1 Fuel Cycle Chemistry Chemistry in the fuel cycle §Uranium àSeparation àFluorination §Fission...

1

Fuel Cycle Chemistry

• Chemistry in the fuel cycle Uranium

Separation Fluorination

Fission products

• Advanced Fuel Cycle Fuel development Separations

• Environmental behavior Waste forms

Focus on chemistry and radiochemistry in the fuel cycle

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.

3

Uranium chemistry

• Separation and enrichment of U• Uranium separation from ore

Solvent extractionIon exchange

• Separation of uranium isotopesGas centrifugeLaser

5

Mining

• Uranium is reduced (tetravalent)

• Introduction of oxygen produces hexavalent uranium

• 222Rn daughter

• Ore mining or solution mining solution mining uses injection of sulfuric acid to

dissolve U and solution is removed not all solution is removed minerals are solubilized seepage into aquifer (Dresden, Sachsen)

6

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+

7

Uranium Ore

http://www.cogema.fr/photos_gb/

8

Yellowcake production

http://www.cogema.fr/photos_gb/

9

U Fluorination

U ore concentrates

Conversion to UO3

UO2

H2 Reduction

UF4

U metalUF6

HNO3Solvent extraction purification

HF

Mg

MgF2

F2

10

Fuel FabricationEnriched UF6

UO2Calcination, Reduction

Tubes

Pellet Control40-60°C

Fuel Fabrication

11

Aside: Fluorination of UO2 by NH4HF2

• Degradation of TRISO Fuel and Kernel Matrices using Ammonium BifluorideChemical treatment of TRISO

• Concept: Fluorination of graphite, SiC and actinide kernel by NH4HF2

• Solid-solid reactions have been observed between ammonium bifluoride and oxides of vanadium, zirconium, thorium, uranium, and plutonium

• Reaction with uranium dioxide at 25 °CUO2(s) + 4 NH4HF2(s) → (NH4)4UF8∙2H2O(s)

12

Fluorination of UO2: Ball Mill at 25 °C

UO2 + 4 NH4HF2 → (NH4)4UF8∙2H2O~50 g finely-divided (30 m) UO2 and 10% excess NH4HF2

20 minutes in a ball mill

→→

13

U enrichment

• Utilizes gas phase UF6

Gaseous diffusion lighter molecules have a higher velocity at same

energy

* Ek=1/2 mv2

For 235UF6 and 238UF6

• 235UF6 impacts barrier more often

14

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• 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

15

Fission Product Chemistry

• Chemistry dictated by oxidation state Importance of isotopes

related to half life Shorter half-lives

important in reactor maintenance

Longer lived isotopes important waste treatment and disposal* Dose and heat

18

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

19

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

20

Cesium and Strontium

• High yield from fission• Both beta

Some half-lives similar• Similar chemistry

Limited oxidation statesComplexationReactions

• Can be separated or treated together

22

Alkali Elements• 1st group of elements

Li, Na, K, Rb, CsSingle s electron outside noble gas coreChemistry dictated by +1 cation

no other cations known or expectedMost bonding is ionic in nature

Charge, not sharing of electronFor elemental series the following decrease

melting of metalssalt lattice energyhydrated radii and hydration energyease of carbonate decomposition

23

Solubility

• Group 1 metal ions soluble in some non-aqueous phases

• Liquid ammoniaAqueous electron

very high mobility• Amines• Tetrahydrofuran• Ethylene glycol dimethyl ether• Diethyl ether with cyclic polyethers

24

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

26

Group 2 Elements• 2nd group of elements

Be, Mg, Ca, Sr, Ba, RaTwo s electron outside noble gas coreChemistry dictated by +2 cation

no other cations known or expectedMost bonding is ionic in nature

Charge, not sharing of electronFor elemental series the following decrease

melting of metals* Mg is the lowest

ease of carbonate decompositionCharge/ionic radius ratio

27

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

30

Trivalent Actinides

• Am, CmAm does have oxidation states 3-6most prevalent state is 3+

• Similar chemical behaviorTrivalent lanthanides can be used as homologsThermodynamic data can be interchanged

• Alpha emittersProduced through neutron capture

32

A=241 Isotopes

33

Solution chemistry

• Oxidation statesAm (3-6)

Am3+, AmO2+, AmO2

2+ can be made

Am4+ rapidly disproportionates in solution except concentrated fluoride or phosphate

Cm (3 and 4)Cm4+

is a strong oxidizing agent

Cm4+ can be stabilized in high fluoride concentrations forming CmF5

- or CmF62-

34

Trivalent State

• In solution formsCarbonatesHydroxidesOrganic complexes

• Easily separated from other actinides by redox properties

• Behaves similar to trivalent lanthanides

38

0

10

20

30

Wav

enu

mb

er (

103 c

m-1

)Absorption and fluorescence process of Cm3+

Optical Spectra

HGF

7/2A

Z 7/2

Fluorescence Process

Excitation

EmissionlessRelaxation

FluorescenceEmission

40

Waste from Reactor• For a typical 1000 MWe reactor

30 tons of spent fuel are produced each year 4-11 m3 of HLW up to 400 m3 of non-HLW

Medium Level Waste or Low Level Waste• Generally waste not from spent fuel• Only LLW in USA (no MLW)

Radionuclide Inventory• Concerned about:

amount half-live decay mode

42

Power Plants

• Spent Fuel Actinides, Fission, Activation Products

Radionuclides from Fuel (in Kg)Isotope Starting Ending ∆235U 33 7.9 -25.1236U 0 4.0 4238U 967 942.9 -24.1237Np 0 0.75 0.75Am, Cm 0 0.2 0.2Pu 0 9.05 9.05FP 0 35.1 35.1

45

Solvent Extraction: PUREX• Based on separating aqueous phase from organic phase• Used in many separations

U, Zr, Hf, Th, Lanthanides, Ta, Nb, Co, NiCan be a multistage separationCan vary aqueous phase, organic phase, ligandsUncomplexed metal ions are not soluble in organic

phaseMetals complexed by organics can be extracted into

organic phaseConsidered as liquid ion exchangers

46

Extraction Reaction• Phases are mixed• Ligand in organic phase complexes metal ion in

aqueous phaseConditions can select specific metal ions

oxidation stateionic radiusstability with extracting ligands

• Phase are separated• Metal ion removed from organic phase

EvaporationBack Extraction

47Effect of nitric acid concentration on extraction of uranyl nitrate with TBP

48

Reactions

• Tributyl Phosphate (TBP)(C4H9O)3P=O

Resonance of double bond between P and OUO2

2+(aq) + 2NO3

-(aq) + 2TBP(org) <--

>UO2(NO3)2.2TBP(org)

Consider Pu4+

51

Extraction Systems

• Automatic systems are availableSeparation of solutions based on density

Organic usually lower density than water* Chlorinated hydrocarbons tend to be

denser than waterNeed to achieve phase separation before

solution extraction

52

Single Solvent Extraction Stage

55

Aside: Third phase formation

• Brief review of third phase formation• Related prior research• Laboratory methods• Np third phase behavior• Comparison with U and Pu• Spectroscopic observations

56

Third Phase Formation

• In liquid-liquid solvent extraction certain conditions cause the organic phase to splitPUREX separations using tributyl phosphate (TBP)Possible with future advanced separations

• Limiting Organic Concentration (LOC) – highest metal content in phase prior to split

• Light phase – mostly diluent• Heavy phase – extractant and metal rich

Problematic to safety!

57

Actinide Third Phases

Light Phase

Heavy Phase

Aqueous

Phase

U(VI)

Pu(IV)

Np(IV) Np(VI)

Pu(VI)

58

Importance to Safety

• Increased risk of criticality• Phase inversion• Difficulty in process fluid separations• Carry-over of high concentration TBP to

heated process units

Possible contribution to Red Oil event at Tomsk, Russia

59

Phase Inverted Plutonium

Inverted Organic

Aqueous

Heavy Organic

Light Organic

60

Prior Research

Majority of work focused on defining LOC boundary (reviewed by Rao and Kolarik)

-Effects of temp., concs., acid, diluent, etc.

Recent work on possible mechanisms

-Reverse micelle evidence from neutron scattering (Osseo-Asare; Chiarizia)

-Spectroscopic studies -UV, IR, EXAFS (Jensen)

61

Reverse Micelle Theory

Classical StoichiometryPossible Reverse Micelle

UOUO222+2+ + 2NO + 2NO33

-- + 2TBP + 2TBP UO UO22(NO(NO33))22 ▪ ▪ 2TBP2TBP

62

Role of the Metal

• LOC behavior well known for U(VI), U(IV), Pu(IV), and Th(IV)

• Little data available on Pu(VI) • No data on any Np systems• Mixed valence systems not understood

63

Mixed Systems

• Observed effect of Pu(VI) in HPT vs. C12

• Large impact of presence of Pu(VI) in HPT

-Indications heavy phase enriched in Pu(VI)• Opposite found with U(VI) inhibiting phase

separation in U(IV) system (Zilberman 2001)

Suggests possible role of trinitrato species

AnO2(NO3)3-

64

Neptunium Study

• Unique opportunity to examine trends in the actinides [LOC curve for U(IV) vs Pu(IV)]- Effective ionic charge- Ionic radii- Stability constants for trinitrato species

• Never been investigated• Ease of preparing both tetravalent and

hexavalent nitrate solutions

65

Neptunium Methods

• Worked performed at Argonne National Laboratory, Argonne, IL

• Stock prepared from nitric acid dissolution of 237Np oxide stock

• Anion exchange purification

-Reillex HPQ resin, hydroquinone (Pu reductant), hydrazine (nitrous scavenger)

• Np(IV) reduction with hydrogen peroxide reduction

• Np(VI) oxidation with concentrated HNO3 under reflux

Np(VI) nitrate salt

Np(IV) nitrate

66

LOC Behavior

• Np(VI) near linear• Np(IV) slight parabolic

Appears between linear U(IV) and parabolic Pu(IV)

• Both curves similar resemblance to distribution values Purex systems Suggests possible link with

metal-nitrate speciation

67

Np Third Phase Boundaries

4 5 6 7 8 9 100.05

0.10

0.15

0.20

0.25

0.30

L

OC

, M

HNO3, M

Np(IV) Np(VI)

68

Comparison with Other Actinides

U Np Pu

An (IV) 0.08

(Wilson 1987)

0.15 0.27

(Kolarik 1979)

An (VI) No 3Φ

(Chiarizia 2003)

0.17 0.10

LOC in 7M HNO3 / 1.1M TBP/dodecane 20-25 °C, M

69

Organic Nitric Acid

• Balance available TBP and organic H+

• Np(IV) = mixture of the monosolvate (TBP · HNO3) and hemisolvate (TBP · 2HNO3) species

• Np(VI) = hemisolvate

Agrees with literature data on U(VI) and Th(IV) acid speciations

70

Valence Trends – An (IV)

• General trend = decreasing LOC as ionic radii increases

Lowest charge density = lowest LOC• Np intermediate between U and Pu• Literature Th(IV) data consistent with trend

71

Valence Trends – An(VI)

• An(VI) = LOC increases as ionic radii decreases

Opposite trend for An(IV), including Th(IV)• Charge density using effective cationic charge and 6-

coordinate radii

No evidence of correlation with charge density within error of effective charge data

• Oxo group interactions not fully considered

Future work required

72

Spectral Study Methods

• Look for spectral trends in Np(VI) system• Examined trends for:

-LOC

-Metal loading

-Nitrate loading (using NaNO3)

• 5 mm quartz cuvette with Cary 5 Spectrometer

73

LOC Spectra

900 1000 1100 1200 13000.0

0.2

0.4

0.6

0.8

1.0

[HNO3], M

4 6 7 8 10

A

bso

rba

nce

Wavelength, nm

Np(VI) 30% TBP / dodecane at LOC

74

Metal Loading

900 1000 1100 1200 13000.0

0.2

0.4

0.6

0.8 [Np(VI)]org, M

LOC = 0.27 0.18 0.09 0.03

Abs

orba

nce

Wavelength, nm

75

Nitrate Effects

Aqueous Organic

1000 1100 1200 13000.0

0.3

0.6

0.9

Abs

orba

nce

Wavelength, nm

[NO-

3], M

4 5 6 saturated

1000 1100 1200 13000.00

0.05

0.10

0.15

0.20

Abs

orba

nce

Wavelength, nm

[H+] = 4M, [Np(VI)] = 0.03M

76

Valence Scoping Experiments

• Examined various mixes of Pu(IV)/Pu(VI)• Solutions prepared by method of slow addition

of concentrated HNO3 to heated syrupy Pu nitrate solution

• Use UV-Vis peak analysis for determination of initial aqueous composition

• Perform mole balance on aqueous phase before and after contact for organic content of each valence species (some samples)

77

Spectrum – Mixed Valence Phases

78

Third phase conclusions

• Third phase behavior measured in Np• LOC trends consistent with U and Pu• Np(IV) LOC trends with charge density• No clear correlation for Np(VI)• Spectroscopic evidence suggests possible role of

trinitrato species in third phase

79

• The Department is engaged in the review and approval process for the NGNP Acquisition Strategy Critical Decision from the Deputy Secretary is expected to be issued in a

matter of weeks• We expect to be able to make awards for NGNP in 2005• Expected NGNP to be gas cooled reactor

TRISO fuel Prismatic

Based on discussion amongst current researchers Not official

• Research coupled with Gen IV, AFCI, nuclear hydrogen and NERI NERI program covers all areas Increase university participation

Current and Future Fuel Cycles: US Approach and R&D Programs

Next Generation Nuclear Plant

80

US DOE Advanced Fuel Cycle Initiative

• Advanced Fuel Cycle Initiative Administered by the Office of Nuclear

Energy, Science and Technology• Stems from National Energy Policy

Development Group, May 2001 Support expansion of nuclear energy in the

United States Develop advanced nuclear fuel cycles and

next generation technologies Develop advanced reprocessing and fuel

treatment technologies

81

AFCI Mission and Goals• Mission

Develop technologies for the transition to a stable, long-term, and politically acceptable advanced fuel cycle Waste Proliferation resistance Economics Safety

* Transition from once-through fuel cycle to an advanced closed fuel cycle

Current focus on aqueous reprocessing; additional research on pyroprocessing

• Goals Develop advanced, proliferation-resistant fuel cycle technologies for current

and next-generation reactors Develop a recommendation on the need for a second repository in the 2007-

2010 timeframe Repository and proliferation needs related to separation Near term focus on utilization of existing reactors for transmutation Reduce cost of geologic disposal by enhancing performance of Yucca

Mountain* Heat loading

82

AFCI Research• National Program on Development of New Nuclear Fuel Cycle

Combines universities, national laboratories, and industry Long term view

To be deployed in the future Training next generation of researchers Development of new facilities

* “Super Atalante” for separations and fuel Development of fuel cycle that combines separations and fuel design

Utilization of separated material for fuel in new or existing reactors

• Address pressing nuclear issues facing the United States: nuclear energy and waste management concerns declining US nuclear infrastructure

Facilities and researchers global nuclear leadership

Cooperation with international partners* France* Russia

84

AFCI Research

• Separations Aqueous-organic Electrochemical separations in molten salt TRISO fuel

Reprocessing and repository behavior of Pu, Np, Am fuel• Fuel

Inert matrix for existing light water reactors Advanced fuels with transuranic elements

Oxides, carbides TRISO fuel

Silica carbide coated fuel for gas reactors Production of coated Pu, Np, Am oxides

• Reactors Advanced light water reactors Gas reactors

High temperature for H2 production TRISO fuel deep burn reactors

* Pu, Np, Am kernel Reactor physics and system analysis

85

Overview of AFC reactors

86

AFCI separations• Bulk U separations

Precipitation Electrochemistry

Disposal or re-enrichment• Separation of actinides and fission products by group

Transuranics (Np, Pu, Am, Cm) Solvent extraction For incorporation into fuel

* Discussion of Am and Cm separation Fuel fabrication

Fission products Cs, Sr

* Separate disposal Lanthanides

* Separation from actinides

87

Separation

• Primarily solvent extraction based on PUREXOrganic phase tributylphosphate in dodecane

Some inclusion of other organic ligand* Acetohydroxamic acid

Aqueous phase nitric acid at varying concentrations

Other processes also examinedPyroprocessingFluoride volatility

88

Current Extraction Scheme

• UREX PUREX modification addition of the acetohydroxamic acid (AHA) reduces Pu

Tetravalent Np and Pu forms aqueous phase AHA complexes U and Tc extracted

• CCD-PEG Cs and Sr extracted with chlorinated cobalt

dicarbollide/polyethylene glycol (CCD/PEG )• NPEX

Np, Pu Nitric acid, acetohydroxamic acid, CH3COOH

• TRUEX Remaining fission products except lanthanides TBP with Diphenyl-N,N-dibutylcarbamoyl phosphine oxide

(CMPO), oxalic acid

• CYANEX-301 Am and Cm TBP, CYANEX-301

89

Separation flowsheet

CCD-PEG

Tc, U

TRUEXCYANEX-301

UREX

Cs, Sr

NPEX

Pu, Np

FPAm, Cm

Cs, Sr, Np, Pu, Am, Cm, FP, Ln

Np, Pu, Am, Cm, FP, Ln

Am, Cm, FP, Ln

Am, Cm, LnLn

90

High Level Waste and AFC

• Separation coupled to extension of repository utility Separation of heat generating isotopes Separation of long lived actinides

• Need 6000 MT reprocessing capacity

Hig

h L

evel

W

aste

91

Expect US Repository Need in 2100

92

Current AFCI Direction

• Stress research for recommendation on second repository in 2007-2010 explore new alternative approaches to provide

confidence in selection advanced recycle research facility

• Investigate other advanced aqueous processes• Defer building and construction• Increase systems analysis and modeling• Align separations with current US non-proliferation

policy May need to emphasize group actinide separations

Remove U, keep rest of actinides as group

• FY 2005 Budget at $ 68 M

93

Evaluation of Fuel Cycles

• 6 cases were studied for 3 growth rates (0%, 1.8%, 3.8%) Once through LWR LWR + Inert Matrix Fuel (IMF)(TRU) recycle in

LWR start separations in 2025

LWR + MOX (Np, Pu, Am) one pass in LWR start separations in 2025

LWR + IMF (Pu, Np) + Fast Reactor (FR) (TRU) LWR + MOX (Pu, Np) one pass + FR (TRU) LWR + FR (TRU)

94

Pu Inventory

95

Repository Heat Loading

96

Implementation of Fuel Cycle

• Separation facility identified as limiting factor Early large scale separation enterprise reactor capacity for recycled materials is not issue

Delay in separation causes large inventory delay in 2nd tier reactor

• Exact separation scheme open to debate Ideally only fission products go to repository Separation and storage of Cm

Decay of 244Cm will leave 245Cm Time before separation

Analysis supports both 5 year and 30 year waiting period* Different results based on heat loading and

proliferation Issue is decay of 241Pu Need to prevent placement of 241Am, 238,240Pu and

237Np in repository

97

Separation Needs• Both repository and proliferation resistance needs to be

addressed Repository

Reduction of heat loading

* Separation of Cs, Sr

* Removal of 241Pu and 241Am Environmental behavior of 237Np

Proliferation Build up of fissile isotopes in fuel cycle Separation of Pu during reprocessing

* Procedure should not produce separated Pu stream

98

Separations• Current extraction research (Scheme 1)

U and Tc with UREX U precipitation as nitrate

Cs and Sr with CCD-PEG Np, Pu with NPEX Remaining fission products except lanthanides with TRUEX Am and Cm with CYANEX-301 and TPB

Am and Cm separation• New concept separation discussed (Scheme 2)

U Cs, Sr, and long lived fission products No Pu or Np/Pu separation

Actinide group separation• Future separation research and development ongoing

Am and Cm treatment Separation both from recycle or just Cm

• Mass separation can be applied to either scheme Initial U separation needed for both schemes

99

Waste Forms and Packaging

• Components of Waste Radioactive Isotopes Other Materials

• Waste Forms Materials Characteristics Properties and analysis

• Spent Fuel

101

Components of Waste• Radioactive elements common in radioactive waste

monovalent: Cs divalent: Sr, Co trivalent: Am, Cm tetravalent: Zr, Tc, Th, U, Pu heptavalent: Np, Pu hexavalent: U, Pu septvalent: Tc

• Non radioactive elements need to be considered B, lanthanides, Si, Cu, Fe, Ni, Ti, Zr, C, H, O

• Each elements behave differently in the environment and needs to be considered separately

102

Waste Placement and Packaging• A disposal site will contain packaged waste• Waste will have different sections and components

Waste Form Form of the material containing the radioactive waste

Canister Primary container of the waste form

* Consider robust canister Overpack

Barrier surrounding canisters for up to 1 meter May not be used

103

Waste Placement and Packaging

Backfill Material placed into gallery Different possible backfill materials

* Bentonite, crushed geologic material High exchange capacity or low

permeability Sleeve for removal may be included Drip shield

Divert water from package

104

Waste Package Requirements•From 10CFR60

“Substantially complete” (assuming anticipated processes and events) containment for 300 to 1000 years after repository closure

Release rate after 1000 years < 10 ppm/year for inventory at 1000 years

Retrievability at any time up to 50 years after emplacement starts

105

Waste Forms

• In US, two existing high level waste forms Spent fuel

Zircaloy clad ≈5% UO2

Borosilicate glass SiO2-B2O3-Na2O

1-30% waste in the glass

• A number of other waste forms are being considered

106

Waste Forms

• Ceramics For disposal of weapons grade Pu Very durable material Based on TiO2, ZrO2 Up to 20% incorporation of waste

For Weapons grade Pu, up to 10% Pu Zeolite ceramics examined for disposal of liquid

metal reactor waste High Na contain precluded normal ceramics

and glass

107

Zeolite

108

Waste Forms

• Other Glass Developed as potential candidates

Pb-Fe phosphate Lanthanide borosilicate

* For weapons grade Pu Monazite Sulfur glass

* For Hanford wasteWaste loading determines volume, radiation dose, and

thermal property of glass

109

Glass

• Inorganic product of fusion Cooled to a rigid condition No crystallization

Amorphous material Any substance with rapid cooling

110

Glass Structure• Compound forming structural network

Oxides of Si, B, and P• Modifiers

Decrease melting temperature Add favorable processing properties

Can degrade stability, increase solubility* Oxides of Na, K, Ca, Ba

• Intermediates Can be present in waste May act as network former, increase durability

Oxides of Al, Fe

111

Thermal Stability of Glass• Devitrification

Formation of crystals in glass

Lower chemical stability Increased leaching

Reduced waste loading• Phase separation

Liquid-liquid phase separation during formation

112

Glass Radiation Stability

• Atomic displacement by heavy particle radiation Volume change

Density changes by 1% Depends upon glass chemical composition

Crystallization Concern over stored energy in glass leading to cracking or

crystallization• Ionizing effect from ß and • Chemical effects

Disordering Breaking of bonds causes increased corrosion

Radiolytic processes in aqueous medium in contact with glass

113

Glass Corrosion• Formation of altered phase on glass surface• Can inhibit diffusion of radionuclide out of

glass• Two possible method of radionuclide release

Diffusion of radionuclide out of glassDepends upon chemical behavior of

radionuclide Corrosion of glass with release of

radionuclideRelease depends upon glassSecondary phase formation varies for

radionuclide

114

• Need to consider colloids

• Chemical changes in near field can also effect glass dissolution

Basic Dissolution Rate EquationRate=Sk(1-(Q/K)

S=surface area, k=rate coefficient, Q= activity, K=Ksp

=stoichiometric number for rate-limiting reaction

115

Spent Fuel• Barrier

Zr cladding Zr corrosion

* Zr + 2H2O -> ZrO2 + 2H2

Weakening of cladding Drop in thermal conductivity

• Radiation Atomic Displacement Neutron activation of Zr and Ni

• 3 phases of release Gap release, grain release, UO2 dissolution

116

Geochemistry• Principles which control the behavior of dissolved groundwater constituents• Behavior based on equilibrium concepts• Provide insight into behavior

Groundwater Constituents• Concentration Units

Molality (m) (mol/kg solute) Molarity (M) (mol/L)for dilute concentrations (<0.2 M), m≈M Mass concentration (g/L, mg/L) Equivalent (valence per unit)Used for resins or humic substances (moles H+/g)Often written in milliequivalents/g (meq/g)

117

Speciation and Transport

118

Temperature Effects• Temperature effects can be described by enthalpy (∆H) and entropy (∆S)• Gibbs Free Energy (∆G) relates ∆H and ∆S

∆G=∆H-T∆S∆G=-RTlnß

T in K, R=8.314 J/molK• Temperature effect on ß can be described as:

Rlnß HT

S

119

Oxidation-Reduction• Charge of ions in solution

Fe, Mn, Co, As, Cr, U, Np, Pu are some redox sensitive metal ions

• Eh-pH diagrams can show the oxidation states based on oxygen and hydrogen Eh is also written as pE

O2(g) + 4H+ + 4e <--> 2 H2O

2H+ + 2e <--> H2(g) • At 25 °C

pE = 20.8 - pH + 0.25 logPO2

pE = - pH - 0.5 logPH2

120

Eh-pH diagram for Gohy Groundwater

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

7.5 8.0 8.5 9.0 9.5

Eh

(V

)

pH

U(VI)

Gohy-532Gohy-573

Gohy-1271

Gohy-2211

Gohy-2227

U(IV)

121

Am and Cm at ORNL WAG-5 site

• pH near 7

• Carbonate system

• Use modified Gohy data FA stability less than HA

• [FA(III)] =2 µmol/L

• [An3+]t = [Cm3+] + [Am3+] = 20 pmol/L

• aqueous carbonate concentration evaluated from the measured alkalinity

• ionic strength at 0.02 M

122

Data of WAG-5 Site

• Species logß• AnFA(III) 6.09±0.12• An(OH)FA(II) 13.04±0.20

• An(OH)2FA(I) 17.24±0.30

• An(CO3)FA(I) 12.74±0.30

• Also carbonate and hydrolysis data• LC = 0.279pH - 1.01

maximum =1, minimum = 0

123

0.0

0.2

0.4

0.6

0.8

1.0

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

Mo

le F

rac

tio

n

pH

An3+

AnHA(III)

AnOHHA(II)

AnCO3HA(I)

AnCO3

water range

An(CO3)

2

Speciation Calculation for WAG-5 SiteSpeciation Calculation for WAG-5 Site

1 Fuel Cycle Chemistry Chemistry in the fuel cycle §Uranium àSeparation àFluorination §Fission...

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Transcript of 1 Fuel Cycle Chemistry Chemistry in the fuel cycle §Uranium àSeparation àFluorination §Fission...