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Something about nuclear graphite
Tongxiang LIANG
Nuclear Graphite Research Group
Institute of Nuclear and New Energy Technology
Tsinghua University
北京 陶瓷
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OVERVIEW
• History
• Requirements
• Manufacturing
• Microstructure
• Irradiation performance
• Disposal of spent graphite
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History of nuclear graphite
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• In 1919 Rutherford discovered the proton, and put out the idea that there could be a particle with mass but no charge. He called it a neutron.
• James Chadwick discovered the neutron in 1932, received the Nobel Prize for his discovery in 1935.
Neutron is the golden key to open the gate
of nuclear energy
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• Physicists soon found that the neutron is an ideal "bullet" for
bombarding other nuclei. In 1934, Enrico Fermi found that
slow (thermal) neutrons striking the target will be more likely
to collide with silver atoms; the increased collisions result in
higher radioactivity.
• 1938, Otto Hahn, Lise Meitner and Fritz Strassmann
discovered that uranium nuclei split when bombarded with
neutrons.
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• The nuclear cross section of U235 for slow neutrons is about 1000 barns, while for fast neutrons it is in the order of 1 barn.
Therefore thermal neutrons are more likely to cause U235 to fission than to be captured by U238.
• How to obtain slow neutron? use a neutron moderator such as graphite, Be, water, to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutron.
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• In 1942 the Manhattan Engineer Project was set up in
the United. Scientists recruited to produce an atom bomb
included Robert Oppenheimer (USA), David Bohm (USA), Leo
Szilard (Hungary), Eugene Wigner (Hungary), Rudolf Peierls
(Germany), Otto Frisch (Germany), Niels Bohr (Denmark), Felix
Bloch (Switzerland), James Franck (Germany), James Chadwick
(Britain), Emilio Segre (Italy), Enrico Fermi (Italy), Klaus Fuchs
(Germany) and Edward Teller (Hungary).
• In order to produce Pu, Chicago Pile 1 (CP-1) reactor was built.
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Fuel of CP1 CP-1 reactor was a pile of uranium and graphite blocks, graphite is neutron moderator
Graphite from CP 1
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Then, graphite has been widely used as a
moderator, reflector and fuel matrix in various
types of nuclear reactors, such as gas cooled
reactor (e.g. AGR, MAGNOX), Russian RBMK
reactors, high temperature gas cooled reactor
(Dragon, Peach Bottom, AVR, THTR-300, Fort
St. Vain, HTTR, HTR-10 ) and so on.
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Magnox AVR
THTR 300
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Dragon reactor
HTR-10
M.S.T. Price, Nucl Eng and Design, 251 (2012) 60-68
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Requirements
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Nuclear designer requires:
six high and four low •High purity (neutronic and waste point of view, Boron
content)
•High density (The greater the density, the greater its
moderation of neutron flux)
•High irradiation stability (The irradiation behavior is
strongly influenced by the source of the pitch, the coke
and the manufacturing process)
•High thermal conductivity
•High strength
•High oxidation resistivity
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• low anisotropy, less than 1.1, defined by
coefficient of thermal expansion (CTE) in
orthogonal directions
• low CTE, ~4 x 10-6 K-1 (20 - 120℃)
• low elastic modulus
• low cost
The graphite crystal is anisotropic; i.e. its properties are different in perpendicular and parallel directions relative to the principal alignment of the basic planes
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Nuclear graphite IG11 NBG18
Filler petroleum coke Pitch coke
Mean particle size ( µm) 20 300
process isostatic pressing vibration molding
Density (g/cm3) 1.78 1.85
Flexural strength (N/mm2) 37 30
Tensile strength (N/mm2) 25 24
Compressive strength (N/mm2) 77 77
Thermal conductivity (W/mK) 78
CTE (10-6/℃) 4.1
Anisotropy 1.05 1.03
Basic properties of nuclear graphite
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Philippe Beghein et al, Nuclear Eng and Design, 251 (2012) 146-149
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Philippe Beghein et al, Nuclear Eng and Design, 251 (2012) 146-149
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Nuclear Graphite Manufacturing
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• cold isostatic pressing or vibration molding is used for shaping in order to get low anisotropy.
• After baking (carbonization), the artifact is typically impregnated with a petroleum pitch and re-baked to get densification. Impregnation and re-bake may perform several times to attain the required density.
• Graphitization typically occurs at temperature >2,500°C.
• Additional halogen purification may be required.
W Winders, et al, Project 23747, Graphite Technology Development Plan, PLN-2497. Idaho Falls:
Idaho National Lab.2007
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Raw materials: 1) Calcined coke-----petroleum or pitch coke (size and distribution) 2) Binder----coal tar pitch (soft point and carbon yield, thermoplastic) 3) impregnants---coal tar pitch or polymer 4) additives
mixing
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How to get isotropic graphite?-----coke/manufacture
process
•A needle coke known as AGOT graphite was used in the Hanford
piles and achieved some undesirable results. Needle coke is
anisotropic, and when irradiation it develops large internal stresses
in one direction, which results in cracking and a decreased
irradiation lifetime.
Magnox, windscale, WAGR
piles used needle coke and
extrusion processed graphite
as moderator
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• Gilsonite coke from natural mine in Utah USA,
particles have “onion skin” structure, molding
process gives isotropic property.
Needle coke
Isotropic cokes are favored for nuclear graphite
AGR, THTR 300 used, no longer exist
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• Germany developed a “second coke” method:
coke + binder----blended----baked----milled,
mix with binder----blended----baked---
impregnated----graphitized
• In order to get isotropic behavior, Isostatic
pressing or Vibration molding is used
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Isostatically presser
oil
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Vibration molding machine
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Baking or carbonization 1000~1250℃, Binder to carbon
A N Jones and B J Marsden, CARBOWASTE Abbeye St Jacut 25th -28th Oct 2010
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Autoclave Impregnation By pitch to improve density
Using low soft point, high carbon yield pitch
A N Jones and B J Marsden, CARBOWASTE Abbeye St Jacut 25th -28th Oct 2010
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Acheson and lengthwise graphitization
furnace
Acheson lengthwise
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Acheson and LWG
semi-products themselves act as resistor
Philippe Beghein et al, Nuclear Eng and Design, 251 (2012) 146-149
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machining
The final product
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• There is a big shrink rate difference between pitch
and coke particles during the baking period, as a
result internal stress will be produced that may lead to
the cracking of graphite products.
• The green coke’s or mesocarbon microbead (MCMB)
surface is chemically active, which makes it easy to
combine with binder. Furthermore, coke and binder
have a same shrink rate during the heat treatment
process. Graphol series graphite designed by Oak
Ridge National Laboratory belong to green coke
graphite, the damage strain of graphol graphite is
larger than that of conventional graphite.
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Coke size: •Particle size, pore or crack size and distribution have great effect on the performance of graphite. Isostatic pressing graphite (e.g. IG110) used fine-particle-coke, about 20 m, vibration forming graphite (e.g. NBG18) has a large particle size, 300 m, and they have different fracture behavior.
•For coarse-particle graphite, it did not break immediately when the crack grow to the critical value. But the fine-particle graphite damaged quickly as the crack began to grow.
• The fracture toughness is increased due to energy dissipating mechanisms such as crack deflection, crack bridging and pull out.
•Coarse-particle graphite exhibits low sensitivity on the crack than the fine-particle graphite. Upon repeatedly loading, coarse graphite fracture toughness increased but fine-particle graphite decreased.
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Microstructure
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J. Kane, et al, Boise State University
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Very complex structure
C Karthik, et al, Boise State University
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G: Giloscarbon filler particle
B: binder phase
C: calcination cracks
E: gas entrapment pores
needle coke filler particle
Optical micrograph of two graphite
Paul J Hacker, J. Phys. D: Appl. Phys. 33 (2000) 991–998
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Mrozowski cracks
Mrozowski cracks, with lengths from tens of nanometres to more than 10 µm and width from several nanometres to about 200 nm.
TEM of Virgin Graphite
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(a) Graphite lattice; (b) turbostratic graphite
Upon heating, graphite increases
its order of crystallinity
Mrozowski cracks were formed due to the thermal expansion coefficient difference along and normal to the grain of graphite during slow cooling process of graphitisation.
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Irradiation performance
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• Under irradiation, graphite undergo changes in its thermo-
mechanical properties, especially via swelling and irradiation-
induced creep, which affects the graphite’s in-service life time.
• understanding of these life-limiting phenomena is very
important for the development of new nuclear graphite. The
current understanding is that the ballistic displacement of carbon
atoms caused by irradiation results in the accumulation of
interstitial atoms between the basal planes. These interstitial
clusters eventually rearrange to form new basal planes resulting in
the expansion along the c-axis; however, this explanation has been
disputed by several researchers.
• The historic nuclear graphite no longer exist; must
characterize the microstructures of new nuclear graphite
and demonstrate that they exhibit acceptable properties in
both the non-irradiated and irradiated state.
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Upon neutron irradiation a neutron will knock carbon
atoms from the basal plane and cause the formation of a
vacancy
New plane forms, c-axis expansion
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Irradiation : the <a>-axis shrinkage and <c>-axis growth
46
B.T. Kelly et al, IAEA-TECDOC-1154, 2000
The in-plane c-c bond is very strong, the interplanar bonding is weak
Atom is difficult insert into the basal plane
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C. Karthik, et al, Boise State University
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the cracks tend to close upon heating
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closure of the cracks by electron irradiation
Keyun Wen, et al, Journal of Nuclear Materials 381 (2008) 199–203
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In-situ e-Beam Irradiation Effects of NBG 18 graphite Crack closing by the swelling of c direction
J. Kane, et al, Boise State University
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(1) (2)
(3)
In-situ HRTEM of NBG-18
graphite under e-Beam
irradiation
Mrozowksi cracks
J. Kane, et al, Boise State University
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At the beginning of irradiation, a-axis shrinking and c-axis swelling, but the swelling is taken up by the accommodation cracks.
The dimensional change is shrink
After the cracks closed, the c-axis accommodation has been filled, turnaround
swell
Ne
utro
n d
ose
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How to determine the lifetime of reflector graphite?
Shrink—swell, at the point of “0” changes
53
dim
ensi
on
al v
aria
tio
ns
Neutron dose
Lifetime
low anisotropy
has a longer
lifetime
Turnaround. After turnaround pore generation and expansion will continue until internal stresses are large enough to propagate cracks
Dimensional instability under irradiation is one of the main problems for nuclear graphite
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For the moderator and reflector of HTGR, the
lifetime of graphite is about 40 years, it must
have a higher irradiation stability.
The irradiation behavior is strongly influenced
by its crystallographic structure, i.e., by the
source of the pitch, the coke and the
manufacturing process.
Which kind of raw materials and process is
benefit for irradiation? Unknown! need further
research.
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has strong bonds in the basal plane, a-axis; weak bonds in “c” direction
Modulus is greater in “a” direction
The coefficient of thermal expansion is much greater in “c” direction
Heat is transferred by lattice vibration (phonon) along the basal plane, thermal conductivity is greater in “a” direction
Other behavior Graphite crystal Polycrystalline graphite
Always have some degree of preferred orientation,---with grain (WG) or against grain (AG)
Higher modulus in WG direction
Lower CTE in WG direction
Higher thermal conductivity in WG direction
Irradiation induce defects, phonon-phonon scattering, thermal conductivity decrease
Cracks closure lead to the CTE rise
Irradiation defects pinning dislocation, and crack closure, so modulus increase; high dose, large pores and crack, modulus decrease
Irradiation
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(200)peak as the irradiation dose (at 30 ℃), irradiation reduce the graphitization degree
56
Irr. dose
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Change in Young’s modulus
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 100 200
CTE
α/α
0
Dose n/cm2 EDND x 102
430oC
600oC
940oC
1240oC
Just like Young’s modulus, Initial increase of CTE attributed to closure of Mrozowski cracks. Subsequent decrease attributed to opening of new porosity.
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Thermal conductivity decreases as the neutron dose increasing
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Disposal of spent graphite
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After decommissioning, a HTGR with 200MW will
produce about 600 tons radioactive nuclear graphite,
and during the annual operation, the discharged spent
fuel contains about 60 tons graphite.
Up till now about 250,000 tons irradiated graphite
has to be disposed as radioactive waste, though with
low radioactivity. How to dispose the large volume of
contaminated graphite reasonably needs to be addressed
if we want to develop HTGR reactor sustainably.
Low Level Waste (LLW).
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• Most radioisotopes can be removed by high
temperature annealing.
• 14C has a half-life of 5730 years, and is a
biologically hazardous substance because of its
readily assimilation by the human body, how
to dispose 14C becomes the key issue for
management of nuclear graphite during
decommissioning of graphite moderated
reactors.
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J. Fachinger et al,Nuclear Engineering and Design 238 (2008) 3086
Experimentally determined 14C specific activity
was 130±20 kBq/g for the RBMK-1500 reactor
14C activity measured by LSC as a function
of fast neutron fluence. (D. Lexa, J of
Nuclear Materials 2006,348,122)--the
ASTRA research reactor
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There are several principal solutions possible:
•Packing and disposal of graphite in appropriate repositories
•Incineration of graphite with the exhaust of CO2 gas
•Recycling.
Every management route has its strong and weak aspects.
Disposal option is simple and cheap, but this requires building specialized repositories. Disposal is not very attractive from the point of view of nuclear energy sustainability.
Incineration means that all 14C would be emitted to the atmosphere. Incineration of one RBMK-1500 reactor graphite (1700 tons of graphite, ∼7×1014 Bq of 14C)would increase the amount of 14C in the atmosphere by 0.6%.
is the most challenging option
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• Recycling of nuclear graphite is of great practical value. However, it is very difficult to separate 14C from the graphite matrix.
• Steam pyrolysis is an effective technology for the remove of 14C
• The mechanism of steam pyrolysis for removing 14C is based on the fact: most of 14C is located on the graphite surface, the inner surface of pores or grain boundary, then it becomes 14CO2 and 14C O when oxidized by water or oxygen.
14 N + n ----14C
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Schematic drawing of steam pyrolysis
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Summary • HTGR graphite’s life is about 40 years, if prolong to 60 years, much
more research works should be done on graphite.
• Research should focus on the following materials: Graphite using
green coke as raw material has a high fracture strain, vibration
molding of coarse particle graphite shows special fracture
toughness.
• In recent decades, materials science has made great progress, but
nuclear graphite research came to a standstill since the 1980s.
Improvement on performances of nuclear graphite may be inspired
by introducing new ideas, including nano-carbon materials, anti-
oxidation coating, mesophase carbon, and so on.
• Steam pyrolysis method is the effective technology for removing
14C from spent graphite. Recycling of waste graphite for nuclear
graphite fabrication is of great practical value due to sustainability.
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Nuclear graphite research institutions
• The University of Manchester
• University of Bath
• Oak Ridge National Laboratory
• University of Oxford
• SGL
• INET---will publish some papers in three years
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INET’s plan
Research program in my group:
•Manufacture and properties of nuclear graphite
•Irradiation of graphite, including modeling and simulation by the first principle, Molecular Dynamics, MC…..
•Disposal of nuclear graphite waste