How Nuclear Power Works

How Nuclear Power Works

Nuclear power plants provide about 17 percent of the world's electricity. Some countries depend more on nuclear power for electricity than others. In France, for instance, about 75 percent of the electricity is generated from nuclear power, according to the International Atomic Energy Agency. In the United States, nuclear power supplies about 15 percent of the electricity overall, but some states get more power from nuclear plants than others. There are more than 400 nuclear power plants around the world, with more than 100 in the United States.

The dome-shaped containment building at the Shearon Harris Nuclear Power Plant near Raleigh, NC

Have you ever wondered how a nuclear power plant works or how safe nuclear power is? In this article, we will examine how a nuclear reactor and a power plant work. We'll explain nuclear fission and give you a view inside a nuclear reactor.

Uranium

Uranium is a fairly common element on Earth, incorporated into the planet during the planet's formation. Uranium is originally formed in stars. Old stars exploded, and the dust from these shattered stars aggregated together to form our planet. Uranium-238 (U-238) has an extremely long half-life> (4.5 billion years), and therefore is still present in fairly large quantities. U-238 makes up 99 percent of the uranium on the planet. U-235 makes up about 0.7 percent of the remaining uranium found naturally, while U-234 is even more rare and is formed by the decay of U-238. (Uranium-238 goes through many stages or alpha and beta decay to form a stable isotope of lead, and U-234 is one link in that chain.)

Uranium-235 has an interesting property that makes it useful for both nuclear power production and for nuclear bomb production. U-235 decays naturally, just as U-238 does, by alpha radiation. U-235 also undergoes spontaneous fission a small percentage of the time. However, U-235 is one of the few materials that can undergo induced fission. If a free neutron runs into a U-235 nucleus, the nucleus will absorb the neutron without hesitation, become unstable and split immediately. See How Nuclear Radiation Works for complete details.

Nuclear Fission

The animation below shows a uranium-235 nucleus with a neutron approaching from the top. As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom happens to split). The two new atoms then emit gamma radiation as they settle into their new states. There are three things about this induced fission process that make it especially interesting:

* The probability of a U-235 atom capturing a neutron as it passes by is fairly high. In a reactor working properly (known as the critical state), one neutron ejected from each fission causes another fission to occur.

* The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (1x10-12 seconds).

* An incredible amount of energy is released, in the form of heat and gamma radiation, when a single atom splits. The two atoms that result from the fission later release beta radiation and gamma radiation of their own as well. The energy released by a single fission comes from the fact that the fission products and the neutrons, together, weigh less than the original U-235 atom. The difference in weight is converted directly to energy at a rate governed by the equation E = mc2.

Something on the order of 200 MeV (million electron volts) is released by the decay of one U-235 atom (if you would like to convert that into something useful, consider that 1 eV is equal to 1.602 x 10-12 ergs, 1 x 107 ergs is equal to 1 joule, 1 joule equals 1 watt-second, and 1 BTU equals 1,055 joules). That may not seem like much, but there are a lot of uranium atoms in a pound of uranium. So many, in fact, that a pound of highly enriched uranium as used to power a nuclear submarine or nuclear aircraft carrier is equal to something on the order of a million gallons of gasoline. When you consider that a pound of uranium is smaller than a baseball, and a million gallons of gasoline would fill a cube 50 feet per side (50 feet is as tall as a five-story building), you can get an idea of the amount of energy available in just a little bit of U-235.

In order for these properties of U-235 to work, a sample of uranium must be enriched so that it contains 2 percent to 3 percent or more of uranium-235. Three-percent enrichment is sufficient for use in a civilian nuclear reactor used for power generation. Weapons-grade