.

In summary: Fast neutrons bounce off the moderator and become thermal (slow) neutrons.

The slow neutrons are far more likely to be absorbed on U-235. The reason is simple: a slowly moving neutron feels the nuclear force for a longer time, and is more readily deflected towards the U-235 nucleus. So having slow neutrons means you can use 3% U-235 instead of 80%.

In Canada, they use D2O. This is more expensive, but even more effective in slowing neutrons. As a result, they can use natural unenriched uranium, which is only 0.7% U-235.

Can a reactor turn into an atomic bomb?

No. The real reason is that a reactor depends on slow neutrons. If the chain reaction begins to run away (because the number of absorbed neutrons in each generation becomes greater than 1) then the fuel heats up. Pretty soon it is hot enough to explode. This will happen as soon as the fuel is a few thousand degrees. That will blow up the reactor, but the energy released will be about the same that you would get from TNT. It's an explosion, but it is a million times smaller than an nuclear bomb.

In the atomic bomb, they had to use fast neutrons (not moderated) in order to have the entire 80 generations over with before the bomb blew itself apart. After 80 generations, the temperature was many millions of degrees. The only reason is hasn't yet blown apart is that there wasn't enough time! With moderated neutrons, the chain reaction is much slower, since the neutrons are slower.

Nuclear waste

As the uranium fuel fissions, the reactor gradually fills up with the fission fragments. Most of these are radioactive. They are the same particles that caused radioactive fallout. Some of them have half-lives of a few seconds. Some have half lives of years. We already discussed Strontium-90, which makes up 5% of the fission fragments, and has a half-life of 28 years.

Suppose we "turn off" the chain reaction. We can do this by removing the moderator, or by putting in special "control rods" that absorb neutrons. Then the reactor will still produce heat from the radioactive decay of the remaining fission fragments. So the reactor continues to produce power, although the power level continues to decrease.

The following interesting plot shows this decrease. It was made specifically to address the dangers of the lingering radioactivity. So the vertical axis shows the deaths that would occur this radiation were all put inside people. Obviously that can't happen, but the plot is interesting anyway.

It is interesting to study this. Look at the dark line that lies on top of all the others. Notice the scale on both axes is not linear; each tick mark increases the amount by a factor of ten. In the units of this plot, right after the material is removed from the reactor there is enough radioactivity to kill between 10 and 100 million people. (Let's take the value to be 50 million.) Check: can you see that on the plot?

One hundred years later, there is still radioactivity. It has dropped to a level of about 1 million. In other words, it has decreased from 50 to 1, i.e. it is down by a factor of 50, to about 2% of its original level.

After 10,000 years, the radioactivity has dropped to about 0.1 million. At this point it is no more radioactive then the original uranium that was taken out of the ground to produce it. So the net effect, if this radioactivity is buried, is that the average radioactivity of the ground has been reduced by the "burning of uranium."

The China Syndrome

The term "China Syndrome" was originally invented by someone with a strange sense of humor to describe the worst possible nuclear reactor accident. (Most people seem to think there is something worse: a reactor becoming a nuclear bomb. But, as I described above, that is not possible because the uranium is not sufficiently enriched.)

In the China Syndrome, the water that is usually being boiled by the chain reaction, suddenly leaks away. There is no water to boil. What would happen in this "loss of coolant" accident? Can you guess?

The first thing is surprising to most people: the chain reaction stops. The reason is that the cooling water is also a moderator; it slows neutrons. So when the water is gone, the neutrons are not moderated. That means that most neutrons are absorbed on U-238, which does not give a chain reaction. So the chain reaction stopped.

Interesting flub by Senator: When the Chernobyl Nuclear Reactor underwent a similar accident, the Russians announced that the chain reaction had stopped. The chairman of the Senate Intelligence Committee announced on television that this was a "blatant lie." I cringed. He was confusing the chain reaction with the decay of the remaining fission fragments. He knew the radioactivity hadn't stopped, but didn't realize that the Soviet's were being completely honest. The fact that the chain reaction had stopped was important; it meant that the level of power being produced had dropped enormously. (Remember this, if you become a Senator!)

The chain reaction stops, but there is still the "waste heat" from the fission fragments. Without the cooling water, the reactor gets hotter and hotter. The fuel finally melts. It melts through its containers and forms a puddle at the bottom of the steel reactor vessel. The fuel puddle keeps on getting hotter and hotter. The steel reactor vessel melts. The fuel falls into the ground. It keeps on getting hotter. The soil and rock melts. The fuel just keeps on going -- all the way "to China".

No, obviously it won't reach China. (Besides, China isn't on the other side of the Earth.) It won't get too far, because it spreads out, and that allows it to cool. But in doing this, it has broken through the steel vessel that is supposed to keep it from the environment. Any gases that are in the fuel pellets will escape into the atmosphere. It is these gases (and some volatile elements, such as iodine) that caused the most damage at Chernobyl.

There is a huge amount of radioactivity in the reactor -- enough to kill 50 million people (if they ate it). Even a small amount leaked into the atmosphere can do enormous damage. As we stated in an earlier chapter, the number of expected deaths from Chernobyl is about 24,000. It is difficult to imagine a worse accident than the Chernobyl one, so the 24,000 is a much more reasonable estimate than the 50 million.

Very interesting discussion topic: 24,000 deaths is a pretty frightening number. Is nuclear power worth it? Why not just use something else, such as solar? Well -- people may not want to use solar until it is as cheap as oil. (That will happen sometime in your lifetime -- a Muller prediction.) So in the meantime, let's just use something safe: oil.

Is oil really safe? It pours lots of carbon dioxide into the atmosphere. The consequences of that are debated, but most people think the result will be serious global warming. How bad is that? How would you compare it to 24,000 deaths? Some people might argue that the Afghan war is one of the consequences of our use of oil. Why would we have bases in Saudi Arabia, if oil weren't so important to us?

Incidentally, the Chernobyl power plant had a terrible design. It didn't even have a containment building, like we have in the US. If it did, there may very well have been virtually no deaths. So is it fair to think of US Nuclear Power plants in terms of Chernobyl?

Other things are dangerous too. If you are unfamiliar with the tragedy of Bhopal, look it up on the web. In 1984, a gas leak from a chemical plant killed 5,000 people in the town of Bhopal in India. Some people have estimated that the total number of deaths from this accident will eventually reach 20,000.

Waste Storage

What should we do with the waste, the fission fragments left over from burning U-235? Some people say "bury it." Put it back into the ground. But what if it get into the ground water? Most people assume that that would be horribly bad. Therefore, they argue, it must be put in a very stable geologic mine, some place where it will undergo no disturbance for 10,000 years. Such a location has been prepared in Nevada, but opponents say that even this site can't be certified for 10,000 years. Who knows what kind of government we will have then!

Other people argue that the danger is greatly exaggerated. If all the radioactivity were released, in a fire, for example,then the number of deaths is about 24,000. (It happened -- Chernobyl. And these deaths aren't even observable, as we showed in the chapter on Radioactivity.) If instead, we bury the waste in the ground, it will easily be at least 24,000 times safer! So we don't even need a special location.

What do you think? Keep in mind that there is a real public fear of radioactivity that makes it very difficult to make a rational decision. Any governor who accepts radioactive waste storage in his state, is very likely to be challenged by people who feel that any level of radioactivity is too much. As a potential future president, how would you handle this? How can you balance the real risks vs. the perceived risks, and still be reelected?

Some people say, avoid all this complication! Just put the waste into rockets, and send it into the sun! But people who say this are ignoring the possibility of an accident. What is the probability that the rocket will fail, and fall back to earth, releasing all the radioactivity.

Is waste storage a technical problem? Many scientists think it is, and are trying to find a clever technical solution. But I think that the issue is dominated by the public perception problem. The politician must find a solution that seems safe even to those who are unaware of the natural radioactivity in the environment. It is a very tough political problem

Coal burning plants bury their waste in the ground. They are not radioactive, but the ashes are very high in carcinogens. What if these get into the ground water? How safe is coal, as an alternative to nuclear?