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Nuclear Power Station. A generating station in which nuclear energy is converted into electrical energy.
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Nuclear Power Station • A generating station in which nuclear energy is converted into electrical energy. • Nuclear reactors, which produce heat by splitting uranium atoms, do the same job as conventional power producing equipment (furnace) in the generation of electricity – they produce heat to convert water into steam, which spins a turbine or generator to make electricity.
Nuclear Power Station • Instead of coal, oil or natural gas, nuclear reactors use natural uranium for fuel. But the uranium is not burned. Uranium atoms make heat by splitting – the technical term is fissioning. • “ The process of splitting of a nucleus of a heavy atom into a number of light nuclei with the liberation of large amount of energy and two or three neutrons is called nuclear fission’’.
Nuclear Power Station • How the Heat Energy is produced: • When a neutron (a tiny sub-atomic particle that is one of the components of almost all atoms) strikes an atom of uranium, the uranium atom splits into two lighter atoms (which are called fission products) and releases heat at the same time. • The fissioning process also releases from one to three more neutrons that can split other uranium atoms.
Nuclear Power Station • This is the beginning of a "chain reaction" in which more and more uranium atoms are split, releasing more and more neutrons (and heat). • In a power reactor, the chain reaction is tightly controlled to produce only the amount of heat needed to generate a specific amount of electricity.
Nuclear Power Station • Components of Nuclear Power Plant: • 1. Shielding • A nuclear reaction is a source of intense radiation apart from the heat generated in the exothermic process. • When a nucleus gets split into two parts during the fission process it results in the production of large amounts of heat energy since the reaction is exothermic in nature.
Nuclear Power Station 1. Shielding……… But this is not the only product of nuclear fuel "combustion" but there are several other by-products such as alpha rays, beta rays, gamma rays and of course the fast moving neutrons. Because of the risk, radiation shielding is required to prevent this harmful radiation from leaving the reactor and affecting the outside men and materials.
Nuclear Power Station • 2. Reactor Vessel • It acts to enclose the various parts inside the reactor including the core, shield, reflector etc. • The coolant needs a passage to flow through the reactor so that it can be used to transfer the heat to the working fluid or the turbine directly, as the case may be, and this passage is provided by the reactor vessel.
Nuclear Power Station • 2. Reactor Vessel……….. • To withstand the high pressure with exists inside the reactor and could be of the order of 200 kgf/cm. • To provide a safe working environment for all concerned.
Nuclear Power Station • 3. Coolant • A nuclear reactor is a source of intense heat which is generated through the exothermic fission reactions taking place inside the core. • Therefore a coolant is necessary to ensure that this heat is taken away and utilized in a proper manner. • The immense amount of heat energy present in the nuclear reactor core needs to be transferred in some manner so that it is converted into electrical energy.
Nuclear Power Station • 3. Coolant…….. • This also helps to keep the working temperature of the core within safe limits for the materials used in the construction of the reactor. • Hence a coolant plays an important role in components of nuclear power plant and serves the dual purpose of removing the heat from the reactor as well as transferring it to the electricity generation circuit either directly or indirectly depending on the type of nuclear reactor being used for the purpose.
Nuclear Power Station Properties of an Ideal Coolant • A coolant should not absorb neutrons. • Since a coolant is exposed to high temperatures and well as severe levels of radiation, it is obvious that it should posses excellent resistance to both high temperatures as well as high levels of radiation.
Nuclear Power Station Properties of an Ideal Coolant • A coolant should be non-corrosive in nature otherwise it might tend to damage and corrode the very core which is meant to be protected by it through proper removal of heat. • Coolants used in nuclear reactors could be either in the liquid state or in the solid state. In case the coolant is a liquid it should have a high boiling point so that it does not get evaporated due to the high heat inside the reactor.
Nuclear Power Station Properties of an Ideal Coolant • If it is a solid it should have a relatively low melting point . Commonlyused Coolants • Light water (H2O) • Heavy water (D2O) • Carbondioxide (CO2) • Helium • Nitrogen • Sodium • Sodium-potassium mixture
Nuclear Power Station 4.Reflector • The reactor consists of the fission process which occurs when a thermal energy neutron is absorbed by the target nucleus leading to its division into two nuclei and emission of 2 or 3 neutrons apart from the heat energy. • These neutrons fly randomly in all directions and are usually in the region of fast moving energy neutrons.
Nuclear Power Station 4.Reflector……………. • Many of these neutrons may simply get lost by flying off the reactor core and thus serving no useful purpose. • In order to reduce this process of neutron loss the inner surface of the reactor core is surrounded by a material which helps to reflect these escaping neutrons back towards the core of the reactor and these materials are known as reflecting materials.
Nuclear Power Station 4.Reflector……………. • Properties of Reflectors • Low absorption • High reflection • Radiation stability • Resistance to Oxidation • Materials used as Reflectors • Light water (H2O) • Heavy water (D2O) • Carbon
Nuclear Power Station • 5.Moderator • Whenever a thermal neutron causes fission it also leads to the release of fast neutrons. • These fast neutrons have to be slowed down and brought to lower energy levels if they have to cause successful fission. • A moderator is a medium which is used to absorb a portion of the kinetic energy of fast neutrons so that they come in the category of thermal neutrons which help to sustain a controlled chain reaction.
Nuclear Power Station 5.Moderator • Materials used for Moderator • Normal or Light Water • Deuterium (Heavy water ) • Graphite • Beryllium • Lithium
Nuclear Power Station • 6.Control Rods • Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. • Capable of absorbing many neutrons without fissioning themselves. • A control rod is removed from or inserted into the central core of a nuclear reactor in order to increase or decrease the neutron flux. • Affects the thermal power of the reactor, the amount of steam produced, and hence the electricity generated.
Nuclear Power Station 6.Control Rods……….. • Materials used for Control Rods • Boron • Silver • Indium • Cadmium • Hafnium
Nuclear Power Station • 7. Reactor • An apparatus or structure in which fissile material can be made to undergo a controlled, self-sustaining nuclear reaction with the consequent release of energy.
Nuclear Power Station • Boiling Water Reactor (BWR)
Nuclear Power Station • 1.Boiling Water Reactor (BWR)…….. • Fuel: Enriched uranium(2.4% U235) • Moderator: Light Water • Coolant: Light Water • Advantages: • Small size pressure vessel, • High steam pressure, • Simple construction and elimination of heat exchanger circuit resulting in reduction cost and gain in thermal efficiency.
Nuclear Power Station • 1.Boiling Water Reactor (BWR)…….. • Drawbacks: • There is a danger of radioactive contamination of steam. • More elaborate safety measures are to be provided increasing the cost. • It cannot meet a sudden increase in load.
Nuclear Power Station 1.Pressurised Water Reactor (PWR)……..
Nuclear Power Station • 1.Pressurised Water Reactor (PWR)…….. • Fuel: Enriched uranium (3.2% U235) • Moderator: Light Water • Coolant: Light Water
Nuclear Power Station 1.Pressurised Water Reactor (PWR)…….. • In a PWR, the primary coolant(water) is pumped under high pressure to the reactor core where it is heated by the energy generated by the fission of atoms. • The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator
Nuclear Power Station • 1.Pressurised Water Reactor (PWR)…….. • Advantages: • PWR reactors are very stable, this makes the reactor easier to operate from a stability standpoint. • PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials.
Nuclear Power Station • 1.Pressurised Water Reactor (PWR)…….. • Advantages: • The control rods are held by electromagnets and fall by gravity when current is lost; full insertion safely shuts down the primary nuclear reaction. • PWR technology is favoured by nations seeking to develop a nuclear navy; the compact reactors fit well in nuclear submarines and other nuclear ships.
Nuclear Power Station • 1.Pressurised Water Reactor (PWR)…….. • Drawbacks: • The coolant water must be highly pressurized to remain liquid at high temperatures. • Additional high pressure components such as reactor coolant pumps, pressurizer, steam generators, etc. are also needed. • Increases the capital cost and complexity of a PWR power plant
