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14-1 CHEM 312: Part 2 Lecture 14 Plutonium Chemistry Readings §Pu chemistry à 710/files/plutonium.pdf

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Presentation on theme: "14-1 CHEM 312: Part 2 Lecture 14 Plutonium Chemistry Readings §Pu chemistry à 710/files/plutonium.pdf"— Presentation transcript:


2 14-1 CHEM 312: Part 2 Lecture 14 Plutonium Chemistry Readings §Pu chemistry à 710/files/plutonium.pdf 710/files/plutonium.pdf §Challenges of Pu chemistry à 710/lanl Pu book/LASCIENCE.PDF 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 228≤A≤247 Important isotopes § 238 Pu  237 Np(n,  ) 238 Np * 238 Pu from beta decay of 238 Np *Separated from unreacted Np by ion exchange àDecay of 242 Cm à0.57 W/g àPower source for space exploration *83.5 % 238 Pu, chemical form as dioxide *Enriched 16 O to limit neutron emission Ø6000 n s -1 g -1 Ø0.418 W/g PuO 2 à150 g PuO 2 in Ir-0.3 % W container

3 14-2 Metallic Pu Interests in processing-structure- properties relationship Reactions with water and oxygen Impact of self- irradiation Density19.816 g·cm −3 Liquid density at m.p.densitym.p.16.63 g·cm −3 Melting point912.5 KK Boiling point3505 KK Heat of fusion2.82 kJ·mol −1kJ·mol −1 Heat of vaporization333.5 kJ·mol −1kJ·mol −1 Heat capacity(25 °C) 35.5 J·mol −1 ·K −1 Formation of Pu metal Ca reduction Pyroprocessing §PuF 4 and Ca metal àConversion of oxide to fluoride àStart at 600 ºC goes to 2000 ºC àPu solidifies at bottom of crucible §Direct oxide reduction àDirect reduction of oxide with Ca metal àPuO 2, Ca, and CaCl 2 §Molten salt extraction àSeparation of Pu from Am and lanthanides àOxidize Am to Am 3+, remains in salt phase àMgCl 2 as oxidizing agent * Oxidation of Pu and Am, formation of Mg *Reduction of Pu by oxidation of Am metal

4 14-3 Pu metal Electrorefining §Liquid Pu oxidizes from anode ingot into salt electrode §740 ºC in NaCl/KCl with MgCl 2 as oxidizing agent àOxidation to Pu(III) àAddition of current causes reduction of Pu(III) at cathode àPu drips off cathode Zone refining (700-1000 ºC) §Purification from trace impurities àFe, U, Mg, Ca, Ni, Al, K, Si, oxides and hydrides §Melt zone passes through Pu metal at a slow rate àImpurities travel in same or opposite direction of melt direction §Vacuum distillation removes Am §Application of magnetic field levitates Pu levitation.html

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

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. ànarrower for large volumes

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

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 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

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

10 14-9 Pu phase transitions demonstrates change in f-electron behavior at Pu

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

12 14-11 Phase never observed, slow kinetics

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 àWith Fe, mp=410 °C, 10 % Fe àUsed 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 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 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 àSmall energy difference between orbital can overlap in solids àAccounts for different configurations

15 14-14 Metallic Pu Modeling to determine electronic structure and bonding properties §Density functional theory àDescribes an interacting system of fermions via its density not via many-body wave function à3 variables (x,y,z) rather than 3 for each electron *For actinides need to incorporate ØLow symmetry structures ØRelativistic effects ØElectron-electron correlations §local-density approximation (LDA) àInclude external potential and Coulomb interactions àapproximation based upon exact exchange energy for uniform electron gas and from fits to correlation energy for a uniform electron gas §Generalized gradient approximation (GGA) àLocalized electron density and density gradient Total energy calculations at ground state

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

17 14-16 Pu metal mechanical properties Stress/strain properties §High strength properties bend or deform rather than break àBeyond 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 àSimilar 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

18 14-17 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 γ- phase and δ-phase data represent work of several investigators §different purity materials, and different testing rates àAccounts for variations in values, especially for α-Pu phase

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 àMetallic bonding is not very directional since valence electrons are shared throughout crystal lattice àResults 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

20 14-19 Pu metal corrosion and oxidation Formation of oxide layer §Can include oxides other than dioxide §Slow oxidation in dry air àGreatly 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 àPromotes 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

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 4–5 μm in room temperature surface stresses cause oxide particles to spall §oxide layer reaches a steady-state thickness àfurther oxidation and layer removal by spallation Eventually thickness of oxide layer remains constant

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 àPu 2 O 3 reacts with diffusing O 2 to form dioxide Oxidation kinetics in dry air at room temperature

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 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 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

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 (25°C) §(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 25°C, transformation is slow àtime required for complete reduction of PuO 2 depends on initial thickness of PuO 2 layer àhighly uncertain because reaction kinetics are not quantified above 150°C, 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 μm At a metal temperature of 50°C 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 150°C–200°C in dry air, rate of autoreduction reaction increases relative oxidation reaction §steady-state condition in oxide shifts toward Pu 2 O 3,

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 àPu(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 –55°C and 350°C and a H 2 pressure of 1 bar §reaction at fully active surface consumes Pu at a constant rate of 6–7 g/cm 2 min §Advances into metal or alloy at about 20 cm/h

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 àoxide may be Pu 2 O 3 or PuO 2–x (0< x <0.5) §Oxygen reacts with PuH x to form heat (~160 kcal/mol of Pu) and H 2

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