11-1 Fuel Cycle Chemistry • Chemistry in the fuel cycle § Uranium à Separation à 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
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 - PowerPoint PPT Presentation
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11-1
Fuel Cycle Chemistry• Chemistry in the fuel cycle
§ Uraniumà Separationà 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
11-2
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.
11-3
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
à 234: 0.005±0.001à 235: 0.720±0.001à 238: 99.275±0.002
• 233U from 232Th
11-4
Uranium chemistry• Separation and enrichment of U• Uranium separation from ore
§ Solvent extraction§ Ion exchange
• Separation of uranium isotopes§ Gas centrifuge§ Laser
11-5
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
11-6
Natural U chemistry
• U is also found in younger rocks at higher concentrations called “ore bodies”§ Ore bodies are located downstream from mountain ranges
à Atmosphere became oxidizing about 1E9 years agoà Rain penetrated into rock fractures, oxidizing the uranium to
U(VI) à Dissolving it as an anionic carbonate or sulfate complexes à 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
11-7
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.
11-8
Uranium solution chemistry
• Uranyl(VI) most stable in solution§ Uranyl(V) and U(IV) can also be in solution
à U(V) prone to disproportionation § Stability based on pH and ligands§ Redox rate is limited by change in species
à 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
11-9
Aqueous solution complexes• Strong Lewis acid• Hard electron acceptor
§ F->>Cl->Br-I-
§ Same trend for O and N groupà 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
à Inductive effect* Electron redistribution of coordinated ligand* Exploited in synthetic chemistry
• U(III) and U(V)§ No data in solution
à Base information on lanthanide or pentavalent actinides
11-10
Uranyl chemical bonding• Bonding molecular orbitals
§ sg2 su
2 pg4 pu
4
à Order of HOMO is unclear* pg< pu< sg<< su
proposedØ Gap for s based on 6p orbitals interactions
§ 5fd and 5f f LUMO§ Bonding orbitals O 2p characteristics§ Non bonding, antibonding 5f and 6d§ Isoelectronic with UN2
• Pentavalent has electron in non-bonding orbital
11-11
11-12
11-13
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
à Net negative 0.43 on oxygensà 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
11-14
11-15
Acid-Leach Process for U Milling
U ore
Crushing & GrindingWater
Acid Leaching
SlurryH2SO4
SteamNaClO3
40-60°C
SeparationTailings
Solvent Extraction
Recovery, Precipitation
Drying (U3O8)
Organic Solvent
NH4+
11-16
In situ mining
Acidic solution (around pH 2.5)
11-17
Uranium purification• TBP extraction
§ Based on formation of nitrate species
§ UO2(NO3)x2-x + (2-x)NO3
- + 2TBP UO2(NO3)2(TBP)2
11-18
Solvent Extraction• Two phase system for separation
§ Sample dissolved in aqueous phaseà 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 neededà 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
11-19
Solvent extraction• Distribution coefficient
§ [M]org/[M]aq=Kd§ Used to determine separation factors for
a given metal ionà Ratio of Kd for different metal ions
• Distribution can be used to evaluate stoichiometry§ Plot log Kd versus log [X], slope is
stoichiometry
11-20
U Fluorination
U ore concentrates
Conversion to UO3
UO2
H2 Reduction
UF4
U metalUF6
HNO3Solvent extraction purification
HF
Mg
MgF2
F2
11-21
Fuel FabricationEnriched UF6
UO2Calcination, Reduction
Tubes
Pellet Control40-60°C
Fuel Fabrication
Other species for fuelnitrides, carbides
Other actinides: Pu, Th
11-22
U enrichment
• Utilizes gas phase UF6
§ Gaseous diffusionà lighter molecules have a higher velocity at same
energy
* Ek=1/2 mv2
à For 235UF6 and 238UF6
• 235UF6 impacts barrier more often
11-23
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
• 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
11-31
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 potentialà High metallic Mo
indicates O:M of 2à O:M above 2, more
Mo in UO2 lattice Relative partial molar Gibbs free energy of oxygen of the fission product oxides and UO2
11-32
Properties of fission products in oxide fuel
11-33
Burnup• Measure of extracted energy
§ Fraction of fuel atoms that underwent fissionà %FIMA (fissions per initial metal atom)
§ Actual energy released per mass of initial fuel à Gigawatt-days/metric ton heavy metal (GWd/MTHM)à 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.
à typical value is 200 MeV/fissionà 100 % burnup around 1000 GWd/MTHM
• Determine burnup§ Find residual concentrations of fissile nuclides after irradiation
à Burnup from difference between final and initial valuesà Need to account for neutron capture on fissile nuclides
§ Find fission product concentration in fuelà Need suitable half-lifeà Need knowledge of nuclear data