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

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

Final Product

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 alloyà 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

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

materialà Initiate stress corrosion cracking

§ Chemical knowledge useful in events where fuel is outside of cladding

• Some radionuclides generated in structural material

11-26

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

11-27

Fission process• Recoil length about 10 microns, diameter of 6 nm

§ About size of UO2 crystal

§ 95 % of energy into stopping powerà Remainder into lattice defects

* Radiation induced creep§ High local temperature from fission

à 3300 K in 10 nm diameter• Delayed neutron fission products

§ 0.75 % of total neutronsà 137-139I and 87-90Br as examples

• Some neutron capture of fission products eff

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

11-29

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

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

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

* cumulative fission yield, neutron capture cross sectionà Simple analytical procedureà 137Cs(some migration issues) 142Nd(stable isotope), 152Eu are suitable fission products

§ Neutron detection also usedà Need to minimize 244Cm

11-34

Fuel variation during irradiation

11-35

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

11-36

Plutonium

• Isotopes from 228≤A≤247• Important isotopes

§ 238Puà 237Np(n,g)238Np

* 238Pu from beta decay of 238Np* Separated from unreacted Np by ion exchange

à Decay of 242Cmà 0.57 W/g à Power source for space exploration

* 83.5 % 238Pu, chemical form as dioxide* Enriched 16O to limit neutron emission

Ø 6000 n s-1g-1

Ø 0.418 W/g PuO2

à 150 g PuO2 in Ir-0.3 % W container

11-37

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

11-38

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 235U?

3. What are the similarities and differences between lanthanides and actinides?

4. What are some trends in actinide chemistry?

11-40

Pop Quiz

• What are the influences of 5f electrons on the chemistry of the actinides?