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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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 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

  • 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

Pu phase transitions

demonstrates change in f-electron behavior at Pu

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

Phase never observed, slow kinetics

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

  • 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 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

  • 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

  • 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

  • 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 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

  • 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

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

  • 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

  • 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

  • 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 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-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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