Nuclear Power in the Present and Future

During the last century, nuclear power has been established as a reliable source of energy in the major industrialized countries. Nuclear power plants provide about 17 percent of the world's electricity. In the United States, nuclear power supplies about 15 percent of the electricity overall. Although no new plants are scheduled to be built in the United States, nuclear power is growing to be a popular producer of power. It has recently enjoyed a revival in attention and research due to the environmental concerns surrounding current conventional energy sources. Issues of regulation and safety are at the forefront of all discussions involving nuclear power. (Lillington) One of the major concerns is the radioactive waste that is produced during the fission of uranium.

Uranium is an element that was integrated into the planet during the Earth's formation from the dust of shattered stars. It was discovered by Martin Heinrich Klaproth in 1789 and although Klaproth thought the compound he extracted was pure uranium, it was actually uranium dioxide. Today, uranium is obtained from uranium ores such as pitchblende, uraninite, carnotite, and autunite. It can also be found in phosphate rock, lignite (brown coal) and monazite sand. There three different types of isotopes that can be found: uranium-234, uranium-235 and uranium-238. All three isotopes are radioactive, but uranium-235 is the only fissionable isotope that can be used for nuclear power. (Gagnon)

Uranium-235 makes up about 0.7 percent of the uranium that can be found naturally. It can be used for both nuclear power production and for nuclear bomb production. Uranium-235 decays naturally by alpha radiation and undergoes spontaneous fission a small percentage of the time. But it is its ability to undergo induced fission that makes it a good compound for use in nuclear power. That means if a free neutron runs into a uranium-235 nucleus, the nucleus would absorb the neutron without hesitation, become unstable and split immediately.

When the nucleus splits other atoms form and two or three new neutrons are thrown off. The two new atoms then emit gamma radiation as they settle into their new states. The probability of a uranium-235 atom capturing a neutron as it passes by is fairly high. In a reactor one neutron, ejected from each fission, causes another fission to occur. This process of capturing the neutron and splitting the nucleus happens in a matter of picoseconds.

The energy released when a single atom splits is massive and gives off heat and gamma radiation. The two atoms that result from the fission later release beta radiation and gamma radiation of their own as well. 3.204 x 10-11 joules of energy is released from the decay of one uranium-235 atom which may not seem like much, but there are a lot of uranium atoms in a pound of uranium. So many that a pound of highly enriched uranium can be used to power a nuclear submarine or nuclear aircraft carrier and is equal to about a million gallons of gasoline. (Ong) Enriched uranium contains 2-3 percent of uranium-235, this enrichment is sufficient for use in a civilian nuclear reactor. Highly enriched weapons-grade uranium is composed of 90 percent or more uranium-235.

For a nuclear reactor you need mildly enriched uranium. This is typically formed into pellets, approximately the same diameter as a dime and an inch in length. These pellets are arranged into long rods, and then collected together into bundles. The bundles are submerged in water inside a pressure vessel, the water acting as a coolant. To prevent the uranium from overheating and melting control rods, made out of a material that absorbs neutrons, are inserted into the bundle.

A mechanism attached to the rods, allowing the operators to raise and lower the control rods, controlling

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. If less heat is wanted the rods are lowered into the center of the uranium bundle. To shut the reactor down the rods can be lowered completely, this is done in the event an accident or to change the fuel.

The uranium core is a high-energy heat source that is then used to turn water it to steam to turn a turbine. The turbine spins a generator, turning the original heat energy in to usable electricity. In some reactors the steam also goes through a secondary heat exchanger to convert more of the water to steam. The advantage to this design is that the radioactive water/steam never contacts the turbine. Carbon dioxide or liquid metal can also be used as the coolant fluid, which is in contact with the reactor core, allowing the core to be operated at higher temperatures. (Gonyeau)

After about 18 months in a reactor, fission begins to slow down, and the uranium rods must be replaced. It takes