A device that initiates and controls a sustained nuclear chain reaction is known as nuclear reactor. The thermal energy released from nuclear fission is used for generating electricity. Heat energy is also used for moving submarines and aircraft carriers. They are also used to produce medical isotopes for cancer treatment, imaging and for research purposes. Many industrial power plants run on nuclear power.
Nuclear reactors are a great source of thermal energy. The fuel for the nuclear reactor comprises heavy atoms such uranium-235 or plutonium-239. These atoms are placed inside the reactor vessel along with a small neutron source. Each atom absorb a neutron and undergo nuclear fission. A large amount of energy is generated each time an atom is split. Each heavy nucleus decompose into 2 or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons. All these are collectively known as fission products. Enormous heat is expelled out of the turbine with the help of a coolant( such as water). Evaporated coolant reaches the turbine and spin the generator shaft.
The various components of a nuclear reactor are:
Moderator: Moderator is implanted in the core of the reactor. It helps to slow down the neutrons release from fission chain reaction. Heavy water or graphite is commonly used.
Control Rods: These are made with neutron absorbing material such as boron or cadmium. They are used to control the reaction rate.
Coolant: Potable water, heavy water, liquid sodium, or helium is used as coolant. It absorbs heat and passes it to the turbine.
Turbine: It converts heat energy to electricity.
Containment: It is a dome-shaped structure made of steel that separates the reactor from the environment.
Cooling Towers: Some amount of heat energy cannot be converted to electricity. Cooling tower is specifically used to absorb this excess heat and convert them to pure water vapor. Later it is released to the atmosphere.
Many fuel pins put together form a fuel assembly. Fuel is lifted into and out of the reactor in assemblies. In a PWR reactor(Pressurised Water Reactor), a fuel assembly consists of 179-246 fuel rods. Approximately121-193 fuel assemblies are loaded in each nuclear reactor.The assemblies are packed with structural materials that keeps the pins without touching them. After every 5 years, one third of the assemblies are removed and stored in what are called storage pools. They lose their radioactivity over a period of 10 years. These assemblies are called Spent Nuclear Fuel. When the spent Nuclear fuel is reprocessed, it is known as High Level Waste. A 100MW reactor ends up with a discharge of 2 metric tons of High Level waste.
Several hundred of assemblies put together form a full core. A nuclear full core is the part that contains nuclear fuel. This fuel is used to trigger nuclear reactions. Different assemblies can have different fuels. They can vary in their composition and in their age. The core of the nuclear reactor generates thermal power. It contains low-enriched uranium or plutonium, control systems and structural materials. In a boiling water reactor, the nuclear fuel rods are equivalent to the diameter of a large gel type ink-pen, each measuring about 12 feet (3.7 m) long. These fuel rods are grouped in bundles known as "fuel assemblies". Each fuel rod contains pellets of uranium, or uranium oxide, and are stacked end to end. The control rods inside the core are filled boron or cadmium which readily capture neutrons. The control rods absorb the neutrons and hence slow down the fission reaction. The absence of control rods intensify the chain reaction. An alternative nuclear fuel like fissile uranium-233 (U-233) made by the neutron-bombardment of the common thorium-232 can also be used.
There are about six types of nuclear reactors. Each vary in terms of fuels and coolants used.
Pressurized Water Reactor(PWR)
Boiling Water Reactor(BWR)
Sodium Cooled Fast Reactor
Canada Deuterium-Uranium Reactor (CANDU)
High Temperature Gas Cooled Reactor
Fukushima Daiichi Nuclear Plant Disaster is one of the largest man-made hazards in recent years. It occured on 11th March 2011 in Japan. The disaster involved nuclear meltdowns, equipment failures, and release of radioactive materials at the Fukushima nuclear plant. It happpened due to tsunami and earthquake. This is the largest nuclear accident since the 1986 Chernobyl disaster.
The Chernobyl disaster occurred in 1986 in Ukraine. An explosion released large quantities of radioactive materials to the atmosphere. Around 9,85,000 people died of cancer between 1986 and 2004 due to this disaster.