1Fri, Apr. 20, 2007 Phy107 Lecture 331From last time…• Radioactive decay: alpha, beta, gamma• Radioactive half-life• Decay type understood in terms of numberneutrons, protons.Today:• Nuclear Fission and FusionFri, Apr. 20, 2007 Phy107 Lecture 332Hour Exam 3• Average = 14.3/200246810120 2 4 6 8 10 12 14 16 18 20Phy107 Spr07 HE3# PEOPLE# CORRECTFri, Apr. 20, 2007 Phy107 Lecture 333Energy stored in the nucleusNPThesepulledtogether bystrong forceNPRequires lots ofwork to pullthem apartSo energy of nucleus is LESS than that of isolated nucleons……and energy is released when nucleons bind together.Fri, Apr. 20, 2007 Phy107 Lecture 334Binding energy of different nucleiEnergy of separatednucleonsMass difference / nucleon(MeV/c2)Fri, Apr. 20, 2007 Phy107 Lecture 335Energy production•Coal-burning plant–10,000 tons coal/dayto produce 1,000 MW•Fission reactor–Uses 100 tons uranium/yrto produce 1,000 MW•Hydroelectric plant–Uses 60,000 tons/sec waterto produce 1,000 MWFri, Apr. 20, 2007 Phy107 Lecture 336Another comparisonHow can we release this energy?2Fri, Apr. 20, 2007 Phy107 Lecture 337QuestionEnergy ofseparatednucleonsMass difference /nucleon (MeV/c2)Suppose we could split the Iron (Fe) nucleus into two equal parts. Does this require energy input, or is energy released?A. ReleasedB. InputC. Same before & afterFri, Apr. 20, 2007 Phy107 Lecture 338Differences between nuclei• Schematic view ofprevious diagram•56Fe is most stable• Move toward lowerenergies by fission orfusion.• Energy releasedrelated to differencein binding energy.Fri, Apr. 20, 2007 Phy107 Lecture 339Nuclear fission• A heavy nucleus is split apart into twosmaller ones.• Energy is released because the lighter nucleiare more tightly bound, less mass• E=mc2, energy is releasedFri, Apr. 20, 2007 Phy107 Lecture 3310Fission• Fission occurs when a heavy nucleus breaks apartinto smaller pieces.• Does not occur spontaneously, but is induced bycapture of a neutronFri, Apr. 20, 2007 Phy107 Lecture 3311Neutron capture• When neutron is captured, 235U becomes 236U– Only neutron # changes, same number of protons.Nucleus distorts andoscillate, eventuallybreaking apart(fissioning)Fri, Apr. 20, 2007 Phy107 Lecture 3312Neutron production• Fission fragmentshave too manyneutrons to bestable.• So free neutrons areproduced in additionto the large fissionfragments.• These neutrons caninitiate more fissionevents# protons# neutrons3Fri, Apr. 20, 2007 Phy107 Lecture 3313Chain reaction• If neutrons produced by fissioncan be captured by othernuclei, fission chain reactioncan proceed.Fri, Apr. 20, 2007 Phy107 Lecture 3314Neutrons• Neutrons may be captured by nuclei that donot undergo fission– Most commonly, neutrons are captured by 238U– The possibility of neutron capture by 238U is lowerfor slow neutrons.• The moderator helps minimize the capture ofneutrons by 238U by slowing them down, makingmore available to initiate fission in 235U.Fri, Apr. 20, 2007 Phy107 Lecture 3315The critical mass• An important detail is the probability ofneutron capture by the 235U.• If the neutrons escape before being captured,the reaction will not be self-sustaining.• Neutrons need to be slowed down to encouragecapture by U nucleus• The mass of fissionable material must be largeenough, and the 235U fraction high enough, tocapture the neutrons before they escape.Fri, Apr. 20, 2007 Phy107 Lecture 3316The first chain reaction• Construction of CP-1,(Chicago Pile NumberOne) under the footballstadium in an abandonedsquash court.• A ‘pile’ of graphite,uranium, and uraniumoxides.• Graphite = moderator,uranium for fission.• On December 2, 1942:chain reaction produced1/2 watt of power.• 771,000 lbs graphite,80,590 pounds of uranium oxide and12,400 pounds of uranium metal,• Cost ~ $1 million.• Shape was flattened ellipsoid25 feet wide and 20 feet high.Fri, Apr. 20, 2007 Phy107 Lecture 3317Pile assemblyLevel 3Graphite layers formthe base of the pile.Level 7Uranium oxidepseudospheresstart at level 7Level 10Tenth layer ofgraphite blockscontainingpseudospheres ofuranium oxideLevel 19The 19th layer ofgraphite coveringlayer 18 containingslugs of uraniumoxide.Fri, Apr. 20, 2007 Phy107 Lecture 3318Binding energy /nucleonMass number235UraniumFissionfragmentsHow much energy?Bindingenergy/nucleon ~1MeV lessfor fission fragmentsthan for originalnucleusThis differenceappears as energy.Energy/nucleonreleased by fission4Fri, Apr. 20, 2007 Phy107 Lecture 3319Energy released•235U has 235 total nucleons,so ~240 MeV released in one fusion event.•235U has molar mass of ~235 gm/mole– So 1 kg is ~ 4 moles = 4x(6x1024)=2.5x1025 particles• Fission one kg of 235U– Produce ~6x1033 eV = 1015 Joules– 1 kilo-ton = 1,000 tons of TNT = 4.2x1012 Joules– This would release ~250 kilo-tons of energy!!!• Chain reaction suggests all this could bereleased almost instantaneously.Fri, Apr. 20, 2007 Phy107 Lecture 3320Uranium isotopes•235U will fission.• does not. When it absorbs neutron, it becomes , beta decays(neutron changes to proton) to , t1/2=23 min• This quickly beta decays to , t1/2=2.3 days ! 92239U ! 92238U ! 93239Np ! 94239PuUranium, Neptunium, Plutonium1941: discovered that Pu will fission.Fission limited to 235U, 239PuFri, Apr. 20, 2007 Phy107 Lecture 3321Uranium isotopes• Only the lessabundant 235U willfission.• Natural abundanceis less than 1%,most is 238UFri, Apr. 20, 2007 Phy107 Lecture 3322Where does uranium come from?•Uranium is one most abundantelements, but in lowconcentration•E.g. uranium is mixed withgranite, covering 60% of theEarth’s crust.•But only four parts of uraniumper million million parts ofgranite.Fri, Apr. 20, 2007 Phy107 Lecture 3323Uranium ore processing• Mechanically crush the ore• Acid treatment to separate the uranium metal from rock.• Purified with chemicals to leach out (dissolve) the uranium.• Chemically precipitate uranium-rich fraction.• Dry the uranium-rich solution, giving U3O8, (yellowcake)• Yellowcake packaged into special steel drums, 400 kg when full.• Hauled by truck to uranium refinery.Barrel ofyellowcakeFri, Apr. 20, 2007 Phy107 Lecture 3324Uranium Hexafluoride• Yellowcake converted to Uranium Hexafluouride• Main use is separation of two main isotopes of uranium;235U has only 0.71% natural abundance.• What is UF6 like?• White crystalline solid at room temperatureSublimes (turns to gas) at
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