As mentioned on the previous page, nuclear power is power that is generated by a nuclear reactor and is produced by a chain reaction of nuclear fission (or nuclear fusion). Nuclear fission is a process where an atom, generally an isotope of uranium, is split by a neutron into two lighter atoms and emits both heat and radiation, in addition to ejecting two or three new neutrons, which in turn split other uranium atoms causing a chain reaction. Nuclear fusion, on the other hand, is a process where two atoms are forcibly joined, usually isotopes of hydrogen, using large amounts of energy that in turn produces a helium atom plus a neutron plus energy that is absorbed or release (which is considerably larger than the amount used to join the atoms together).
A Nuclear Fission Chain Reaction:
The picture above was taken from the following site: NRC: Chain Reaction
Nuclear Reactors
(The image above is from Energy Kid's Page, for a better picture of a nuclear reactor go to New Physics at Work)
Nuclear power plants operate the same way as plants that operate using fossil fuels (oil, natural gas, coal, etc), the only difference is what produces the heat. As mentioned earlier nuclear power plant's heat is produced by the nuclear fission chain reaction. The heat released then boils the water and which then makes steam that turns the turbine-generator to give electricity. The heat is produced in the nuclear reactor, or rather the reactor core.
Nuclear reactors are the machines that control and contain the nuclear fission chain reaction as well as releasing the heat at a controlled rate. The reactor core is made up four main parts: the fuel, the control rods, the coolant, and the moderator. The fuel consists of pellets of enriched uranium dioxide encased in rods called fuel rods (this is the stuff that is needed to start the whole process). The control rods have a type of material that regulates the rate of the chain reaction (so when they are in place, the reaction slows and when they are removed the reaction speeds up). The coolant is generally water and is pumped into the reactor to carry away the heat that is produced in the reactor (this process is similar to the water in the a car's cooling system) . The moderator which is also generally water helps slow down the speed the atoms travel, which is important since the slower the neutrons travel the more likely they will be captured by the uranium nucleus and then continue the nuclear fission process.
So what kind of reactors are there?
While there are many different kinds of reactors with different designs, the two most common in the United States are the Boiling Water Reactor (BWR) and the Pressurized Water Reactor (PWR).
Boiling Water Reactors
Boiling Water Reactors (BWR) are the second most common type of reactors in the US. This type of reactor operates the same way a fossil fuel reactor does and does not have a steam generator. There are two different designs for the BWR, the dual cycle and the single cycle, where there are two loops for the coolant in the dual cycle and only one in the single cycle. Since it doesn't have a steam generator, the water in the reactor boils inside a pressure vessel and the steam is produced when the reactor coolant (pure water) moves upward through the reactor core to absorb the heat. When that happens, steam is produced which then goes to the turbine generator to turn the turbines for the single loop BWR. In the dual loop reactor, the steam is sent to the steam generator where the heat is used to produce steam and then that is sent to the turbines.
Pressurized Water Reactor
The Pressurized Water Reactor is similar in design with the dual cycle of the Boiling Water Reactor, and the only difference is the extent of high pressure in the PWR. The setup is very similar to a pressurized cooker, and as the water can't escape the pressure in the cooker increases, this is the same format for the PWR. As the pressure increases, so does the temperature of the water in the pot. In the PWR plant, a pressurizer unit keeps the water that is flowing through the reactor vessel under very high pressure to prevent it from boiling. The hot water then flows into the steam generator where it is converted to steam. The steam passes through the turbine which produces electricity.
*To look at a BWR schematic, go here: NRC: BWR Schematic
*To look at a PWR schematic, go here: NRC: PWR Schematic
~Most of the information on this page was found on the following pages:
If you have any questions or concern, please email me at kjdesai@email.unc.edu
©2005