14-1 Plutonium Chemistry From: Chemistry of actinides §Nuclear properties and isotope production...
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Transcript of 14-1 Plutonium Chemistry From: Chemistry of actinides §Nuclear properties and isotope production...
![Page 1: 14-1 Plutonium Chemistry From: Chemistry of actinides §Nuclear properties and isotope production §Pu in nature §Separation and Purification §Atomic properties.](https://reader036.fdocuments.in/reader036/viewer/2022062321/56649e605503460f94b5ab41/html5/thumbnails/1.jpg)
14-1
Plutonium Chemistry• From: Chemistry of actinides
§ Nuclear properties and isotope production
§ Pu in nature§ Separation and
Purification§ Atomic properties§ Metallic state§ Compounds § Solution chemistry
• Isotopes from 228≤A≤247• Important isotopes
§ 238Puà 237Np(n,g)238Np
* 238Pu from beta decay of 238Np
* Separated from unreacted Np by ion exchange
à Decay of 242Cmà 0.57 W/g à Power source for space
exploration* 83.5 % 238Pu, chemical
form as dioxide* Enriched 16O to limit
neutron emissionØ 6000 n s-1g-1
Ø 0.418 W/g PuO2
à 150 g PuO2 in Ir-0.3 % W container
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14-2
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 oreà n,g reaction
* Neutrons fromØ SF of UØ neutron
multiplication in 235U
Ø a,n on light elements
* 24.2 fission/g U/hr, need to include neutrons from 235U
• 244Pu§ Based on Xe isotopic ratios
à SF of 244Pu§ 1E-18 g 244Pu/g bastnasite mineral
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14-3
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)à Small energy separations between oxidation statesà All states can be prepared
* Pu(III) and (IV) more stable in acidic solutions* Pu(V) in near neutral solutions
Ø Dilute Pu solutions favored* Pu(VI) and (VII) favored in basic solutions
Ø Pu(VII) stable only in highly basic solutions and strong oxidizing conditions
§ Some evidence of Pu(VIII)
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14-4
Pu solution spectroscopy• A few sharp bands
§ 5f-5f transitionsà More intense than 4f of
lanthanidesà Relativistic effects accentuate
spin-orbit couplingà Transitions observed
spectroscopically* Forbidden transitions* Sharp but not very intense
• Pu absorption bands in visible and near IR region§ Characteristic for each oxidation
state
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14-5
Pu Hydrolysis/colloid formation
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14-6
Pu solution chemistry• Nitrates
§ Bidentate and planar geometryà Similar to carbonates but much
weaker ligand§ 1 or more nitrates in inner sphere
• Peroxide§ No confirmed structure§ Pu2(m-O2)2(CO3)6
8- contains doubly bridged Pu-O core
• Halides§ Studies related to Pu separation and
metal formation§ Solid phase double salts discussed
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14-7
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
à Precipitation of BiPO4 in acid carries tri- and tetravalent actinides* Bismuth nitrate and phosphoric acid* Separation of solid, then oxidation to Pu(VI)
à Sulfuric acid forms solution U sulfate, preventing precipitation
§ Used after initial purification methods§ LaF3 for precipitation of trivalent and tetravalent actinides
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14-8
Metallic Pu• Interests in
processing-structure-properties relationship
• Reactions with water and oxygen
• Impact of self-irradiation
Density 19.816 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−1Formation of Pu metal
• Ca reduction• Pyroprocessing
§ PuF4 and Ca metalà Conversion of oxide to fluorideà Start at 600 ºC goes to 2000 ºCà Pu solidifies at bottom of crucible
§ Direct oxide reductionà Direct reduction of oxide with Ca metalà PuO2, Ca, and CaCl2
§ Molten salt extractionà Separation of Pu from Am and
lanthanidesà Oxidize Am to Am3+, remains in salt phaseà MgCl2 as oxidizing agent
* Oxidation of Pu and Am, formation of Mg
* Reduction of Pu by oxidation of Am metal
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14-9
Pu metal• Electrorefining
§ Liquid Pu oxidizes from anode ingot into salt electrode
§ 740 ºC in NaCl/KCl with MgCl2 as oxidizing agentà Oxidation to Pu(III)à Addition of current causes reduction of
Pu(III) at cathodeà Pu drips off cathode
• Zone refining (700-1000 ºC)§ Purification from trace impurities
à Fe, U, Mg, Ca, Ni, Al, K, Si, oxides and hydrides
§ Melt zone passes through Pu metal at a slow rateà 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
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14-10
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
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14-11
Phase never observed, slow kinetics
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14-12
Metallic Pu
• Electronic structure shows competition between itinerant and localized behavior§ Boundary between magnetic
and superconductivity§ 5f electrons 2 to 4 eV bands,
strong mixingà Polymorphismà Solid state instabilityà Catalytic activity
• Isolated Pu 7s25f6, metallic Pu 7s26d15f5
§ Lighter than Pu, addition f electron goes into conducting band
§ Starting at Am f electrons become localizedà Increase in atomic
volume
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14-13
Pu phase transitions
demonstrates change in f-electron behavior at Pu
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14-14
Relativistic effects
• bandwidth narrows with increasing orbital angular momentum§ Larger bands increase
probability of electrons movingà 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
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14-15
• 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
Arrhenius Curves for Oxidation of Unalloyed and Alloyed Plutonium in Dry Air and Water Vapor
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14-16
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 à 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,
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14-17
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
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14-18
Hydride-Catalyzed Oxidation of Pu
• After the hydride-coated metal or alloy is exposed to O2, oxidation of the pyrophoric PuHx forms 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 interfaceà 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
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14-19
Pu oxide• Pu storage, fuel, and power
generators• Important species
§ Corrosion§ Environmental behavior
• Different Pu oxide solid phases§ PuO§ Pu2O3
à Composition at 60 % O
à Different forms at PuOx
* x=1.52, bcc* x=1.61, bcc
§ PuO2
à fcc, wide composition range (1.6 <x<2)
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14-20
Pu oxide preparation• Pu2O3
§ Hexagonal (A-Pu2O3) and cubic (C-Pu2O3)à Distinct phases that can co-existà No observed phase transformation
* Kinetic behavior may influence phase formation of cubic phaseØ C-Pu2O3 forms on PuO2 of d-stabilied metal when
heated to 150-200 °C under vacuumØ Metal and dioxide fcc, favors formation of fcc Pu2O3
Ø Requires heating to 450 °C to produce hexagonal form
Ø Not the same transition temperature for reverse reaction
Ø Indication of kinetic effect§ Formed by reaction of PuO2 with Pu metal, dry H2, or C
à A-Pu2O3 formedà PuO2+Pu2Pu2O3 at 1500 °C in Ta crucible
* Excess Pu metal removed by sublimation à 2PuO2+CPu2O3 + CO
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14-21
Pu oxide preparation• Hyperstoichiometric sesquioxide (PuO1.6+x)
§ Requires fast quenching to produce of PuO2 in meltà Slow cooling resulting in C-Pu2O3 and PuO2-x
à x at 0.02 and 0.03• Substoichiometric PuO2-x
§ From PuO1.61 to PuO1.98
à Exact composition depends upon O2 partial pressure§ Single phase materials
à Lattice expands with decreasing O
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14-22
Pu oxide preparation• PuO2
§ Pu metal ignited in air§ Calcination of a number of Pu compounds
à No phosphatesà 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 solidà Rate of heating can effect composition due to decomposition and gas evolution
§ PuO2 is olive greenà Can vary due to particle size, impurities
§ Pressed and sintered for heat sources or fuel§ Sol-gel method
à Nitrate in acid injected into dehydrating organic (2-ethylcyclohexanol)à Formation of microspheres
* Sphere size effects color
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14-23
U-Pu-Oxides
• MOX fuel§ 2-30 % PuO2
• Lattice follows Vegard’s law
• Different regions§ Orthorhombic U3O8
phase§ Flourite dioxide
à Deviations from Vegard’s law may be observed from O loss from PuO2 at higher temperature