Nuclear power plant
Nuclear power plant
Introduction
- The key difference between controlled and uncontrolled chain reactions is that controlled chain reactions do not lead to any explosive effects whereas uncontrolled chain reactions lead to explosive energy release.
- :::Controlled vs Uncontrolled Chain Reaction::::::
(1)A controlled chain reaction is a chain of nuclear reactions that take place subsequently under controlled conditions.
An uncontrolled chain reaction is a chain of nuclear reactions that take place subsequently, but not under controlled conditions.
(2) Components::::A controlled chain reaction is carried out in the presence of moderators.
An uncontrolled chain reaction is carried out in the absence of moderators.
(3) Control Measures::::A nuclear chain reaction is converted into a controlled chain reaction via regulating the amount of fissile isotopes present, reducing the time of reaction and using moderators.
- An uncontrolled chain reaction has no control measures.
Uncontrolled chain reactions are used in nuclear bombs.
Principles of nuclear power
In nuclear power plants, neutrons collide with uranium atoms, splitting them. This split releases neutrons from the uranium that in turn collide with other atoms, causing a chain reaction. This chain reaction is controlled with "control rods" that absorb neutrons.
The fission in a nuclear reactor heats the reactor coolant. The coolant may be water or gas, or even liquid metal, depending on the type of reactor. The reactor coolant then goes to a steam generator and heats water to produce steam. The condenser is a heat exchanger which is connected to a secondary side such as a river or a cooling tower. The water is then pumped back into the steam generator and the cycle begins again.
The nuclear reactor is the heart of the station. In its central part, the reactor's core produces heat due to nuclear fission. With this heat, a coolant is heated as it is pumped through the reactor and thereby removes the energy from the reactor. Heat from nuclear fission is used to raise steam, which runs through turbines, which in turn power the electrical generators.
Nuclear reactors usually rely on uranium to fuel the chain reaction. Uranium is a very heavy metal that is abundant on Earth and is found in sea water as well as most rocks. Naturally occurring uranium is found in two different isotopes: uranium-238 (U-238), accounting for 99.3% and uranium-235 (U-235) accounting for about 0.7%. Isotopes are atoms of the same element with a different number of neutrons. Thus, U-238 has 146 neutrons and U-235 has 143 neutrons.
Different isotopes have different behaviors. For instance, U-235 is fissile which means that it is easily split and gives off a lot of energy making it ideal for nuclear energy. On the other hand, U-238 does not have that property despite it being the same element. Since nuclear fission creates radioactivity, the reactor core is surrounded by a protective shield. This containment absorbs radiation and prevents radioactive material from being released into the environment.
In addition, many reactors are equipped with a dome of concrete to protect the reactor against both internal casualties and external impacts.
- functions of control rods in nuclear reactor
Control rods are used in nuclear reactors to control the fission rate of uranium or plutonium. Their compositions includes chemical elements such as boron, cadmium, silver, or indium, that are capable of absorbing many neutrons without themselves fissioning .
- function of moderator in nuclear reactor
The moderator helps slow down the neutrons produced by fission to sustain the chain reaction. Heavy water and graphite are used as moderator in nuclear reactor. Explanation: Heavy water is made from the isotope of hydrogen which is deutarium. Heavy water is used in nuclear reaction because it acts as a moderator as well as a coolant.
Monazite…—— Thorium
In a boiling water reactor, shown above, the water is allowed to boil into steam, and is then sent through a turbine to produce electricity.
In pressurized water reactors, the core water is held under pressure and not allowed to boil. The heat is transferred to water outside the core with a heat exchanger (also called a steam generator), boiling the outside water, generating steam, and powering a turbine. In pressurized water reactors, the water that is boiled is separate from the fission process, and so does not become radioactive.
After the steam is used to power the turbine, it is cooled off to make it condense back into water. Some plants use water from rivers, lakes or the ocean to cool the steam, while others use tall cooling towers. The hourglass-shaped cooling towers are the familiar landmark of many nuclear plants. For every unit of electricity produced by a nuclear power plant, about two units of waste heat are rejected to the environment.
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