How Nuclear Power Works


Nuclear Fission: The Heart of the Reactor

Despite all the cosmic energy that the word "nuclear" invokes, power plants that depend on atomic energy don't operate that differently from a typical coal-burning power plant. Both heat water into pressurized steam, which drives a turbine generator. The key difference between the two plants is the method of heating the water [source: Mnsu.edu].

While coal-powered plants burn fossil fuels, nuclear-powered plants depend on the heat that occurs during nuclear fission, when one atom splits into two and releases energy. Nuclear fission happens naturally every day. Uranium, for example, constantly undergoes spontaneous fission at a very slow rate. This is why the element emits radiation, and why it's a natural choice for the induced fission that nuclear power plants require [source: World-nuclear.org].

Uranium is a common element on Earth and has existed since the planet formed. While there are several varieties of uranium, uranium-235 (U-235) is the one most important to the production of both nuclear power and nuclear bombs.

U-235 decays naturally by alpha radiation: It throws off an alpha particle, or two neutrons and two protons bound together. It's also one of the few elements that can undergo induced fission. Fire a free neutron into a U-235 nucleus and the nucleus will absorb the neutron, become unstable and split immediately.

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The animation above 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 splits). The process of capturing the neutron and splitting happens very quickly.

The decay of a single U-235 atom releases approximately 200 MeV (million electron volts). That may not seem like much, but there are lots of uranium atoms in a pound (0.45 kilogram) of uranium [source: World-nuclear.org]

The splitting of an atom releases heat and gamma radiation, or radiation made of high-energy photons. The two atoms that result from the fission later release beta radiation (superfast electrons) and gamma radiation of their own, too [source: World-nuclear.org].

But for all of this to work, scientists have to first enrich a sample of uranium so that it contains 2 to 3 percent more U-235 [source: World-nuclear.org]. Three percent enrichment is sufficient for nuclear power plants, but weapons-grade uranium is composed of at least 90 percent U-235. The process of enriching uranium is done via a centrifuge after a gas has been created from the uranium. The force of the centrifuge separates the U-235 atoms from the U-238 atoms. At first, there is only a slight increase in the concentration of U-235 atoms, so the process has to be repeated several times in the centrifuge to increase the enrichment. Making weapons-grade uranium is very difficult and expensive, which is one reason so few countries have nuclear weapons. But these barriers are not insurmountable [source: Zielinski].

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