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Lecture 8: Plutonium Chemistry

• From: Chemistry of actinides Nuclear properties and isotope production Pu in nature Separation and Purification Atomic properties Metallic state Compounds Solution chemistry

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Pu nuclear properties

• Isotopes from 228≤A≤247• Important isotopes

238Pu 237Np(n,)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

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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

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Pu in nature• Most Pu due to anthropogenic sources• 239,244Pu can be found in nature

239Pu from nuclear processes occurring in U ore n, reaction

* Neutrons from SF of U neutron multiplication in 235U ,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

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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

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Pu separations• Solvent extraction

Some novel chemistry with third phase formation

http://www.nap.edu/books/0309052262/html/41.html

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Pu separations• Extraction chromatography

Extractant on solid support• Ion-exchange

Both cation and anion exchange Anion exchange based on formation of appropriate species in acidic

solution Change of solution impact sorption to column

• Pu separation Sorb Pu(IV,VI) in 6 M acid, reduce to Pu(III)

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Pu anion exchange

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Pu cation exchange

• General cation exchange trends for Pu HN03, H2S04, and HC104 show stronger influence than HC1 Strong increase in distribution coefficient in HClO4 at high

acidities exhibited for Pu(III) and Pu(VI)

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Pu separations• Alkaline solutions

Need strong ligands that can compete with hydroxide to form different species F-, CO3

2-, H2O2

* High solubility, based on oxidation state* Stabilize Pu(VII)

• Room temperature ionic liquids Quaternary ammonium with anions

AlCl4-, PF6

-

Liquid-liquid extraction Electrochemical disposition

N R

NTf2

N NNTf2 N NTf2N

O

NTf2

NS S

O

O O

O

CF3F3C

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Pu separations

• Halide volatility (PuF6, PuCl6) PuO2 in fluidized bed reactor with fluorine at 400°

C Can substitute NH4HF2

for some fluorination Also use of O2F2

PuF6 decomposes to PuF4 and F2 in a thermal decomposition column

• Supercritical fluid extraction Most research with CO2

Use complexants dissolved in SCF TBP.HNO3, TTA for extraction from soil

Change of pressure to achieve separations

8-18

Pu atomic properties• Ground state configuration [Rn]5f67s2 • Term symbol 7F0 • Optical emission Pu I spectra

Within 3 eV (24000 cm-1) 5f56d7s2, 5f66d7s, 5f56d27s, 5f67s7p, 5f57s27p, and 5f56d7s7p

* Large number of electronic states and thousands of spectral lines

Isotopic influence on spectra

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Pu atomic properties

• Moessbauer spectroscopy 238,239,240Pu

238Np beta decay, 44 keV photon 239Np beta decay, 57.3 keV photonAlpha decay of 244Cm, 42.9 keV photon

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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−1

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Preparation 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

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Pu metal

• Electrorefining Liquid Pu oxidizes from anode ingot into molten salt electrode 740 ºC in NaCl/KCl with MgCl2 as oxidizing

agentOxidation to Pu(III)Addition of current causes reduction of

Pu(III) at cathodePu drips off cathode

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Pu metal• 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

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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 the allotropes vary significantly

dramatic volume changes with phase transitions• Crystal structure of the 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 the melting point.• Pu lattice is very soft vibrationally and very nonlinear

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Pu metal phases• Low symmetry ground state for 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 the actinides Bandwidths are a function of volume.

narrower for large volumes

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Pu metal phase• ground-state as a function

of bandwidth for Nb and U and bct (body-centered

tetragonal) and ort (orthorhombic), bcc (body-centered cubic)

• When the f band in uranium is forced to be broader than 7 eV, the high-symmetry bcc structure is stable

• Demonstrates narrow bands favor lower-symmetry structures for U, not that niobium

• true equilibrium bandwidths (Weq) are narrow (larger volumes) for the light actinides.

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Pu metal phase• atomic-sphere approximation

calculations for contributions to orbitals fcc phase

• If Pu had only an f band contribution equilibrium lattice constant smaller than measured

• Contribution from s-p band stabilizes larger volume

• f band is narrow at larger volume (low symmetry)

• strong competition between the repulsive s-p band contribution and the attractive f band term induces instability near the ground state

• density-of-states functions for different low-symmetry crystal structures

• are very similar total energies for different low-

symmetry crystal structures are very close to each other

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Pu metal phase• For actinides f electron bonding increases up to Pu

Pu has the highest phase instability• At Am the f electrons localize completely and become nonbonding

At Am coulomb forces pull f electrons inside the valence shell leaving 2 or 3 in the s-p and d bands

• f-f interaction varies dramatically with very small changes in interatomic distances lattice vibrations or heating

• f-f and f-spd interactions with temperature may results in localization as Pu transforms from the α- to the 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

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Pu metallic radii based on 12 coordinate and extrapolated to room temperatures

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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

phase Ga stabilizes phase Complicated phase

diagram

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Phase never observed, slow kinetics

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Metallic Pu• Other elements that stabilize 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 phase due to Ga diffusion in cooling

• Np expands the and phase region 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

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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

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Metallic Pu

• Modeling to determine electronic structure and bonding properties Density functional theory

Describes an interacting system of fermions via its density not via the 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 the correlation energy for a uniform electron gas

Generalized gradient approximation (GGA) Localized electron density and density gradient

• Total energy calculations at ground state

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Modeling Pu metal electronic configuration

• Pu metal configuration 7s26d15f5

From calculations, all eight valence electrons are in the conduction band,

5f electrons in α-plutonium behave like the 5d electrons of the transition metals than the 4f of the 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

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• 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

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Transition at Pu

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• For Pu, degree of f electron localization varies with phase

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• 5f electrons extend relatively far from the nucleus compared to the 4f electrons 5f electrons

participate in chemical bonding

• much-greater radial extent of the probability densities for the 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 the ionic cores in relativistic calculations

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Pu metal physical and thermodynamic properties

• Already reviewed density and thermal expansion

• Heat capacity Difficulties in measurement due to self-heating

and damage 239Pu 2.2 mW/g

Low temperature measurements do not permit annealing

Use of 242Pu helps overcome decay related issues

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The Specific Heat of Plutonium and Other Metals

• The low-temperature specific heat of a metal is the sum of a lattice term an electronic term

• In this figure, the line for copper represents the behavior of most metals whereas the lines for α- and δ-plutonium have the highest values of γ (intercept values) of any pure element Indicating that

conduction electrons have an enhanced effective mass

• The compound UBe13 has an extremely high electronic specific heat, which continues to increase until it is cut off by the compound’s transition to superconductivity just below 1 K The

superconductivity of UBe13 proves that its large heat capacity must be associated with the conduction electrons

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Pu metal magnetic behavior

• Magnetic susceptibility () Internal response of material to applied

magnetic field M = χB,

M is magnetization of the materialB is the magnetic field intensity

• Large values for Pu and alloys

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• Susceptibilities of Pu higher than most metals,

• lower than materials with local moment

• Variation in susceptibility as plutonium changes phase

• increase slightly as temperature decreases

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Pu metal mechanical properties

• Related to crystal structure and melting point Pu has a range of structures with different

melting pointsResults in a variety of mechanical

properties* Stress, oxidation, corrosion,

pyrophoricity, self-irradiation• Sensitive to chemistry (alloying) and processing

(microstructure)

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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

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Pu metal mechanical properties

• 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 the 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

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• For α-Pu elastic limit is basically fracture strength

• The Pu-Ga alloy behaves more like Al Fails by ductile fracture after elongation

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• 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 the α-Pu phase

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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 the crystal lattice Results in metal atoms surrounding themselves with as

many neighbors as possible* close-packed, relatively simple crystal structures

• The 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

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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 PuO2 in the presences of water enhances bulk corrosion of Pu metal in moist air

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• 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

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• steady-state layer of Pu2O3 at oxide-metal interface Pu2O3 thickness is small compared with the oxide

thickness at steady state Autoreduction of dioxide by the metal at the oxide

metal interface produces Pu2O3

Pu2O3 reacts with the diffusing O2 to form dioxide

Steady state in dry air at room temperature

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• 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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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,

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PuO2+x Study

• Examined Pu oxides by two methods X-ray diffraction (XRD)

Gives information about structure* Lattice parameters

X-ray photoelectron spectroscopy (XPS)Used to evaluate binding of oxygen

• Examined reaction of PuO2 with H2O from 25 °C to 350 °C

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Results• Mass spectrometric analysis shows production

of H2(g)

PuO2(s)+xH2O(abs) <--> PuO2+x(s)+ xH2(g)

Time (hr)

350°C

300°C250°C

200°C

H2 pressure formation

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Results

• Lattice parameter change Attributed to increase in Pu:O ratio

O/Pu ratio

Cubic lattice parameter variation

ao=5.3643+0.01746 (O:Pu)

Relative insensitivity attributed to formation of Pu(VI)

Extra O forms plutonyl

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Results

• X-ray photoelectric spectroscopy data High binding energies for the oxide

442 eV, 429 eV

* 4f5/2, 4f7/2

Pu(VI) or Pu(VII) No Pu(V)

O 1s spectrum consistent with oxideAbsence of OH- attributed to continued

reaction of water

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Results• PuO2+x formed via catalytic cycle

Driven by H2O sorbed to surface

If O2 present, H reforms water

Formation of water drives catalytic cycle

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Pu oxide coating reaction with H2

• Plutonium hydride (PuHx) fcc phase forms a continuous solid solution for 1.9 < x < 3.0

Pu(s) + (x/2)H2(g) → PuHx(s)• observed value of x depends on hydrogen pressure and temperature• hydride is readily oxidized by air• decomposes back to its component• elements when heated in continuously pumped vacuum• Hydriding occurs only after the ubiquitous dioxide layer on the metal is penetrated• Unlike oxidation the reaction of hydrogen initiates at a limited number of

nucleation sites• a single nucleation site typically appears only after a lengthy, but unpredictable,

induction period• Once formed sites are the most reactive areas of the surface

Hydriding rate is proportional to the active area covered by the hydride Increases exponentially over time to a maximum value as sites grow and

ultimately cover the surface . At that point, the rate

• Temperatures between –55°C and 350°C and a H2 pressure of 1 bar reaction at a fully active surface consumes plutonium at a constant rate of

6–7 g/cm2 min Advances into metal or alloy at about 20 cm/h

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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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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 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

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rupture in inner container for Pu metal

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• increase in the innervessel’s diameter near the ruptured endshows the extent of hydride-catalyzedcorrosion during a 3-hour period

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Radiation damage• Decay rate for 239Pu is sufficient to produce radiation

damage Buildup of He and radiation damage within the

metal • radiation damage is caused mainly by the uranium

nuclei recoil energy from the decay to knock plutonium

atoms from their sites in the crystal lattice of the metal Vacancies are produced

• Effect can produce void swelling• On the microscopic level, the vacancies tend to diffuse

through the metal and cluster to form voids• Macroscopically, the net effect the metal swells

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Pu Decay and the Generation of Defects• α particle has a range of about 10 μm through the Pu• uranium nucleus range is only about 12 nm• Both particles produce displacement damage

Frenkel pairs namely vacancies and interstitial atoms

Occurs predominantly at the end of their ranges• Most of the damage results from the uranium nucleus and is

confined to the collision cascade region

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Stages for Radiation-Induced Void Swelling

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Predictions for Radiation-Induced Damage in Pu

• predicted contributions to volume distortion in stabilized plutonium at 70°C

• Distortions due to void swelling are likely to be much larger than those due to helium-bubble formation

• large uncertainty in the transient period prevents estimating when the void swelling should begin its linear growth rate

• figure shows several possible swelling curves

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Pu Compounds• Original difficulties in producing compounds

Amount of Pu Purity

• Aided by advances in microsynthesis and increase in amount of available starting material

• Much early effort in characterization by XRDPu Hydrides• PuHx

x varies from 1.9< x <3.0 Pu + x/2 H2PuHx

H2 partial pressure used to control exact stoichiometry Variations and difficulties rooted in desorption of H2

• Pu hydride crystallizes in a fluorite structure

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Pu hydride• Pu hydride oxidation state

PuH2 implies divalent Pu, an unstable oxidation state Pu(II) measurements show Pu as trivalent and PuH2 is metallic

Pu(III), 2 H- and 1e-

Electron in conduction band Consistent with electrical conductivity measurements showing PuHx progressively

changes from a metallic to semiconductor with increasing x Electrons removed from conduction band and bound as H– on octahedral sites as the

hydride increases• Phase relationships

Two differing phase diagrams Temperature, pressure, reaction rates differ Dependence on the formation of saturated Pu hydride

* Hydrogen saturated Pu (PuHs) forms and co-exists with Pu hydride* 5f electrons localized in Pu(III) compounds

High pressure hydride synthesis results in more complex diagram Coexistence of cubic (PuH2.77) and hexagonal PuH2.88 (Region III and IV).

Orthorhombic PuH2.95 (Region V) Region VI in high pressure hydride synthesis phase diagram is unknown

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Pu hydride• Solid state structure

Similar to lanthanide trifluoride system Cubic from PuHx 1.9 to 2.7

Decrease in lattice parameter with increasing x

* 5.36 to 5.34 Å Hexagonal beyond x=2.9 Hydrogen mobility in structure

H found at octahedral and tetrahedral sites Replacement with deuterium

Neutron scattering to correlate H location with stoichiometry

Structure becomes complicated at x near 3

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Aries process• Hydride used to prepare

metal Formation of

hydride from metal Heated to 400 °C

under vacuum to release hydrogen

Can convert to oxide (with O2) or nitride (N2) gas addition during heating

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Pu borides• Range of compounds

PuBx x= 2, 4, 6, 12, 66 Potential storage or waste form for Pu High melting points

Little work performed on compounds• Prepared from heating elements

Under vacuum between 900 °C and 1200 °C Arc melting under Ar

• Pu hydride can also act as starting material• Structure

Dominated by B-B bonding Similar to most metal borides Pu occupies vacant sites

* Power XRD, no single crystal data• Little data on properties

Some data on magnetic properties Suggest tetravalent Pu for diborides

* Based on comparison to Np complexes

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Pu carbides• Four known compounds

Pu3C2, PuC1-x, Pu2C3, and PuC2

PuC exists only as substoichiometric compound PuC0.6 to PuC0.92

Compound considered candidate for fuels• Synthesis

At high temperatures elemental C with: Pu metal Pu hydrides Pu oxides

* Oxygen impurities present with oxide starting material* High Pu carbides can be used to produce other carbides

PuC1-x from PuH2 and Pu2C3 at 700 °C Final product composition dependent upon synthesis temperature,

atmosphere (vacuum or Ar) and time

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Pu carbides• Structure

Lattice constant depends upon composition As C content increases, C are replaced by C2 units

Pu2C3 from Pu4(C2)3

* Cubic structure with 12 C2 units in cell* Studied by XRD and neutron scattering

XRD not accurate for C Neutron scattering shows C-C of 1.295

Å in Pu2C3

C2 bond in acetylene is 1.20 Å PuC2

Variation of XRD data with temperature* High temperature (1710 °C) cubic * Room temperature tetragonal unit cell

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Pu carbides• Chemical properties

PuC1-x oxidizes in air starting at 200 °C Slower reaction with N2

Formation of PuN at 1400 °C Pu2C3 has reactions similar to PuC1-x

All Pu carbides dissolve in HNO3-HF mixtures Liberation of CO2 with oxidizing acids With lower carbides formation of other organics

* Mellitic and oxalic acids• Thermodynamic properties

PuC1-x evaporates upon heating• Ternary phases prepared

Pu-U-C M3C2, MC1-x, M2C3, and MC2 observed

Pu-Th-C Mixed carbide-nitrides, carbide-oxides, and carbide

hydrides have been prepared

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Pu-silicon system• Five known Pu-Si compounds

5:3, 3:2, 1:1, 3:5, and 1:2 (Pu:Si) Highest melting point for 3:5 at 1646 °C

• Synthesis Reaction with PuF3 at 1200 °C under vacuum

4 PuF3 + (3+4x)Si4 PuSix + 3 SiF4

* SiF4 is volatile and removed Arc melting of Si and Pu or PuHx under Ar PuO2 with Si or SiC at 1400 °C under vacuum

• Structures Commonalities with borides Production of isolated Si2 units or structures (chains, layers, networks) with

increased Si content• Properties

Metallic appearance Pyrophoric Oxidize in air to form PuO2

Reacts with water High melting point and high densities

8.96 g/cm3 (Pu3Si5) 10.151 g/cm3

(PuSi) 11.98 g/cm3 (Pu5Si3) 8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Density

Pu/Si

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Pu pnictides• Basic compounds

Highest order PuX2

Prepared by reaction of Pu metal or hydride in sealed quartz tube at 400-750 °C

• Pu-nitrogen system Only PuN known with certainty

Narrow composition range Liquid Pu forms at 1500 °C

* PuN melting point not observed Preparation

Pu hydride with N2 between 500 °C and 1000 °C Can react metal, but conversion not complete Formation in liquid ammonia

* PuI3 + NH3 +3 M+ PuN + 3 MI+ 1.5 H2

Intermediate metal amide MNH2 formation Pu precipitates

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Pu nitride• Structure

fcc cubic NaCl structure Lattice 4.905 Å

Data variation due to impurities, self-irradiation Pu-N 2.45 Å Pu-Pu 3.47 Å

• Properties High melting point (estimated at 2830 °C) Compatible with steel (up to 600 °C) and Na (890 °C, boiling point) Reacts with O2 at 200 °C Reaction rates increase with H2O vapor Dissolves in mineral acids

Rapidly with HNO3

Moderately delocalized 5f electrons Increases with atomic number of ligand Behavior consistant with f5 (Pu3+) Supported by correlated spin density calculations

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Pu-P system

• Formed from Pu hydride with PH3 at elevated

temperature Pu hydride with excess red phosphorus in

pressure vessel at 600-800 °C under ArExcess P removed by distillation at 300

°C • Melts with decomposition at 2600 °C • Pu As and Pu Sb compounds also form

Pu4Sb3 formed in addition to mono species

8-103

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)

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8-105

• PuO Existence of phase uncertain

Definitely identified in gas phase* IR spectrum

Not indicated in phase diagram Surface film on Pu metal Molten Pu metal with stoichiometric Ag2O Reduction of PuO2 with C at 1500-1800 °C Reduction of PuOCl or PuO2 with Ba vapor

• PuO reacts violently with O2

Some discussion on PuO actually PuOC Oxycarbide does not react violently with O2

Pu oxide preparation

8-106

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 -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+Pu2Pu2O3 at 1500 °C in Ta crucible

* Excess Pu metal removed by sublimation 2PuO2+CPu2O3 + CO

8-107

Pu oxide preparation

• Hyperstoichiometric sesquioxide (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

8-108

8-109

Pu oxide preparation

• PuO2

Pu metal ingited 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

8-110

Pu oxide preparation

• PuO2+x, PuO3, PuO4

Tetravalent Pu oxides are favored Unable to oxidize PuO2

* High pressure O2 at 400 °C* Ozone

PuO2+x reported in solid phase Related to water reaction

* PuO2+xH2OPuO2+x + xH2

* Final product PuO2.3, fcc PuO3 and PuO4

reported in gas phase From surface reaction with O2

* PuO4 yield decreases with decreasing O2 partial pressure

8-111

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8-113

Pu oxide structures

• Lattice changes with O/Pu ratio fcc commonality with PuO2

Related to fluorite structure

8-114

8-115

8-116

8-117

Pu oxide• Oxygen at 1.84 Å

Similar to Pu=O in Pu(V) complexes 1.85 Å

Interpreted as mixture of Pu(IV) and Pu(V) Oxidation by electron transfer to O f4 to f3

transition• Properties

Interstitial excess O and O vacancies are mobile Interstitials are more mobile

Similar to O behavior in U Studied by gas phase isotope exchange

• Vaporization complicated Some composition change upon heating

C-Pu2O3 decomposes to A-Pu2O3 and PuO1.6+x

Data on vaporization conflicting* Dependent upon technique, interaction with matrix

PuO2 goes to PuO1.831

Gas phase PuO2+ and PuO+

8-118

8-119

Pu oxide

• Chemical properties Thermodynamic parameter available for Pu oxides Dissolution

High fired PuO2 difficult to dissolve Rate of dissolution dependent upon temperature and

sample history* Irradiated PuO2 has higher dissolution rate with

higher burnup Dissolution often performed in 16 M HNO3 and 1 M HF

* Can use H2SiF6 or Na2SiF6

KrF2 and O2F2 also examined Electrochemical oxidation

* HNO3 and Ag(II) Ce(IV) oxidative dissolution

8-120

Pu S, Se, and Te systems• Forms PuX, Pu2X3, PuX2-x

PuTe3 • Prepared from stoichiometric reaction of PuHx and

elements in sealed quartz 1 week at 350-750 °C PuX2-x

Decomposition to form other ratios• Direct from elements• Structures

All PuX are fcc All Pu2X3 are bcc or orthorhombic PuX2-x are tetragonal with a PuS2 monoclinic PuTe3 pseudo-tetragonal

8-121

8-122

Pu S, Se, Te

• Properties Metallic luster

PuS: Au colorPuSe: Cu colorPuTe: Black

NonmagneticSemiconductors

f-d hybridization

8-123

Alkali metal oxoplutonates• Formed from PuO2 and alkali metal oxides, hydroxides, peroxides, or

carbonates Variation in atmosphere

O2, inert gas, vacuum• Pu(IV)

Li8PuO6

• Pu(V) Li7PuO6, Li3PuO4, Na3PuO4

Oxidizing atmosphere• Pu(VI)

M6PuO6, M4PuO5 (Li, Na) M2PuO4 (K, Rb, Cs)

From oxides in oxygen atmosphere• Pu(VII)

M5PuO4 (Li, Na) M3PuO5 (Rb, Cs)

8-124

Group II Pu oxides• Pu(III)

BaPu2O4

BaO, Pu, and PuO2 in H2

* Formation of PuHx from Pu metal* Atmosphere switched to inert

• Pu(IV) Sr and Ba compounds only MPuO3

• Pu(V) Ba3PuO5.5

From Ba3PuO6, PuO2, and BaO Oxidation state of compound is uncertain

• Pu(VI) MPuO4, M3PuO6 (Ca, Sr, and Ba) Ba2MPuO6 (Sr, Mn, Pb, Mg, Ca)

8-125

Structures

• Perovskites CaTiO3 structure (ABO3)

Pu(IV, VI, or VII) in octahedral PuO6n-

Cubic lattice BO6 octahedra with A cations at center unit

cell

• Double perovskites (Ba,Sr)3PuO6 and Ba(Mg,Ca,Sr,Mn,Zn)PuO6

M and Pu(VI) occupy alternating octahedral sites in cubic unit cell

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8-128

8-129

• Pu-Ln oxides PuO2 mixed with LnO1.5

Form solid solutionsOxidation of Pu at higher levels of Ln

oxides to compensate for anion defects Solid solutions with CeO2 over entire range

8-130

Ternary oxides of Pu and actinides

• Prepared with Th, Pa, U, and Cm

• ThO2

Solid solutions over entire range Follows Vegard’s Law

At 1000 °C Melting points constant up to 25 % wt ThO2,

increase linearly with increasing ThO2

At 1650 °C under Ar partial phase separtion C-Pu2O3

8-131

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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8-133

• Prepared by precipitation process or co-milling• Properties examined

O potential Thermal

8-134

Plutonium halides

• General formula PuX3

PuF4 stable solid, PuCl4 can be found in gas phase PuF6 gas phase

f2 electron configuration• Trivalent oxyhalides

PuOX Some different oxyfluorides can be formed

PuOF3, PuOF4, PuO2F2

A range of fluoride salts with monovalent cation General formula MxPuF4+x

* x = 1, 2, 3, 4* M7Pu6F31 and MPuF6

8-135

Pu fluoride preparation• Used in the preparation of Pu metal• 2PuO2 + H2 +6 HF 2 PuF3 + 4 H2O at 600 °C• Pu2(C2O4)3 + 6 HF2 PuF3 + 3 CO + 3 CO2 + 3 H2O at

600 °C At lower temperature (RT to 150 °C) Pu(OH)2F2

or Pu(OH)F3 forms PuF3 from HF and H2

PuF4 from HF and O2

Other compounds can replace oxalates (nitrates, peroxides)

• Stronger oxidizing conditions can generate PuF6

PuO2 + 3 F2 PuF6 + O2 at 300 °C PuF4 + F2 PuF6 at 300 °C

8-136

Pu fluoride preparation• PuF3

Insoluble in water Prepared from addition of HF to Pu(III) solution

Reduce Pu(IV) with hydroxylamine (NH2OH) or SO2

Purple crystals PuF3

.0.40H2O

* Anhydrous PuF3 formed by heating in HF gas at 200-300 °C

* heating starting material in H2 from 150 to 600 °C, then HF at 200-300 °C

* Heating PuF4 in H2 from 600 °C

8-137

Pu fluoride preparation• PuF4

In soluble in H2O From the addition of HF to Pu(IV) solution

* Pale pink PuF4.2.5H2O

* Soluble in nitric acid solutions that form fluoride species

Zr, Fe, Al, BO33-

Heating under vacuum yields trifluoride Formation of PuO2 from reaction with water

* PuF4+2H2OPuO2+4HF Reaction of oxide with fluoride

* 3PuF4+2PuO24PuF3+O2

Net: 4PuF4+2H2O4PuF3+4HF+O2

* High vacuum and temperature favors PuF3 formation

Anhydrous forms in stream of HF gas

8-138

Pu fluoride preparation• PuF6

Formation from reaction of F2 and PuF4

Fast rate of formation above 300 °C Reaction rate

* Log(rate/mg PuF4 cm-2hr-1=5.917-2719/T)

Faster reaction at 0.8 F2 partial pressure

Condensation of product near formation Liquid nitrogen in

copper condenser near PuF4

Can be handled in glass

8-139

Pu fluoride structures

• PuF3

Each Pu surrounded by 9 F

8-140

Pu fluoride structures

• PuF4

Isostructural with An and Ln tetraflourides

Pu surrounded by 8 FDistorted square

antiprism

• PuF6

Gas phase Oh

symmetry

8-141

Pu fluoride properties

• PuF3

Melting point: 1425 °C Boiling point: decomposes at 2000 °C

• PuF4

Melting point: 1037°C • PuF6

Melting point: 52°C Boiling point: 62°C ΔsublH°=48.65 kJ/mol, ΔfH°=-1861.35 kJ/mol IR active in gas phase, bending and stretching

modes Isotopic shifts reported for 239 and 242

8-142

8-143

• PuF6

• Equilibrium constant measured for PuF6PuF4+F2

ΔG=2.55E4+5.27T

At 275 °C, ΔG=28.36 kJ/mol

ΔS=-5.44 J/K mol

ΔH=25.48 kJ/mol

8-144

Pu halides

• PuF6 decomposition Alpha decay and temperature

Exact mechanism unknown Stored in gas under reduced pressure

• Higher halide preparation PuCl3 from hydrochlorination

Pu2(C2O4)3.10H2O+6HCl2PuCl3+3CO2+3CO+13H2O

Reaction of oxide with phosgene (COCl2) at 500 °C Evaporation of Pu(III) in HCl solution

PuCl4

PuCl3+0.5Cl2PuCl4

* Gas phase* Identified by peaks in gas phase IR

8-145

Pu halides

• PuBr3

Combination of elements HBr with PuHx

Pu(III) oxalate with HBr, 400-600 °C• PuI3

Pu metal with HI at 400 °C 2Pu+3HgI2 2PuI3+3Hg

• Structures (PuCl3) 9 Cl for each Pu Tricapped trigonal prism

8-146

Pu halides

• PuBr3 and PuI3 structure Isostructural 6 Pu-Br of 3.08 Å 2 Pu-Br caps of 3.06 Å

8 coordinate PuBr and I larger than Cl

* Enhanced electron repulsion• Properties

PuCl3 free energy of formation determinedΔG=-924.7+0.22292T

PuOX from reaction with H2O

8-147

Pu oxyhalides• Only prepared with Pu(III) and

Pu(VI) PuOX for all halides from

water reaction with PuX3

Pu(VI) species Excess PuF6 with water

and HF

* PuO2F2

PuO2Cl2 from vacuum evaporation of PuCl6 at room temperature

8-148

Ternary halogenoplutonates

• Pu(III-VI) halides with ammonia, group 1, group 2, and some transition metals

• Preparation Metal halides and Pu halide dried in solution Metal halides and PuF4 or dioxide heat 300-600 °C

in HF stream PuF4 or dioxide with NH4F heated in closed vessel

at 70-100 °C with repeated treatment PuF6 or PuF4 with group 1 or 2 fluorides

8-149

8-150

8-151

8-152

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)

8-153

Pu solution chemistry

• Pu3+ and Pu4+ simple hydrates free species

• Plutonyl oxo species for Pu(V) and Pu(VI) Pu(V) effective charge 2.2 Pu(VI) effective charge 3.2

• PuO4-

• Redox chemistry instrumental in identifying species

8-154

Pu solution chemistry

• Coordination number varies Large values, 8 to 10 for water coordination

• Spectroscopic properties 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

8-155

8-156

8-157

8-158

8-159

8-160

8-161

8-162

8-163

Pu solution chemistry• Other spectroscopic methods employed in Pu

analysis Photoacoustic spectroscopy Thermal lensing

• Vibrational spectroscopy Oxo species

Asymmetric stretch 930-970 cm-1

* 962 cm-1 in perchloric acidLinear arrangement of oxygen

Raman shifts observedSensitive to complexation

* Changes by 40 cm-1

8-164

Pu solution chemistry

• Redox chemistry Potentials close to 1 V for 4 common states Kinetics permit coexistance of oxidation states

Pu(IV) and Pu(V) tend toward disproportionation * 3Pu4++2H2O2Pu3++PuO2

2++4H+

K=0.0089 at 1.0 M I* 3PuO2

++4H+Pu3++2PuO22++2H2O

Pu concentration Ionic strength pH

Kinetics for disproportionation based on time and Pu concentration Moles seconds (M s)

• Some redox couples are quasi- or irreversible Breaking or forming oxo bonds

i.e., Pu(V)/Pu(III), Pu(VI)/Pu(III)• Equilibrium between redox states

K=Pu(III)Pu(VI)/Pu(IV)Pu(V) K=13.1, corrected for hydrolysis

8-165

8-166

8-167

8-168

Pu solution chemistry• Preparation of pure oxidation states

Pu(III) Generally below pH 4 Dissolve -Pu metal in 6 M HCl Reduction of higher oxidation state with Hg or Pt cathode

* 0.75 V vs NHE Hydroxylamine or hydrazine as reductant

Pu(IV) Electrochemical oxidation of Pu(III) at 1.2 V

* Thermodynamically favors Pu(VI), but slow kinetics due to oxo formation

Pu(V) Electrochemical reduction of Pu(VI) at pH 3 at 0.54 V (vs SCE)

* Near neutral in 1 micromole/L Pu(V) Pu(VI)

Treatment of lower oxidation states with hot HClO4

Ozone treatment Pu(VII)

Oxidation in alkaline solutions* Hexavalent Pu with ozone, anodic oxidation

8-169

Pu solution chemistry

• Pu(VI) oxo oxygen exchange with water 18O enriched water exchange

need to maintain hexavalent oxidation state* Exchange rate increases with lower oxidation state

Exchange half life = 4.55E4 hr at 23 °C Two reaction paths

* Reaction of water with Pu(VI)* Breaking of P=O bonds by alpha decay

Faster exchange rate measured with 238Pu• Pu redox by actinides

Similar to disproprotionation Rates can be assessed against redox potentials

Pu4+ reduction by different actinides shows different rates* Accompanied by oxidation of An4+ with yl bond formation

Reduction of Pu(VI) by tetravalent actinides proceeds over pentavalent state

Reactions show hydrogen ion dependency

8-170

8-171

Pu solution chemistry

• Pu reduction by other metal ions and ligands Rates are generally dependent upon proton and ligand concentration

Humic acid, oxalic acid, ascorbic acid Poor inorganic complexants can oxidize Pu

Bromate, iodate, dichromate Reactions with single electron reductants tend to be rapid

Reduction by Fe2+

Complexation with ligands in solution impacts redox Different rates in carbonate media compared to perchlorate Mono or dinitrate formation can effect redox

* Pu(IV) formation or reaction with pentavalent metal ions proceeds faster in nitrate than perchlorate

* Oxidation of Pu(IV) by Ce(IV) or Np(VI) slower in nitrate Pu(VI) reduction can be complicated by disproportionation Hydroxylamine (NH2OH), nitrous acid, and hydrazine (N2H4)

Used in PUREX for Pu redox control Pu(III) oxidized

* 2Pu3++3H++NO3-2Pu4++HNO2+H2O

* Re-oxidation adds nitrous acid to the system which can initiate an autocatalytic reaction

8-172

Pu 2 phase redox system

transfers

HAN

N2

HNO2

HNO3

Fe(II)

Fe(III)

Pu(IV)

Pu(III)

HNO2

HN3N2H4

Fe(II) Fe(III)

H2O

PuO2+

PuO22+

N2O4 NO2

NO2-

H+/NO3-

N2/NH4+

AzidesNaAgPu

Inter-Phase Layer

8-173

Pu aqueous chemistry• Reduction of Pu(IV) to Pu(III) by HAN is fast

• Two Reactions are possible:

• Preferred Reaction depends on the ratio R

HOHONPuPuOHNH 6442 2234

3

HOHNPuPuOHNH 42222 2234

3

03

0

][

)]([

OHNH

IVPuR

343

3 NOPuPuNO dK

HPuOHOHPu hK 32

4

HOHNHOHNH aK23

OHONHPuOHNHPuOH kkK22

3/2

3 333

OHNONH k222 22 4

8-174

Pu aqueous chemistry

• kinetic of the reaction derived using the steady state approximation applied to NH2O

• Reoxidation of Pu

rate control

• With

222234 adh KKKKkk

23

42

23

2

])[(][)]([

][)]([)]([

NOKHIIIPu

OHNHIVPuk

dt

IVPud

d

224

423 NONOPuONPu

24

33 HNONOPuHNOHPu

OHONNOHNOH 24232

8-175

Pu aqueous chemistry

• In addition to the scavenging of nitrous acid, hydrazine also may reduce Pu(IV) to Pu(III) Excess of Pu

Excess of hydrazine

Net

• Autocatalytic reaction may result from Pu redox cycling in HAN/N2H4 system

HNPuHNPu 444 23

424

243

424 2222 NNHPuHNPu

HNHNPuPuHN 4234

52 2

1

8-176

Pu aqueous chemistry

• Autoradiolysis Formation of radicals and redox agents Low reaction if concentrations below 1 M

With nitrate can form other reactive species (HNO2)

Formation of Pu(IV).H2O2

Rate proportional to Pu concentration and dose rate

Pu(VI) reduction proceeds over Pu(V) Formation of HNO2 and disproportionation

8-177

Pu hydrolysis• Size and charge

Smaller ions of same charge higher hydrolysis For tetravalents

* Pu>Np>U>Pa>Th

8-178

Pu hydrolysis 10 mM

8-179

Pu(III) 10 mM

8-180

Pu(IV) 10 mmol/L

8-181

Pu(V) 10 mmol/L

8-182

Pu(VI) 10 mmol/L

8-183

Pu aqueous chemistry• Hydrolysis/colloid formation

In many systems solubility derived Pu(IV) concentrations vary due to colloid formation

Colloids are 1- to 1000-nm size particles that remain suspended in solution

x-ray diffraction patterns show Pu(IV) colloids are similar to the fcc structure of PuO2 Basis for theory

that colloids are tiny crystallites PuO2,

* May include some water saturated of hydrated surface

Prepared by addition of base or water to acidic solutions

8-184

Pu colloid model

8-185

Pu aqueous chemistry: colloids• Characterization

SANS Long, thin rods 4.7 nm x 190 nm

Light scattering Spherical particles 1 nm to 370 nm

Laser induced breakdown 12 nm to 25 nm

• XAFS studies of Pu(IV) colloids demonstrated that average fcc structure is overly simplistic additional chemical forms are present that affect solubility Variations in measured Pu(IV) concentrations may be related to the local structure colloids displays many discrete Pu–O distances

2.25 Å Pu-OH to 3.5 Å amplitude of Pu–Pu is reduced, decrease in number of nearest neighbors

four H atoms incorporated into the Pu(IV) colloid structure could result in one Pu vacany.

EXAFS reveals that many atoms in the colloid structure are distributed in a non-Gaussian way when several different oxygen containing groups are present

* O2–,, OH-, and OH2

8-186

8-187

Pu aqueous chemistry

• Complexing ions General oxidation state trends for complexation constants

Pu(IV)>Pu(VI)≈Pu(III)>Pu(V)• Oxoanions

Pu complexes based on charge and basicity of ligand ClO4

-<IO3-<NO3

-<SO42-<<CO3

2-<PO43-

* 7 to 12 ligands (higher value for Pu(IV)• Carbonate

Inner and outer sphere complexation with water Outer interaction form chains and layer structures

Bidentate with small bite angle Pu(III) carbonate

Oxidize rapidly to tetravalent state Complexation values consistent with Am(III)

Pu(IV) carbonate Pu(CO3)n

4-2n, n from 1 to 5* n increases with pH and carbonate concentration

8-188

8-189

Pu aqueous chemistry

• Pu(V) carbonates Carbonates to Pu(V) solution Reduction of Pu(VI) carbonates

Mono and triscarbonato species • Pu(VI) extension of U(VI) chemistry

8-190

8-191

8-192

8-193

Pu solution chemistry• Pu nitrates

First Pu complexes and important species in reprocessing and separations

Bidentate and planar geometry Similar to carbonates but much weaker ligand

1 or more nitrates in inner sphere Pu(III) species have been prepared but are unstable Pu(IV) species

Pu(NO3)n4-n, n=1-6

* Tris and pentanitrato complexes not as prevalent Removal of water from coordination sphere with nitrate

complexation* Pu-O; 2.49 Å for Nitrate, 2.38 Å for H2O

Spectrophotometric determination of complexation constants with nitrate and perchlorate

Pu(NO3)66- complexes with anion exchange resin

For Pu(IV) unclear if penta- or hexanitrato species Evidence suggests hexanitrato species in the presence of

resins

8-194

Pu solution chemistry• Pu nitrates

Nitrate solids from precipitation from nitric acid solutions Orthorhombic Pu(NO3)4.

.5H2O

M2Pu(NO3)6.2H2O; M=Rb, Cs, NH4

+, pyridinium in 8 to 14 M HNO3

* Pu-O 2.487 Å Mixed species

TBP complexes, amide nitrates No inner sphere Pu(V) nitrate complexes found Only Pu(VI) mononitrate in solution

Solid phase PuO2(NO3)2.xH2O; x=3,6

characterized

8-195

Pu solution chemistry• Sulfate

Pu(III) Mono and disulfate complexes Solid K5Pu(SO4)4

.8H2O* Indicates Pu(SO4)4

5- in solution* Likely Pu(SO4)n

3-2n in solution Pu(IV)

High affinity for sulfate complexes Mono and bisulfate solution species Solid K4Pu(SO4)4

.2H2O hydrated Pu(SO4)2 n=4, 6, 8, 9 Mixed Pu2(OH)2(SO4)3(H2O)4

* Should be in basic solution with high sulfate Pu(V) species not well characterized Pu(VI) forms mono- and bisulfate from acidic solutions

Examined by optical and IR spectroscopy Solids of M2PuO2(SO4)2

8-196

Pu solution chemistry• Phosphate complexes

Low solubility Range of solid species, difficult characterization

* Range of protonated phosphates* P2O7

4-, (PO3)nn-

* Ternary complexes Halides, organics, uranium

Pu(III) Not characterized but proposed Pu(H2PO4)n

3-n n=1-4 Pu(IV)

Wide range of complexes Only Pu(HPO4)2

.xH2O examined in solution phase Pu(V)

Ammonium monohydratephosphate Pu(V) tetrahydrate species Evidence of PuO2HPO4

-

Pu(VI) MPuO2PO4

.yH2O* Solution complexes from Pu(VI) hydroxide and H3PO4

8-197

Pu solution chemistry• Iodate

Pu(IO3)4 precipitate Not well characterized Prepared by hydrothermal methods

* Preparation of Pu(VI) diiodate species Mixed Pu(VI) trishydroxide species

From Pu(IV) and H5IO6 in hydrothermal reaction, forms (PuO2)2(IO3)(-OH)3

Pu(V) forms Pu(IV/VI) species• Perchlorate

No pure solution or solid phases characterized Most likely does not form inner sphere complexes in aqueous solution

• Oxalates Previously discussed, forms microcrystals Mono and bidentate forms Pu(III) form trivalent oxalates with 10 and 6 hydrates Pu(IV) forms with 2, 4, and 5 oxalates with n waters (n=0,1,2,or 6)

Tetra and hexa monovalent M salts Mono hydroxide mixed solid species formed

Pu(V) disproportionates Pu(VI)O2 oxalates

8-198

Pu solution chemistry• Peroxide

Used to form Pu(IV) from higher oxidation states Further reduction of Pu(IV), mixed oxidation states

Pu(IV) peroxide species determined spectroscopically Two different absorbances with spectral change in

increasing peroxide No confirmed structure

Pu2(-O2)2(CO3)68- contains doubly bridged Pu-O core

Formation of peroxide precipitate that incorporates surrounding anions High acidity and ionic strength In alkaline media, Pu(VI) reduced to Pu(V) with

formation of 1:1 complex

8-199

Pu solution chemistry

• Carboxylate complexes Single or multiple carboxylate ligands for strong complexes with Pu

with typical oxidation state stability trend Tend to stabilize Pu(IV) Pu(III)

Oxidation to Pu(IV) at pH > 5 Range of mixed species

* Degree of protonation (HxEDTA)* Mixed hydroxide species

Pu(IV) Stabilized by complexation Solution phase at relatively high pH 1:1 Pu to ligand observed (Pu:EDTA, Pu:DTPA)

* Range of mixed species can be formed EDTA used in the dissolution of Pu(IV) oxide or hydroxide

solids Pu(V) complexes to be unstable

Oxidation or reduction solution dependent Pu(VI) species observed

8-200

Pu solution chemistry• Halides

Studies related to Pu separation and metal formation Solid phase double salts discussed

• Cation-cation complexes Bridging over yl oxygen form plutonyl species Primarily examined for Neptunyl species Observed for UO2

2+ and PuO2+

6 M perchlorate solution Formation of CrOPuO4+ cation from oxidation of Pu(IV) with

Cr(VI) in dilute HClO4

8-201

Pu non-aqueous chemistry• Very little Pu non-aqueous and organometallic chemistry

Limited resources• Halides useful starting material

Pu halides insoluble in polar organic solvents Formation of solvated complexes

PuI3(THF)x from Pu metal with 1,2-diiodoethane in THF* Tetrahydrofuran

Also forms with pyridine, dimethylsulfoxide Also from the reaction of Pu and I2

Solvent molecules displaced to form anhydrous compounds Single THF NMR environment at room temperature

Two structures observed at -90 °C

8-202

Pu non-aqueous chemistry• Pu oxidation with Tl or Ag

hexafluorophosphate in acetonitrile (CH3CN) Pu(CH3CN)9(PF6)3

.CH3CNPu trivalent cation complex

surrounded by PF6- anion

9 coordinate tricapped trigonal prism

• PuCl4 can be stabilized PuCl4L2or3 from Cs2PuCl6 with amide

(RCONR’2) or phosphine oxide (R3PO)Oh based symmetry for L2

8-203

Pu non-aqueous chemistry

• Amides Uses PuI3(THF)4 starting material

3 NaN(SiMe3)2 yields Pu complex with 3 NaI* IR shows asymmetric PuNSi2 stretch at

986 cm-1 Structure based on U and La

complexes• Alkoxides

Reaction of Pu(N(SiMe3)2)3 with 3 2,6-Bu2C6H3OH yields Pu(O-2,6-Bu2C6H3)3

Will coordinate Lewis base (donate electron pair)

8-204

Pu non-aqueous chemistry• Borohydrides

PuF4 + 2Al(BH4)3Pu(BH4)4+ 2Al(BH4)F2

Separate by condensation of Pu complex in dry ice IR spectroscopy gives pseudo Td

12 coordinate structure• Cyclooctatraene (C8H8) complexes

[NEt4]2PuCl6 + 2K2C8H8 Pu(C8H8)2+4KCl + 2[NEt4]Cl in THF Slightly soluble in aromatic and chlorinated

hydrocarbons D8h symmetry 5f-5f and 5f-6d mixing

* Covalent bonding, molar absorptivity approaching 1000 L mol-1cm-1

8-205

Pu non-aqueous chemistry

• Cyclopentadienyl (C5H5), Cp PuCl3 with molten (C5H5)2Be

trisCp Pu* Reactions also possible with Na, Mg,

and Li Cp Cs2PuCl6+ 3Tl(C5H5) in acetonitrile Formation of Lewis base species

CpPuCl3L2

* From PuCl4L2 complex Characterized by IR and Vis spectroscopy

8-206

Pu electronic structure• Ionic and covalent bonding models

Ionic non-directional electrostatic bonds Weak and labile in solution

* Core 5f Covalent bonds are stronger and exhibit stereochemical orientation All electron orbitals need to be considered

Evidence of a range of orbital mixing• PuF6

Expect ionic bonding Modeling shows this to be inadequate

Oh symmetry Sigma and pi bonds

t2g interacts with 6d t2u interacts with 5f or 6p and 7p for sigma bonding t1g non-bonding

Range of mixing found 3t1u 71% Pu f, 3% Pu p, 26% F p characteristics

Spin-orbital coupling splits 5f state Necessary to understand full MO, simple electron filling does not

describe orbital* 2 electrons in 5f orbital

Different arrangements, 7 f states

8-207

8-208

Pu electronic structure• PuO2

n+

Linear dioxo Pu oxygen covalency Linear regardless of number of valence 5f electrons D∞h, no

Pu oxygen sigma and pi bonds Sigma from 6pz2 and hybrid 5fz3 with 6pz

Pi 6d and 5f pi orbitals Valence electrons include non-bonding orbital and higher than pi and sigma in energetics

5f add to non bonding orbitals Weak ionic bonds in equatorial plane Spin-orbital calculations shown to lower bond energy

8-209

8-210

8-211

Pu electronic structure• Plutonocene

8 C 2 pi orbitals form pi bonds Combine with 6d Pu atomic

orbitals 5f form and

orbitals* 5f

directed toward C8 rings

Bond 49 % f character

Demonstrates covalency

Other interactions weaker

Stronger d interactions* Orbital 11% d character

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8-213

Pu electronic structure

• Plutonocene Experimental

evidence of splitting

Need to consider spin-orbital coupling

Different occupancy of orbitals