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RFSS: Part 2 Lecture 14 Plutonium Chemistry

RFSS: Part 2 Lecture 14 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

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RFSS: Part 2 Lecture 14 Plutonium Chemistry

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  1. RFSS: Part 2 Lecture 14 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: Pu chapter • http://radchem.nevada.edu/classes/rdch710/files/plutonium.pdf • Nuclear properties and isotope production • Pu in nature • Separation and Purification • Atomic properties • Metallic state • Compounds • Solution chemistry

  2. 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

  3. 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

  4. Pu phase stability • 6 different Pu solid phases • 7th phase at elevated pressure • fcc phase least dense • Energy levels of allotropic phases are very close to each other • Pu extremely sensitive to changes in temperature, pressure, or chemistry • Densities of allotropes vary significantly • dramatic volume changes with phase transitions • Crystal structure of allotropes closest to room temperature are of low symmetry • more typical of minerals than metals. • Pu expands when it solidifies from a melt • Low melting point • Liquid Pu has very large surface tension with highest viscosity known near melting point • Pu lattice is very soft vibrationally and very nonlinear

  5. Pu metal phases • Low symmetry ground state for a phase due to 5f bonding • Higher symmetry found in transition metals • f orbitals have odd symmetry • Basis for low symmetry (same as p orbitals Sn, In, Sb, Te) • odd-symmetry p orbitals produce directional covalent-like bonds and low-symmetry noncubic structures • Recent local density approximation (LDA) electronic-structure calculations show narrow width of f bands leads to low-symmetry ground states of actinides • Bandwidths are a function of volume. • narrower for large volumes

  6. Pu metal phase • atomic-sphere approximation calculations for contributions to orbitals • d fcc phase • If Pu had only f band contribution equilibrium lattice constant would be smaller than measured • Contribution from s-p band stabilizes larger volume • f band is narrow at larger volume (low symmetry) • strong competition between repulsive s-p band contribution and attractive f band term induces instability near ground state • density-of-states functions for different low-symmetry crystal structures • total energies for crystal structures are very close to each other

  7. Pu metal phase • f-f interaction varies dramatically with very small changes in interatomic distances • lattice vibrations or heating • f-f and f-spd interactions with temperature results in localization as Pu transforms from α- to δ-phase • Low Pu melting temperature due to f-f interaction and phase instability • Small temperature changes induce large electronic changes • small temperature changes produce relatively large changes in free energy • Kinetics important in phase transitions

  8. For actinides f electron bonding increases up to Pu • Pu has highest phase instability • At Am f electrons localize completely and become nonbonding • At Am coulomb forces pull f electrons inside valence shell • 2 or 3 electrons in s-p and d bands • For Pu, degree of f electron localization varies with phase

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

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

  11. Phase never observed, slow kinetics

  12. Metallic Pu • Other elements that stabilize d phase • Al, Ga, Ce, Am, Sc, In, and Tl stabilize phase at room temperature • Si, Zn, Zr, and Hf retain phase under rapid cooling • Microstructure of d phase due to Ga diffusion in cooling • Np expands a and b phase region • b phase stabilized at room temperature with Hf, Ti, and Zr • Pu eutectics • Pu melting point dramatically reduced by Mn, Fe, Co, or Ni • With Fe, mp=410 °C, 10 % Fe • Used in metallic fuel • Limit Pu usage (melting through cladding) • Interstitial compounds • Large difference in ionic radii (59 %) • O, C, N, and H form interstitial compounds

  13. Modeling Pu metal electronic configuration • Pu metal configuration 7s26d15f5 • From calculations, all eight valence electrons are in conduction band, • 5f electrons in α-plutonium behave like 5d electrons of transition metals than 4f of lanthanides • Bonding and antibonding orbitals from sum and differences of overlapping wavefunctions • Complicated for actinides • Small energy difference between orbital can overlap in solids • Accounts for different configurations

  14. Metallic Pu • Modeling to determine electronic structure and bonding properties • Density functional theory • Describes an interacting system of fermions via its density not via many-body wave function • 3 variables (x,y,z) rather than 3 for each electron • For actinides need to incorporate • Low symmetry structures • Relativistic effects • Electron-electron correlations • local-density approximation (LDA) • Include external potential and Coulomb interactions • approximation based upon exact exchange energy for uniform electron gas and from fits to correlation energy for a uniform electron gas • Generalized gradient approximation (GGA) • Localized electron density and density gradient • Total energy calculations at ground state

  15. Relativistic effects • Enough f electrons in Pu to be significant • Relativistic effects are important • 5f electrons extend relatively far from nucleus compared to 4f electrons • 5f electrons participate in chemical bonding • much-greater radial extent of 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

  16. Pu metal mechanical properties • Stress/strain properties • High strength properties bend or deform rather than break • Beyond a limit material abruptly breaks • Fails to absorb more energy • α-plutonium is strong and brittle, similar to cast iron • elastic response with very little plastic flow • Stresses increase to point of fracture • strength of unalloyed α-phase decreases dramatically with increasing temperature • Similar to bcc and hcp metals. • Pu-Ga δ-phase alloys show limited elastic response followed by extensive plastic deformation • low yield strength • ductile fracture • For α-Pu elastic limit is basically fracture strength • Pu-Ga alloy behaves more like Al • Fails by ductile fracture after elongation

  17. Pu mechanical properties • Tensile-test results for unalloyed Pu • Related to temperature and resulting change in phases • Strengths of α- and β-phase are very sensitive to temperature • Less pronounced for γ-phase and δ-phase • data represent work of several investigators • different purity materials, and different testing rates • Accounts for variations in values, especially for α-Pu phase

  18. Pu metal mechanical properties • Metal elastic response due to electronic structure and resulting cohesive forces • Metallic bonding tends to result in high cohesive forces and high elastic constants • Metallic bonding is not very directional since valence electrons are shared throughout crystal lattice • Results in metal atoms surrounding themselves with as many neighbors as possible • close-packed, relatively simple crystal structures • Pu 5f electrons have narrow conduction bands and high density-of-states • energetically favorable for ground-state crystal structure to distort to low-symmetry structures at room temperature • Pu has typical metal properties at elevated temperatures or in alloys

  19. Pu metal corrosion and oxidation • Formation of oxide layer • Can include oxides other than dioxide • Slow oxidation in dry air • Greatly enhanced oxidation rate in presence of water or hydrogen • Metal has pyrophoric properties • Corrosion depends on chemical condition of Pu surface • Pu2O3 surface layer forms in absence or low amounts of O2 • Promotes corrosion by hydrogen • Pu hydride (PuHx, where 1.9 < x < 3) increases oxidation rate in O2 by 1013 • PuO2+x surface layer forms on PuO2in presence of water • enhances bulk corrosion of Pu metal in moist air

  20. Pu oxidation in dry air • O2 sorbs on Pu surface to form oxide layer • Oxidation continues but O2 must diffuse through oxide layer • Oxidation occurs at oxide/metal interface • Oxide layer thickness initially increases with time based on diffusion limitation • At oxide thickness around 4–5 μm in room temperature surface stresses cause oxide particles to spall • oxide layer reaches a steady-state thickness • further oxidation and layer removal by spallation • Eventually thickness of oxide layer remains constant

  21. Oxidation kinetics in dry air at room temperature • steady-state layer of Pu2O3 at oxide-metal interface • Pu2O3 thickness is small compared with oxide thickness at steady state • Autoreduction of dioxide by metal at oxide metal interface produces Pu2O3 • Pu2O3 reacts with diffusing O2 to form dioxide

  22. Arrhenius Curves for Oxidation of Unalloyed and Alloyed Plutonium in Dry Air and Water Vapor • ln of reaction rate R versus 1/T • slope is proportional to activation energy for corrosion reaction • Curve 1 oxidation rate of unalloyed plutonium in dry air or dry O2 at a pressure of 0.21 bar. • Curve 2a to water vapor up to 0.21 bar • Curves 2b and 2c temperature ranges of 61°C–110°C and 110°C–200°C, respectively • Curves 1’ and 2’ oxidation rates for δ-phase gallium-stabilized alloy in dry air and moist air • Curve 3 transition region between convergence of rates at 400°C and onset of 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 gas-solid interface • Curve 5 temperature-dependent oxidation rate of ignited droplets of metal or alloy during free fall in air

  23. Oxide Layer on Plutonium Metal under Varying Conditions • corrosion rate is strongly dependent on metal temperature • varies significantly with isotopic composition, quantity, geometry, and storage configuration • steady-state oxide layer on plutonium in dry air at room temperature (25°C) • (a) Over time, isolating PuO2-coated metal from oxygen in a vacuum or an inert environment turns surface oxide into Pu2O3 by autoreduction reaction • At 25°C, transformation is slow • time required for complete reduction of PuO2 depends on 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 steady-state oxide layer to air results in continued oxidation of metal • Kinetic data indicate a one-year exposure to dry air at room temperature increases oxide thickness by about 0.1 μm • At a metal temperature of 50°C in moist air (50% relative humidity), 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, rate of autoreduction reaction increases relative oxidation reaction • steady-state condition in oxide shifts toward Pu2O3,

  24. Rates for Catalyzed Reactions of Pu with H2, O2, and Air • Plutonium hydride (PuHx) • fcc phase • forms a continuous solid solution for 1.9 < x < 3.0 • Pu(s) + (x/2)H2(g) → PuHx(s) • x depends on hydrogen pressure and temperature • Pu hydride is readily oxidized by air • Hydriding occurs only after dioxide layer is penetrated • Hydrogen initiates at a limited • hydriding rates values are constant • indicate surface compounds act as catalysts • hydride sites are most reactive location • Hydriding rate is proportional to active area covered by hydride • Temperatures between –55°C and 350°C and a H2 pressure of 1 bar • reaction at fully active surface consumes Pu at a constant rate of 6–7 g/cm2 min • Advances into metal or alloy at about 20 cm/h

  25. Hydride-Catalyzed Oxidation of Pu • hydride-coated Pu exposed to O2 • oxidation of PuHx forms surface layer of oxide with heat evolution • Produced H2 reforms PuHx at hydride-metal interface • Exothermic, helps drive reaction • sequential processes in reaction • oxygen adsorbs at gas-solid interface as O2 • O2 dissociates and enters oxide lattice as anionic species • thin steady-state layer of PuO2 may exist at surface • oxide ions are transported across oxide layer to 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

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