Nuclear Reactor: A nuclear reactor is the central component of a nuclear power station that generates nuclear energy under controlled conditions for use as a source of electrical power. Power reactors generally consist of three main parts. They are (1) the reactor, or pressure, vessel; (2) the core; and (3) control rods. The reactor vessel holds the other reactor parts. It is installed near the base of the reactor building. The vessel has steel walls at least 15 centimetres thick,. Steel pipes lead into and out of the vessel to carry water and steam. The core contains the nuclear fuel and so is the part of the reactor where fission occurs. The core is near the bottom of the reactor vessel. It consists mainly of the nuclear fuel held in place between an upper and a lower support plate. Control rods are long metal rods that contain such elements as boron or cadmium. These elements absorb free neutrons and thus help control a chain reaction. The control rods are inserted into the core or withdraw to slow down or speed up a chain reaction. Moderators and Coolants: Reactor operations also depend on substances called moderators and coolants. A moderator is a substance, such as water or carbon, that slows down neutrons which pass through it. Reactors require a moderator because the neutrons released by fission are fast neutrons. But slow neutrons are needed to cause a chain reaction in the mixture of U-238 and U-235 that reactors use as fuel. A coolant is a substance, such as water or carbon dioxide, that conducts heat well but does not easily absorb free neutrons. The coolant carries heat from the chain reaction. By doing so, the coolant servers both to prevent the reactor core from melting and to produce steam. Many power reactors are light water reactors, which use light (ordinary) water as both the moderator and the coolant. Heavy water reactors use deuterium oxide, or heavy water, as both the moderator and the coolant. Graphite is another moderator. Indian reactors (except the one at Tarapur) use heavy water. Fuel Preparation: The uranium used in light waters reactors must be enriched—that is, the percentage of U-235 must be increased. Free neutrons then have a better chance of striking a U-235 nucleus. Steam Production: The reactor achieves criticality when a chain reaction in the fuel has been induced to provide, on an average, one more reaction for every fission reaction. The light water reactors are of two main types. One type, the pressurised water reactor, produces steam outside the reactor vessel. The other type, boiling water reactor, makes steam inside the vessel. Most nuclear plants use pressurised water reactors. These reactors heat the moderator-water in the core under extremely high pressure. The pressure allows the water to heat past its normal boiling point of 100 °C without actually boiling. The chain reaction heats the water to about 320 °C. Pipes carry this extremely hot, though not boiling, water to steam generators outside the reactors. Heat from the pressurised water boils water in the steam generator and so produces steam. In a boiling water reactor, the chain reaction boils the moderator- water in the core. Pipes carry the steam produced from the reactor to the plant’s turbines. In India, the standard reactor type is the pressurised heavy water reactor. The fuel rods have to be removed and reprocessed from time to time to separate radioactive waste products and small amounts of plutonium-239 from unused uranium. Plutonium-239 is produced in the reactor when uranium-238 absorbs fast fission neutrons; like uranium-235, it undergoes fission and is used in fast-breeder reactors and to make nuclear weapons. Experimental Breeder Reactors: The most important type of experimental breeder uses the plentiful uranium isotope-U-238 as its basic fuel. The reactor changes the U-238 into the isotope plutonium 239 (Pu- 239) by radioactive decay. Like U-235, Pu-239 can create a chain reaction and so can be used for energy production. Another breeder uses the natural element thorium as its basic fuel. It changes the thorium into the isotope U-233, which can also produce a chain reaction. India has developed an experimental breeder reactor at Kalpakkam, Chennai, using mixed carbide fuel and sodium as coolant.