The Fires of Nuclear Fission

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2/28/2012 What Are the Advantages and Disadvantages of Nuclear Energy? ã Nuclear power has a low environmental impact and a very low accident risk, but high costs, a low net energy yield, longlived radioactive wastes, vulnerability to sabotage, and the potential for spreading nuclear weapons technology have limited its use. The Fires of Nuclear Fission Questions to be answered today 1. What are the steps to using nuclear fission to generate electricity? 2. Advantages and disadvantages of usin
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  2/28/20121 The Fires of Nuclear Fission What Are the Advantages and Disadvantages of Nuclear Energy?  ã 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 tosabotage, and the potential for spreadingnuclear weapons technology have limited its use. Questions to be answered today 1.What are the steps to using nuclear fission togenerate electricity?2.Advantages and disadvantages of using nuclearfission as a power source?3.Why are Japan’s reactors in trouble?4.Compare Chernobyl to Japan’s current situation! 4-meter-tall sculpture of Einstein's 1905 E  = mc  2 formula at the 2006 Walk of Ideas, Berlin, Germany The Marvelous Nuclear Power ã What is nuclear power? Nuclear power plantsrun on the principle of  nuclear fission : The process of splitting a large nucleus intosmaller ones, usually by bombarding thetarget nucleus with neutrons. ã Important to be noted; The products of thisreaction actually possess slightly less massthan the reactants. 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 inenergy.  2/28/20122 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 eachother, 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. ã Uranium (element number 92) has several isotopes,and U-235 has a mass number of 235, written: ã In order for Uranium-235 to undergo fission, it mustbe struck by a neutron. ã A bare neutron has no protons, but a mass number of one, and can thus be written: U  23592 n 10 ã Uranium nucleus undergoes fission , breaking apartinto smaller nuclei. One such reaction is: ã Note that we put one neutron in, and got three out –and those three are important! They will initiate a chain reaction . n Kr  BanU  109236141561023592 3  ãA chain reaction is one in which the products of an initial step undergofurther reaction.ãHere, the three neutrons emitted bythe fission process can strike other  nearby U-235 atoms, and inducefission in them.ã... Producing more neutrons, whichcan go on to strike more nearby U-235 atoms...ãAn important concept with regard tochain reactions is that of  criticalmass ; The amount of fissionablematerial which is necessary tosustain the chain reaction. For U-235, this is 15 kg; If 15 kg of U-235is contained in the same place, it willundergo spontaneous fission. Thisis the principle behind nuclear bombs . ã Recall: ã If the mass numbers on both sides are equal, where isthe energy coming from? ã Recall that the actual mass of a nucleus is not simplythe mass number. n Kr  BanU  109236141561023592 3  ã In fact, an atom of U-235 weighs 235.043924 amu. ã An atom of Kr-92 weighs 91.926156 amu. ã An atom of Ba-141 weighs 140.914412 amu. ã A neutron weighs 1.00866 amu. ã So, the reactants weigh 236.052584 amu. ã The products weigh 235.866548 amu. ã Over the course of this reaction, 0.186036 amuof matter is converted into energy  . n Kr  BanU  109236141561023592 3   2/28/20123 ã Over the course of this reaction, 0.186036 amuof matter is converted into energy  . ã That’s about 1/1000 th of the total mass. ã How much energy is produced from the fission of 1 kgof U-235? E = mc 2 = [(1/1000)(1 kg)](9.0x10 16 m 2 /s 2 )= 9.0 x 10 13 Joules! ã This is the same amount of energy as from 33,000tons of TNT, or 3300 tons of coal. n Kr  BanU  109236141561023592 3  What are the steps tousing the massive energy of nuclear fission togenerate electricity?  How Does a Nuclear Fission Reactor Work? (1) ã Controlled nuclear fission reaction in a reactor  – Light-water reactors . ã Fueled by uranium ore and packed as pelletsin fuel rods and fuel assemblies . ã Control rods absorb neutrons. How Does a Nuclear Fission Reactor Work? (2) ã Water is the usual coolant . ã Containment shell around the core forprotection. ã Water-filled pools or dry casks for storage of radioactive spent fuel rod assemblies. Light water reactors ã 85% of world’s nuclear generated electricity(100% in US). ã High inefficient in terms of energy conversion (upto 83% lost as waste heat). ã There are three varieties of light water reactors:the pressurized water reactor (PWR), the boilingwater reactor (BWR), and (most designs of) thesupercritical water reactor (SCWR). The Koeberg nuclear power station located in South Africa, consisting of twopressurized water reactors fueled with uranium.  2/28/20124 The Palo Verde nuclear generating station located in Wintersburg, Arizona, about45miles (80km) west of central Phoenix. It is the largest nuclear generationfacility in the United States, averaging over 3.3gigawatts(GW) of electrical power production in 2008 to serve approximately 4million people. Nuclear fuel ã Made from uranium ore. ã Enriched to 3% of radioactive isotope U-235. ã Made into pellets, size of pencil, energyequivalent to 1 ton of coal. ã Pellets are packed into large pipes-fuel rods. ã Rods are grouped together into fuel assemblies,these assemblies are placed into reactor core. (Left) A typical nuclear fuel pellet.(Right) Nuclear fuel pelletsthat are ready for fuelassembly completion. Controlling the reaction ã Control rods placed between rods. ã Control rods moved in and out of the assemblies,absorbing neutrons which trigger the chainreaction. ã Water circulates through the assemblies,removing the heat, keeping the rods frommelting.  A pressurized water reactor head, with the control rods visible on the top. What happens if thereis no water? 
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