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

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

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)

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

14-5

Pu Hydrolysis/colloid formation

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

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

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

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

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

14-11

Phase never observed, slow kinetics

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

14-13

Pu phase transitions

demonstrates change in f-electron behavior at Pu

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

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

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,

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

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

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)

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

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

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

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