Fuel Cycle Chemistry. Chemistry in the fuel cycle Uranium Separation Fluorination and enrichment Chemistry in fuel speciation Fundamental of fission products and actinides Production Solution chemistry Speciation Spectroscopy Focus on chemistry in the fuel cycle
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Fuel Cycle Chemistry • Chemistry in the fuel cycle • Uranium • Separation • Fluorination and enrichment • Chemistry in fuel • speciation • Fundamental of fission products and actinides • Production • Solution chemistry • Speciation • Spectroscopy • Focus on chemistry in the fuel cycle • Speciation (chemical form) • Oxidation state • Ionic radius and molecular size
Reactor basics • Utilization of fission process to create heat • Heat used to turn turbine and produce electricity • Requires fissile isotopes • 233U, 235U, 239Pu • Need in sufficient concentration and geometry • 233U and 239Pu can be created in neutron flux • 235U in nature • Need isotope enrichment induced fission cross section for 235U and 238U as function of the neutron energy.
Nuclear properties • Fission properties of uranium • Defined importance of element and future investigations • Identified by Hahn in 1937 • 200 MeV/fission • 2.5 neutrons • Natural isotopes • 234,235,238U • Ratios of isotopes established • 234: 0.005±0.001 • 235: 0.720±0.001 • 238: 99.275±0.002 • 233U from 232Th
Uranium chemistry • Separation and enrichment of U • Uranium separation from ore • Solvent extraction • Ion exchange • Separation of uranium isotopes • Gas centrifuge • Laser
Natural U chemistry • Natural uranium consists of 3 isotopes • 234U, 235U and 238U • Members of the natural decay series • Earth’s crust contains 3 - 4 ppm U • As abundant as As or B • U is also chemically toxic • Precautions should be taken against inhaling uranium dust • Threshold limit is 0.20 mg/m3 air • About the same as for lead • U is found in large granitic rock bodies formed by slow cooling of the magma about 1.7 - 2.5 E 9 years ago
Natural U chemistry • U is also found in younger rocks at higher concentrations called “ore bodies” • Ore bodies are located downstream from mountain ranges • Atmosphere became oxidizing about 1E9 years ago • Rain penetrated into rock fractures, oxidizing the uranium to U(VI) • Dissolving it as an anionic carbonate or sulfate complexes • Water and the dissolved U migrated downstream, reducing material was encountered forming ore bodies • Reduction to insoluble U(IV) (U4+) compounds • Most important mineral is uraninite (UO2+x, x = 0.01 to 0.25) • Inorganic (pyrite) or organic (humic) matter • Uranium concentration is 50 - 90% • Carnotite (a K + U vanadate) 54% U • U is often found in lower concentrations, of the order of 0.01 - 0.03% in association with other valuable minerals such as apatite (phosphate rock), shale, or peat
Uranium minerals URANINITE UO2 uranium oxide CARNOTITE K2(UO2)2(VO4)2• 1-3 H2O hydrated potassium uranyl vanadate AUTUNITE Ca(UO2)2(PO4)2•10 H2O hydrated calcium uranyl phosphate.
Uranium solution chemistry • Uranyl(VI) most stable in solution • Uranyl(V) and U(IV) can also be in solution • U(V) prone to disproportionation • Stability based on pH and ligands • Redox rate is limited by change in species • Making or breaking yl oxygens • UO22++4H++2e-U4++2H2O • yl oxygens have slow exchange • Half life 5E4 hr in 1 M HClO4 • Rate of exchange catalyzed by UV light • yl forms from f orbitals in U
Aqueous solution complexes • Strong Lewis acid • Hard electron acceptor • F->>Cl->Br-I- • Same trend for O and N group • based on electrostatic force as dominant factor • Hydrolysis behavior • U(IV)>U(VI)>>>U(III)>U(V) • Uranium coordination with ligand can change protonation behavior • HOCH2COO- pKa=17, 3.6 upon complexation of UO2 • Inductive effect • Electron redistribution of coordinated ligand • Exploited in synthetic chemistry • U(III) and U(V) • No data in solution • Base information on lanthanide or pentavalent actinides
Uranyl chemical bonding • Bonding molecular orbitals • sg2 su2 pg4 pu4 • Order of HOMO is unclear • pg<pu<sg<< suproposed • Gap for s based on 6p orbitals interactions • 5fd and 5ff LUMO • Bonding orbitals O 2p characteristics • Non bonding, antibonding 5f and 6d • Isoelectronic with UN2 • Pentavalent has electron in non-bonding orbital
Uranyl chemical bonding • Linear yl oxygens from 5f characteristic • 6d promotes cis geometry • yl oxygens force formal charge on U below 6 • Net charge 2.43 for UO2(H2O)52+, 3.2 for fluoride systems • Net negative 0.43 on oxygens • Lewis bases • Can vary with ligand in equatorial plane • Responsible for cation-cation interaction • O=U=O- - -M • Pentavalent U yl oxygens more basic • Small changes in U=O bond distance with variation in equatoral ligand • Small changes in IR and Raman frequencies • Lower frequency for pentavalent U • Weaker bond
Acid-Leach Process for U Milling U ore Water Crushing & Grinding Slurry H2SO4 Steam NaClO3 40-60°C Acid Leaching Separation Tailings Organic Solvent Solvent Extraction NH4+ Recovery, Precipitation Drying (U3O8)
In situ mining Acidic solution (around pH 2.5)
Uranium purification • TBP extraction • Based on formation of nitrate species • UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2
Solvent Extraction • Two phase system for separation • Sample dissolved in aqueous phase • Normally acidic phase • Aqueous phase contacted with organic containing ligand • Formation of neutral metal-ligand species drives solubility in organic phase • Organic phase contains target radionuclide • May have other metal ions, further separation needed • Variation of redox state, contact with different aqueous phase • Back extraction of target radionuclide into aqueous phase • Distribution between organic and aqueous phase measured to evaluate chemical behavior
Solvent extraction • Distribution coefficient • [M]org/[M]aq=Kd • Used to determine separation factors for a given metal ion • Ratio of Kd for different metal ions • Distribution can be used to evaluate stoichiometry • Plot log Kd versus log [X], slope is stoichiometry
U Fluorination HNO3 U ore concentrates Solvent extraction purification Conversion to UO3 H2 Reduction UO2 HF UF4 F2 Mg U metal UF6 MgF2
Fuel Fabrication Enriched UF6 Calcination, Reduction UO2 Pellet Control 40-60°C Tubes Fuel Fabrication Other species for fuel nitrides, carbides Other actinides: Pu, Th
U enrichment • Utilizes gas phase UF6 • Gaseous diffusion • lighter molecules have a higher velocity at same energy • Ek=1/2 mv2 • For 235UF6and 238UF6 • 235UF6impacts barrier more often
Final Product Gas centrifuge • Centrifuge pushed heavier 238UF6 against wall with center having more 235UF6 • Heavier gas collected near top • Enriched UF6 converted into UO2 • UF6(g) + 2H2OUO2F2 + 4HF • Tc follows light U fraction if present • Ammonium hydroxide is added to the uranyl fluoride solution to precipitate ammonium diuranate • 2UO2F2 + 6NH4OH (NH4)2U2O7 + NH4F + 3 H2O • Calcined in air to produce U3O8 and heated with hydrogen to make UO2
Laser Enrichment • Based on photoexcitation • Atomic Vapor Laser Isotope Separation (AVLIS) • Molecular Laser Isotope Separation (MLIS) • Separation of Isotopes by Laser Excitation (SILEX). • All use laser systems, optical systems, and separation module system • AVLIS used a uranium-iron (U-Fe) metal alloy • Three excitation wavelengths used • SILEX and MLIS use UF6 • 238U absorption peak 502.74 nm, 235U is 502.73 nm • Use of tunable lasers so only 235U is excited • Then excited to ion state • Charge separation by electrostatic
Radiochemistry in reactor • Speciation in irradiated fuel • Utilization of resulting isotopics • Fuel confined in reactor to fuel region • Potential for interaction with cladding material • Initiate stress corrosion cracking • Chemical knowledge useful in events where fuel is outside of cladding • Some radionuclides generated in structural material
Radionuclides in fresh fuel • Actual Pu isotopics in MOX fuel may vary • Activity dominated by other Pu isotopes • Ingrowth of 241Am • MOX fuel fabrication in glove boxes
Fission process • Recoil length about 10 microns, diameter of 6 nm • About size of UO2 crystal • 95 % of energy into stopping power • Remainder into lattice defects • Radiation induced creep • High local temperature from fission • 3300 K in 10 nm diameter • Delayed neutron fission products • 0.75 % of total neutrons • 137-139I and 87-90Br as examples • Some neutron capture of fission products
Fuel variation during irradiation • Chemical composition • Radionuclide inventory • Pellet structure • Higher concentrations of Ru, Rh, and Pd in Pu fuel • Total activity of fuel effected by saturation • Tends to reach maximum • Radionuclide fuel distribution studied • Fission gas release • Axial distribution by gamma scanning • Radial distribution to evaluate flux
Perovskite phase (A2+B4+O3) • Most fission products homogeneously distributed in UO2 matrix • With increasing fission product concentration formation of secondary phases possible • Exceed solubility limits in UO2 • Perovskite identified oxide phase • U, Pu, Ba, Sr, Cs, Zr, Mo, and Lanthanides • Mono- and divalent elements at A • Mechanism of formation • Sr and Zr form phases • Lanthanides added at high burnup
Epsilon phase • Metallic phase of fission products in fuel • Mo (24-43 wt %) • Tc (8-16 wt %) • Ru (27-52 wt %) • Rh (4-10 wt %) • Pd (4-10 wt %) • Grain sizes around 1 micron • Concentration nearly linear with fuel burnup • 5 g/kg at 10MWd/kg U • 15 g/kg at 40 MWd/kg U
Epsilon Phase • Formation of metallic phase promoted by higher linear heat • high Pd concentrations (20 wt %)indicate a relatively low fuel temperature • Mo behavior controlled by oxygen potential • High metallic Mo indicates O:M of 2 • O:M above 2, more Mo in UO2 lattice Relative partial molar Gibbs free energy of oxygen of the fission product oxides and UO2
Burnup • Measure of extracted energy • Fraction of fuel atoms that underwent fission • %FIMA (fissions per initial metal atom) • Actual energy released per mass of initial fuel • Gigawatt-days/metric ton heavy metal (GWd/MTHM) • Megawatt-days/kg heavy metal (MWd/kgHM) • Burnup relationship • Plant thermal power times days of dividing by the mass of the initial fuel loading • Converting between percent and energy/mass by using energy released per fission event. • typical value is 200 MeV/fission • 100 % burnup around 1000 GWd/MTHM • Determine burnup • Find residual concentrations of fissile nuclides after irradiation • Burnup from difference between final and initial values • Need to account for neutron capture on fissile nuclides • Find fission product concentration in fuel • Need suitable half-life • Need knowledge of nuclear data • cumulative fission yield, neutron capture cross section • Simple analytical procedure • 137Cs(some migration issues) 142Nd(stable isotope), 152Eu are suitable fission products • Neutron detection also used • Need to minimize 244Cm
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
Plutonium • 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
Pu nuclear properties • 239Pu • 2.2E-3 W/g • Basis of formation of higher Pu isotopes • 244-246Pu first from nuclear test • Higher isotopes available • Longer half lives suitable for experiments
Questions • What drives the speciation of actinides and fission products in spent nuclear fuel? What would be the difference between oxide and metallic fuel? • Describe two processes for enriching uranium. Why does uranium need to be enriched? What else could be used instead of 235U? • What are the similarities and differences between lanthanides and actinides? • What are some trends in actinide chemistry?
Pop Quiz • What are the influences of 5f electrons on the chemistry of the actinides?