Plutonium chemistry
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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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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


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


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)


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


Pu Hydrolysis/colloid formation


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


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


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

http://arq.lanl.gov/source/orgs/nmt/nmtdo/AQarchive/98fall/magnetic_levitation.html


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


Phase never observed, slow kinetics


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


Pu phase transitions

demonstrates change in f-electron behavior at Pu


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


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


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,


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


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


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)


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


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


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


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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