session 10 – nuclear power

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  • 1. Session 10 Nuclear PowerT. Ferguson, University of

2. Session 10 Nuclear Power Recall that nuclear supplies ~ 8 Quads to US annually (8% of total; 20% of electricity) Terminology Fuels, reserves, wastes Energy Release, Efficiencies Costs Status and Policy T. Ferguson, University of 3. Nuclear Power Basics Nuclear vs. chemical energy All energy derives from nuclear! Resulting sum of Fission: Splitting heavy atoms products has slightly less mass than sum Fusion: Combining lighter atoms of original reactants Fissionable isotope captures neutron, yields: Unstable isotope Fragments with high kinetic energy Neutrons Beta, gamma, neutrino emissions Moderator Control Rods T. Ferguson, University of 4. Terminology Nucleon Nuclide Radionuclide Isotope Alpha, Beta, Gamma Rays Fissionable Material Fertile Material EnrichmentT. Ferguson, University of 5. Radioactive Decay Fission might be best understood by first looking at how the most abundant, naturally occurring isotope of Uranium, U-238, decays: First, elements with atomic number above Lead tend to decay Decay implies transitioning to a stable element with smaller neutron-proton ratio U-238 has 146 neutrons, for an n/p of 1.587 This neutron ratio is the highest for any natural isotope U-238 decays by first emitting an alpha particle: 2n + 2p An alpha particle is identical to the Helium nucleus So U-238 loses 2n and 2p, reducing it to Thorium-234 But Thorium is also unstable, and emits a Beta particle: nuclear electron This, in effect, increases the proton count by 1, forcing the release of a neutron to keep the nucleon count constant T. Ferguson, University ofSo, Thorium-234 becomes Protactinium-234 (Z=91), which is also unstable . . . And eventually ends at Pb 6. Radioactive Decay Radioactive Half-life: time for half of atoms to decay If N=number of atoms present, and N0 = number of atoms initially, and = decay rate constant, Then N = N0 e t Set N=0.5N0 to solve for T1/2 U-238 half-life is 4.5 E 9 years (age of universe) T. Ferguson, University of 7. Radioactive Decay Radioactive Half-life: time for half of atoms to decay Radium: Discovered by Curies in 1898, T 1/2 of 1600 years, part of U-238 decay chain Decay rate of 1 gram of Radium is basis for unit of decay, the curie. So, the curie is a measure of the radioactivity of a material T. Ferguson, University of 8. Radioactive Decay Derivation of curie: If N = N0 e t, then = 0.693/T1/2. Given T1/2 for Ra-226 = 1600 yrs, = 1.375 E -11 sec-1. To obtain the decay rate, we need the number of atoms in one gram of Ra-226 T. Ferguson, University of 9. Radioactive Decay Ra-226 has atomic weight of about 226, so 1 kg-mol = 226 kg has Avogadros number of atoms (6.02 E 26), which becomes N0. 1 gram, therefore, contains 2.66 E 21 atoms, which is N The decay rate is N = 3.66 E 10 disintegrations per second (almost 40 billion events per second) (The curie is formally 3.7 E 10 disintegrations/s) T. Ferguson, University of 10. Nuclear Fission Energy (scattered) (absorption &)Uranium-235+NeutronsFission (absorption & capture)Radioactive fission productsNeutrons (about 2.5)Uranium-235T. Ferguson, University ofProcess repeats 11. Nuclear Fission 1. Neutrons are the key ingredient EnergyUranium-235+Neutrons3. Critical: Steady rate of chain reaction Subcritical: Decreasing reaction rate Supercritical: Increasing reaction rateFissionRadioactive fission products2. If at least one Neutrons of these results (about 2.5) in a second event, a self-sustaining fission chain reaction ensuesUranium-235T. Ferguson, University ofProcess repeats 12. Quote of the Week If we have in the future an accident where the reactors go critical, I would only pray for MiamiDade County since there is no way to evacuate the population today compared with in 1972, when the reactors were originally permitted," the president Rhonda Roff of an environmental group called "Save It Now, Glades" told AFP. Comment from article from AFP on Floridas electrical blackout of 2/26/08 T. Ferguson, University of 13. Nuclear Fission 1. Moderator EnergyUranium-235+NeutronsFissionRadioactive fission productsNeutrons (about 2.5) 2. Neutrons: W/O moderator: 2 MeV With moderator: 1/40 eV 3. Neutrons start with high energy, but are then thermalized by moderator T. Ferguson, University ofUranium-235Process repeats 14. Thermal Nuclear Fission vs. Fast Fission EnergyUranium-235+Neutrons1. U-235 only natural fuel that works with thermal neutrons 2. Probability of spontaneous fission of U-235 very, very small (1 per second, or 200 MeV=3.2E-11 J/s/kg) 3. Fission starts with absorption of neutron 4. Prob of absorption decreases with neutron energy (so moderator used in thermal reactors) 5. Fast fission reactors use other fuels able to fission with high energy neutronsT. Ferguson, University ofFissionRadioactive fission productsNeutrons (about 2.5)Uranium-235Process repeats 15. Thermal FissionT. Ferguson, University ofFrom Wikipedia: Accessed 2/28/08 16. Energy of Fission Fission of U-235 releases about 200 MeV per atom (recall that 1 electron volt = 1.6 E -19 J, or 200 MeV = 3.2 E -11 Joules) Compare to combustion of Carbon with Q=94E6 cal/kg-mol 4.1 eV per atom 50 million times more energy on atom-atom basis 2.5 million times more energy on weight basis Instead of 3 million tons of coal per year for 1000 MW plant, nuclear fission would require 1.2 tons of U-235T. Ferguson, University of 17. PWR Fuel Assembly Sample PWR Fuel Assembly Array of 14X14 rods 179 fuel rods 16 control rods - ganged 1 instrumentation rod Assembly is 7 X 7, 12 ft tallFuel: U-235 enriched from natural concentration of 0.71% to a few % Fission of U-238 possible only with fast neutrons T. Ferguson, University ofFrom Accessed 2/28/08; and instructor notes 18. The Uranium Fuel Cycle - Sources -T. Ferguson, University ofSource: International Atomic Energy Agency 19. Fuel Cycle Annual mass flows for 1000 MWe LWR Ore 86,000 tonsStorage (US) Reprocessing (UK, France)T. Ferguson, University ofUF6 gas 203 tonsU3O8 solid 162 tonsSpent Fuel 36 tonsEnriched UF6 53 tonsReactorUO2 Fuel 36 tonsLow Level Waste 50 tonsAdapted from Tester, et al, Sustainable Energy. Figure 8.6 20. Reactor Designs Designs Currently in Operation (Generation II) PWR Pressurized Water Reactor (Westinghouse) BWR Boiling Water Reactor (GE) GCR Gas Cooled Reactor LMFBR Liquid Metal Fast Breeder Reactor PHWR Pressurized Heavy Water Reactor RBMK Similar to BWR T. Ferguson, University of 21. New Reactor Designs Designs Submitted in Recent Applications (Generation III and III+): AP1000 (Westinghouse) EPR (Areva) ESBWR (GE) ABWR (GE) US-APWR (Mitsubishi) 1T. Ferguson, University of(6 COLs)1 (3) (5) (1) (1)Combined Licenses, as of 10/21/08. Covers 25 new units 22. New Reactor Locations, UST. Ferguson, University ofSource: NRC 23. T. Ferguson, University of 24. Boiling Water ReactorSource: US NRC T. Ferguson, University of 25. Pressurized Water ReactorT. Ferguson, University of 26. Nuclear Power Performance Water in liquid state limited to 705 F Reactors (PWR, BWR) limited to