Transcript 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 238Pu   237Np(n,g)238Np * 238Pu from.

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
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
14-1
•
239Pu
Pu nuclear properties


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
14-2
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)
14-3
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
14-4
Pu Hydrolysis/colloid formation
14-5
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
14-6
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
14-7
• Interests in
processing-structureproperties relationship
• Reactions with water
and oxygen
• Impact of selfirradiation
Formation of Pu metal
•
•
Metallic Pu
Density
−3
19.816 g·cm
−3
Liquid density at m.p. 16.63 g·cm
Melting point
912.5 K
Boiling point
3505 K
Heat of fusion
2.82 kJ·mol
Heat of vaporization
333.5 kJ·mol
Ca reduction
Heat
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
capacity
−1
−1
−1
−1
(25 °C) 35.5 J·mol ·K
14-8
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
14-9
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
14-10
Phase
never
observed,
slow
kinetics
14-11
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
14-12
Pu phase transitions
demonstrates change in f-electron behavior at Pu
14-13
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
14-14
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 moistureenhanced 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 δphase 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 metal14-15
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
14-16

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 PuHxcoated metal or alloy in air
• the hydriding rates of
PuHx- or Pu2O3-coated
metal or alloy at 1 bar of
pressure,
• oxidation rates of PuHxcoated metal or alloy in O2
• rates are extremely rapid,
• values are constant

indicate the surface
compounds act as
catalysts
14-17
Hydride-Catalyzed
Oxidation of Pu
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•
•
•
•
•
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
14-18
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)
14-19
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
14-20
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
14-21
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
14-22
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
14-23