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RFSS: Part 3 Lecture 14 Plutonium Chemistry

RFSS: Part 3 Lecture 14 Plutonium Chemistry. Isotopes from 228≤A≤247 Important isotopes 238 Pu 237 Np(n, g ) 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

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RFSS: Part 3 Lecture 14 Plutonium Chemistry

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  1. RFSS: Part 3 Lecture 14 Plutonium 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 • From: Pu chapter • http://radchem.nevada.edu/classes/rdch710/files/plutonium.pdf • Nuclear properties and isotope production • Pu in nature • Separation and Purification • Atomic properties • Metallic state • Compounds • Solution chemistry

  2. 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

  3. 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

  4. 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 H2PuHx • 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- and1e- 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

  5. 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

  6. 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 MNH2formation, PuN precipitates • 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 • Dissolves in mineral acids • Moderately delocalized 5f electrons • Behavior consistent with f5 (Pu3+) • Supported by correlated spin density calculations

  7. Pu oxide • Pu storage, fuel, and power generators • PuO (minor species) • Pu2O3 • Forms on PuO2of d-stabilized 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 • PuO2 with Pu metal, dry H2, or C • 2PuO2+CPu2O3 + 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

  8. 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

  9. 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

  10. 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+xH2OPuO2+x + xH2 • Final product PuO2.3, fcc • PuO3 and PuO4reported in gas phase • From surface reaction with O2 • PuO4 yield decreases with decreasing O2 partial pressure

  11. 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

  12. 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

  13. Pu fluoride preparation • Used in preparation of Pu metal • 2PuO2 + H2 +6 HF 2 PuF3 + 4 H2O at 600 °C • Pu2(C2O4)3 + 6 HF2 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

  14. Pu fluoride preparation • PuF4 • 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+2H2OPuO2+4HF • Reaction of oxide with fluoride • 3PuF4+2PuO24PuF3+O2 • Net: 4PuF4+2H2O4PuF3+4HF+O2 • High vacuum and temperature favors PuF3 formation • Anhydrous forms in stream of HF gas

  15. 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=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 Fluorination of PuF4 by fluorine diluted with He/O2 mixtures to produce PuF6 (Steindler, 1963).

  16. Pu fluoride structures • PuF4 • Isostructural with An and Ln tetraflourides • Pu surrounded by 8 F • Distorted square antiprism • PuF6 • Gas phase Oh symmetry absorption spectrum of gaseous PuF6from Steindler and Gunther (1964a)

  17. 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 • Equilibrium constant measured for PuF6PuF4+F2 • ΔG=2.55E4+5.27T • At 275 °C, ΔG=28.36 kJ/s • ΔS=-5.44 J/K mol • ΔH=25.48 kJ/mol

  18. 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+6HCl2PuCl3+3CO2+3CO+13H2O • Reaction of oxide with phosgene (COCl2) at 500 °C • Evaporation of Pu(III) in HCl solution • PuCl4 • PuCl3+0.5Cl2PuCl4 • Gas phase • Identified by peaks in gas phase IR

  19. 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 NH4Fheated in closed vessel at 70-100 °C with repeated treatment • PuF6 or PuF4 with group 1 or 2 fluorides phase diagram of KCl–PuCl3system

  20. 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

  21. Pu non-aqueous chemistry • Borohydrides • PuF4 + 2Al(BH4)3Pu(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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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.

  27. Pop Quiz • What is the reason that Pu metal has multiple phases? • Bring to class or submit via e-mail • Provide comments in blog

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