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CHEM 312: Part 2 Lecture 14 Plutonium Chemistry

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1 CHEM 312: Part 2 Lecture 14 Plutonium Chemistry
Readings Pu chemistry Challenges of Pu chemistry Pu book/LASCIENCE.PDF Nuclear properties and isotope production Pu in nature Pu solution chemistry Separation and Purification Atomic properties Metallic state Compounds 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

2 Metallic Pu Interests in processing-structure-properties relationship
Reactions with water and oxygen Impact of self-irradiation Density g·cm−3 Liquid density at m.p. 16.63 g·cm−3 Melting point 912.5 K Boiling point 3505 K Heat of fusion 2.82 kJ·mol−1 Heat of vaporization 333.5 kJ·mol−1 Heat capacity (25 °C) 35.5 J·mol−1·K−1 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 ( 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

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 Pu, degree of f electron localization varies with phase
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 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 PuO2 in 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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