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Plutonium Chemistry

Plutonium Chemistry. Isotopes from 228≤A≤247 Important isotopes 238 Pu 237 Np(n, g ) 238 Np 238 Pu from beta decay of 238 Np Separated from unreacted Np by ion exchange Decay of 242 Cm 0.57 W/g Power source for space exploration 83.5 % 238 Pu, chemical form as dioxide

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Plutonium Chemistry

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  1. Plutonium Chemistry • Isotopes from 228≤A≤247 • Important isotopes • 238Pu • 237Np(n,g)238Np • 238Pu from beta decay of 238Np • Separated from unreacted Np by ion exchange • Decay of 242Cm • 0.57 W/g • Power source for space exploration • 83.5 % 238Pu, chemical form as dioxide • Enriched 16O to limit neutron emission • 6000 n s-1g-1 • 0.418 W/g PuO2 • 150 g PuO2 in Ir-0.3 % W container • From: Chemistry of actinides • Nuclear properties and isotope production • Pu in nature • Separation and Purification • Atomic properties • Metallic state • Compounds • Solution chemistry

  2. Pu nuclear properties • 239Pu • 2.2E-3 W/g • Basis of formation of higher Pu isotopes • 244-246Pu first from nuclear test • Higher isotopes available • Longer half lives suitable for experiments • Most environmental Pu due to anthropogenic sources • 239,244Pu can be found in nature • 239Pu from nuclear processes occurring in U ore • n,g reaction • Neutrons from • SF of U • neutron multiplication in 235U • a,n on light elements • 24.2 fission/g U/hr, need to include neutrons from 235U • 244Pu • Based on Xe isotopic ratios • SF of 244Pu • 1E-18 g 244Pu/g bastnasite mineral

  3. Pu solution chemistry • Originally driven by the need to separate and purify Pu • Species data in thermodynamic database • Complicated solution chemistry • Five oxidation states (III to VII) • Small energy separations between oxidation states • All states can be prepared • Pu(III) and (IV) more stable in acidic solutions • Pu(V) in near neutral solutions • Dilute Pu solutions favored • Pu(VI) and (VII) favored in basic solutions • Pu(VII) stable only in highly basic solutions and strong oxidizing conditions • Some evidence of Pu(VIII)

  4. Pu solution spectroscopy • A few sharp bands • 5f-5f transitions • More intense than 4f of lanthanides • Relativistic effects accentuate spin-orbit coupling • Transitions observed spectroscopically • Forbidden transitions • Sharp but not very intense • Pu absorption bands in visible and near IR region • Characteristic for each oxidation state

  5. Pu Hydrolysis/colloid formation

  6. Pu solution chemistry • Nitrates • Bidentate and planar geometry • Similar to carbonates but much weaker ligand • 1 or more nitrates in inner sphere • Peroxide • No confirmed structure • Pu2(m-O2)2(CO3)68- contains doubly bridged Pu-O core • Halides • Studies related to Pu separation and metal formation • Solid phase double salts discussed

  7. Pu separations • 1855 MT Pu produced • Current rate of 70-75 MT/years • 225 MT for fuel cycle • 260 MT for weapons • Large scale separations based on manipulation of Pu oxidation state • Aqueous (PUREX) • Non-aqueous (Pyroprocessing) • Precipitation methods • Basis of bismuth phosphate separation • Precipitation of BiPO4 in acid carries tri- and tetravalent actinides • Bismuth nitrate and phosphoric acid • Separation of solid, then oxidation to Pu(VI) • Sulfuric acid forms solution U sulfate, preventing precipitation • Used after initial purification methods • LaF3 for precipitation of trivalent and tetravalent actinides

  8. Metallic Pu • Interests in processing-structure-properties relationship • Reactions with water and oxygen • Impact of self-irradiation Formation of Pu metal • Ca reduction • Pyroprocessing • PuF4 and Ca metal • Conversion of oxide to fluoride • Start at 600 ºC goes to 2000 ºC • Pu solidifies at bottom of crucible • Direct oxide reduction • Direct reduction of oxide with Ca metal • PuO2, Ca, and CaCl2 • Molten salt extraction • Separation of Pu from Am and lanthanides • Oxidize Am to Am3+, remains in salt phase • MgCl2 as oxidizing agent • Oxidation of Pu and Am, formation of Mg • Reduction of Pu by oxidation of Am metal

  9. Pu metal • Electrorefining • Liquid Pu oxidizes from anode ingot into salt electrode • 740 ºC in NaCl/KCl with MgCl2 as oxidizing agent • Oxidation to Pu(III) • Addition of current causes reduction of Pu(III) at cathode • Pu drips off cathode • Zone refining (700-1000 ºC) • Purification from trace impurities • Fe, U, Mg, Ca, Ni, Al, K, Si, oxides and hydrides • Melt zone passes through Pu metal at a slow rate • Impurities travel in same or opposite direction of melt direction • Vacuum distillation removes Am • Application of magnetic field levitates Pu http://arq.lanl.gov/source/orgs/nmt/nmtdo/AQarchive/98fall/magnetic_levitation.html

  10. Metallic Pu • Pu liquid is denser that 3 highest temperature solid phases • Liquid density at 16.65 g/mL • Pu contracts 2.5 % upon melting • Pu alloys and the d phase • Ga stabilizes phase • Complicated phase diagram

  11. Phase never observed, slow kinetics

  12. Metallic Pu • Electronic structure shows competition between itinerant and localized behavior • Boundary between magnetic and superconductivity • 5f electrons 2 to 4 eV bands, strong mixing • Polymorphism • Solid state instability • Catalytic activity • Isolated Pu 7s25f6, metallic Pu 7s26d15f5 • Lighter than Pu, addition f electron goes into conducting band • Starting at Am f electrons become localized • Increase in atomic volume

  13. Pu phase transitions demonstrates change in f-electron behavior at Pu

  14. Relativistic effects • bandwidth narrows with increasing orbital angular momentum • Larger bands increase probability of electrons moving • d and f electrons interact more with core electrons • Narrowing reflects • decreasing radial extent of orbitals with higher angular momentum, or equivalently • decrease in overlap between neighboring atoms • Enough f electrons in Pu to be significant • Relativistic effects are important • 5f electrons extend relatively far from nucleus compared to the 4f electrons • 5f electrons participate in chemical bonding • much-greater radial extent of the probability densities for 7s and 7p valence states compared with 5f valence states • 5f and 6d radial distributions extend farther than shown by nonrelativistic calculations • 7s and 7p distributions are pulled closer to ionic cores in relativistic calculations

  15. Arrhenius Curves for Oxidation of Unalloyed and Alloyed Plutonium in Dry Air and Water Vapor • ln of the reaction rate R versus 1/T • slope of each curve is proportional to the activation energy for the corrosion reaction • Curve 1 oxidation rate of unalloyed plutonium in dry air or dry O2 at a pressure of 0.21 bar. • Curve 2a increase in the oxidation rate when unalloyed metal is exposed to water vapor up to 0.21 bar, equal to the partial pressure of oxygen in air • Curves 2b and 2c show the moisture-enhanced oxidation rate at water vapor pressure of 0.21 bar in temperature ranges of 61°C–110°C and 110°C–200°C, respectively • Curves 1’ and 2’ oxidation rates for the δ-phase gallium-stabilized alloy in dry air and moist air (water vapor pressure ≤ 0.21 bar), respectively • Curve 3 transition region between the convergence of rates at 400°C and the onset of the autothermic reaction at 500°C • Curve 4 temperature-independent reaction rate of ignited metal or alloy under static conditions • rate is fixed by diffusion through an O2-depleted boundary layer of N2 at the gas-solid interface • Curve 5 temperature-dependent oxidation rate of ignited droplets of metal or alloy during free fall in air

  16. Oxide Layer on Plutonium Metal under Varying Conditions • corrosion rate is strongly dependent on the metal temperature • varies significantly with the isotopic composition,quantity, geometry, and storage configuration • steady-state oxide layer on plutonium in dry air at room temperature (25°C) is shown at the top • (a) Over time, isolating PuO2-coated metal from oxygen in a vacuum or an inert environment turns the surface oxide into Pu2O3 by the autoreduction reaction • At 25°C, the transformation is slow • time required for complete reduction of PuO2 depends on the initial thickness of PuO2 layer • highly uncertain because reaction kinetics are not quantified • above 150°C, rapid autoreduction transforms a several micrometer-thick PuO2 layer to Pu2O3 within minutes • (b) Exposure of the steady-state oxide layer to air results in continued oxidation of the metal • Kinetic data indicate that a one-year exposure to dry air at room temperature increases the oxide thickness by about 0.1 μm • At a metal temperature of 50°C in moist air (50% relative humidity), the corrosion rate increases by a factor of approximately 104 • corrosion front advances into unalloyed metal at a rate of 2 mm per year • 150°C–200°C in dry air, the rate of the autoreduction reaction increases relative to that of the oxidation reaction • steady-state condition in the oxide shifts toward Pu2O3,

  17. Rates for Catalyzed Reactions of Pu with H2, O2, and Air • Diffusion-limited oxidation data shown in gray compared to data for the rates of reactions catalyzed by surface compounds • oxidation rates of PuHx-coated metal or alloy in air • the hydriding rates of PuHx- or Pu2O3-coated metal or alloy at 1 bar of pressure, • oxidation rates of PuHx-coated metal or alloy in O2 • rates are extremely rapid, • values are constant • indicate the surface compounds act as catalysts

  18. Hydride-Catalyzed Oxidation of Pu • After the hydride-coated metal or alloy is exposed to O2, oxidation of the pyrophoric PuHxforms a surface layer of oxide and heat • H2 formed by the reaction moves into and through the hydride layer to reform PuHx at the hydride-metal interface • sequential processes in reaction • oxygen adsorbs at the gas-solid interface as O2 • O2 dissociates and enters the oxide lattice as an anionic species • thin steady-state layer of PuO2 may exist at the surface • oxide ions are transported across the oxide layer to the oxide-hydride interface • oxide may be Pu2O3 or PuO2–x (0< x <0.5 • Oxygen reacts with PuHx to form heat (~160 kcal/mol of Pu) and H2 • H2 produced at the oxide-hydride interface moves • through the PuHx layer to the hydride-metal interface • reaction of hydrogen with Pu produces PuH2 and heat

  19. Pu oxide • Pu storage, fuel, and power generators • Important species • Corrosion • Environmental behavior • Different Pu oxide solid phases • PuO • Pu2O3 • Composition at 60 % O • Different forms at PuOx • x=1.52, bcc • x=1.61, bcc • PuO2 • fcc, wide composition range (1.6 <x<2)

  20. Pu oxide preparation • Pu2O3 • Hexagonal (A-Pu2O3) and cubic (C-Pu2O3) • Distinct phases that can co-exist • No observed phase transformation • Kinetic behavior may influence phase formation of cubic phase • C-Pu2O3 forms on PuO2 of d-stabilied metal when heated to 150-200 °C under vacuum • Metal and dioxide fcc, favors formation of fcc Pu2O3 • Requires heating to 450 °C to produce hexagonal form • Not the same transition temperature for reverse reaction • Indication of kinetic effect • Formed by reaction of PuO2 with Pu metal, dry H2, or C • A-Pu2O3 formed • PuO2+Pu2Pu2O3 at 1500 °C in Ta crucible • Excess Pu metal removed by sublimation • 2PuO2+CPu2O3 + CO

  21. Pu oxide preparation • Hyperstoichiometricsesquioxide (PuO1.6+x) • Requires fast quenching to produce of PuO2 in melt • Slow cooling resulting in C-Pu2O3 and PuO2-x • x at 0.02 and 0.03 • Substoichiometric PuO2-x • From PuO1.61 to PuO1.98 • Exact composition depends upon O2 partial pressure • Single phase materials • Lattice expands with decreasing O

  22. Pu oxide preparation • PuO2 • Pu metal ignited in air • Calcination of a number of Pu compounds • No phosphates • Pu crystalline PuO2 formed by heating Pu(III) or Pu(IV) oxalate to 1000 °C in air • Oxalates of Pu(III) forms a powder, Pu(IV) is tacky solid • Rate of heating can effect composition due to decomposition and gas evolution • PuO2 is olive green • Can vary due to particle size, impurities • Pressed and sintered for heat sources or fuel • Sol-gel method • Nitrate in acid injected into dehydrating organic (2-ethylcyclohexanol) • Formation of microspheres • Sphere size effects color

  23. U-Pu-Oxides • MOX fuel • 2-30 % PuO2 • Lattice follows Vegard’s law • Different regions • Orthorhombic U3O8 phase • Flourite dioxide • Deviations from Vegard’s law may be observed from O loss from PuO2 at higher temperature

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