HTGR Fuel Fabrication and quality control
Bing Liu
IAEA Course on
High Temperature Gas Cooled Reactor Technologies
Institute for Nuclear and New Energy Technology,
Tsinghua University, Beijing, China
October 22-26, 2012
INET, Tsinghua University
„
„
„
Bing Liu
New Materials Division
INET, Institute of Nuclear and New Energy Technology,
Tsinghua University
Nuclear fuel and fuel cycle,
HTR fuel,
coated particle,
manufacture and model
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Outline
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1. Introduction of HTR fuel
2. Fabrication Technology and Equipments
3. Examination, Quality control and quality assurance
4. References
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1. Introduction of HTR fuel
① Full ceramic fuel
② Normal 1200℃,
Accident 1600℃
③ Burn up 10-19%FIMA
④ High level retard
ability of FP（10-6）
⑤ High outlet
temperature（750950℃）
⑥ Inherent safety
character and the
first barrier to FP
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The concept of coated particle fuel was invented by Huddle of the UKAEA
and patented in March 1959
Design functions of the ceramic coating layers of the TRISO fuel particle.
Kernel
Buffer
(density ≤1.10 g.cm-3)
•
•
Composed of UO2, UCO, ThO2, U-Th, U-Pu
Provide fission energy
•
Provides void volume for gaseous fission products and carbonoxygen reaction products (CO, CO2) released from fuel kernel.
Accommodates fuel kernel swelling.
Protects PyC and SiC layers from recoil damage.
•
•
•
Inner PyC (iPyC)
(density ≥1.85 g.cm-3)
•
•
SiC
(near theoretical
density of 3.21 g.cm-3 )
•
•
•
Outer PyC (oPyC)
(density > 1.85 g.cm-3)
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•
•
A diffusion barrier to fission products, retains gaseous fission
products.
An impermeable layer prevents Cl2 from reaching kernel during
SiC deposition, and prevents CO from interacting with SiC during
irradiation.
Provides mechanical substrate for deposition of SiC layer.
Primary barrier to fission products, retains all gaseous and solid
fission products at normal operating temperatures (<1250°C)
except 110mAg
Load bearing layer for particle
Creates compressive stress on SiC during irradiation due to PyC
shrinkage
Retains gaseous fission products
Provides bonding layer with carbonaceous fuel element matrix
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History of HTR fuel elements
1.
2.
3.
4.
5.
AERE, Harwell and the DRAGON project in the U.K.
GA and the Battelle Memorial Institute (BMI) in the U.S.A.
Spherical fuel element preceded the coated fuel particle in
NUKEM, GmbH. Germany.
More recently, development also in Japan, France, China, South
Korea and South Africa.
carbide/oxide, UC, UC2, (Th,U)C2, (U,Zr)C, and UO2,
BISO/TRISO, HEU/LEU, several types of the fuel
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Outline
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1. Introduction of HTR fuel
2. Fabrication Technology and Equipments
3. Examination, Quality control and quality assurance
4. References
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2. Fuel Fabrication process of HTR fuel (cont.)
„ Kernel technology and equipment
„ Coated particle technology and equipment
„ Compacts (prismatic), and pebble technology and
equipment
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2-1Kernel fabrication Process and technology
1 . Sol preparation
2. Gel particles forming
3 . Aging, washing and drying
Ammonia
4 . Calcining to UO3 particles
5 . Reducing and sintering
6 . Sieving and sorting
7. Kernel
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Kernel manufacture Process and technology
(cont.)
1. Powder Metallurgical method
2. Sol-Gel Process (with good sphericity, and high
density)
3. Gel control (internal, external, and total)
4. The reaction of gel, G/L interface, L/L interface
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1.
Prepare Feed Solution:
Feed material is in the form of U3O8 powder which is dissolved
in nitric acid to form a uranyl nitrate solution according to:
3U3O8 (s) + 20HNO3 (aq) → 9UO2(NO3)2 (aq) + 10H2O + 2NO (g).
The uranyl nitrate solution is pre-neutralized with ammonium
hydroxide just prior to precipitation:
2UO2(NO3)2 (aq) + NH4OH (aq) →
2UO2(NO3)1.5(OH)0.5 (aq) + NH4NO3 (aq).
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2.
3.
Prepare Casting Solution:
A casting solution is prepared by adding small
amounts of polyvinyl alcohol and
tetrahydrofurfuryl alcohol to the pre-neutralized
uranyl nitrate solution. This adjusts the surface
tension and viscosity to ensure proper droplet
formation and also assists later with uniform
shrinkage and crystal growth.
Cast ADUN (acid-deficient uranyl nitrate in
aqueous solution) Microspheres:
This step is carried out in a glass column filled
with the concentrated ammonium hydroxide
precipitation solution.
The casting solution is fed to the nozzles (typically
up to five or six) at the top of the column, where
a vibrator shakes off droplets from the feed
stream.
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4.
5.
6.
7.
Age: The microspheres and the accompanying precipitation solution
are transferred from the casting column to a flat tank for aging. During
ageing the vessel is heated with steam to 80°C. The process fully
converts the gel spheres to solid ammonium diuranate kernels, and
ensures complete crystal growth.
Wash: After ageing the solution is drained from the vessel. The ADUN
kernels in the vessel are washed with water to remove the
ammonium nitrate as well as ammonium hydroxide and
tetrahydrofurfuryl alcohol. Then the kernels are washed.
Dry: The final step is to dry the kernels. The diameter of a dried ADU
kernel is about 1 mm and the bulk density 1 g.cm-3.
Calcine: After drying the ADUN kernels are calcined in air up to
430°C. The remaining organic additives are cracked and driven off
during a gradual temperature increase.
Above 400°C the ADU is converted to UO3:
(NH4)U2O7 (s) + O2 (g) → 2UO3 (s) + 2H2O (g)+ NO (g).
The diameter of a calcined kernel is ~0.8 mm and the bulk density ~2
g.cm-3.
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8.
Reduce and Sinter: After calcining follows reduction and
sintering at high temperature to remove remaining impurities
and densify the kernels. The process is carried out under
hydrogen atmosphere to reduce the UO3 to UO2:
UO3 (s) + H2 (g) → UO2 (s) + H2O (g).
The temperature is taken up to 1600°C in order to form dense,
stoichiometric UO2 kernels that have a diameter of 500 µm and
a density just below the theoretical value of 10.96 g.cm-3.
Sieve: The final production steps are sieving to remove any
under and over sized kernels.
10. Sort: The sieved kernels are then sorted to remove any oddshapes. This is performed on a vibrating sorting table,
slightly inclined to allow spherical kernels to roll down-hill while
odd-shaped particles are vibration transported along a
perpendicular direction and collected for recycling.
9.
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The diagram of kernel process
*From Dr. Georg Braehler and Dr. Volker Dürr, NUKEM
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Kernel manufacture Process and technology (cont.)
UCO and PuO2 kernel
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The key manufacture equipments of UO2 kernel in INET
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The key equipments ----nozzle
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2-2 Coated particle fabrication process and equipments
Deposition Diagram of coated particle
UO2 kernels
C2H2+Ar
Buffer layer
Laminar
C2H2+C3H6+Ar
MTS+H2＋Ar
Inner PyC layer
SiC layer
Fine axial
C2H2+C3H6+Ar
Out PyC layer
sieving
sorting
CP product
Sampling examination
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Large columnar
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Coated particle fabrication (cont.)
1.
2.
3.
4.
TRISO coated fuel particles are fabricated using spouted-bed chemical
vapor deposition process (uninterrupted and sequential process).
Specified properties of the 4 layers are influenced by the process
conditions :
Deposition temperature
Coating gas fraction
Coating gas ratio (PyC dense)
Deposition rate
Deposition time
Technology and size of CVD coater are also to be taken into account
because coating mechanisms are depending on hydrodynamics, heat
and mass transfer…
Key problems: particles cycling characters in high temperature
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Coated particle fabrication process
1. Two key technology: particle Fluidization and chemical vapor
deposition (CVD)
2. Four layers uninterrupted and sequential coating process
3. Pyrolysis and compound, heat transfer and mass transfer.
4. All the Control Parameters temperature, gas pressure and gas
flow reach “the best conditions”
It is a complex “black box” process
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Coated particle fabrication (cont.)
The fluidization technology
1. The cold simulation and
calculation
2. The fluidization tube
and the gas distributor
3. Particles cycle
performance:
uniform, Reliable
and maintainable
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Coated particle fabrication (cont.)
1.
Buffer layer (porous carbon layer)
Properties
Density/Porosity
Thickness: 95um
Chemical process
C2H2 (g)
2C (s) + H2 (g) diluted in Ar
1250°C < T < 1450 °C
Fluidizing gas : Ar
Deposition rate of ~ 15-25 μm/min
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Coated particle fabrication (cont.)
2.
OPyC layer (outer dense carbon layer)
Properties
Density/Porosity
Thickness
Anisotropy
Permeability
Chemical process
1250°C < T < 1400 °C
C2H2 (g)
2C (s) + H2 (g)
C3H6 (g)
3C (s) + 3H2 (g)
Fluidizing gas : Ar
Deposition rate of ~ 3-7 μm/min
diluted in Ar
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I-OPyC Density and anisotropy
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Coated particle fabrication (cont.)
3. SiC
Properties
Density
Thickness
Microstructure Structure (Grain size, fine axial)
Chemical process
1500℃< T < 1650 °C
CH3SiCl3 (g) + H2 (g)
SiC (s) + 3HCl (g) + H2
(g)
Fluidizing gas : H2
Deposition rate of ~ 0.2 – 0.5 μm/min
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Coated particle fabrication equipment
The coater and sorting table
Vibrator sorting table
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The coaters:
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The coated particle manufacture equipment in INET
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2-3 The fuel elements for HTR (compact, and pebble)
1. Process diagram of COMPACK
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2. Process diagram of spherical fuel element
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3. The manufacture of raw materials----graphite
power for fuel element
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4. Matrix power manufacture
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5. Overcoating of TRISO coated particle
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6. Press with temperature in a steel mould---compact
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Press in a room temperature with rubber module (silica gel)
some key equipments in NUKEM for pebble fabrication
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Dispersion of coated particles, Size measure, U free zone (X-RAY),
in compact and pebble—the final examination of fuel
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Pebble manufacture equipments in INET
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Outline
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1. Introduction of HTR fuel
2. Fabrication Technology and Equipments
3. Examination, Quality control and quality
assurance
4. References
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Quality control, quality assurance and examination methods
1. quality assurance (QA) and quality control (QC) of asmanufactured fuel will be of utmost importance.
2. Quality is defined as conformity of properties and conditions of
items or activities with the specifications.
3. In the fuel qualification, the procedure is such that quality
characteristics are specified and then proven by examination.
fuel kernels and the coated particles and the fuel spheres, and,
first of all, to the raw materials.
4. QC and QA are related to the examination methods closely.
5. QC and QA are the main characters of the fuel industrial
manufacture.
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1. The main design specifications of pebble
UO2 kernel
diameter/µm
density/ (g·cm-3)
sphericity (Dmax/Dmin)
O/U ratio
B content /(g·g-1)
Graphite matrix and the fuel
Density /(g·cm-3)
Total ash /(μg·g-1)
Li content /(μg·g-1)
Coated fuel particle
B content /(μg·g-1)
Thickness of Buffer layer /µm
Thermal conduction 1000°C
/(Wcm-1·K-1)
Thickness of IPyC /µm
Thickness of SiC /µm
Thickness of OPyC/µm
Density of buffer layer /(g·cm-3)
Density if IPyC
/(g·cm-3)
Density of SiC /(g·cm-3)
Density of OPyC /(g·cm-3)
OAF of IPyC and OPyC
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Anisotropy α⊥/α∥
Corrosion 1000°C, He+1vol,%H2O
/(mg·cm-2h-1)
Wear /(mg·ball-1 h-1)
Drop strength (4m in height)
Crushing strength /KN
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Characterization methods applied in Germany for HTR fuel *
Inspection Item
Method
Sampling Rate
Raw Materials
Compression density of
graphite powder
Density measurement under defined load
Powder in forging die
Rebound of graphite
powder
Measurement of height difference of powder
column during and after load
Powder in forging die
Spec. electric
resistance of graphite
powder
Measurement of voltage drop along powder
column
Powder in forging die
Impurities in graphite
powder
Chemical analysis after incineration, emission
and absorption spectrometry, photometry,
fluorimetry
Representative
quantity
Impurities in uranyl
(U3O8 powder)
Chemical analysis after incineration, emission
and absorption spectrometry, photometry,
fluorimetry
Representative
quantity
Isotope composition
Mass spectrometry with regard to
238U
Representative
quantity
234U, 235U,
*from IAEA-TECDOC-1674
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Fuel Kernel
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Heavy metal loading
Transfer of kernels into a stoichiometrically well defined state and do
chemical analysis
O/HM ratio
Potential controlled coulometry
Isotope composition
Mass spectrometry with regard to
234U, 235U, 238U
Carbon content
Oxidation of kernels and do chemical analysis of CO2
Oxygen content (UCO)
Hot extraction of oxygen, transfer into CO and do chemical analysis of
CO, infrared spectrometry
Dope material content
Spectral photometry, Atom absorption spectrometry
Diameter
Optical imaging with particle size analyzer, X-ray microradiography
Sphericity
Counting of fraction of odd-shaped particles,
Multiple measurement of maximum and minimum diameter,
Micro-radiography, stereo-microscope
Density
Optical particle size analyzer or V-slot to measure mean
diameter;
Mercury pycnometer or air pycnometer to measure volume
Phase structure
Measurement of reflection on certain lattice planes and comparison of
intensities with Debye-Scherrer goniometer
Structure
Measurement of reflection on defined lattice planes,
X-ray with Debye-Scherrer goniometer
Sieve fraction
100 % sieving with DIN sieves
Weight
Weight of counted number of kernel and determine mean weight
Impurities
Spectral photometry, Atom absorption spectrometry
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Larger
number of
kernels
Grinded
kernels
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Coated Fuel Particle
Diameter
Optical particle size analyzer
Layer thickness
X-ray projection micro-radiography (only oPyC and
SiC),
X-ray contact macroradiography,
Microscopy analysis of ceramographic sections,
Optical particle size analyzer, Fluid pycnometer
Density
Weight of counted number of particles and
determine mean weight
Density of highly
dense layers
Liquid density gradient column with calibration
bodies
Gas pycnometer
Optical anisotropy
factor
OAF (in air) or
OPTAF (in oil),
Bacon anisotropy
factor BAF
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Ceramographic cuts exposed to polarized light,
OPTAF is ratio of reflected light intensity vertically
to deposition direction over reflected light
intensity in deposition direction; Correlation
between OPTAF and BAF
Volume of ~50 g
Growth features size
and distribution
Etching of ceramographic sections by wet oxidation, plasma
oxidation, or ion bombardment,
Scanning electron microscopy (SEM),
Transmission electron microscopy (TEM)
Polygonity of layers
X-ray diffraction, Stereo-microscope,
Measurement of layer thickness in 300 – 900 position
Heavy metal content
Grinding of particles and transfer into distinct compounds of U by
oxidation, quantitative chemical analysis of U
Surface
contamination
Leaching of particles with HNO3, quantitative chemical analysis of U
Defective SiC layers
Burn-Leach method
Heavy metal migration
Micro-radiography, visual inspection of buffer layer
Tightness of iPyC
Micro-radiography, visual inspection of buffer and iPyC layers after
leaching with HNO3 compared to before
Samples taken after
iPyC coating process
Micro-porosity
Determination of fractions of layer, fibre, mosaic components in PyC
by X-ray small-angle diffraction
Fragments of PyC
layers
Pore structure
Quantitative image analysis and determination of pore size
distribution
Ultimate tensile strength
of SiC and PyC
Determination of fracture load by crushing between sapphire plates,
Hemispherical bursting, Ring compression test,
Micro-hardness
Vickers or Brinell hardness
Young's modulus: PyC,
SiC
Crushing between sapphire plates and recording stress-strain curve
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SEM on fractured
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coating,
TEM on thinned
coating specimens
Single SiC or PyC rings
prepared from layers,
Single SiC half shells
PyC or SiC specimens
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Some key techniques of examination
1. Diameter of kernel and coated particle
2. Sphericity of kernel and coated particle
3. Anisotropy of I-OPyC layers
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Fuel Sphere
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Matrix density
Dimension and weight measurements
Matrix specimens 5 x 5 x
35 mm3
Thermal expansion
coefficient and
anisotropy
Measurement of temperature and dimensional change
with dilatometer;
Anisotropy is ratio of coefficient (parallel) over
coefficient (vertical)
Matrix specimens 5 x 5 x
35 mm3
Dynamic elasticity
modulus
Elastomat or frequency generator
E = 4 f2 l2 ρ
where f – resonance frequency, l – length, ρ density
Matrix specimens 5 x 5 x
35 mm3
Bending strength
3-point test on bending device
σ = (Fm l)/W where Fm – fracture strength, l –
support span, W – resistance momentum
Matrix specimens 5 x 5 x
35 mm3
Compressive
strength
σ = Fc/Q
where Fc – crushing strength, Q – cross section of
specimen
Matrix specimens 5 x 5 x
35 mm3
Tensile strength
σ = Ft/Q where Ft – breaking force, Q – cross section of
specimen
Matrix specimens 8 mm
diameter x 30 mm
Specific electrical
resistance
R = (U Q)/(I l) where U – voltage drop, Q – cross
section, I – electric current, l - length
Matrix specimens 5 x 5 x
35 mm3
Thermal conductivity
@ RT
Direct measurement
Matrix specimens 5 x 5 x
35 mm3
Thermal conductivity
@ 40°C
Thermo-conductometer after Schröder setting a
stationary temperature difference by means of the
boiling temperatures of two liquids and measuring the
time required for the vaporization of a certain quantity
of liquid, comparison with calibration standard
Matrix specimens 5 x 5 x
35 mm3
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Radial flux method:λ = (Q ln(r2/r1))/(2 l ΔT)
Matrix specimens 40 mm
where Q – power of central heater, r1, r2 – distances of
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diameter x 25 mm
TC from specimen axis, l – active length of specimen,
Matrix specimens 6 mm
Thermal conductivity
ΔT – temperature difference between TC
diameter x 32 mm
@ 1000°C
Modified Kohlrausch procedure by setting an almost
with axial bore hole
parabolic axial temperature profile with maximum in
of 1 mm diameter
specimen centre and small drop to the sides (< 10°)
Impurities, ash
20-50 g of matrix
contents, B
Spectral photometer, atomic absorption spectrometry
material
equivalent
Number of falls
Falls onto pebble bed of the same spheres until fracture
Sphere
Fracture load
Direct measurement
Sphere
Determination of mass loss after 10 h annealing @ 900°C
or 1000°C in flowing helium at 0.1 MPa and 1% steam:
Corrosion velocity
Corrosion velocity K = Δm/(F t)
where Δm – mass loss, F – sphere surface, t - time
Sphere
Fuel-free zone
Examination of particle-free shell by X-ray and visual
inspection
Sphere
Abrasion
Determination of mass loss in mg/h in abrasion drum
Sphere
Surface appearance
Visual inspection
Sphere
Released heavy metal
(matrix
contamination)
Electrolytical deconsolidation of matrix material with HNO3
and quantitative chemical analysis of U in electrolyte and
leach solution
Sphere
Defective SiC layers
Burning of spheres in muffle furnace and leaching of
uranium- see description of burn-leach test in chapter 3.1.4.
Sphere
Heavy metal content
Burning of spheres in muffle furnace, destruction of SiC
layers
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Free U content (coated particle) Failure fraction and U contamination
Burn leach test
1. Particles are heated in air to burn off exposed carbon.
2. Hot nitric acid dissolves kernels in particles with defective SiC.
3. Performed before and after compacting and press.
4. Low defect fractions require large QC samples
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Examination equipments in INET
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Equipments for chemical examination
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Batch treatment in QA/QC
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QA/QC diagram in the fabrication of Chinese pebble
natural graphite
1.checking
certif.
2.enrichment
3.U content
4.impurity
5.Beq
1.density
2.diameter
3.portion of
Dmax/Dmin≥
1.2
4.ratio of O/U
5.portion of
odd-shaped
particles
6.impurity
1.thickness
2.density
3.PyC OPTAF
4.surface U
contamination
5.free U fraction
HP:Hold Point
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U3O8
HP1
fuel kernel
fabricating
electro graphite
HP2
HP3
binder
HP4
graphite matrix sphere fabricating
graphite matrix sphere
fuel kernel
HP7
HP5
coating
spherical fuel element fabricating
coated fuel
particle
spherical fuel element
HP6
HP8
fuel element product
1.checking certif.
2.impurity
3.Beq
4.density
5.grain size
6.BET surface
1.checking certif.
2.viscosity
3.molecular weight
4.softness point
5.impurities
1.density
2.total ash content
3.Li content
4.Beq
5.thermal conductivity
6.anisotropic degree of
thermal expansion
7.oxidation rate
8.crush strength
9.drop number
10.wear rate
1.U content in FE
2.diameter
3.fuel-free shell
4.U contamination rate
of graphite matrix
5.free U portion
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Basic
design
research
Examinati
on and
QA/QC
Lamina
r
grain
Ammonia
Fine
columnar
grain
Large columnar
grain
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Technology
and facility
Mass
production
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Other topics of fuel fabrication:
1. Effluent treatment and recovery of Uranium
Liquid waste from process (ammonia
removal for fuel processing
wastewater)
2. Waste from cp and compact and pebble
(Separation of the fuel particles from
the graphite matrix, Fragmentation of
the fuel particles)
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New development------- GFR
------- MSR
------- VHTR
exhibit excellent safety performance at accident
temperatures of at least 1600°C, and should
ultimately tolerate burn-ups of about 10 to 20%
fissions per initial metal atom, and fluences up to
6×1025 n/m2 (E > 0.1 MeV).
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References
1. IAEA-TECDOC-1674 Advances in High Temperature Gas Cooled Reactor Fuel
Technology
2. Georg Braehler, HTR Fuel fabrication: processes and equipment, NUkEM
3. Scott E. Niedzialek Demonstration of Commercial Scale Compacting at The
Babcock & Wilcox Company, VHTR R&D FY12 Technical Review Meeting, Salt
Lake City, Utah, May 22 - 24, 2012
4. Michael Trammell Overview of AGR-2 NGNP Fuel Compact Fabrication, VHTR
R&D FY10 Technical Review Meeting, Denver, Colorado, April 27-29, 2010
5. C. Ablitzer CVD in fluidized-bed furnace Pyrolytic Carbon and SiC deposition,
Eurocourse on coated particle fuel, Petten, The Netherlands, December 4th –
7th, 2007.
6. John Hunn Quality Assurance and Quality Control for Coated Particles and Fuel
Compacts Eurocourse on Coated Particle Fuel, Petten, The Netherlands,
December 4th –7th, 2007
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Thank you for your attention！
Bing Liu
©Division of New Materials
bingliu@tsinghua.edu.cn