Nuclear fission: Nuclear fission is the splitting of atoms to produce energy in the form of heat. Uranium, a naturally occurring metal, is radioactive. The atoms of uranium are the largest and the heaviest known to occur on earth. Being heavy, they are unstable. The nucleus of a uranium atom can easily break up into smaller pieces. This process is called fission. Uranium is the only element in which fission takes place easily, setting off a chain reaction or a self-sustained splitting of atoms. The energy produced in fission is called nuclear energy or atomic energy. When fission generates heat in uranium, there is no flame or smoke. Besides uranium, the atoms of plutonium are fissionable. But plutonium does not occur in nature. It is a by-product of uranium used as fuel in nuclear reactors.
Criticality: When the chain reaction takes place for the first time in a nuclear electricity reactor, it means that the reactor has reached its first criticality.
Nuclear power reactor: A nuclear power reactor is a device that converts nuclear energy into heat energy. Heat is produced in the reactor when the uranium atom splits, The heat released by chain reaction in the reactor is used to convert water into steam. This steam drives a turbine, which is connected to an electricity generator. Turbines have blades. The movement of the blades in the turbine powers the generator to produce electricity, which is wheeled into the grid and thence on to factories, homes, hospitals, schools, and so on. In a nuclear bomb, an extremely rapid, multiplying chain reaction causes the explosive release of energy. In a reactor, the pace of chain reaction is controlled to produce heat.
Fuel: A nuclear reactor can use different types of fuel to generate electricity. The fuel can be natural uranium, low-enriched uranium, plutonium-uranium oxide/carbide and, thorium-uranium 233.
Coolant: Light water, that is, ordinary water, and heavy water are normally used as coolants in a reactor. A coolant is a substance circulated through a nuclear reactor to remove or transfer heat.
Moderator: Light water, heavy water, graphite or liquid sodium can be a moderator in a reactor. It surrounds the fuel core of the reactor and slows down neutrons, thus increasing their chances of fissioning.
Heavy water: In heavy water, the two hydrogen atoms are replaced by two atoms of deuterium, that is, its formula is D 2 O.
Pressurised Heavy Water Reactor (PHWR): In a PHWR, the fuel used is natural uranium, and heavy water is both the coolant and the moderator. It is called “pressurised” because the coolant is under high pressure.
Natural uranium is used as a fuel in a PHWR. Natural uranium contains two kinds of isotopes: uranium-238 and uranium-235. It contains 99.3 per cent of uranium-238 which is not fissile, and only 0.7 per cent of uranium-235, which is fissile. So it is the less abundant U-235 isotope that splits and generates energy.
Light Water Reactor (LWR): In an LWR, the fuel is low-enriched uranium, and light water is used both as a coolant and as a moderator. The Kudankulam reactors are LWRs.
Enriched uranium: If the non-fissile U-238 is removed from natural uranium, the U-235 concentration will go up. This is known as uranium enrichment. Enrichment is achieved by a series of chemical and physical processes in centrifuges. In India, the enrichment of uranium is done at Rare Materials Plant, Ratnahalli, Mysore.
If enriched uranium is used as fuel in a reactor, the availability of neutrons is high enough to generate electricity, and light water can be used as both coolant and moderator.
T.S. Subramanian
Source: Nuclear Energy by Gratian Vas; The New Nuclear Nations by Leonard Spector; NPCIL publications; S.K. Malhotra, Head, Environmental and Public Awareness Division, DAE; and N. Nagaich, Executive Director (Corporate Planning and Corporate Communications), NPCIL.
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