Fabrication of Uranium Oxide Fuels
Transcript of Fabrication of Uranium Oxide Fuels
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U N C L A S S I F I E D
Operated by Los Alamos National Security, LLC for the U.S. Department of Energy’s NNSA
U N C L A S S I F I E D Slide 1
Fabrication of Uranium Oxide Fuels
Erik Luther
ATR User Week 2012
LA-UR-12-21451
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Outline
Slide 2
Performance
Properties Structure
Composition
n Ceramic Nuclear Fuel
n Powder Processing Science • Powder feedstock • Compaction • Sintering
n Summary
MICROSTRUCTURE; e.g., grain size and porosity, AFFECTS PROPERTIES; e.g., thermal conductivity and creep
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Uranium Fuel Cycle
n Exploration for uranium n Mining and milling of uranium ore to produce uranium concentrate known as
‘yellow cake’ n Purification and conversion of yellow cake into gaseous uranium hexafluoride
(UF6) suitable for enrichment to increase the proportion of 235U to 2-5%
Slide 3
n Conversion of enriched UF6 to UO2 powder suitable for making oxide fuel pellets
n Fabrication of uranium dioxide fuel pellets
n Fabrication of fuel pins made from stacks of UO2 fuel pellets encapsulated in cladding, grouped in clusters, termed fuel assemblies
n Service n Used fuel storage – recycle?
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World LWR fuel fabrication capacity, metric tons/yr
Slide 4 World Nuclear Association 2011!
Countries Fabricator Location Conversion Pelletizing Rod/assembly Belgium AREVA NP-FBFC Dessel 0 700 700 Brazil INB Resende 160 160 280 China CNNC Yibin 400 400 450 France AREVA NP-FBFC Romans 1800 1400 1400 Germany AREVA NP-ANF Lingen 800 650 650 India DAE Nuclear Fuel Complex Hyderabad 48 48 48 Japan NFI (PWR) Kumatori 0 360 284
NFI (BWR) Tokai-Mura 0 250 250 Mitsubishi Nuclear Fuel Tokai-Mura 475 440 440 GNF-J Kurihama 0 750 750
Kazakhstan Ulba Ust Kamenogorsk 2000 2000 0 Korea KNFC Daejeon 600 600 600 Russia TVEL-MSZ* Elektrostal 1450 1200 120
TVEL-NCCP Novosibirsk 250 200 400 Spain ENUSA Juzbado 0 300 300 Sweden Westinghouse AB Västeras 600 600 600 UK Westinghouse** Springfields 950 600 860 USA AREVA Inc Richland 1200 1200 1200
Global NF Wilmington 1200 1200 750 Westinghouse Columbia 1500 1500 1500
Total 13433 14558 12662
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Types of Nuclear Fuel Assemblies n Numerous fuel assembly types n UO2 pellets clad with zircalloy or stainless n ~27 tons of fuel is required each year by a
1000 MWe reactor
Slide 5
PWR fuel assembly 17x17 ~4 meters ~66% of world capacity
PHWR or CANDU fuel assembly 50cm x 10cm ø ~6% of world capacity
BWR fuel assembly 6x6 to 10x10 ~22% of world capacity
AGR fuel assembly 36 pin ~2% of world capacity
World Nuclear Association 2011!
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U N C L A S S I F I E D
Ceramic Nuclear Fuel n Specifications based on 40+ years of
experience • Chemistry • Geometry • Microstructure • Physical
n Fuel assemblies still fail due to pellet failure n Science
• Separate effects of thermal gradients, radiation, microstructure etc. on thermal diffusivity, fission product distribution, pore generation and migration
• Safer fuels – better scientific understanding
n Other fuels • Fast reactors • MOX, MA-MOX, thoria? • UN, UC, composites, targets? • Recycle?
Slide 6
Garzarolli et al., 1979 !
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Fuel Failure – so what?
n High temperature, chemical corrosion, radiation damage, physical stresses
n Cladding is breached – radioactive material leaks into coolant water
n Not a significant plant safety risk
n “Very minor” leak – ignored leaking rod is removed at next refueling
n “Small” leak – allowable thermal transients are restricted
n “Significant” leak – reactor shutdown, faulty assembly removed
n Cost penalty to operating at reduced power or shutdown & replace failed fuel with matching remaining enrichment
n Balance economics and longer fuel burn
Slide 7
Bottom line: pellet failure affects the bottom line
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Minimizing fuel failure
n Fuel failure rates improved ~ 60% from 1986 to 2006
n ~14 leaks per million rods loaded (IAEA 2010)
n Exclusion of foreign material from primary coolant water • Sophisticated debris filters
n Improved cleanliness in fuel assembly
n Pellet – Clad Interaction (PCI) • Power transients • PCMI: Pellet – Clad contact pressure • PCCI: Chemical reaction – build up of e.g. iodine, cesium, cadmium • Fragmented “relocated” fuel pellets • Localized heating
Slide 8 Suspect missing pellet surface
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Fuel Pellet Specifications n 87.7% uranium n Total impurities 1500µg/g n O/M from 1.99 to 2.02 n Equivalent Boron Content (EBC) < 4.0 µg/g (B, Gd, Eu, Dy, SM, Cd) n Dimensions TBD (diameter, length, perpendicularity, surface finish)
• ~1 cm x 1.2 cm (+/- ~0.001) n Density TBD
• 95% of theoretical – 10.96 g/cc n Grain size and pore morphology TBD
• ~30 µm n Pellet integrity
• Visually acceptable n Cracks
• ½ the pellet length, 1/3 the pellet circumference n Chips
• 1/3 of the pellet end • > 5% of cylindrical area
Slide 9
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Fabrication of UO2 Fuel Pellets “Turning big rocks into little rocks then turning little rocks into big rocks”
n Powder synthesis • IDR (Integrated Dry Route) • ADU (Ammonium Diuranate) • AUC (Ammonium Uranium
Carbonate)
n Powder conditioning
n Compaction
n Thermal treatment
Slide 10
UO2 Powder
Milling
Additives – binder, lubricant, poisons
Granulation
Pressing
Burnout
Sintering
Grinding
Inspection
Service
Recycled Powder
Sieving
ADU
IDR
AUC
Path Dependent
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U N C L A S S I F I E D Slide 11
Black Art or Science? n Process knowledge is not directly transferrable
• Specifications for powder feedstock has limitations • Feedstock will change • Small changes to feedstocks will affect processing parameters • How do you compensate for variations?
n Chemistry/powder additions for MOX, MA-MOX n Material recycled from used fuel will come in many forms Would you treat this
powder the same as this powder?
What would you do differently to get the
same pellet?
AUC ADU
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U N C L A S S I F I E D
The crime…
Slide 12
Porosity that will alter heat transfer and
fission gas transport properties
ABB DUO2 sintered at 1750 ˚C
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I suspect the powder…
Slide 13
Particle agglomerates
E. Liniger & R. Raj
Seemingly uniform packing n UO2 powder particle variables • Morphology • Agglomeration
— Hard — Soft
• Size distribution • Powder flow into die
— Uniformity n Solutions to some powder
particle variables • Milling • Sieving
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Guilty…
Slide 14
Poor Powder Flow & Microporosity
Isolated “stranded” pores affect sintering rate
Pellet sintered at 1750 ˚C
As-received powder
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Reforming the powder…
Slide 15
Surface area ABB as-received: 5.5 m2/g 2 hr milled: 7.3 m2/g
0
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0.01 0.1 1 10 100
% fi
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equivalent spherical diameter (µm)
ABB as-recd
ABB 15 min mill
ABB 2 hr mil
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A repeat offender…
n No cracking • Die filling improved • Stresses reduced
n Still porous • Pore shape improved
Slide 16
Want higher density
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Digging deeper… n Compaction
• Air removal • Friction
• Die wall • Powder/powder
• Solutions • Lubricants
• Oil for tooling • Stearic acid
• Tooling • e.g., carbide
liners
Slide 17
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Green density
Slide 18
n DUO2 powder (various sources) • As received • Milled • Milled for 15 min with EBS
n Pellets pressed in dual action punch and die set • Nominal 5.7 mm diameter by 5 mm thick
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0 100 200 300 400
% th
eore
tical
den
sity
compaction pressure (MPa)
As-recd
MDD Agg
Mill/EBS
Volumetric densities sensitive to O/M ratio Trend line added to aid eye
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0 100 200 300 400
% th
eore1cal den
sity
MPa
Sigma
ABB
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Pressure – enough is enough
n Delamination
n Cracks follow density gradients • Higher gradient when thicker
Slide 19
~250 MPa
~150 MPa
Defects don’t go away
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Sintering Theory
Slide 20
Introduction to Ceramics, W Kingery, H Bowen, D Uhlmann, John Wiley & Sons, 1976
Sintering requires mass diffusion § Pore removal can be driven by
chemical potential due to geometry § Surface area § Particle size § Chemical heterogeneity § Stoichiometry (O/M ratio) § Macroscopic heterogeneity § Microscopic heterogeneity § Heat transport
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Sintering Theory
Slide 21
n Initial • Surface smoothing • Grain boundaries/necks form • Rounding of open pores
n Intermediate • Pore shrinkage at boundaries • Mean porosity decreases • Slow grain growth
n Final • Closed pores • Pores at boundaries shrink or disappear • Large pores shrink slowly • Fast grain growth • Trapped pores shrink slowly
Powder Metallurgy Science, R. German, Metal Powder Industries Foundation, 1984
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U N C L A S S I F I E D Slide 22 10/25/11 § TFCIG Meeting
§ 22
§ A: 800˚C § B: 900˚C § C: 1000˚C
§ D: 1100˚C § E: 1200˚C § F: 1300˚C
§ F: 1400˚C § G: 1500˚C § H: 1600˚C
Decreasing Porosity P
oros
ity C
oars
ens
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Slide 23
Grain Evolution During Sintering Electron Backscatter Diffraction of evolving microstructure
1100C
1200C
1300C 1400C
1500C
1600C
Shows grain growth evolution and defects
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U N C L A S S I F I E D Slide 24
Grain size “control”
1650˚C 4 hrs – 93.4% 1350˚C 4 hrs – 91.8% n EBSD images of pellets sintered under gettered argon
n Grains grown from 3 microns to 21 microns
n Density relatively constant 92% and 93% respectively
n No strong texture
0
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aver
age
grai
n si
ze (µ
m)
sintering temperature (˚C)
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Grain size control
n Addition of aluminum from ADS (aluminum distearate) ~100 ppm
n Al2O3 is known to pin grain boundaries
n 1650˚C for 4 hrs n Grain size decreased
from 21 µm to 11 µm
Slide 25
As rec’d – 94.5% 0.25wt% ADS – 92.2%
Minimizing grain
boundary energy
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O/M effects on sintering
Slide 26
-1.20E-01
-1.00E-01
-8.00E-02
-6.00E-02
-4.00E-02
-2.00E-02
0.00E+00
2.00E-02
0 200 400 600 800 1000 1200 1400 1600 1800
∂L/L
temperature ˚C
2.00
2.10
2.26
-1.20E-01
-1.00E-01
-8.00E-02
-6.00E-02
-4.00E-02
-2.00E-02
0.00E+00
2.00E-02
0 200 400 600 800 1000 1200 1400 1600 1800
∂L/L
temperature ˚C
Ar
ArH
n UO2 sintering is enhanced by hyperstoichiometry (O/U >2.00) n Can adjust O/M of powder prior to processing or adjust with
atmosphere
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U N C L A S S I F I E D Slide 27
O/M control n Guided by
thermodynamic modeling at ORNL
n Achieved with sophisticated gas control system
n Argon carrier with sub ppm hydrogen and oxygen control
n Allows precise sintering control
0
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-‐5.0
-‐4.5
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tempe
rature (˚C)
weight loss (mg)
weight
O/M
temperature
2.16
2.13
2.10
2.09
2.06
2.03
1.E-15 1.E-12 1.E-09 1.E-06 1.E-03 1.E+00 1.E+03
0 100 200 300 400 500
ppm
O2
time (min)
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Dimensional changes due to sintering
Slide 28
Green pellet shows uniform
diameter
Sintered pellet shows
“hourglassing”
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Processing modifications to minimize hourglassing
Slide 29
GREEN pellets:
SINTERED Pellets:
Less Hourglassing
Milled/lubricated Control Milled
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Pellet Geometry
n Dished and chamfered • Dish to accommodate thermal expansion • Chamfer to minimize chipping
n Why not barreled?
n Controlled porosity?
Slide 30
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Microstructure Affects Properties - Pore structure
Slide 31
Diffusivity of MDD Pellet Compared to Typical Curve
AREVA
MDD
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Alternative Fabrication Methods There’s more than one way to skin a cat…
n Spark Plasma Sintering (SPS) • Rapid, near net shaping • Large volume production, microstructure
n Extrusion • Homogeneous microstructure, continuous,
complex shapes • Liquid processing
n Microwave sintering • Rapid, remote equipment • Microstructure
Slide 32
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“Safer” designs n High temp – crack steam to generate
hydrogen
n 1/30th powder density
n Helium coolant
n Negative feedback stabilizes core around 1600C
Slide 33
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Summary
n Uranium oxide fuel pellets widely used in PWR, BWR, PHWR, AGR
n Fabrication by traditional cold press and sinter • Powder conditioning • Compaction • Sintering
n Properties of pellet are path dependent
n Most failure attributed to “missing pellet surface”
n Specifications, QC and process control
n Advanced characterization (EBSD, laser flash, 3D SEM)
n Alternative fabrication methods
Slide 34
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Acknowledgments
Slide 35
Ken McClellan Darrin Byler
Andrew Nelson Bogdan Mihaila
Pallas Papin Denny Guidry Anna Llobet
John Dunwoody Steve Willson
Stewart Voit Ray Vedder
Claudia Rawn Chinthaka Silva
Dixie Barker
LANL ORNL
Pedro Peralta ASU
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Perspective n All the used nuclear fuel produced by the U.S. in 50 years of operation
—approximately 62,500 metric tons—would only cover a football field to a depth of about 7 yards. – Nuclear Energy Institute 2010!
n A 1000 MW(e) nuclear power station – produces ~ 30 tons of high level waste per year. In comparison, a 1000 MW(e) coal plant produces 300,000 tons of ash per year. – International Atomic Energy Agency!
n Some coal deposits contain uranium at concentrations greater than 100 ppm. Burning coal concentrates this by a factor of ten.
n The fly ash emitted by a coal power plant carries into the surrounding environment 100 times more radiation than a nuclear power plant producing the same amount of energy. – Scientific American!
n One fuel pellet provides as much energy as 1 ton of coal, 149 gallons of oil, 17,000 ft3 of natural gas – Ameren!
Slide 36
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UO2 Reconversion Process
Slide 37
- Nuclear Fuel Cycle Information System, IAEA 2009!
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U N C L A S S I F I E D Slide 38
Everything you ever wanted to know about hoppers…* *but were afraid to ask
Slide 38
image from stellar manufacturing!
Cohesive Arch plugs
hopper
Issues § Ratholing/Piping § Funnel Flow § Arching/Doming § Insufficient Flow § Segregation § Flushing
Stable annular region
Active flowing region
Air in
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Sintering Reality n Sintering
• Elemental diffusion • Impurities and/or sintering aids • Heat transport • O/M ratio
n Small, high surface area, uniformly packed, “active” particles n Processing space
• Powder conditioning — Milling, sieving, O/M, etc.
• Pressing conditions — Density, density variations, etc.
• Atmosphere (O/M) — Hydrogen, oxygen, water — Initial reduction, post reduction, constant O/M
• Temperature and time
Slide 39
How do you grow grains?
How do you prevent grains from growing?
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O/M controlled sintering
Slide 40
1.E-14
1.E-13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
-3.0E-01
-2.5E-01
-2.0E-01
-1.5E-01
-1.0E-01
-5.0E-02
0.0E+00
5.0E-02
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ppm
O2
dL/L
o
time (min)
n Sintering a pellet under controlled atmosphere
n Complex PO2 profile to maintain constant O/M
n Guided by thermodynamic modeling at ORNL
n Achieved with sophisticated gas control system
n Argon carrier with sub ppm hydrogen and oxygen control