Transcript Nuclear
Energy Systems & Climate Change Thus. 5 Nov. 2009 Ch.7: Nuclear Dr. E.J. Zita (& Cheri Lucas Jennings) [email protected] http://academic.evergreen.edu/curricular/energy/0910/home.htm What’s happening today: • Questions? Announcements? • Ch.7: Nuclear • Brief Reports at 2:30 • 3:15 Seminar – finishing McKibben Responses due this week to Brief Reports: Percent of electricity from nuclear Nuclear-generating capacity Fundamental Forces Gravity Electromagnetism Nuclear Unification http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html Discovery of the atomic nucleus 1909 Rutherford Nuclear strong force (vs. electric) Isotopes U 238 99.3 U 235 0.7 Isotopes Same number of protons = same chemistry pn protons Solve for m2 Element Nuclear binding energy Nuclear binding energy E=Dmc2 Fission → radioactive waste Fusion is safe, but works only in stars, so far Magnetic confinement fusion E=Dmc2: The nuclear difference Nuclear energy ~ 10 million x chemical energy 1 truckload Uranium/yr ~ 100 trainloads coal/wk E=Dmc2 really only applies to mass-energy transformations (not stretched rubber bands…) Nuclear Fusion in the Sun: 4H He + Dm Fusion: 4H He + Dm Nuclear fission Heavy, unstable nuclei can fall apart naturally. Throwing neutrons at them can make them split faster: Neutron-induced fission (Lise Meitner) Discovery of fission 1938 Hahn + Strassmann Meitner + Frisch Nuclear chain reaction: critical mass ~ 30 lb for U235 ~ 30 tonnes coal Controlled fission reaction: Moderator keeps neutron multiplication factor = 1 Moderator slows neutrons so they can fission U. Fast neutrons can’t do the job. Removal of graphite rods stops fission. Atomic mass Ex.7.5 showed that using a 5 kW electric dryer (powered by a 33% efficient nuclear plant) for an hour produces N=1.2x1018 nuclei of 239Pu (plutonium). Mass per nucleon = mn = 1.67 x 10-27 kg The mass of each 239Pu nucleus = m = 239 mn = _____ Total 239Pu mass produced = M = N m = ______ Nuclear reactors Light-Water reactors (LWR) need enriched U235 (ordinary water steam turbine electricity) •Boiling-water reactor (simple, 1/3 of LWRs) •Pressurized-water reactor (primary doesn’t boil) Pro: Safety: loss of coolant = loss of moderator Con: difficult to refuel CANDU (Deuterated, or heavy water + natural U238) •Continuous refueling capability, easy to steal More Nuclear reactors Graphite moderator Pro: continuous refueling capability Con: loss of coolant ≠ loss of moderator Chernobyl HTGR (High Temperature Gas-cooled Reactor) Pro: high safety Con: low performance Breeder reactors: first discuss beta decay… Beta decay (weak force) n p + e- + neutrino C N e neutrino 14 6 14 7 Breeder reactors Rare U235 is fissile when hit with neutrons Common U238 can transmute Pu contributes to fission power generation in old U reactors Breeder reactors Pro: * use up common U238 * operate at higher temperature (efficiency) Con: • higher temperature, higher risk of nuclear accident • Liquid sodium coolant – flammable with air contact • Plutonium = potent bomb fuel • Critical mass ~ 5 kg (see Example 7.5) Even France only uses one breeder. Plutonium reprocessing (Union of Concerned Scientists: www.ucsusa.org) • Reprocessing would increase the risk of nuclear terrorism • Reprocessing would increase the ease of nuclear proliferation • Reprocessing would hurt U.S. nuclear waste management efforts • Reprocessing would be very expensive Advanced reactor designs Standard LWR: coolant = moderator Advanced LWR: passive safety features Standardized design – easier to build Maximum nuclear efficiency: 36% Advanced HTGR: pebble-bed reactor pebbled fuel He gas coolant heat exchanger turbine Could burn Pu from old nuclear weapons Design efficiency 50% (not yet operational) Nuclear power plants Pressure vessel limits Thigh and efficiency Otherwise, much like other power plants Radioactivity Gamma rays: very high energy photons – zero mass (produced by excited nuclei) Alpha particles: very high mass (Helium nuclei) can have high or low kinetic energy If they penetrate matter, can do great damage. Most dangerous if ingested. Beta particles: electrons (or anti-electrons) Can have high or low kinetic energy Can slightly penetrate matter. (weak force) Alpha decay Alpha particle = helium nucleus 4 nn nucleons nucleons4 4 2 He pp protons Element protons2 X 2 He Radioactivity Gamma decay Alpha decay C14 from cosmic rays Cosmic rays excite N14 → decays to C14 Solar max: magnetic solar wind sweeps away cosmic rays → less *N14 → less C14 http://www.nuclearonline.org/newsletter/Oct05.htm Lower recent C14 /C12 from fossil fuel burning Little Ice Age: low solar magnetic activity more cosmic rays and C14 Evidence of anthropogenic source for greenhouse gases Nuclear Policy • High subsidies supported growth in industry in decades past • Safety regulations plus major cost and schedule overruns made nuclear start-ups increasingly diffiult • 1979 Three Mile Island accident “seriously damaged public confidence in nuclear power” • US nuclear in decline – no new plants in 30 years • 1986 Chernobyl near-meltdown, major irradiation of local area, contamination spreading to lesser extent throughout USSR, Europe, Asia. Undetermined # of lives lost Radioactive decay: l=decay rate DN l N Dt DN l N Dt dN l Ndt dN l dt N dN l Ndt dN l dt N N t dN N N l t dt 0 0 N ln l t N0 N lt e N0 N (t ) N 0e lt Half-life = T1/2 N (t ) N 0e lt N0 N (T1 2 ) 2 N0 lT N 0e 1 2 2 1 lT e 12 2 1 lT1 2 1 ln ln 2 ln e 2 ln 2 ln e T1 2 ln 2 l lT1 2 lT 12 Half-life Solve for n and then t… Measuring radiation Bequerel = 1 decay per second: but what kind of decay? How much energy? Curie = radioactivity of 1 g of 226Ra Consider effects on biological tissue: Rad = 0.01 J of radiation absorbed by 1 kg Also consider what kind of particles – alpha, beta, gamma? Most useful measure: Sv = Sievert = dose (in rad) * quality factor (QF) Radiation quality factor (QF) Higher QF = more dangerous radiation Type QF X and gamma rays ~1 Beta ~1 Fast protons 1 Slow neutrons ~3 Fast neutrons up to 10 Alpha particles and up to 20 heavy ions Chernobyl: how many deaths? http://www.nirs.org/ch20/index.htm http://www.nirs.org/reactorwatch/accidents/accidentshome.htm How many accidents unreported? http://www.iht.com/articles/2007/03/15/business/nuke.php More Nuclear Policy Advocates call for nuclear renaissance because: • Technology is well-established • We know it can produce high-density electric power • Since we are not willing to give up quality of life dependent on high-density power, nuclear and hydro are the only current options • Hydro is essentially fully developed in countries like the US, and has ecological costs of its own • Vitrification can address waste issues Waste disposal: Yucca Mountain? http://library.thinkquest.org/17940/texts/nuclear_waste_storage/nuclear_waste_storage.html Waste disposal: Vitrification? http://environment.pnl.gov/brochures/WTP.pdf http://picturethis.pnl.gov/PictureT.nsf/All/3U2S5D?opendocument UCS on nuclear 1. 2. 3. 4. 5. Need cheap, effective solutions to GW quickly Nuclear power is not the “silver bullet” Rapid major expansion of nuclear is not feasible Nuclear security is a major concern Research should continue, especially on nuclear waste issues UCS: Nuclear is not the solution to GW http://www.ucsusa.org/global_warming/solutions/nuclear-power-and-climate.html Brief Reports Please get / put homework from/on the front table Break… Seminar on last half of McKibben