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11-1 Fuel Cycle Chemistry Chemistry in the fuel cycle §Uranium àSeparation àFluorination and enrichment Chemistry in fuel §speciation Fundamental of fission.

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Presentation on theme: "11-1 Fuel Cycle Chemistry Chemistry in the fuel cycle §Uranium àSeparation àFluorination and enrichment Chemistry in fuel §speciation Fundamental of fission."— Presentation transcript:

1 11-1 Fuel Cycle Chemistry Chemistry in the fuel cycle §Uranium àSeparation àFluorination 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

2 11-2 Reactor basics Utilization of fission process to create heat §Heat used to turn turbine and produce electricity Requires fissile isotopes § 233 U, 235 U, 239 Pu §Need in sufficient concentration and geometry 233 U and 239 Pu can be created in neutron flux 235 U in nature §Need isotope enrichment induced fission cross section for 235 U and 238 U as function of the neutron energy.

3 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 § 234,235,238 U §Ratios of isotopes established à234: 0.005±0.001 à235: 0.720±0.001 à238: ± U from 232 Th

4 11-4 Uranium chemistry Separation and enrichment of U Uranium separation from ore §Solvent extraction §Ion exchange Separation of uranium isotopes §Gas centrifuge §Laser

5 11-5 Natural U chemistry Natural uranium consists of 3 isotopes § 234 U, 235 U and 238 U Members of the natural decay series §Earth’s crust contains 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/m 3 air §About the same as for lead U is found in large granitic rock bodies formed by slow cooling of the magma about E 9 years ago

6 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 àAtmosphere became oxidizing about 1E9 years ago àRain penetrated into rock fractures, oxidizing the uranium to U(VI) àDissolving it as an anionic carbonate or sulfate complexes àWater and the dissolved U migrated downstream, reducing material was encountered forming ore bodies *Reduction to insoluble U(IV) (U 4+ ) compounds Most important mineral is uraninite (UO2+x, x = 0.01 to 0.25) Inorganic (pyrite) or organic (humic) matter Uranium concentration is % Carnotite (a K + U vanadate) 54% U U is often found in lower concentrations, of the order of % in association with other valuable minerals such as apatite (phosphate rock), shale, or peat

7 11-7 Uranium minerals URANINITE UO 2 uranium oxide CARNOTITE K 2 (UO 2 ) 2 (VO 4 ) H 2 O hydrated potassium uranyl vanadate AUTUNITE Ca(UO 2 ) 2 (PO 4 ) 2 10 H 2 O hydrated calcium uranyl phosphate.

8 11-8 Uranium solution chemistry Uranyl(VI) most stable in solution §Uranyl(V) and U(IV) can also be in solution àU(V) prone to disproportionation §Stability based on pH and ligands §Redox rate is limited by change in species àMaking or breaking yl oxygens *UO H + +2e -  U 4+ +2H 2 O yl oxygens have slow exchange §Half life 5E4 hr in 1 M HClO 4 §Rate of exchange catalyzed by UV light yl forms from f orbitals in U

9 11-9 Aqueous solution complexes Strong Lewis acid Hard electron acceptor §F - >>Cl - >Br -  I - §Same trend for O and N group à 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 §HOCH 2 COO - pKa=17, 3.6 upon complexation of UO 2 àInductive effect *Electron redistribution of coordinated ligand *Exploited in synthetic chemistry U(III) and U(V) §No data in solution àBase information on lanthanide or pentavalent actinides

10 11-10 Uranyl chemical bonding Bonding molecular orbitals   g 2  u 2  g 4  u 4 àOrder of HOMO is unclear *  g <  u <  g <<  u proposed  Gap for  based on 6p orbitals interactions  5f  and 5f  LUMO §Bonding orbitals O 2p characteristics §Non bonding, antibonding 5f and 6d §Isoelectronic with UN 2 Pentavalent has electron in non-bonding orbital

11 11-11

12 11-12

13 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 UO 2 (H 2 O) 5 2+, 3.2 for fluoride systems àNet negative 0.43 on oxygens àLewis 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

14 11-14

15 11-15 Acid-Leach Process for U Milling U ore Crushing & Grinding Water Acid Leaching Slurry H 2 SO 4 Steam NaClO °C Separation Tailings Solvent Extraction Recovery, Precipitation Drying (U 3 O 8 ) Organic Solvent NH 4 +

16 11-16 In situ mining Acidic solution (around pH 2.5)

17 11-17 Uranium purification TBP extraction §Based on formation of nitrate species §UO 2 (NO 3 ) x 2-x + (2-x)NO TBP  UO 2 (NO 3 ) 2 (TBP) 2

18 11-18 Solvent Extraction Two phase system for separation §Sample dissolved in aqueous phase àNormally 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 àVariation 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

19 11-19 Solvent extraction Distribution coefficient §[M] org /[M] aq =K d §Used to determine separation factors for a given metal ion àRatio of K d for different metal ions Distribution can be used to evaluate stoichiometry §Plot log K d versus log [X], slope is stoichiometry

20 11-20 U Fluorination U ore concentrates Conversion to UO 3 UO 2 H 2 Reduction UF 4 U metal UF 6 HNO 3 Solvent extraction purification HF Mg MgF 2 F2F2

21 11-21 Fuel Fabrication Enriched UF 6 UO 2 Calcination, Reduction Tubes Pellet Control 40-60°C Fuel Fabrication Other species for fuel nitrides, carbides Other actinides: Pu, Th

22 11-22 U enrichment Utilizes gas phase UF 6 §Gaseous diffusion àlighter molecules have a higher velocity at same energy *E k =1/2 mv 2 àFor 235 UF 6 and 238 UF UF 6 impacts barrier more often

23 11-23 Gas centrifuge Centrifuge pushed heavier 238 UF 6 against wall with center having more 235 UF 6 §Heavier gas collected near top Enriched UF 6 converted into UO 2 §UF 6 (g) + 2H 2 O  UO 2 F 2 + 4HF §Tc follows light U fraction if present Ammonium hydroxide is added to the uranyl fluoride solution to precipitate ammonium diuranate §2UO 2 F 2 + 6NH 4 OH  (NH 4 ) 2 U 2 O 7 + NH 4 F + 3 H 2 O Calcined in air to produce U 3 O 8 and heated with hydrogen to make UO 2 Final Product

24 11-24 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 àThree excitation wavelengths used §SILEX and MLIS use UF U absorption peak nm, 235 U is nm Use of tunable lasers so only 235 U is excited Then excited to ion state Charge separation by electrostatic

25 11-25 Radiochemistry in reactor Speciation in irradiated fuel Utilization of resulting isotopics Fuel confined in reactor to fuel region §Potential for interaction with cladding material àInitiate stress corrosion cracking §Chemical knowledge useful in events where fuel is outside of cladding Some radionuclides generated in structural material

26 11-26 Radionuclides in fresh fuel Actual Pu isotopics in MOX fuel may vary §Activity dominated by other Pu isotopes §Ingrowth of 241 Am §MOX fuel fabrication in glove boxes

27 11-27 Fission process Recoil length about 10 microns, diameter of 6 nm §About size of UO 2 crystal §95 % of energy into stopping power àRemainder into lattice defects *Radiation induced creep §High local temperature from fission à3300 K in 10 nm diameter Delayed neutron fission products §0.75 % of total neutrons à I and Br as examples Some neutron capture of fission products

28 11-28 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

29 11-29 Perovskite phase (A 2+ B 4+ O 3 ) Most fission products homogeneously distributed in UO 2 matrix With increasing fission product concentration formation of secondary phases possible §Exceed solubility limits in UO 2 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

30 11-30 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

31 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 potential àHigh metallic Mo indicates O:M of 2 àO:M above 2, more Mo in UO 2 lattice Relative partial molar Gibbs free energy of oxygen of the fission product oxides and UO 2

32 11-32 Properties of fission products in oxide fuel

33 11-33 Burnup Measure of extracted energy §Fraction of fuel atoms that underwent fission à%FIMA (fissions per initial metal atom) §Actual energy released per mass of initial fuel àGigawatt-days/metric ton heavy metal (GWd/MTHM) àMegawatt-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. àtypical value is 200 MeV/fission à100 % burnup around 1000 GWd/MTHM Determine burnup §Find residual concentrations of fissile nuclides after irradiation àBurnup from difference between final and initial values àNeed to account for neutron capture on fissile nuclides §Find fission product concentration in fuel àNeed suitable half-life àNeed knowledge of nuclear data *cumulative fission yield, neutron capture cross section àSimple analytical procedure à 137 Cs(some migration issues) 142 Nd(stable isotope), 152 Eu are suitable fission products § Neutron detection also used àNeed to minimize 244 Cm

34 11-34 Fuel variation during irradiation

35 11-35 Radionuclide Inventories Fission Products §generally short lived (except 135 Cs, 129 I)  ß,  emitters §geochemical behavior varies Activation Products §Formed by neutron capture ( 60 Co)  ß,  emitters §Lighter than fission products §can include some environmentally important elements (C,N) Actinides §alpha emitters, long lived

36 11-36 Plutonium 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

37 11-37 Pu nuclear properties 239 Pu §2.2E-3 W/g §Basis of formation of higher Pu isotopes § Pu first from nuclear test Higher isotopes available §Longer half lives suitable for experiments

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39 11-39 Questions 1.What drives the speciation of actinides and fission products in spent nuclear fuel? What would be the difference between oxide and metallic fuel? 2.Describe two processes for enriching uranium. Why does uranium need to be enriched? What else could be used instead of 235 U? 3.What are the similarities and differences between lanthanides and actinides? 4.What are some trends in actinide chemistry?

40 11-40 Pop Quiz What are the influences of 5f electrons on the chemistry of the actinides?


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