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The Fires of Nuclear Fission Diambil dari berbagai sumber
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What Are the Advantages and Disadvantages of Nuclear Energy? Nuclear power has a low environmental impact and a very low accident risk, but high costs, a low net energy yield, long- lived radioactive wastes, vulnerability to sabotage, and the potential for spreading nuclear weapons technology have limited its use.
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The Marvelous Nuclear Power What is nuclear power? Nuclear power plants run on the principle of nuclear fission: The process of splitting a large nucleus into smaller ones, usually by bombarding the target nucleus with neutrons. Important to be noted; The products of this reaction actually possess slightly less mass than the reactants.
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If we accept that mass can be converted into energy – which is to say that mass is just another form of energy – there must be a way to express that conversion. The Einstein Equation: E = mc 2 Energy = mass x (speed of light) 2 Note the units here... c is a large number! (3.0 x 10 8 m/s) 2 = 9.0 x 10 16 m 2 /s 2 1 Joule = 1 kg m 2 /s 2 Small changes in mass make for HUGE changes in energy.
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Let’s take a specific example: The fission of Uranium-235 Recall that atoms can exist as different isotopes, each of which must contain the same number of protons as each other, but which contain a different number of neutrons. Protons = atomic number, this defines the element. Neutrons, together with protons, make up the mass of the nucleus.
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Uranium (element number 92) has several isotopes, and U-235 has a mass number of 235, written: In order for Uranium-235 to undergo fission, it must be struck by a neutron. A bare neutron has no protons, but a mass number of one, and can thus be written:
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Uranium nucleus undergoes fission, breaking apart into smaller nuclei. One such reaction is: Note that we put one neutron in, and got three out – and those three are important! They will initiate a chain reaction.
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A chain reaction is one in which the products of an initial step undergo further reaction. Here, the three neutrons emitted by the fission process can strike other nearby U-235 atoms, and induce fission in them.... Producing more neutrons, which can go on to strike more nearby U- 235 atoms... An important concept with regard to chain reactions is that of critical mass; The amount of fissionable material which is necessary to sustain the chain reaction. For U- 235, this is 15 kg; If 15 kg of U-235 is contained in the same place, it will undergo spontaneous fission. This is the principle behind nuclear bombs.
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Recall: If the mass numbers on both sides are equal, where is the energy coming from? Recall that the actual mass of a nucleus is not simply the mass number.
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In fact, an atom of U-235 weighs 235.043924 amu. An atom of Kr-92 weighs 91.926156 amu. An atom of Ba-141 weighs 140.914412 amu. A neutron weighs 1.00866 amu. So, the reactants weigh 236.052584 amu. The products weigh 235.866548 amu. Over the course of this reaction, 0.186036 amu of matter is converted into energy.
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That’s about 1/1000 th of the total mass. How much energy is produced from the fission of 1 kg of U-235? E = mc 2 = [(1/1000)(1 kg)](9.0x10 16 m 2 /s 2 ) = 9.0 x 10 13 Joules! This is the same amount of energy as from 33,000 tons of TNT, or 3300 tons of coal.
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How Does a Nuclear Fission Reactor Work? (1) Controlled nuclear fission reaction in a reactor – Light-water reactors. Fueled by uranium ore and packed as pellets in fuel rods and fuel assemblies. Control rods absorb neutrons.
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How Does a Nuclear Fission Reactor Work? (2) Water is the usual coolant. Containment shell around the core for protection. Water-filled pools or dry casks for storage of radioactive spent fuel rod assemblies.
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Light water reactors 85% of world’s nuclear generated electricity (100% in US). High inefficient in terms of energy conversion (up to 83% lost as waste heat). There are three varieties of light water reactors: the pressurized water reactor (PWR), the boiling water reactor (BWR), and (most designs of) the supercritical water reactor (SCWR).
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The Koeberg nuclear power station located in South Africa, consisting of two pressurized water reactors fueled with uranium.
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The Palo Verde nuclear generating station located in Wintersburg, Arizona, about 45 miles (80 km) west of central Phoenix. It is the largest nuclear generation facility in the United States, averaging over 3.3 gigawatts (GW) of electrical power production in 2008 to serve approximately 4 million people.
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Nuclear fuel Made from uranium ore. Enriched to 3% of radioactive isotope U-235. Made into pellets, size of pencil, energy equivalent to 1 ton of coal. Pellets are packed into large pipes-fuel rods. Rods are grouped together into fuel assemblies, these assemblies are placed into reactor core.
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(Left) A typical nuclear fuel pellet. (Right) Nuclear fuel pellets that are ready for fuel assembly completion.
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Controlling the reaction Control rods placed between rods. Control rods moved in and out of the assemblies, absorbing neutrons which trigger the chain reaction. Water circulates through the assemblies, removing the heat, keeping the rods from melting.
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A pressurized water reactor head, with the control rods visible on the top.
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How nuclear reactors generate electricity Superheated water turns into steam. Steam passed through turbine. Physical motion of the turbine is converted into electrical energy.
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A typical Light-Water-Moderated and -Cooled Nuclear Power Plant with Water Reactor.
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Spent Fuel rods After about 3-4 years of use, the Fuel rods become spent-level of fission drops beneath a certain level. Rods are taken out of reactor stored nearby in water filled pools or dry casks. Stored until they cool down enough to be shipped for permanent storage or to be recycled. These storage facilities are next to the reactor plants, vulnerable to terrorist attack or accidents.
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After 3 or 4 Years in a Reactor, Spent Fuel Rods Are Removed and Stored in Water
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Worst Commercial Nuclear Power Plant Accident in the U.S. Three Mile Island – March 29, 1979. – Near Harrisburg, PA, U.S. – Nuclear reactor lost its coolant. – Led to a partial uncovering and melting of the radioactive core. – Unknown amounts of radioactivity escaped. – People fled the area. – Increased public concerns for safety. Led to improved safety regulations in the U.S.
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Viewed from the west, Three Mile Island currently uses only one nuclear generating station, TMI-1, which is on the left. TMI-2, to the right, has not been used since the accident. Note that this is a pre-accident photo taken when TMI-2 was in operation.
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Worst Nuclear Power Plant Accident in the World Chernobyl – April 26, 1986. – In Chernobyl, Ukraine. – Series of explosions caused the roof of a reactor building to blow off. – Partial meltdown and fire for 10 days. – Huge radioactive cloud spread over many countries and eventually the world. – 350,000 people left their homes. – Effects on human health, water supply, and agriculture.
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Aerial view of the damaged core on May 3, 1986. Roof of the turbine hall is damaged (image center). Roof of the adjacent reactor 3 (image lower left) shows minor fire damage.
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The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right).
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Human casualties 56 people lost their lives as a direct result of radiation poisoning or fire. Thyroid cancer From drinking Milk 10-12 thousand.
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Recent Nuclear Power Plant Accident in the World Fukushima – March 11, 2011. – In Fukushima, Japan. – A series of ongoing equipment failures and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the 9.0 magnitude Tohoku earthquake and tsunami on 11 March 2011. Partial meltdown and fire for 10 days. – Experts consider it to be the second largest nuclear accident after the Chernobyl disaster, but more complex as multiple reactors are involved.
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Unit 1 of Fukushima Reactors before the explosion. The join can be seen between the lower concrete building and upper lighter cladding which was blown away in the explosion. The trees and lamp posts indicate its size.
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TRADE-OFFS Conventional Nuclear Fuel Cycle Large fuel supply Advantages Low environmental impact (without accidents) Emits 1/6 as much CO 2 as coal Moderate land disruption and water pollution (without accidents) Moderate land use Low risk of accidents because of multiple safety systems (except for Chernobyl-type reactors) Cannot compete economically without huge government subsidies Disadvantages Low net energy yield High environmental impact (with major accidents) Environmental costs not included in market price Risk of catastrophic accidents No widely acceptable solution for long-term storage of radioactive wastes Spreads knowledge and technology for building nuclear weapons Subject to terrorist attacks
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Fig. 15-22, p. 392 TRADE-OFFS Coal vs. Nuclear High land use Coal Ample supply High net energy yield Very high air pollution High CO 2 emissions High land disruption from surface mining Low cost (with huge subsidies) Nuclear Ample supply of uranium Low net energy yield Low air pollution Low CO 2 emissions Much lower land disruption from surface mining Moderate land use High cost (even with huge subsidies)
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Dealing with Radioactive Wastes Produced by Nuclear Power Is a Difficult Problem High-level radioactive wastes – Must be stored safely for 10,000–240,000 years. Where to store it? – Deep burial: safest and cheapest option. – Transportation concerns. – Would any method of burial last long enough? – There is still no facility: NIMBY scenario. Can the harmful isotopes be changed into harmless isotopes? (working on it, $$$).
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Will Nuclear FUSION Save Us? “Nuclear fusion is the power of the future and always will be”. Still in the laboratory phase after 50 years of research and $34 billion dollars. 2006: U.S., China, Russia, Japan, South Korea, and European Union; – Will build a large-scale experimental nuclear fusion reactor by 2040.
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With great power comes great responsibility
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