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Nuclear Forensics Summer School Chemical behavior of isotopes and radioelements

Nuclear Forensics Summer School Chemical behavior of isotopes and radioelements

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Nuclear Forensics Summer School Chemical behavior of isotopes and radioelements

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  1. Nuclear Forensics Summer SchoolChemical behavior of isotopes and radioelements • Radioisotopes and radioelements of concern • Fission products • Actinides • Actinide decay products • Speciation in fuel • Trends by periodic group • Cs • Sr • Lanthanides • Halides • Noble Gases • Polonium • Actinides • Provide basis for understanding chemical behavior

  2. Fission Products • Fission yield curve varies with fissile isotope • 2 peak areas for U and Pu thermal neutron induced fission • Variation in light fragment peak • Influence of neutron energy observed 235U fission yield

  3. Burnup: LWR UO2 39 MWd/kg (Siemens/KWU) Ignore oxygen contribution

  4. Burnup: Fast reactorelement distribution for different burnup

  5. Elements in fuel at burnup • From oxide (39 MWd/kg) • Actinides: U, Np, Pu • Noble gases: Xe, Kr • Group 1: Cs, Rb • Group 2: Sr, Ba • Group 4: Zr • Lanthanides: (Y), Nd, Ce, La, Pr, Sm, Pm, Eu • Metal phase: Mo, Ru, Pd, Tc, Rh • Degree in metal phase varies • Non-metals: Te, I • From fast reactor • Actinides: U, Pu • Noble Gases: Xe, Kr, He • Group 1: Cs, Rb • Group 2: Ba, Sr • Group 4: Zr • Lanthanides: Nd, Ce, La, Pr, Sm, (Y) • Metal phase: Mo, Ru, Pd, Rh, Tc • Non-metals: Te, I

  6. Speciation in Spent Fuel • Chemical form of actinides and fission products vary with fuel • Oxide fuel • Fuel for thermal reactors • Speciation dictated by reaction with oxygen • Noble gases • Oxides as solid solutions in UO2 (Ln, Group 1, Group 2, Zr, Nb, Mo, Te) • Separate oxide phase (Group 1, Group 2, Zr, Nb, Mo,Te) • Metallic phases (Mo, Tc, Ru, Rh, Pd) • Specific behavior dependent upon concentration • Related to burnup and fuel composition • Metallic fuel • Fuel for fast reactors, non-aqueous cooling of reactor • Elemental species most common • Solid solutions and intermetallic phases • Reactions with halides (I-)

  7. Oxide Volatility • For treatment of oxide fuel • UO2 oxidized to U3O8 • Heating to 400-600 °C in O2 containing atmosphere • Around 30% volume increase • U3O8 reduction by addition of H2 • Kr, Xe, I removed • Some discrepancies 1. AECL Technologies, Inc. “Plutonium Consumption Program-CANDU Reactor Projects,” Final Report, July 1994. 2. SCIENTECH, Inc., Gamma Engineering Corp., “Conceptual Design and Cost Evaluation for the DUPIC Fuel Fabrication Facility,” Final Report, SCIE-COM-219-96, May 1996. Recycling of Nuclear Spent Fuel with AIROX Processing, D. Majumdar Editor, DOE/ID-10423, December 1992. Bollmann, C.A., Driscoll, M.J., and Kazimi, M.S.: Environmental and Economic Performance of Direct Use of PWR Spent Fuel in CANDU Reactors. MIT-NFC-TR-014, 44-45, June 1998.

  8. Element Volatility Melting Points • Melting points correlate with vapor pressure • Zone refining can have applications • Data for elements • Need to consider solid solutions and intermetallics in fuel

  9. Radionuclide Inventories • Fission Products • generally short lived (except 135Cs, 129I) • ß,emitters • geochemical behavior varies • Activation Products • Formed by neutron capture (60Co) • ß,emitters • Lighter than fission products • can include some environmentally important elements (C,N) • Actinides • alpha emitters, long lived

  10. Fission products • Kr, Xe • Inert gases • Xe has high neutron capture cross section • Lanthanides • Similar to Am and Cm chemistry • High neutron capture cross sections • Tc • Redox state (Tc4+, TcO4-) • I • Anionic • 129I long lived isotope

  11. Cesium and Strontium • High yield from fission • Both beta • Some half-lives similar • Similar chemistry • Limited oxidation states • Complexation • Reactions • Can be separated or treated together

  12. Alkali Elements • 1st group of elements • Li, Na, K, Rb, Cs • Single s electron outside noble gas core • Chemistry dictated by +1 cation • no other cations known or expected • Most bonding is ionic in nature • Charge, not sharing of electron • For elemental series the following decrease • melting of metals • salt lattice energy • hydrated radii and hydration energy • ease of carbonate decomposition

  13. Solubility • Group 1 metal ions soluble in some non-aqueous phases • Liquid ammonia • Aqueous electron • very high mobility • Amines • Tetrahydrofuran • Ethylene glycol dimethyl ether • Diethyl ether with cyclic polyethers

  14. Complexes • Group 1 metal ions form oxides • M2O, MOH • Cs forms higher ordered chloride complexes • Cs perchlorate insoluble in water • Tetraphenylborate complexes of Cs are insoluble • Degradation of ligand occurs • Forms complexes with ß-diketones • Crown ethers complex Cs • Cobalthexamine can be used to extract Cs • Zeolites complex group 1 metals • In environment, clay minerals complex group 1 metal ions

  15. Group 2 Elements • 2nd group of elements • Be, Mg, Ca, Sr, Ba, Ra • Two s electron outside noble gas core • Chemistry dictated by +2 cation • no other cations known or expected • Most bonding is ionic in nature • Charge, not sharing of electron • For elemental series the following decrease • melting of metals • Mg is the lowest • ease of carbonate decomposition • Charge/ionic radius ratio

  16. Complexes • Group 2 metal ions form oxides • MO, M(OH)2 • Less polarizable than group 1 elements • Fluorides are hydroscopic • Ionic complexes with all halides • Carbonates somewhat insoluble in water • CaSO4 is also insoluble (Gypsum) • Nitrates can form from fuming nitric acid • Mg and Ca can form complexes in solution • Zeolites complex group 2 metals • In environment, clay minerals complex group 2 metal ions

  17. Technetium • Electronic configuration of neutral, gaseous Tc atoms in the ground • [Kr]4d55s2[l] with the term symbol 6S5/2 • Range of oxidation states • TcO4-, TcO2 • Tc chemical behavior is similar to Re • Both elements differ from Mn • Tc atomic radius of 1.358 Å • 0.015 Åsmaller than Re

  18. Technetium • Tc and Re often form compounds of analogous composition and only slightly differing properties • Compounds frequently isostructural • Tc compounds appear to be more easily reduced than analogous Re species • Tc compounds frequently more reactive than Re analogues • 7 valence electrons are available for bonding • formal oxidation states from +7 to -1 have been synthesized • Potentials of the couples TcO4-/TcO2and TcO4/Tc are intermediate between those of Mn and Re • TcO4 – is a weakoxidizing agent

  19. Polonium • Chemistry of Po is similar to Te and Bi • dissolves readily in dilute acids • PoH2 volatile • melting point −36.1°C • Boiling point 35.3°C) • Halides have structure PoX2, PoX4 and PoX6 • 2oxides PoO2 and PoO3 are the products of oxidation of polonium.[14] • Some microbes can methylate Po • Similar to Hg, Se and Te • electron configuration of Po ground state atoms • 5s25p65d106s26p4(3P2) • analogous to the configurations of Se and Te • stable oxidation states of -2, +2, +4, and +6 would be expected

  20. Lanthanides • Electronic structure of the lanthanides tend to be [Xe]6s24fn • ions have the configuration [Xe]4fm • Lanthanide chemistry differs from main group and transition elements due to filling of 4f orbitals • 4f electrons are localized • Hard acid metals • Actinides are softer, basis of separations • Lanthanide chemistry dictated by ionic radius • Contraction across lanthanides • 102 pm (La3+) to 86 pm (Lu3+), • Ce3+ can oxidized Ce4+ • Eu3+ can reduce to Eu2+ with the f7 configuration which has the extra stability of a half-filled shell

  21. Lanthanides • Difficult to separate lanthanides due to similarity in ionic radius • Multistep processes • Crystallization • Solvent extraction (TBP) • Counter current method • larger ions are 9-coordinate in aqueous solution • smaller ions are 8-coordinate • Complexation weak with monodentate ligands • Need to displace water • Stronger complexes are formed with chelating ligands

  22. Actinides • Occurrence • Ac, Th, Pa, U natural • Ac and Pa daughters of Th and U • Traces of 244Pu in Ce ores • Properties based on filling 5f orbitals

  23. Actinide Electronic Structure

  24. Electronic structure • Electronic Configurations of Actinides are not always easy to confirm • atomic spectra of heavy elements are very difficult to interpret in terms of configuration • Competition between 5fn7s2 and 5fn-16d7s2 configurations • for early actinides promotion 5f  6d occurs to provide more bonding electrons much easier than corresponding 4f  5d promotion in lanthanides • second half of actinide series resemble lanthanides more closely • Similarities for trivalent lanthanides and actinides • 5f orbitals have greater extension with respect to 7s and 7p than do 4f relative to 6s and 6p orbitals • The 5 f electrons can become involved in bonding • ESR evidence for bonding contribution in UF3, but not in NdF3 • Actinide f covalent bond contribution to ionic bond • Lanthanide 4f occupy inner orbits that are not accessible • Basis for chemical differences between lanthanides and actinides

  25. Electronic Structure • 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers • especially U - Am • Bonding can include any orbitals since energetically similar • Explains tendency towards variable valency • greater tendency towards (covalent) complex formation than for lanthanides • Lanthanide complexes tend to be primarily ionic • Actinide complexes complexation with p-bonding ligands • Hybrid bonds involving f electrons • Since 5f / 6d / 7s / 7p orbital energies are similar orbital shifts may be on the order of chemical binding energies • Electronic structure of element in given oxidation state may vary with ligand • Difficult to state which orbitals are involved in bonding

  26. Ionic Radii • Trends based on ionic radii

  27. Absorption Spectra and Magnetic Properties • Electronic Spectra • 5fn transitions • narrow bands (compared to transition metal spectra) • relatively uninfluenced by ligand field effects • intensities are ca. 10x those of lanthanide bands • complex to interpret • Magnetic Properties • hard to interpret • spin-orbit coupling is large • Russell-Saunders (L.S) Coupling scheme doesn't work, lower values than those calculated • LS (http://hyperphysics.phy-astr.gsu.edu/hbase/atomic/lcoup.html) • Weak spin orbit coupling • Sum spin and orbital angular momentum • J=S+L • ligand field effects are expected where 5f orbitals are involved in bonding

  28. Pu absorbance spectrum

  29. Oxidation states and stereochemistry

  30. Hybrid orbitals • Various orbital combinations similar to sp or d orbital mixing • Linear: sf • Tetrahedral: sf3 • Square: sf2d • Octahedral: d2sf3 • A number of orbital sets could be energetically accessible • General geometries • Trivalent: octahedral • Tetravalent: 8 coordination

  31. Stereochemistry

  32. Stereochemistry

  33. Stereochemistry

  34. Actinide metals • Preparation of actinide metals • Reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca or Ba at 1100 – 1400 °C • Other redox methods are possible • Thermal decomposition of iodine species • Am from Am2O3 with La • Am volatility provides method of separation • Metals tend to be very dense • U 19.07 g/mL • Np 20.45 g/mL • Am lighter at 13.7 g/mL • Some metals glow due to activity • Ac, Cm, Cf

  35. Pu metal • Some controversy surrounding behavior of metal http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818030.pdf

  36. Oxidation states • +2 • Unusual oxidation state • Common only for the heaviest elements • No2+ and Md2+ are more stable than Eu2+ • 5f6d promotion • Divalent No stabilize by full 5f14 • Element Rn5f147s2 • Divalent actinides similar properties to divalent lanthanides and Ba2+ • +3 • The most common oxidation state • The most stable oxidation state for all trans-Americium elements except No • Of marginal stability for early actinides Pa, U (But: Group oxidation state for Ac) • General properties resemble Ln3+ and are size-dependent • Binary Halides, MX3 easily prepared, & easily hydrolyzed to MOX • Binary Oxides, M2O3 known for Ac, Pu and trans-Am elements

  37. Oxidation states • +4 • Principal oxidation state for Th • similar to group 4 • Very important, stable state for Pa, U, Pu • Am, Cm, Bk & Cf are increasingly easily reduced - only stable in certain complexes e.g. Bk4+ is more oxidizing than Ce4+ • MO2 known from Th to Cf (fluorite structure) • MF4 are isostructural with lanthanide tetrafluorides • MCl4 only known for Th, Pa, U & Np • Hydrolysis / Complexation / Disproportionation are all important in aqueous phase • +5 • Principal state for Pa (similar to group 5) • For U, Np, Pu and Am the AnO2+ ion is known • Comparatively few other AnV species are known • fluorides fluoro-anions, oxochlorides, uranates, • +6 • AnO22+ ions are important for U, Np, Pu, Am UO22+ is the most stable • Few other compounds e.g. AnF6 (An = U, Np, Pu), UCl6, UOF4 etc..., U(OR)6 • +7 • Only the marginally stable oxo-anions of Np and Pu, e.g. AnO53-

  38. Redox chemistry (Frost diagrams)

  39. Redox chemistry

  40. Redox chemistry • actinides are electropositive • Pa - Pu show significant redox chemistry • all 4 oxidation states of Pu can co-exist in appropriate conditions • stability of high oxidation states peaks at U (Np) • redox potentials show strong dependence on pH (data for Ac - Cm) • high oxidation states are more stable in basic conditions • even at low pH hydrolysis occurs • tendency to disproportionation is particularly dependent on pH • at high pH 3Pu4+ + 2H2O PuO22+ + 2Pu3+ + 4H+ • early actinides have a tendency to form complexes • complex formation influences reduction potentials • Am4+(aq) exists when complexed by fluoride (15 M NH4F(aq)) • radiation-induced solvent decomposition produces H• and OH• radicals • lead to reduction of higher oxidation states e.g. PuV/VI, AmIV/VI

  41. Actinide complexes