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How Nuclear Power Works
by Marshall Brain

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.

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

The dome-shaped containment building at the Shearon Harris Nuclear Power Plant near Raleigh, N.C., dwarfs a large truck parked nearby.

Nuclear Fission
Uranium is a fairly common element on Earth and was incorporated into the planet during the planet's formation. Uranium is originally formed in stars. Old stars explode, and the dust from these shattered stars aggregated together to form our planet. Uranium-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. Uranium-235 makes up about 0.7 percent of the remaining uranium found naturally, while uranium-234 is even more rare and is formed by the decay of uranium-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.

Figure 1

Figure 1 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 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 (1 x 10-12 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 two to three percent or more of uranium-235. Three percent enrichment is sufficient for use in a civilian nuclear reactor used for power generation. Weapons-grade uranium is composed of 90 percent or more U-235.

Inside a Nuclear Power Plant
To build a nuclear reactor, what you need is some mildly enriched uranium. Typically, the uranium is formed into pellets with approximately the same diameter as a dime and a length of an inch or so. The pellets are arranged into long rods, and the rods are collected together into bundles. The bundles are then typically submerged in water inside a pressure vessel. The water acts as a coolant. In order for the reactor to work, the bundle, submerged in water, must be slightly supercritical. That would mean that, left to its own devices, the uranium would eventually overheat and melt.

To prevent this, control rods made of a material that absorbs neutrons are inserted into the bundle using a mechanism that can raise or lower the control rods. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the rods are raised out of the uranium bundle. To create less heat, the rods are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.

The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a steam turbine, which spins a generator to produce power. In some reactors, the steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated at higher temperatures. Once you get past the reactor itself, there is very little difference between a nuclear power plant and a coal-fired or oil-fired power plant except for the source of the heat used to create steam.

Electricity for homes and businesses comes from this generator at the Shearon Harris plant. It produces 870 megawatts.

Pipes carry steam to power the generator at the power plant.

The reactor's pressure vessel is typically housed inside a concrete liner that acts as a radiation shield. That liner is housed within a much larger steel containment vessel. This vessel contains the reactor core as well the hardware (cranes, etc.) that allows workers at the plant to refuel and maintain the reactor. The steel containment vessel is intended to prevent leakage of any radioactive gases or fluids from the plant. Finally, the containment vessel is protected by an outer concrete building that is strong enough to survive such things as crashing jet airliners. These secondary containment structures are necessary to prevent the escape of radiation/radioactive steam in the event of an accident such as that at Three Mile Island. The absence of secondary containment structures in Russian nuclear power plants allowed radioactive material to escape in an accident at Chernobyl.

Steam rises from the cooling tower at the Harris plant.

Workers in the control room at the nuclear power plant can keep an eye on the nuclear reactor and take action if something goes wrong.

Uranium-235 is not the only possible fuel for a power plant. Another fissionable material is plutonium-239. Plutonium-239 can be created easily by bombarding U-238 with neutrons -- something that happens all the time in a nuclear reactor.

Subcriticality, Criticality and Supercriticality
When a U-235 atom splits, it gives off two or three neutrons (depending on the way the atom splits). If there are no other U-235 atoms in the area, then those free neutrons fly off into space as neutron rays. If the U-235 atom is part of a mass of uranium so that there are other U-235 atoms nearby, then one of three things happens:
  • If, on average, exactly one of the free neutrons from each fission hits another U-235 nucleus and causes it to split, then the mass of uranium is said to be critical. The mass will exist at a stable temperature. A nuclear reactor must be maintained in a critical state.
  • If, on average, less than one of the free neutrons hits another U-235 atom, then the mass is subcritical. Eventually, induced fission will end in the mass.
  • If, on average, more than one of the free neutrons hits another U-235 atom, then the mass is supercritical. It will heat up. For a nuclear bomb, the bomb's designer wants the mass of uranium to be very supercritical so that all of the U-235 atoms in the mass split in a microsecond. In a nuclear reactor, the reactor core needs to be slightly supercritical so that plant operators can raise and lower the temperature of the reactor. The control rods give the operators a way to absorb free neutrons so that the reactor can be maintained at a critical level.

The amount of uranium-235 in the mass (the level of enrichment) and the shape of the mass control the criticality of the sample. You can imagine that if the shape of the mass is a very thin sheet, most of the free neutrons will fly off into space rather than hitting other U-235 atoms. A sphere is the optimal shape. The amount of uranium-235 that you must collect together in a sphere to get a critical reaction is about two pounds. This amount is therefore referred to as the critical mass. For plutonium-239 the critical mass is about 10 ounces.

What Can Go Wrong
Well-constructed nuclear power plants have an important advantage when it comes to electrical power generation -- they are extremely clean. Compared with a coal-fired power plant, nuclear power plants are a dream come true from an environmental standpoint. A coal-fired power plant actually releases more radioactivity into the atmosphere than a properly-functioning nuclear power plant. Coal-fired plants also release tons of carbon, sulfur and other elements into the atmosphere (see Question 481 for details).

Unfortunately, there are significant problems with nuclear power plants:

  • Mining and purifying uranium has not, historically, been a very clean process
  • Improperly-functioning nuclear power plants can create big problems. The Chernobyl disaster is the best recent example. Chernobyl was poorly designed and improperly operated, but it dramatically shows the worst-case scenario. Chernobyl scattered tons of radioactive dust into the atmosphere.
  • Spent fuel from nuclear power plants is toxic for centuries and, as yet, there is no safe permanent storage facility for it.
  • Transporting nuclear fuel to and from plants poses some risk, although to date, the safety record in the U.S. has been good.
These problems, at least in the U.S., have largely derailed the creation of new nulear power plants. Society seems to have decided that the risks outweigh the rewards.

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