Rfss part 2 lecture 14 plutonium chemistry
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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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Rfss part 2 lecture 14 plutonium chemistry

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

Metallic pu

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

Pu metal

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


Pu phase stability

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

Pu metal phases

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

Pu metal phase

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

Pu metal phase1

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

Rfss part 2 lecture 14 plutonium chemistry

  • 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

Rfss part 2 lecture 14 plutonium chemistry

Pu phase transitions

demonstrates change in f-electron behavior at Pu

Metallic pu1

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

Rfss part 2 lecture 14 plutonium chemistry

Phase never observed, slow kinetics

Metallic pu2

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

Modeling pu metal electronic configuration

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

Metallic pu3

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

Relativistic effects

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

Pu metal mechanical properties

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

Pu mechanical properties

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

Pu metal mechanical properties1

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

Pu metal corrosion and oxidation

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

Rfss part 2 lecture 14 plutonium chemistry

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

Oxidation kinetics in dry air at room temperature

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

Arrhenius curves for oxidation of unalloyed and alloyed plutonium in dry air and water vapor

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

Oxide layer on plutonium metal under varying conditions

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,

Rates for catalyzed reactions of pu with h 2 o 2 and air

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

Hydride catalyzed oxidation of pu

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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