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CHEM 312: Part 3 Lecture 14 Plutonium Chemistry
Readings Pu chemistry Challenges of Pu chemistry 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 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
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Radiation damage Decay rate for 239Pu is sufficient to produce radiation damage Buildup of He and radiation damage within metal radiation damage is caused mainly by uranium nuclei recoil energy from decay to knock plutonium atoms from their sites in crystal lattice of metal Vacancies are produced Effect can produce void swelling On microscopic level, vacancies tend to diffuse through metal and cluster to form voids Macroscopic metal swelling observed
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Pu Decay and Generation of Defects
α particle has a range of about 10 μm through Pu U recoil nucleus range is only about 12 nm Both particles produce displacement damage Frenkel pairs namely vacancies and interstitial atoms Occurs predominantly at end of their ranges Most of damage results from U nucleus Distortions due to void swelling are likely to be larger than those from helium-bubble formation
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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 XRD Pu 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 Pu hydride oxidation state PuH2 implies divalent Pu, measurements show Pu as trivalent and PuH2 is metallic Pu(III), 2 H- and 1e- in conduction band Consistent with electrical conductivity measurements Hydride used to prepare metal (basis of Aries process) 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 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 Chemical properties PuC1-x oxidizes in air starting at 200 °C Slower reaction with N2 Formation of PuN at 1400 °C All Pu carbides dissolve in HNO3-HF mixtures Ternary phases prepared Pu-U-C and Pu-Th-C Mixed carbide-nitrides, carbide-oxides, and carbide hydrides
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Pu nitride 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, PuN precipitates Structure fcc cubic NaCl structure Lattice 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 Dissolves in mineral acids Moderately delocalized 5f electrons Behavior consistent with f5 (Pu3+) Supported by correlated spin density calculations
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Pu oxide Pu storage, fuel, and power generators PuO (minor species)
Forms on PuO2 of d-stabilized metal when heated to °C under vacuum Metal and dioxide fcc, favors formation of fcc Pu2O3 Requires heating to 450 °C to produce hexagonal form PuO2 with Pu metal, dry H2, or C 2PuO2+CPu2O3 + CO PuO2 fcc, wide composition range (1.6 <x<2) Pu metal ignited in air Calcination of a number of Pu compounds No phosphates 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
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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
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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
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Mixed Pu oxides 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 Pu-Ln oxides PuO2 mixed with LnO1.5 Form solid solutions Oxidation of Pu at higher levels of Ln oxides to compensate for anion defects Solid solutions with CeO2 over entire range
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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
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Pu fluoride preparation
Used in 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 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
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Pu fluoride preparation
Insoluble in H2O From 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
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PuF6 preparation Formation from reaction of F2 and PuF4
Fast rate of formation above 300 °C Reaction rate Log(rate/mg PuF4 cm-2hr-1= /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 Fluorination of PuF4 by fluorine diluted with He/O2 mixtures to produce PuF6 (Steindler, 1963).
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Pu fluoride structures
absorption spectrum of gaseous PuF6 from Steindler and Gunther (1964a) PuF4 Isostructural with An and Ln tetraflourides Pu surrounded by 8 F Distorted square antiprism PuF6 Gas phase Oh symmetry
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Pu fluoride properties
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°= kJ/mol IR active in gas phase, bending and stretching modes Isotopic shifts reported for 239 and 242 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
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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
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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 °C in HF stream PuF4 or dioxide with NH4F heated in closed vessel at °C with repeated treatment PuF6 or PuF4 with group 1 or 2 fluorides phase diagram of KCl–PuCl3 system
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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 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
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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
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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
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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
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PuO2n+ electronic structure
Linear dioxo Pu oxygen covalency Linear regardless of number of valence 5f electrons D∞h 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 d and f 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
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Review Nuclear properties and isotope production Production from 238U
Fissile and fertile isotopes Pu in nature Location, levels and how produced Separation and Purification Role of redox in aqueous and non-aqueous separations Metallic state Phases, alloys, and reactions with gases Compounds Preparation and properties Solution chemistry Oxidation state Spectroscopic properties Structure and coordination chemistry
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Questions Which isotopes of Pu are fissile, why?
How can one produce 238Pu and 239Pu? How is Pu naturally produced? How is redox exploited in Pu separation? Describe Pu separation in Purex and molten salt systems. What are some alloys of Pu? How does Pu metal react with oxygen, water, and hydrogen? How can different Pu oxidation states in solution be identified? Name a stable Pu(VI) compound in solution, provide its structure.
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Question Comment on blog Respond to PDF Quiz 13
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