Science & Technology

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.      

Nuclear Fission   Nuclear fission is the process whereby an atomic nucleus breaks up into two or more major fragments with the emission of two or three neutrons. It is accompanied by the release of energy in the form of gamma radiation and the kinetic energy of the emitted particles.Fission occurs spontaneously in nuclei of uranium-235, the main fuel used in nuclear reactors. However, the process can also be induced by bombarding nuclei with neutrons because a nucleus that has absorbed a neutron becomes unstable and soon splits. The mass defect is large and appears mostly as k.e. of the fission fragments. These fly apart at great speed, colliding with surrounding atoms and raising their average k.e., that is, their temperature. Heat is therefore produced.If the fission neutrons split other uranium-235 nuclei, a chain reaction is set up. In practice, some fission neutrons are lost by escaping from the surface of the uranium before this happens. The ratio of those escaping to those causing fission decreases as the mass of uranium-235 increases.This must exceed a certain critical mass for a chain reaction to start. Critical mass is thus the minimum mass of fissile material that can undergo a continuous chain reaction. Above the critical mass, the reaction may accelerate into a nuclear explosion if uncontrolled.The U-238 isotope would make an ideal nuclear reactor fuel because it is abundant in nature. But U-238 nuclei usually absorb free neutrons without fissioning. An absorbed neutron simply becomes part of the nucleus. The scarce uranium isotope U-235 is the only natural material that nuclear reactors can use to produce a chain reaction. Uranium with an abundant amount of U-235 is called enriched uranium.    

Nuclear Energy   Nuclear Energy is energy derived from nuclear reactions either by the fission of heavy nuclei into lighter ones or by fusion of light nuclei into heavier ones. In principles, the binding energy of a system of particles forming an atomic nucleus is nuclear energy.It results from changes in the nucleus of atoms. Scientists and engineers have found many uses for this energy, especially in producing electricity. But they do not yet have the ability to make full use of nuclear power. If nuclear energy were fully developed, it could supply all the world’s electricity for millions of years.A nucleus make up most of the mass of every atom and this nucleus is held together by an extremely powerful force. A huge amount of energy is concentrated in the nucleus because of this force.Scientists first released nuclear energy on a large scale at the University of Chicago in 1942, three years after World War II began. This achievement led to the development of the atomic bomb. It is since 1945 that nuclear energy has been put to peaceful uses such as production of electricity.Einstein pointed out that if the energy of a body changes by an amount E, its mass changes by an amount m given by the equation, E = mc2. The implication is that any reaction in which there is a decrease of mass, called a mass defect, is a source of energy.The energy and mass changes in physical and chemical changes are very small; those in some nuclear reactions, such as radioactive decay, are millions of times greater. The sum of the masses of the products of a nuclear reaction is less than the sum of the masses of the reacting particles. This lost mass is converted into energy.    

Radioactivity   Ordinary hydrogen has one proton and no neutrons, so it has mass number 1. Heavy hydrogen, or deuterium, has mass number 2, because it has one proton and one neutron.A radioactive form of hydrogen, tritium, has mass number 3. It has one proton and two neutrons. Ordinary hydrogen, deuterium, and tritium are isotopes of hydrogen. All isotopes of an element have the same chemical properties. The uranium nucleus has 92 protons.The most plentiful isotope of uranium has 146 neutrons. Its mass number is therefore 238 (the sum of 92 and 146). Scientists call this isotope uranium 238 or U-238. The uranium isotope that almost all nuclear reactors use as fuel as 143 neutrons, and so its mass number is 235. This isotope is called uranium 235 or U-235.A nuclear reaction involves changes in the structure of a nucleus. As a result of such changes, the nucleus gains or loses one or more neutrons or protons. It thus changes into the nucleus of a different isotope or element. If the nucleus changes into the nucleus of a different element, the change is called transmutation.Radioactivity is the process by which atoms emit radiation, or atomic particles and rays of high energy, from their nuclei (cores). Of more than 2,300 different kinds of known atoms, more than 2,000 are radioactive. Only about 50 radioactive types exist in nature. Scientists make the rest artificially.Antoine Henri Becquerel of France discovered natural radioactivity in 1896. He found that uranium compounds emitted radiation which affected a photographic plate even when they are wrapped in black paper; they also ionised a gas. Soon afterwards, Marie Curie discovered an even more strongly radioactive substance, namely radium.Every element with an atomic number greater than that of lead (82) is radioactive. The nuclei of some of these elements can decay by splitting in two: this is Spontaneous fission.Natural radioactivity occurs in nine of the lighter elements also. Of these the most important are146C (carbon) and 4019K (Potassium). The isotope was probably formed when the earth was created.Its present existence is due to its long half-life of 1.25 x 109 years; though it only constitutes 0.01% of natural potassium, its presence makes living tissue appreciably radioactive. It may decay either by b-emission or electron capture. It is produced continuously from the action of the neutrons in cosmic rays on atmospheric nitrogen, by a nuclear reaction.Of the seventh row elements, only five are round in nature; radium, actinium, thorium, protactinium and uranium.Emission of Radiation:Different forms of radiation originate in the nuclei of radioactive atoms. There are three kinds of radioactive radiation: alpha particles, which were first identified by Becquerel; beta rays; identified by Ernest Rutherford of New Zealand; and gamma rays, identified by Marie and Pierre Curie of France. Emission of alpha or beta rays causes transmutation, but gamma radiation does not result in transformation.Alpha particles have a positive electrical charge. They consist of two protons and two neutrons, and are identical with the nuclei of helium atoms. Alpha particles are emitted with high energies, but lose energy rapidly when passing through matter. These are stopped by a thick sheet of paper; in air they have a range of a few centimetres, being eventually brought to rest by collisions with air molecules.They cause intense ionisation in a gas (by attracting electrons out of their molecules) and are deflected by electric and very strong magnetic fields. All alpha-particles emitted by a particular radioactive substance have the same speed, about one-twentieth of the speed of light. Americium emits alpha-particles only.Alpha radiation occurs in 238U, an isotope of uranium. After losing an alpha particle, the nucleus has 90 protons and 144 neutrons. The atom with atomic number 90 is no longer uranium, but thorium. The isotope formed is 23490Th.Beta rays are electrons. Some radioactive nuclei emit ordinary electrons, which have negative electrical charges. But others emit positrons, or positively charged electrons. For example, an isotope of carbon, 146C , gives off negative electrons. Carbon 14 has eight neutrons and six protons.When its nucleus transforms, a neutron changes into a proton, an electron, and an antineutrino. After emission of the electron and antineutrino, the nucleus contains seven protons and seven neutrons. Its mass number remains the same, but its atomic number 7 is nitrogen. Thus, 146C changes to 147N after emission of a negative beta particle.A carbon isotope, 116C , emits positrons. Carbon 11 has six protons and five neutrons. When it emits a positron, one proton changes into a neutron, a positron, and neutrino. After emission of the positron and the neutrino, the nucleus contains five protons and six neutrons. The mass number remains the same, but the atomic number drops by one.The element of atomic number 5 is boron. Thus,116C changes into 115B after emission of a positron and a neutrino. Strontium emits beta particles only. Beta particles travel with almost the speed of light. Some can penetrate 13 millimetres of wood.Gamma radiation may occur in several ways. In one process, the alpha or beta particle emitted by a nucleus does not carry off all the energy available. After emission, the nucleus has more energy than in its most stable state. It rids itself of the excess by emitting gamma rays. Gamma rays have no electrical charge. They are similar to X-rays, but they usually have a shorter wavelength.Whereas X-rays are due to energy changes outside atomic nuclei, as are all forms of electromagnetic radiation, gamma-rays, like alpha- and beta-particles, come from inside atomic nuclei. These rays are photons (particles of electromagnetic radiation) and travel with the speed of light. They are much more penetrating than alpha and beta particles.Radium emits alpha-, beta- and gamma-rays. Cobalt is a pure gamma source.Radioactive Decay and Half-Life:Radioactive decay is the process by which a nucleus spontaneously (naturally) changes into the nucleus of another isotope or element. The process releases energy chiefly in the form of nuclear radiation. The decay process happens of its own accord and cannot be controlled; it is unaffected by temperature changes,

Electromagnetic waves    Electromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (a speed value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s. This is depicted in the animation below.The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 108 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below. The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.Waves come in many shapes and forms. While all waves share some basic characteristic properties and behaviors, some waves can be distinguished from others based on some observable (and some non-observable) characteristics. It is common to categorize waves based on these distinguishing characteristics. Longitudinal versus Transverse Waves versus Surface WavesOne way to categorize waves is on the basis of the direction of movement of the individual particles of the medium relative to the direction that the waves travel. Categorizing waves on this basis leads to three notable categories: transverse waves, longitudinal waves, and surface waves.A transverse wave is a wave in which particles of the medium move in a direction perpendicular to the direction that the wave moves. Suppose that a slinky is stretched out in a horizontal direction across the classroom and that a pulse is introduced into the slinky on the left end by vibrating the first coil up and down. Energy will begin to be transported through the slinky from left to right. As the energy is transported from left to right, the individual coils of the medium will be displaced upwards and downwards. In this case, the particles of the medium move perpendicular to the direction that the pulse moves. This type of wave is a transverse wave. Transverse waves are always characterized by particle motion being perpendicularto wave motion. A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction that the wave moves. Suppose that a slinky is stretched out in a horizontal direction across the classroom and that a pulse is introduced into the slinky on the left end by vibrating the first coil left and right. Energy will begin to be transported through the slinky from left to right. As the energy is transported from left to right, the individual coils of the medium will be displaced leftwards and rightwards. In this case, the particles of the medium move parallel to the direction that the pulse moves. This type of wave is a longitudinal wave. Longitudinal waves are always characterized by particle motion being parallel to wave motion. A sound wave traveling through air is a classic example of a longitudinal wave. As a sound wave moves from the lips of a speaker to the ear of a listener, particles of air vibrate back and forth in the same direction and the opposite direction of energy transport. Each individual particle pushes on its neighboring particle so as to push it forward. The collision of particle #1 with its neighbor serves to restore particle #1 to its original position and displace particle #2 in a forward direction. This back and forth motion of particles in the direction of energy transport creates regions within the medium where the particles are pressed together and other regions where the particles are spread apart. Longitudinal waves can always be quickly identified by the presence of such regions. This process continues along the chain of particles until the sound wave reaches the ear of the listener. Waves traveling through a solid medium can be either transverse waves or longitudinal waves. Yet waves traveling through the bulk of a fluid (such as a liquid or a gas) are always longitudinal waves. Transverse waves require a relatively rigid medium in order to transmit their energy. As one particle begins to move it must be able to exert a pull on its nearest neighbor. If the medium is not rigid as is the case with fluids, the particles will slide