Reaction

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Reaction

  1. 1. In an uncontrolled reaction, most of the neutrons released from the nuclei go on to hit other nuclei. Manyatoms are split and enormous amount of energy is released. This is what happens in an atomic bomb.However, to make practical and non-destructive liberation of the energy, we must be able to control thechain reactions so that our nuclear power station would not turn into an atomic bomb.There are several types of nuclear reactors in which controlled reactions can take place but they servesimilar purposes and will be discussed in the following section.Diagram showing the principal components of power stationNuclear fuelsThey are made from uranium oxide [enriched] with extra uranium-235. The fuel is then converted intosmall pellets, packed into tubes called pins. The pins are bundled into larger tubes and are ready to beput into the reactor. One kilogram of this fuel gives as much energy as 55 tons of coal. Back to diagramNeutron ModeratorsIn the reactors, there are always some neutron moderators which slow down the neutrons so that theyhave a higher chance of splitting the next nucleus. Materials used for moderators include graphite andheavy water, containing the isotope deuterium. However, a breeder reactor has no moderator. It dependson the liquid metal coolant to control the temperature.
  2. 2. Back to diagramControl rodsThe core also has neutron-absorbing control rods to control the neutrons released. While the moderatorpromotes stronger interaction of the neutrons with the uranium nuclei, the control rods are there to reducethe concentration of neutrons whenever necessary.The material used for these control rods is cadmium whose nuclei have a large cross-section for thecapture of neutrons. The cadmium rods are inserted into the nuclear reactor to slow down or shut downthe chain reaction whereas withdrawing the rods restarts or speeds up the reaction. Back to diagramCoolantsThe temperature of the reactor is very high although the actual temperature depends on the reactor type.Some reactors operate at temperatures as high as 1500 °C. The coolant flows around the core andcarries heat energy to a steam generator. The coolant materials vary between reactors. They may bewater or carbon dioxide at high pressures. Some reactors such as fast breeders use liquid sodium ascoolant. Back to diagramGeneration of electricityThe coolant carries the heat energy to a heat exchanger (or steam generator), where it is used to makesteam. The steam then drives the turbine, and the turbine drives the generator that produces electricity.Waste steam goes to the cooling towers to be [condensed] back into water. Back to diagramNuclear Reactor DiagramIntroduction to Nuclear Reactor DiagramAn assembly giving large amount of nuclear energy through fission reaction in a controlled rateis called a nuclear reactor. It is a system that contains and controls sustained nuclear chainreactions. The controlled chain reaction and liberation of large amount of energy is the basis ofnuclear reactor. The first fission reaction results in the production of fast moving neutrons. Theseneutrons when passed through moderators become thermal neutrons. Controlled fission reactioncan be done by controlling the thermal neutrons used in reaction. Sustained chain reaction withsteady liberation of energy is maintained in a nuclear reactor. Reactors are classified intodifferent categories based on the fuel used.Features of Nuclear Reactor Diagram:Diagram of Nuclear Reactor mainly consists of
  3. 3. a) Fuel b) Moderators c) Control rods d) Shielding e) CoolantFuel:The fissionable material used in the reactor is called as fuel. The commonly used fuels areUranium, Plutonium or Thorium. It can be U-235, U-238, Pu-236 or Th-232. Uranium is mostlypreferred as it has high melting point.Moderators:Only neutrons of a fairly low speed should be used to have controlled chain reaction. To slowdown the speed fast moving neutrons produced during the fission process, moderators are used.Moderator reduces the speed of the neutron by absorbing its energy but not absorb neutron.Graphite, Heavy water and Beryllium are common moderators.Control Rods:These rods absorb neutrons and stop the chain reaction to proceed further. These are made up ofsteel containing a high percentage of material like cadmium or boron which can absorb neutrons.When control rods are completely inserted into the moderator block then all the neutrons isabsorbed and reaction comes to halt.Shielding:Shielding prevents radiations to reach outside the reactor. Lead blocks and concrete enclosurethat is strong enough of several meters thickness are used for shielding.Coolant:The coolant is substance in a pipe to the steam generator where water is boiled. This is whereheat-exchange process occurs. Heat is absorbed by the coolant that is produced in the reactor.Typical coolants are water, carbon dioxide gas or liquid sodium.Diagram of Nuclear Reactor : Applications
  4. 4. Reactors are used for1) Generating electricity2) Producing radionuclide for industry and medicine3) Conducting research tests4) Military purposes Nuclear ReactorsA nuclear reactor is a device for obtaining and using the energy from a controlled nuclear chainreaction. Controlled means the rate or speed of the nuclear fissions can be changed by theoperator. Most reactors are constructed in a similar manner. The six main parts of a nuclearreactor are fuel, moderator, control rods, coolant, heat exchanger, and safety shields.The fuel in a nuclear reactor is slightly enriched uranium oxide pellets. It is inserted into longrods. Nuclear energy is released from the fuel through a chain reaction.Nuclear Reactor producing heat energyThe moderator is a material that slows the neutrons released during nuclear fission. To keep achain reaction going, the speed of the neutrons must be reduced. The neutrons must be moving ata speed that allows them to be captured by the other nuclei in the fuel. Graphite, a form ofcarbon, is one type of moderator. It surrounds the cans containing the fuel. Ordinary water,heavy water, and beryllium can also be used as moderators. Heavy water contains an isotope ofhydrogen that is heavier than ordinary hydrogen.Control rods regulate the rate of fission in the fuel. They absorb any excess neutrons not requiredto keep the chain reaction going. Control rods are made of boron or cadmium alloyed with
  5. 5. aluminium or steel. These materials absorb neutrons. By pushing the rods into the reactor orpulling them out, the fission rate is controlled. Pushing the rods all the way in stops the fissionreaction.The coolant removes heat energy from the reactor. Examples of coolant are water, carbondioxide, and liquid metals. Liquid sodium is one kind of metal used as a coolant. As coolant ispumped through spaces in the reactor, it absorbs heat released from fission. The coolant carriesthe heat to the heat exchanger.Safety shields are necessary to protect people from the nuclear radiation produced by a reactor.Nuclear radiation can produce burns, cancer, loss of hair, vomiting and destruction of bloodcells. An overdose can cause death. Strict safety regulations are enforced to protect workers andthe general public from dangers of radiation. Lead and thick concrete walls are used as shieldsfor reactors.A radiation badge is worn by those working with radioactive materials. The film inside blackenswhen exposed to gamma rays. The amount of blackening shows the level of radiation to whichthe wearer has been exposed.Nuclear ReactorA nuclear reactor is an installation where a self-sustaining nuclear fission takes place in acontrolled manner and energy released is used for constructive purposes.Sub Topics • Nuclear Fuel • Moderators • Coolant
  6. 6. Nuclear FuelIt consists of the fissionable material namely U233 or U235 or Pu239 Generally U235 is piled inside asealed aluminum cylinder. Thermal neutrons hit these fuel atoms, fission starts and energy isreleased.ModeratorsThey slow down the fast moving secondary neutrons. Usually heavy water, graphite, deuterium,paraffin etc., act as moderators. Control rods have the ability to capture the slow neutrons. Rodsof boron or cadmium are inserted in the holes of the reactor core up to a desirable length forabsorbing the neutrons.CoolantCoolants are substances used to remove and transfer the heat produced to the surrounding. Itpasses this heat over to water in a heat exchanger.This results in superheated steam, which drives a turbine coupled to an electric generator. The wholereactor is protected with concrete walls so that radiations may not produce harmful effects to itssurroundings.

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