Post on 29-Dec-2015
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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
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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.
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Uranium chemistry
• Separation and enrichment of U• Uranium separation from ore
Solvent extractionIon exchange
• Separation of uranium isotopesGas centrifugeLaser
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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)
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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+
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Uranium Ore
http://www.cogema.fr/photos_gb/
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Yellowcake production
http://www.cogema.fr/photos_gb/
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U Fluorination
U ore concentrates
Conversion to UO3
UO2
H2 Reduction
UF4
U metalUF6
HNO3Solvent extraction purification
HF
Mg
MgF2
F2
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Fuel FabricationEnriched UF6
UO2Calcination, Reduction
Tubes
Pellet Control40-60°C
Fuel Fabrication
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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)
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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
→→
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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
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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
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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
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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
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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
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Cesium and Strontium
• High yield from fission• Both beta
Some half-lives similar• Similar chemistry
Limited oxidation statesComplexationReactions
• Can be separated or treated together
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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
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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
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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
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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
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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
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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
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A=241 Isotopes
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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-
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Trivalent State
• In solution formsCarbonatesHydroxidesOrganic complexes
• Easily separated from other actinides by redox properties
• Behaves similar to trivalent lanthanides
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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
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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
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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
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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
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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
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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+
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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
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Single Solvent Extraction Stage
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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
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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!
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Actinide Third Phases
Light Phase
Heavy Phase
Aqueous
Phase
U(VI)
Pu(IV)
Np(IV) Np(VI)
Pu(VI)
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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
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Phase Inverted Plutonium
Inverted Organic
Aqueous
Heavy Organic
Light Organic
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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)
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Reverse Micelle Theory
Classical StoichiometryPossible Reverse Micelle
UOUO222+2+ + 2NO + 2NO33
-- + 2TBP + 2TBP UO UO22(NO(NO33))22 ▪ ▪ 2TBP2TBP
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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
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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-
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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Spectrum – Mixed Valence Phases
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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
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• 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
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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
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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
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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
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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
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Overview of AFC reactors
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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
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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
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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
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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
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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
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Expect US Repository Need in 2100
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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
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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)
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Pu Inventory
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Repository Heat Loading
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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
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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
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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
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Waste Forms and Packaging
• Components of Waste Radioactive Isotopes Other Materials
• Waste Forms Materials Characteristics Properties and analysis
• Spent Fuel
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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
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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
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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
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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
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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
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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
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Zeolite
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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
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Glass
• Inorganic product of fusion Cooled to a rigid condition No crystallization
Amorphous material Any substance with rapid cooling
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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
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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
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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
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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
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• 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
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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)
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Speciation and Transport
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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
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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
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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)
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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
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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