Transcript RFSS: Part 2 Lecture 14 Plutonium Chemistry • From: Pu chapter  http://radchem.nevada.edu/c lasses/rdch710/files/plutoniu m.pdf  Nuclear properties and isotope production  Pu in nature  Separation and Purification  Atomic properties  Metallic state  Compounds  Solution chemistry • • Isotopes from.

RFSS: Part 2 Lecture 14 Plutonium Chemistry
•
From: Pu chapter

http://radchem.nevada.edu/c
lasses/rdch710/files/plutoniu
m.pdf

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
Metallic Pu
• Interests in
processing-structureproperties relationship
• Reactions with water
and oxygen
• Impact of selfirradiation
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
Heat capacity
(25 °C) 35.5 J·mol ·K
−1
−1
−1
−1
Formation of Pu metal
•
•
Ca reduction
Pyroprocessing

PuF4 and Ca metal
 Conversion of oxide to fluoride
 Start at 600 ºC goes to 2000 ºC
 Pu solidifies at bottom of crucible

Direct oxide reduction
 Direct reduction of oxide with Ca metal
 PuO2, Ca, and CaCl2

Molten salt extraction
 Separation of Pu from Am and lanthanides
 Oxidize Am to Am3+, remains in salt phase
 MgCl2 as oxidizing agent
* Oxidation of Pu and Am, formation of Mg
* Reduction of Pu by oxidation of Am metal
14-2
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
14-3
http://arq.lanl.gov/source/orgs/nmt/nmtdo/AQarchive/98fall/magnetic_
levitation.html
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 allotropes vary
significantly

dramatic volume changes
with phase transitions
Crystal structure of 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 melting point
Pu lattice is very soft
vibrationally and very nonlinear
14-4
Pu metal phases
• Low symmetry ground state for a
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) electronicstructure calculations show
narrow width of f bands leads to
low-symmetry ground states of
actinides

Bandwidths are a function
of volume.
 narrower for large
volumes
14-5
Pu metal phase
• atomic-sphere approximation
calculations for contributions to
orbitals

d fcc phase
• If Pu had only f band contribution
equilibrium lattice constant would be
smaller than measured
• Contribution from s-p band
stabilizes larger volume
• f band is narrow at larger volume
(low symmetry)
• strong competition between repulsive
s-p band contribution and attractive
f band term induces instability near
ground state
• density-of-states functions for
different low-symmetry crystal
structures

total energies for crystal
structures are very close to
each other
14-6
Pu metal phase
• f-f interaction varies
dramatically with very small
changes in interatomic distances

lattice vibrations or
heating
• f-f and f-spd interactions with
temperature results in
localization as Pu transforms
from α- to 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
14-7
For actinides f electron bonding
increases up to Pu
Pu has highest phase instability
At Am f electrons localize
completely and become nonbonding
At Am coulomb forces pull f
electrons inside valence shell
2 or 3 electrons in s-p and d
bands
• For Pu, degree of f
electron localization
varies with phase 14-8
Pu phase transitions
demonstrates change in f-electron behavior at Pu
14-9
Metallic Pu
• Pu liquid is denser than
3 highest temperature
solid phases
 Liquid density at
16.65 g/mL
 Pu contracts 2.5 %
upon melting
• Pu alloys and d phase
 Ga stabilizes phase
 Complicated phase
diagram
14-10
Phase
never
observed,
slow
kinetics
14-11
Metallic Pu
•
•
•
•
•
Other elements that stabilize d 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 d phase due to Ga
diffusion in cooling
Np expands a and b phase region

b 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
14-12
Modeling Pu metal electronic configuration
• Pu metal configuration 7s26d15f5
 From calculations, all eight valence electrons are
in conduction band,
 5f electrons in α-plutonium behave like 5d
electrons of transition metals than 4f of
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
14-13
Metallic Pu
•
•
Modeling to determine electronic structure and bonding properties

Density functional theory
 Describes an interacting system of fermions via its density not via
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 correlation energy for a uniform
electron gas

Generalized gradient approximation (GGA)
 Localized electron density and density gradient
Total energy calculations at ground state
14-14
Relativistic effects
•
•
•
•
•
Enough f electrons in Pu
to be significant

Relativistic effects
are important
5f electrons extend
relatively far from
nucleus compared to 4f
electrons

5f electrons
participate in
chemical bonding
much-greater radial
extent of 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
Pu metal mechanical properties
•
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Stress/strain properties

High strength properties bend or
deform rather than break
 Beyond a limit material abruptly
breaks
* Fails to absorb more energy
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 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
For α-Pu elastic limit is basically fracture
strength
Pu-Ga alloy behaves more like Al

Fails by ductile fracture after
elongation
14-16
Pu mechanical properties
•
•
•
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 γphase and δ-phase
data represent work of several
investigators

different purity
materials, and different
testing rates
 Accounts for
variations in
values, especially
for α-Pu phase
14-17
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 crystal lattice
 Results in metal atoms surrounding themselves with as
many neighbors as possible
* close-packed, relatively simple crystal structures
• Pu 5f electrons have narrow conduction bands and high densityof-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
14-18
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 presence of water

enhances bulk corrosion of Pu metal in moist air
14-19
•
•
•
•
•
Pu oxidation in dry air
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
14-20
Oxidation kinetics in dry air at room temperature
• steady-state layer of Pu2O3 at oxide-metal interface
 Pu2O3 thickness is small compared with oxide
thickness at steady state
 Autoreduction of dioxide by metal at oxide metal
interface produces Pu2O3
 Pu2O3 reacts with diffusing O2 to form dioxide
14-21
Arrhenius Curves for Oxidation of Unalloyed and Alloyed Plutonium in
Dry Air and Water Vapor
•
•
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ln of reaction rate R versus 1/T

slope is proportional to activation
energy for corrosion reaction
Curve 1 oxidation rate of unalloyed
plutonium in dry air or dry O2 at a pressure
of 0.21 bar.
Curve 2a to water vapor up to 0.21 bar

Curves 2b and 2c temperature
ranges of 61°C–110°C and 110°C–
200°C, respectively
Curves 1’ and 2’ oxidation rates for δ-phase
gallium-stabilized alloy in dry air and moist
air
Curve 3 transition region between
convergence of rates at 400°C and onset of
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
gas-solid interface
Curve 5 temperature-dependent oxidation
rate of ignited droplets of metal or alloy
during free fall in air
14-22
Oxide Layer on Plutonium Metal under Varying Conditions
•
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•
•
•
•
corrosion rate is strongly dependent on metal
temperature

varies significantly with isotopic
composition, quantity, geometry, and
storage configuration
steady-state oxide layer on plutonium in dry air at
room temperature (25°C)

(a) Over time, isolating PuO2-coated
metal from oxygen in a vacuum or an
inert environment turns surface oxide
into Pu2O3 by autoreduction reaction

At 25°C, transformation is slow
 time required for complete
reduction of PuO2 depends on
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 steady-state oxide layer to
air results in continued oxidation of metal
Kinetic data indicate a one-year exposure to dry air
at room temperature increases oxide thickness by
about 0.1 μm
At a metal temperature of 50°C in moist air (50%
relative humidity), 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, rate of autoreduction
reaction increases relative oxidation reaction

steady-state condition in oxide shifts
toward Pu2O3,
14-23
Rates for Catalyzed Reactions of Pu with H2, O2, and Air
•
•
•
•
•
•
Plutonium hydride (PuHx)

fcc phase

forms a continuous solid
solution for 1.9 < x < 3.0
 Pu(s) + (x/2)H2(g) →
PuHx(s)

x depends on hydrogen
pressure and temperature
Pu hydride is readily oxidized by
air
Hydriding occurs only after
dioxide layer is penetrated

Hydrogen initiates at a
limited
hydriding rates values are
constant

indicate surface
compounds act as catalysts
hydride sites are most reactive
location

Hydriding rate is
proportional to active area
covered by hydride
Temperatures between –55°C and
350°C and a H2 pressure of 1 bar

reaction at fully active
surface consumes Pu at a2
constant rate of 6–7 g/cm
min

Advances into metal or
alloy at about 20 cm/h
14-24
Hydride-Catalyzed
Oxidation of Pu
•
•
•
hydride-coated Pu exposed to O2

oxidation of PuHx forms surface layer
of oxide with heat evolution
Produced H2 reforms PuHx at hydridemetal interface

Exothermic, helps drive reaction
sequential processes in reaction

oxygen adsorbs at gas-solid interface
as O2

O2 dissociates and enters oxide lattice
as anionic species

thin steady-state layer of PuO2 may
exist at surface

oxide ions are transported across
oxide layer to 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
14-25