Nuclear power plants

\Atomic energy has had a mixed history in the half-century or so since the world’s first commercial nuclear power plant opened at Calder Hall (now Sellafield) in Cumbria, England in 1956. Huge amounts of world energy have been produced from atoms ever since, but amid enormous controversy. Some people believe nuclear power is a vital way to tackle climate change; others insist it is dirty, dangerous, uneconomic, and unnecessary. Either way, it helps if you understand what nuclear energy is and how it works—so let’s forget the politics for a moment and take a closer look at the science.

What is atomic energy?

It’s not immediately obvious but tall buildings store energy—potential energy. You have to work hard to lift bricks and other building materials up off the ground into the right position and, as long as they remain where you put them, they can store that energy indefinitely. But a tall, unstable building is bound to collapse sooner or later and, when it does so, the materials from which it was built come crashing back down to the ground, releasing their stored potential energy as heat, sound, and kinetic energy (the bricks could fall on your head!).

Atoms (the building blocks of matter) are much the same. Some large atoms are very stable and quite happy to stay as they are pretty much forever. But other atoms exist in unstable forms called radioactive isotopes. They’re the atomic equivalents of wobbly old buildings: sooner or later, they’re bound to fall apart, splitting into bits like a large building tumbling to the ground and releasing energy on the way. When large atoms split into one or more smaller atoms, giving off other particles and energy in the process, we call it nuclear fission. That’s because the central part of the atom (the nucleus) is what breaks up and fission is another word for splitting apart. Nuclear fission can happen spontaneously, in which we case we call it radioactive decay (the conversion of unstable, radioactive isotopes into stable atoms that aren’t radioactive). It can also be made to happen on demand—which is how we get energy out of atoms in nuclear power plants. That type of fission is called a nuclear reaction.

How much energy can one atom make?

A surprisingly large amount! That was what physicist Albert Einstein meant when he wrote out this simple and now famous equation:

E = mc2

If E is energy, m is mass (the scientific word for the ordinary stuff around us), and c is the speed of light, Einstein’s equation says that you can turn a tiny amount of mass into a huge amount of energy. How come? Looking at the math, c is a really huge number (300,000,000) so c2 is even bigger: 90,000,000,000,000,000. That’s how many joules (the standard measurement of energy) you’d get from a kilogram of mass. In theory, if you could turn about seven billion hydrogen atoms completely to energy, you’d get about one joule (that’s about as much energy as a 10-watt lightbulb consumes in a tenth of a second). Remember, though, these are just ballpark, guesstimate numbers. The only point we really need to note is this: since there are billions and billions of atoms in even a tiny spec of matter, it should be possible to make lots of energy from not very much at all. That’s the basic idea behind nuclear power.

In practice, nuclear power plants don’t work by obliterating atoms completely; instead, they split very large atoms into smaller, more tightly bound, more stable atoms. That releases energy in the process—energy we can harness. According to a basic rule of physics called the law of conservation of energy, the energy released in a nuclear fission reaction is equal to the total mass of the original atom (and all the energy holding it together) minus the total mass of the atoms it splits into (and all the energy holding them together). For a more detailed explanation of why nuclear reactions release energy, and how much they can release, see the article binding energy on Hyperphysics.

What is a chain reaction?

What if you could make lots of atoms split up one after another? In theory, you could get them to release a huge amount of energy. If breaking up billions of atoms sounds like a real bore (like breaking billions of eggs to make an omelet), there’s one more handy thing that helps: some radioactive isotopes will go on splitting themselves automatically in what’s called a chain reaction, producing power for pretty much as long as you want.

Suppose you take a really heavy atom—a stable kind of uranium called uranium-235. Each of its atoms has a nucleus with 92 protons and 143 neutrons. Fire a neutron at uranium-235 and you turn it into uranium-236: an unstable version of the same atom (a radioactive isotope of uranium) with 92 protons and 144 neutrons (remember that you fired an extra one in). Uranium-236 is too unstable to hang around for long so it splits apart into two much smaller atoms, barium and krypton, releasing quite a lot of energy and firing off two spare neutrons at the same time.

Now the brilliant thing is that the two neutrons can crash into two other uranium-235 atoms, making them split apart too. And when each of those atoms splits, it too will produce two neutrons. So a single fission of a single uranium-235 atom rapidly becomes a chain reaction—a runaway, nuclear avalanche that releases a huge amount of energy in the form of heat.

What’s the difference between a nuclear power plant and a nuclear bomb?

In a nuclear bomb, the chain reaction isn’t controlled, and that’s what makes nuclear weapons so terrifyingly destructive. The entire chain reaction happens in a fraction of a second, with one splitting atom producing two, four, eight, sixteen, and so on, releasing a massive amount of energy in the blink of an eye. In nuclear power plants, the chain reactions are very carefully controlled so they proceed at a relatively slow rate, just enough to sustain themselves, releasing energy very steadily over a period of many years or decades. There is no runaway, uncontrolled chain reaction in a nuclear power plant.

How does a nuclear power plant work?

Okay, we’ve figured how to get energy from an atom, but the energy we’ve got isn’t that helpful: it’s just a huge amount of heat! How do we turn that into something much more useful, namely electricity? A nuclear power plant works pretty much like a conventional power plant, but it produces heat energy from atoms rather than by burning coal, oil, gas, or another fuel. The heat it produces is used to boil water to make steam, which drives one or more giant steam turbines connected to generators—and those produce the electricity we’re after. Here’s how:

1. First, uranium fuel is loaded up into the reactor—a giant concrete dome that’s reinforced in case it explodes. In the heart of the reactor (the core), atoms split apart and release heat energy, producing neutrons and splitting other atoms in a carefully controlled nuclear reaction.
   
2. Control rods made of materials such as cadmium and boron can be raised or lowered into the reactor to soak up neutrons and slow down or speed up the chain reaction.
   
3. Water is pumped through the reactor to collect the heat energy that the chain reaction produces. It constantly flows around a closed loop linking the reactor with a heat exchanger.
   
4. Inside the heat exchanger, the water from the reactor gives up its energy to cooler water flowing in another closed loop, turning it into steam. Using two unconnected loops of water and the heat exchanger helps to keep water contaminated with radioactivity safely contained in one place and well away from most of the equipment in the plant.
   
5. The steam from the heat exchanger is piped to a turbine. As the steam blows past the turbine’s vanes, they spin around at high speed.
   
6. The spinning turbine is connected to an electricity generator and makes that spin too.
   
7. The generator produces electricity that flows out to the power grid—and to our homes, shops, offices, and factories.
   

Can a nuclear power plant explode like a nuclear bomb?

One reason many people oppose nuclear power is because they think nuclear plants are like enormous nuclear bombs, just waiting to explode and wipe out civilization. It’s true that nuclear plants and nuclear bombs are both based on nuclear reactions in which atoms split apart, but that’s generally where the similarity begins and ends.

To start with, very different grades of uranium are used in power plants and nuclear bombs (some bombs use plutonium, but that’s another story). Bombs need extremely pure (enriched) uranium-235, which is made by removing contaminants (notably another isotope of uranium, uranium-238) from naturally occurring uranium. Unless the contaminants are removed, they stop a nuclear chain reaction from occurring. Power plants can work with less purified, much more ordinary uranium providing they add another substance called a moderator. The moderator, typically made of carbon or water, effectively “converts” the less pure uranium so it will allow a chain reaction to happen. (I won’t go into the details here, but it works by slowing down neutrons so they are less readily absorbed by any uranium-238 impurities and have a greater chance of causing fission in the all-important uranium-235.) All we really need to know about the moderator is that it makes a chain reaction possible in relatively impure uranium—and without it the reaction stops.

So what happens if the reaction inside a power plant starts to run out of control? If that happens, so much energy is released that the reactor overheats and may even explode—but in a relatively small, entirely conventional explosion, not an apocalyptic nuclear bomb. In that situation, the moderator burns or melts, the reactor is destroyed, and the nuclear reaction stops; there is no runaway chain reaction. The worst situation is called a meltdown: the reactor melts into a liquid, producing a hot, radioactive glob that drops deep down into the ground, potentially contaminating water supplies.

There are various other important differences that stop nuclear power plants from turning into nuclear bombs. In particular, nuclear bombs have to be assembled in a very precise way and detonated so that they implode (pushing the nuclear material together so it reacts properly). These conditions don’t occur in a nuclear power plant.

A different kind of power plant called a fast-breeder reactor works a different way, producing its own plutonium fuel in a self-sustaining process. Its chain reaction is much closer to what happens in a nuclear bomb and it doesn’t work through a moderator. That’s why a fast-breeder reactor could, theoretically, run out of control and cause a nuclear explosion.

Nuclear power—good or bad?

There are plenty of people who support our use of nuclear power, and at least as many who oppose it. Supporters say it’s a less environmentally destructive way of producing electrical energy because, overall, it releases fewer greenhouse emissions (less carbon dioxide gas) than burning fuels such as coal, oil, and natural gas. But opponents are concerned about the dangerous, long-lasting waste that nuclear power stations make, the way nuclear-energy byproducts help people build nuclear bombs, and the risk of catastrophic nuclear accidents.