14-1 CHEM 312: Part 2 Lecture 14 Plutonium Chemistry • Readings § Pu chemistry à http :// radchem.nevada.edu/classes/r dch710/files/plutonium.pdf § Challenges of Pu chemistry à http://radchem.nevada.edu/cl asses/rdch710/lanl Pu book/LASCIENCE.PDF • Nuclear properties and isotope production • Pu in nature • Pu solution chemistry • Separation and Purification • Atomic properties • Metallic state • Compounds • Isotopes from 228≤A≤247 • Important isotopes § 238 Pu à 237 Np(n,g) 238 Np * 238 Pu from beta decay of 238 Np * Separated from unreacted Np by ion exchange à Decay of 242 Cm à 0.57 W/g à Power source for space exploration * 83.5 % 238 Pu, chemical form as dioxide * Enriched 16 O to limit neutron emission Ø 6000 n s -1 g -1 Ø 0.418 W/g PuO 2 à 150 g PuO 2 in Ir-0.3 % W container
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14-1 CHEM 312: Part 2 Lecture 14 Plutonium Chemistry Readings
Pu chemistry http://radchem.nevada.edu/classes/rdch
710/files/plutonium.pdfhttp://radchem.nevada.edu/classes/rdch
710/files/plutonium.pdf Challenges of Pu chemistry
http://radchem.nevada.edu/classes/rdch 710/lanl Pu
book/LASCIENCE.PDFhttp://radchem.nevada.edu/classes/rdch 710/lanl
Pu book/LASCIENCE.PDF Nuclear properties and isotope production Pu
in nature Pu solution chemistry Separation and Purification Atomic
properties Metallic state Compounds Isotopes from 228A247 Important
isotopes 238 Pu 237 Np(n, ) 238 Np * 238 Pu from beta decay of 238
Np *Separated from unreacted Np by ion exchange Decay of 242 Cm
0.57 W/g Power source for space exploration *83.5 % 238 Pu,
chemical form as dioxide *Enriched 16 O to limit neutron emission
6000 n s -1 g -1 0.418 W/g PuO 2 150 g PuO 2 in Ir-0.3 % W
container
Slide 3
14-2 Metallic Pu Interests in processing-structure- properties
relationship Reactions with water and oxygen Impact of self-
irradiation Density19.816 gcm 3 Liquid density at
m.p.densitym.p.16.63 gcm 3 Melting point912.5 KK Boiling point3505
KK Heat of fusion2.82 kJmol 1kJmol 1 Heat of vaporization333.5
kJmol 1kJmol 1 Heat capacity(25 C) 35.5 Jmol 1 K 1 Formation of Pu
metal Ca reduction Pyroprocessing PuF 4 and Ca metal Conversion of
oxide to fluoride Start at 600 C goes to 2000 C Pu solidifies at
bottom of crucible Direct oxide reduction Direct reduction of oxide
with Ca metal PuO 2, Ca, and CaCl 2 Molten salt extraction
Separation of Pu from Am and lanthanides Oxidize Am to Am 3+,
remains in salt phase MgCl 2 as oxidizing agent * Oxidation of Pu
and Am, formation of Mg *Reduction of Pu by oxidation of Am
metal
Slide 4
14-3 Pu metal Electrorefining Liquid Pu oxidizes from anode
ingot into salt electrode 740 C in NaCl/KCl with MgCl 2 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 C) 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
Slide 5
14-4 Pu phase stability 6 different Pu solid phases 7 th 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
Slide 6
14-5 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 actinides Bandwidths are a function of volume.
narrower for large volumes
Slide 7
14-6 Pu metal phase atomic-sphere approximation calculations
for contributions to orbitals 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
Slide 8
14-7 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 - to -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
Slide 9
14-8 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
Slide 10
14-9 Pu phase transitions demonstrates change in f-electron
behavior at Pu
Slide 11
14-10 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 phase Ga stabilizes phase
Complicated phase diagram
Slide 12
14-11 Phase never observed, slow kinetics
Slide 13
14-12 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 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
Slide 14
14-13 Modeling Pu metal electronic configuration Pu metal
configuration 7s 2 6d 1 5f 5 From calculations, all eight valence
electrons are in conduction band, 5f electrons in -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
Slide 15
14-14 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
Slide 16
14-15 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
Slide 17
14-16 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
-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 -phase decreases dramatically
with increasing temperature Similar to bcc and hcp metals. Pu-Ga
-phase alloys show limited elastic response followed by extensive
plastic deformation low yield strength ductile fracture For -Pu
elastic limit is basically fracture strength Pu-Ga alloy behaves
more like Al Fails by ductile fracture after elongation
Slide 18
14-17 Pu mechanical properties Tensile-test results for
unalloyed Pu Related to temperature and resulting change in phases
Strengths of - and -phase are very sensitive to temperature Less
pronounced for - phase and -phase data represent work of several
investigators different purity materials, and different testing
rates Accounts for variations in values, especially for -Pu
phase
Slide 19
14-18 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 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
Slide 20
14-19 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 Pu 2 O 3 surface layer forms in absence or
low amounts of O 2 Promotes corrosion by hydrogen Pu hydride (PuH
x, where 1.9 < x < 3) increases oxidation rate in O 2 by 10
13 PuO 2+x surface layer forms on PuO 2 in presence of water
enhances bulk corrosion of Pu metal in moist air
Slide 21
14-20 Pu oxidation in dry air O 2 sorbs on Pu surface to form
oxide layer Oxidation continues but O 2 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 45 m 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
Slide 22
14-21 steady-state layer of Pu 2 O 3 at oxide-metal interface
Pu 2 O 3 thickness is small compared with oxide thickness at steady
state Autoreduction of dioxide by metal at oxide metal interface
produces Pu 2 O 3 Pu 2 O 3 reacts with diffusing O 2 to form
dioxide Oxidation kinetics in dry air at room temperature
Slide 23
14-22 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 O 2 at a pressure of 0.21
bar. Curve 2a to water vapor up to 0.21 bar Curves 2b and 2c
temperature ranges of 61C110C and 110C 200C, respectively Curves 1
and 2 oxidation rates for -phase gallium-stabilized alloy in dry
air and moist air Curve 3 transition region between convergence of
rates at 400C and onset of autothermic reaction at 500C Curve 4
temperature-independent reaction rate of ignited metal or alloy
under static conditions rate is fixed by diffusion through an O 2
-depleted boundary layer of N 2 at 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
Slide 24
14-23 Oxide Layer on Plutonium Metal under Varying Conditions
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 (25C) (a) Over time, isolating PuO 2
-coated metal from oxygen in a vacuum or an inert environment turns
surface oxide into Pu 2 O 3 by autoreduction reaction At 25C,
transformation is slow time required for complete reduction of PuO
2 depends on initial thickness of PuO 2 layer highly uncertain
because reaction kinetics are not quantified above 150C, rapid
autoreduction transforms a several micrometer-thick PuO 2 layer to
Pu 2 O 3 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 m At a metal temperature of 50C in moist air
(50% relative humidity), corrosion rate increases by a factor of
approximately 10 4 corrosion front advances into unalloyed metal at
a rate of 2 mm per year 150C200C in dry air, rate of autoreduction
reaction increases relative oxidation reaction steady-state
condition in oxide shifts toward Pu 2 O 3,
Slide 25
14-24 Rates for Catalyzed Reactions of Pu with H 2, O 2, and
Air Plutonium hydride (PuHx) fcc phase forms a continuous solid
solution for 1.9 < x < 3.0 Pu(s) + (x/2)H 2 (g) PuH x (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 55C and 350C
and a H 2 pressure of 1 bar reaction at fully active surface
consumes Pu at a constant rate of 67 g/cm 2 min Advances into metal
or alloy at about 20 cm/h
Slide 26
14-25 Hydride-Catalyzed Oxidation of Pu hydride-coated Pu
exposed to O 2 oxidation of PuH x forms surface layer of oxide with
heat evolution Produced H 2 reforms PuH x at hydride- metal
interface Exothermic, helps drive reaction sequential processes in
reaction oxygen adsorbs at gas-solid interface as O 2 O 2
dissociates and enters oxide lattice as anionic species thin
steady-state layer of PuO 2 may exist at surface oxide ions are
transported across oxide layer to oxide-hydride interface oxide may
be Pu 2 O 3 or PuO 2x (0< x