|Nuclear Power Plants
Nuclear power is a type of nuclear technology involving the controlled use of nuclear fission to release energy for work including propulsion, heat, and the generation of electricity. Nuclear energy is produced by a controlled nuclear chain reaction and creates heat—which is used to boil water, produce steam, and drive a steam turbine. The turbine can be used to generate electricity and for mechanical work also.
What is a Nuclear Power Plant?
A Nuclear Power Plant (also referred to as a nuclear generating station or nuclear power station) is an industrial facility for the generation of electric power.
A Nuclear Power plant functions as a nuclear energy center by converting other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy.
At the center of nearly all nuclear power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to.
Nuclear Power Plants
About 17 percent of the world’s electricity is provided by Nuclear power plants. 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.
Advantages of Nuclear Power Plants
-Nuclear power plants have a record of safety excellence that dates back to 1957 when the first commercial nuclear plant began operating.
-Using proven technology, nuclear plants have operated reliably to meet electricity needs.
-Uranium, the fuel for nuclear plants, is a relatively inexpensive fuel that is abundant throughout the world.
-Nuclear power produces no air pollutants.
Disadvantages of Nuclear Power Plants
-Poisonous waste is produced; some of which is highly radioactive.
Disposal of this radioactive waste has not been safely achieved.
-The power station is very expensive to build. When the costs are taken into account, the electricity produced by the power station is relatively expensive. Careless disposal of waste in the past has led to pollution of land, rivers and the ocean.
There are two types of nuclear energy:
1) Nuclear Fission
2) Nuclear Fusion
Nuclear Fission creates energy by splitting atoms while Nuclear Fusion creates energy by combining them. Nuclear Fusion is the safer option between the two but so far, the technology is still under research and development. It has been done in laboratories but not yet so in a larger scale like a power plant.
Nuclear fission is a nuclear reaction in which a heavy nucleus (such as uranium) splits into two lighter nuclei (and possible some other radioactive particles as well).
In such radioactive heavy nuclei, the balance between the strong nuclear force attractive force and the electrostatic repulsive force can be knocked out of equilibrium, by the introduction of energy in the form of an absorbed neutron or photon, the nucleus oscillates in an attempt to regain equilibrium until the electrostatic force gains more power than the shorter-distanced nuclear force, at which point the nucleus splits apart, releasing energy as it does so.
Nuclear fusion is an energy supplying process, which happens in the sun and the stars since billions of years. The fusion process takes place at extraordinary high temperatures at which matter is in the fourth state, the plasma. Plasma is composed of free atomic nuclei (ions) and electrons.
In the inner of the sun fusion reactions occur at some 15 million degrees. Under terristic boundary conditions the plasma is confined by strong magnetic fields at temperatures as high as some 100 million degrees and at extremely low density (about 250.000 times lower than the terristic atmosphere).
Types of Nuclear Fission Power Plants
Two types of U.S. nuclear plants operate on the same principles. Commercial nuclear power plants in the United States are either boiling water reactors or pressurized water reactors. Both are cooled by ordinary water. The coolant—the water—is the main link in the process that converts fission energy to electrical energy.
Boiling water reactors. In boiling water reactors, the water is heated by the nuclear fuel and boils to steam directly in the reactor vessel. It is then piped directly to the turbine. The turbine spins, driving the electric generator, producing electricity. Boiling water reactors are manufactured by General Electric.
Pressurized water reactors. In pressurized water reactors, the water is heated by the nuclear fuel but kept under pressure to prevent it from boiling. Instead, the hot water is pumped from the reactor pressure vessel to a steam generator. There the heat of the water is transferred to a second, separate supply of water, which boils to make steam. The steam spins the turbine, driving the electric generator, producing electricity. Pressurized water reactors are manufactured by Babcock and Wilcox Company; Westinghouse Electric Corporation; and the former Combustion Engineering, Inc., now a part of Westinghouse.
Inside a Nuclear Fission 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 means 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.
Outside a Nuclear Fission Power Plant
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.
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. The absence of secondary containment structures in Russian nuclear power plants allowed radioactive material to escape in an accident at Chernobyl.
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
Sub criticality, Criticality and Super criticality
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 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 sub critical. 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 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 2 pounds (0.9 kg). This amount is therefore referred to as the critical mass. For plutonium-239, the critical mass is about 10 ounces (283 grams).