Nuclear power plants

Last updated: September 22, 2009.

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.

Photo: Nuclear energy—the past or the future? Sleek modern solar panels in the foreground with the now-decommissioned Rancho Seco nuclear plant, Sacramento, California, right behind them. Will nuclear energy tide us over until we can convert the world to renewable energy? Or is it an expensive distraction? Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What is atomic energy?

Photo: Atoms have energy. Picture courtesy of Lawrence Livermore National Laboratory and US Department of 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. It also involves radioactive decay (the conversion of unstable, radioactive isotopes into stable atoms that aren't radioactive).

Photo: Carefully controlled: Before it was closed in the 1970s, NASA's scientific nuclear reactor at Plum Brook Station in Sandusky Ohio was used for developing materials for the space program. The site now does other kinds of cutting-edge space research. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

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, 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 you'd get if you turned a kilogram of mass into pure energy. 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.

Photo: Albert Einstein—godfather of nuclear energy. Photo courtesy of US Library of Congress.

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 omelette), 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.

Photo: Chain reaction! Fire a neutron (1) at a large uranium-235 atom (2). You make an even larger, unstable radioactive isotope of uranium, uranium-236, that promptly splits into two smaller and more stable atoms krypton and barium (3). In the process, heat energy is released and there are two spare neutrons left over (4). The neutrons can go on to react with two more uranium-235 atoms (5) in a hugely energetic chain reaction.

In a nuclear bomb, the chain reaction isn't controlled, and that's what makes nuclear weapons so terrifyingly destructive. In nuclear power plants, however, the reactions can be "switched" on or off relatively easily, so nuclear energy can be produced in a carefully controlled way.

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. You can explore the nuclear issue and people's widely differing views on our page of nuclear power links.

Photo: Nuclear nightmare: In the days following the Chernobyl nuclear power explosion in the Ukraine in 1986, a cloud of radioactive "fallout" spread throughout Europe. In this sequence of pictures, you can see the cloud (the pink area) on day 2, day 6, and day 10 after the accident. Pictures by Lawrence Livermore National Laboratory courtesy of US Department of Energy.