Research of Oxidatio Propertien of Graphits e Used in HTR-10 · 2012. 3. 20. · Outlet stea...
Transcript of Research of Oxidatio Propertien of Graphits e Used in HTR-10 · 2012. 3. 20. · Outlet stea...
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Research of Oxidation Properties of Graphite Used in HTR-10
Xiaowei Luo 1 ),Robin Jean-Charles2)
(1. Institute o f Nuclear and Nezv Enegry Technology, Tsinghua University 9
Beijing 1 0 0 0 8 4,C h i n a 2. CEA Cadarache , DEN/CAD/DTN/STPA,France)
Abstract : The oxidation of graphite influences the graphite performance.
There are many factors to inf luence the graphi te oxidat ion. I n 10 M W
H i g h Temperature Gas-cooled Reactor ( H T R - 1 0 ), t h e graphi te IG-11
was used as moderator and s t ructure material . The dependence of
ox idat ion behaviour on temperature for the graphite I G - 1 1,w a s
invest igated by thermograv imet r ic analysis in the temperature range of
400 to 1 200 °C. The oxidant was dry air (wa te r content < 2 X 1 0 " 6 )
w i t h a f l ow rate of 20 m l / m i n . T h e ox idat ion t ime was 4 hours. The
oxidat ion results exhib i ted three regimes: i n the 400 ~ 600 °C range,
the act ivat ion energy was 158. 56 k j / m o l and ox idat ion was contro l led
by chemical reaction; in the 600〜800 °C range, the act ivat ion energy
was 72. 01 k j / m o l and ox idat ion kinet ics were contro l led by in-pore
diffusion; when the temperature was over 800 °C,the act ivat ion energy
was very smal l and ox idat ion was contro l led by the boundary layer.
Due to CO production, the ox idat ion rate increased at h igh
temperatures. The effect of b u r n - o f f ' on act ivat ion energy was also
investigated. I n the 600〜800 °C range,the act ivat ion energy decreased
w i t h burn-of f . Results in l ow temperature tests were very dispersible
because the ox idat ion behaviour at low temperatures was sensit ive to
inhomogeneous d is t r ibu t ion of impur i t ies and some impur i t ies can
catalyse'graphite oxidat ion.
Key words: G r a g h i t e,O x i d a t i o n,H i g h Temperature Gas-Cooled
Reactor
1. Introduction The graphi te is used w ide ly in reactor , especially in Gas-cooled Reactor
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( G C R ) because of his excellent nuclear propert ies (good moderat ing capacity,
l ow absorpt ion cross section and good i r rad ia t ion performance),chemical i ne r t ,
h igh conduct iv i ty , good mechanical propert ies in h igh temperature and
machining character, good corrosion resistance and mature manufacture process.
I n GCR, the graphi te is used as moderator and st ructure materials. A t the same
t ime, there are much nuclear carbon mater ia l applied in GCR as ref lectors to
ref lect or absorb neutrons and isolate the heat t rans fe r [ i ] . Due to the market,s
demands (sa fe ty , economic and pro l i fe ra t ion and waste disposal。2] ) to reactor ,
the new generation rectors are developed. The H i g h Temperature Gas-cooled
Reactor ( H T G R ) is considered as a classic type in generation JY • H T R - 1 0 is a 10
M W H i g h Temperature Gas-cooled Test Reactor w i t h a pebble bed core to be
bu i l t at Ins t i tu te of Nuclear Energy and New Energy Technology ( I N E T ),
Tsinghua Un ivers i t y in Be i j ing , China? wh ich used graphi te as moderator and
st ructure mater ia l and he l ium as coolant. The H T R - 1 0 was approved by the
State Counci l of China as a part of the China H i g h Technology Programme. The
pr imary system of H T R - 1 0 consists of a Reactor Pressure V e s s e l ( R P V ),a hot
gas duct vessel and a Steam Generator Vessel ( S G V ) . The R P V and S G V are
arranged side by side. The steam Generator (SG) consists of 37 smal l coils pipes
isolate each other wh ich located in a annular cavity of the SGV[3—. The reactor
pressure vessel consists of the fuel elements,graphite block ref lectors,contro l
rods dr iv ing system, smal l absorber bal l system and fuel element handl ing
system. In H T R - 1 0 , there are about 60 tons graphi te and 27 000 fuel elements.
The fuel elements are placed in core and surrounded by graphite ref lectors. The
fuel elements in H T R - 1 0 are successively fed to and removed f r o m the reactor
core by the refuel l ing and discharge tube via a pulse pneumatic single-exit gate,
which is placed inside the pressure vessel. The core has a cylinder body w i th a cone
bottom,whose diameter and effective height are 1. 9 m and 1. 76 m respectively.
Due to impu r i t y of he l ium coolant , there w i l l be an inevitable ox idat ion of
carbon mater ia l ( fuel e lement,graph i te br ick and carbon b r i c k ) at h igh
temperature. A n d the serious ox idat ion of graphite wou ld occur in ingress air
due to rup ture in the p r imary circui t or in ingress water due to rup ture in heat
exchanger. When the fuel element taken place serious ox idat ion, the gaseous
and volat i le f ission products of fai led fuel part icles are released. Furthermore,
the ox idat ion can change the propert ies ( mechanical p rope r t i es, t he rma l
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properties etc. ) of graphite mater ial to influence the safety of rector. So, the
oxidat ion of graphite is very impor tant to safety analysis of reactor and operating
l i fe assessment of graphite components. The plan wo rk w i l l research the
oxidat ion of graphite in H T R - 1 0 on di f ferent conditions and analyse the
oxidat ion rate dependency on temperature, mass rate of coolant,sample shape?
pressure of coolant and gas composit ion by experimental method. The
comparison w i l l be made w i t h oxidat ion outcome of other graphite mater ial
supplied by C E A to f ind some factors influence graphite oxidat ion (such as
porosi ty,ash content and manufacture process). Furthermore? the effect of
oxidation on the mechanical and thermal properties(tension strength,compression
strength, coefficient of thermal conductivity,coefficient of thermal expansion) of
graphite can be studied in plan.
2. Description HRT-10 The designation of H T R - 1 0 is used for development of H i g h Temperature
Gas-cooled Module Reactor ( H T R - M O D U L E ) . H T R - 1 0 used graphite as
moderator and structure material and hel ium as coolant. The spherical fuel
elements are adopted. The spherical fuel elements locate core and are
surrounded by the graphite reflectors. The carbon reflector is arranged at
outside of graphite ref lector. The fuel element is loaded cont inual ly f r om upper
three handl ing tubes w i t h inner diameter 65 m m and removed f rom the bo t tom
suction tube by dynamic gas t ransport in more through out manner. The hel ium
coolant f lows f r om th rough the reactor core in downward direction. The
temperatures of in let and out let he l ium of the core are bout 250 °C and 700 °C
respectively. The pressure of pr imary circuit is 3. 0 MPa. Tab. 1 shows the main
data of the H T R - 1 0 . [3"5]
The structure of spherical fuel element oi H T R - 1 0 is shown in Fig. 1. M
The graphite mat r i x mater ial is a structure material. The tr iso-coated fuel
particles w i t h 0. 9 m m diameter homogeneously disperse in mat r i x in fuel zone,
and in the fuel-free zone i t is used as a shell of the element w i t h the out diameter
60 mm. The graphite mat r i x has to per form a series of tasks in the fuel
elements: moderat ing, transfer f ission heat, loading external force, contain the
f ission produces. The specification of the graphite mat r i x for H T R - 1 0 fuel
elements are l isted in Tab. 2.
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Tab. 1 The main data of the HTR-10
Parameter Un i t Value
Thermal power M W 10
Pr imary hel ium pressure MPa 3. 0
In let hel ium temperature °C 250
Out le t hel ium temperature 。C 700
Pr imary coolant f low k g / s 4. 3
Out le t steam pressure at the S. G. MPa 4. 0
Out le t steam temperature at the S. G. °C 440
Inlet water temperature at the S. G. °C 104
Heat exchange tube I D & 〇D m m 1. 2 & 1 . 8
Number of heat exchange tubes 37
Heat t ransport area m2 56
Secondary steam f low k g / s 3.47
Core volume m3 5
Core diameter cm 180
Core height (average) cm 197
H / D ratio 1. 09
Fuel U 0 2
235 U enrichment of the fresh fuel 17%
Heavy-metal content g / F E 5
Diameter of fuel element m m 60
Number of fuel elements 27 500
Fuel loading scheme Mult-pass mode
Burn-up(average) M W d / t 80 000
Fuel element incore t ime (average) EFPD 1 161
Number of fresh fuel element per day 25
Therma l power of fuel eiementCmaximum) k W / F E 0. 53
Thermal power of fuel element(average) k W / F E 0. 36
Fuel element surface temperatureCmaximum) °C 831
Fuel element centre temperatureC max imum) °C 864
Number of absorber unit in reflector 17
Number of i r radiat ion channels in reactor 3
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Triso Coated Fuel Particle
Tab. 2 Specification of the graphite matrix for the HTR-10 fuel element
Property U n i t Value
Density g /cm 3 1. 7 5 ± 0 . 02
A s h content 1 0 - 6 < 3 0 0
L i 10 " 6 < 0 . 05
Boron equivalent 10 " 6 • < 1 . 3
Thermal conductivi ty ( 1 000 °C) W / ( c m • K ) > 0 . 25
Corrosion rate ( 1 000 ° C , H e + l v o l % H 2 0 ) m g / ( c m 2 • h) < 1 . 5
Erosion rate m g / h > 6
Break load m • k N > 1 8
Member of drop f rom 4 m high onto pebble bed before break > 5 0
Anisot ropy of thermal expansion a 丄 / a " < 1 . 3
3. Fundamental principle of gas oxidation The graphi te is oxidised by the oxygen,water vapour and l i t t l e carbon
dioxide in reactor. Because the graphi te is porous mater ia l , F i r s t l y, t h e
oxid is ing gases must di f fuse to the active site of graphi te, then occur chemical
reaction w i t h carbon atom and f ina l l y the reaction products leave the graphi te by
di f fusion. Dur ing the course of corrosion of the oxid is ing gases, the qual i ty
conservation law must be satisfied. The expression the qual i ty conservat ion law
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is the d i f fus ion-react ion equation. Under one-dimension condi t ion, the equation
becomes fo l l ow ing form [ 7 ],
n 32u RT o 3u A ,,、
D e — j — -5— • K / — — = 0 ( 1 )
dx F t dt
where De is the d i f fus ion coeff icient of ox id is ing gases,RL is the local rate of
chemical react ion, T is temperature of system, u is dimensionless parameter and
defined as fo l lows
=Po/Pj (2)
system,
丄 0 / 丄 V ^ /
P0 is the f ract ional pressure of ox id is ing gases? PT is the to ta l pressure of
Under steady state condi t ions,~~ = 0 and the di f fusion-react ion equation ot
changes to
a 砮 - 营 = Q ⑶
the reaction rate of the ox id is ing gases w i t h the graphite can be calculated by the
Langmu i r -H inshe lwood equation. The inh ib i t i on by the products is very l itt le,
which can be neglected in analysis1-8,9-1. T h e local reaction rate can be expressed
as fo l low ing
R 〖 = r r f e � where k:, k2 are rate parameters dependence on temperature. F r o m the
equation,we f ind the local reaction rate is affected main ly the temperature and
the concentrat ion of ox id is ing gases. I f adopt ing simple fo rm R\ = KP。,the
equation can be express as fo l lowing?
De P^-RTK • w = 0 (5 )
the boundary of the equation (5) is x = 0 , u=u0 the dimensional fractional pressure
of oxidising at exterior surface of graphite block. The solution of equation is
u == u0 • e x p ( — • x) (6 )
the concentrat ion of ox id is ing gases in d i f ferent place in graphi te block depends
on the value of PTK/De. Accord ing to the value,we can classify the ox idat ion
of graphi te in to three regime: chemical reg ime, in -pore d i f fus ion contro l led
regime and boundary layer regime. The value of K and De al l main ly depend on
temperature: De is p ropor t ion to T 1 , 5 [ 1 0 ] ? and K (K ^ C iexp ( — C 2 / T ) [ n ] )
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increases greatly w i t h temperature. So,we also can divide the oxidat ion of
graphite by temperature. A t a low temperature, the value of PTK/De is very
small and the oxidat ion is in “chemical reg ime”. The chemical reactions in this
regime is very s low, the oxidis ing gases can penetrate the graphite in deeply
distance. The concentrat ion of oxid is ing gases and the oxidising attack are
almost un i fo rm in al l penetrable distance (F ig . 2 ) . The oxidising rate is solely
control led by chemical react iv i ty. A t h igh temperature, the value of PTK/De is
very large and the ox idat ion is in “boundary layer control led regime,,. The
chemical react iv i ty in this regime is so h igh that al l oxidising gases penetrat ing
the laminar sublayer reacts w i t h hot graphite at the surface. The concentrat ion
of oxidis ing gases varies quick ly at graphite surface. The oxidis ing attack is
severe at exterior surface of graphite block and changes the geometry of
graphite. I n the inter ior of graphite b lock, the concentration of oxidis ing is
closed to zero ; the ox idat ion of graphite is very l i t t le . Between these two
regimes,the values of PTK and De have the same order. The oxidat ion is in " i n -
pore d i f fus ion contro l led regime”. The gases d i f fus ion in the pore structure of
graphite becomes a react ion-determining factor. Here we f ind a developing
“corros ion profile,,close to the surface of the graphite b lock, the penetrat ion
depth decreasing w i t h increasing temperature. [ u _ 1 2 ]
Penetrating distance
Fig. 2 The variation of the concentration of oxidising gases
wi th the penetrating distance at different temperature
The main oxidis ing gases in H T G R are oxygen and water vapour. For the
oxidat ion of oxygen, th ree oxidat ion regime can be divided as fol lowing:
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T < 7 7 3 . 15 K belongs to chemical regime; 773. 15 K < T < 1 773. 15 K belongs to
in-pore d i f fus ion control led regime; T 〉 1 173. 15 K belongs to boundary layer
regime^1112]. For the oxidation of water vapour,the values of temperature to divide
the oxidation regime were given by V A V I L I N,e t al [13]. For T
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The mat r i x of graphite is made f r o m the nature graphi te, petro leum coke
and binder. For d i f ferent raw mater ia ls, the react iv i ty also are d i f fe rent , for
example, the binder is more reactive than graphite crystal. The temperature of
graphi t isat ion in manufacture process of graphite affects the graphi t isat ion level
and the texture including the crystal and micro-pore structure. The
graphi t isat ion level has effect on the oxidat ion rate of graphite [ 1 5 ] . For the
graphite w i t h di f ferent graphi t isat ion level, the amount of edge site atoms is
d i f ferent , wh ich has effect on the oxidat ion behaviours ( the edge atoms are more
reactive than basal plane a toms) . A t h igh graphi t isat ion temperature, the
graphi t isat ion level of graphite mater ial is h igh and the oxidat ion resistance is
upgraded due to l i t t l e edge atoms. The oxidat ion rates of graphite also vary
correspondingly. For fuel e lement, t he f ina l ly heat-treated is needed for
carbonisation and degasification. The heat-treatment in the fuel manufacture
element have no influence on the impur i t y level and the structure of the
petroleum coke graphi te , but have influence on the nature graphite and
part icular ly on the binder ( res in carbon) [ 1 6 ] 8 ] . The influence of heat-treatment
can change texture of ma t r i x graphite including the crystal and micro-pore
structure. Var ia t ion in st ructure can affect the activity1-16-1 and fur ther affect the
oxidat ion rate of graphite.
The i r radiat ion dose also has effect on graphite oxidat ion. When the
graphite has been irradiated, the structure of crystal was changed. The specific
surface increased whi le the specific pore volume decreased ( the density of the
graphite increased) [ 1 3 ] . The oxidat ion rate of i rradiated graphite is less than the
graphite w i thou t i rradiat ion.
Abou t ox idat ion condi t ion to influence oxidat ion rate,several factors should
be accounted. They are temperature, the content of oxidising gas, f l ow rate,
graphite shape, the pressure of system, i r radiat ion and amount of burn-of f .
The di f fus ion coeff icient and the chemical react iv i ty al l depend great ly on
the temperature. They al l increase w i t h temperature. So,the oxidat ion rate of
graphite also increases w i t h temperature. A t di f ferent t empera tu re, t he
oxidat ion type of k ine t ic (chemica l regime, in-pore d i f fus ion control led regime
and boundary layer regime) is di f ferent.
The oxidat ion rate at given condit ion can calculate by fo l lowing formula:
(7)
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Where W is ox idat ion r a t e,k g / ( m 2 • s) and L is the effective penetrat ion
depth. When the content of ox id is ing gas is large, the chemical react iv i ty is
improved by more effective col l is ion. The inf luence of the oxidis ing gas content
can be exhib i ted in al l temperature.
The large f l ow rate can improve the d i f fus ion of oxidis ing gas and reaction
produces, i t is noted that the large f l ow rate increase cool ing action on graphi te
mater ia l and make the temperature of graphi te reduce. For mainta in ing a
constant temperature, the more heat energy should be provided.
The shape of graphi te component can also affect the d i f fus ion of gas. For
the same quant i ty g raph i te, the d i f fus ion of Gas in graphi te solid w i t h large
geometry surface is easier than in smal l geometry surface area. So,the spherical
graphite component is easily oxidised.
The dependence of ox idat ion rate on the coolant pressure were researched
by Vav i l i n et al [ 1 3 ] . A t a given pressure of oxid is ing gas,the d i f fus ion coeff icient
decrease w i t h increasing the pressure of coo lan t (wh ich packs more molecules in
a given volume,making i t harder for ox id is ing gas to move) . When the coolant
pressure is l o w, t h e inf luence on ox idat ion rate is s l ight. When the coolant
pressure is high, the inf luence is obvious.
The i r rad iat ion can change the chemical react iv i ty by catalyt ic effect to
reduce the act ivat ion energy. The effect of radiolysis w i l l be more apparent at
lower temperature when thermal react ion rates are low [ 1 9 ] .
The amount of burn-o f f has effect on the ox idat ion rate due to var iat ion of
B E T area and d i f fus ion channel. For gas-solid react ion, increasing B E T area
enlarges the gas-solid interface where the reaction occurs^16]. W i t h the
increasing of amount the burn-o f f, the porosi ty of graphi te increases, and the
oxid is ing gases are more easy to di f fuse in graphi te solid. The inf luence of
amount of burn-of f is expected to be apparent at ‘ ‘ in-pore d i f fus ion contro l led
regime,,.
The inf luence extent of each factor on ox idat ion behaviour of graphi te is
d i f ferent. In this exper imental p r o g r a m,w e main ly study the effects of
ox idat ion condi t ion on ox idat ion behaviour, including temperature, gas
composi t ion, f low rate,coolant pressure and geometry, the effects of inherent
parameters of graphi te can be analysed by combining the ox idat ion experiment of
other graphite carried out by Rob in et al. in Cadarache.
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5. Graphite oxidation in reactor I n H T R , the ox idat ion condit ions can be classified into two situations: the
normal operat ion condi t ion and accidents condit ion. On the normal operat ion
condi t ion, the ox idat ion of graphi te or ig inated f r om the impur i t ies of coolant,
adsorpt ion gas by internals and leakage vapour in steam generator. The
ox idat ion of graphi te under accidents condi t ion rises f r om the air ingress or
water ingress accidents.
The ox idat ion behaviours of graphi te under normal operat ion condi t ion are
applied to assess the service l i fe of graphi te component in reactor. The reactor is
designed having a very long operat ion l i fe about 40 〜60 a. The fuel elements
wou ld be replaced by the fuel handl ing system on l ine when the burnup of fuel
element gets a l im i t value. Bu t the ref lector graphite and other graphite
components should be replaced or not th rough al l operating l i fe and if need to
replace,when to replace,which are al l not clear and depend on the graphite
ox idat ion rate under normal operat ion condit ion. The impur i t ies of coolant under
normal condi t ion are shown in Tab. 3.
Tab. 3 Impurities content in coolant
Impur i t ies Content ( 1 0 " 6 )
Water < 2
O2 < 2
H2 < 3 0
N2 < 2
CH4 < 5
CO < 3 0
co2 < 6
Under accidents condi t ion, the signif icant graphite ox idat ion can resul t i n a
release of f ission products [ 2 0 ] and reduce the mechanical s t rength of graphi te
s t ructure in reactor. The air ingress and water ingress accidents are considered
as severe accidents for graphi te corrosion. The air ingress accident rises f r om a
pipe rup ture in the p r imary system. And, the water ingress accident rises f o rm a
rupture of coil pipe of SGV. On basis of ingressive quant i ty of ox id is ing gases,
we can classify the graphi te ox idat ion in to two types. One type is a l im i ted
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ingression of ox id is ing gases due to the reactor pressure vessel to be isolated
effect ively in t ime wh i le accidents occur. Under th is condi t ion, the p r imary
c i rcui t s t i l l stands h igh pressure, the oxid is ing environment changes w i t h the
oxid is ing process. The amount of graphi te ox idat ion depends p r imar i l y on
quanti t ies of ox id is ing gases enter ing the p r imary circui t as we l l as on the
thermodynamic condi t ion in the p r imary circui t . The other is un l im i ted
ingression of ox id is ing gases due to the rector pressure vessel not to isolated
effect ively in t ime in accident. Under this condi t ion, the pressure of p r imary
circui t degrade(air ingress accident) or upgrade (wa te r ingress accident) . The
oxid is ing environment does not change th rough al l course of graphi te oxidat ion.
The amount of graphi te ox idat ion depends main ly on the thermodynamic
condi t ion in reactor. The operat ion parameters of H T R - 1 0 at normal operat ion
are l ist in Tab. 4.
Tab. 4 The operation parameters of HTR-10 at normal operation
Parameter U n i t Va lue
T h e r m a l power M W 10
P r imary c i rcu i t pressure M P a 3. 0
P r imary f l o w rate k g / s 4 . 3
Core in le t temperature °C 250
Core out le t temperature °C 700
Secondary c i rcui t pressure M P a 3. 43
Hea t exchanger in let water temperature °C 104 ‘
Hea t exchanger out let vapour temperature 。c 435
Secondary f l o w rate k g / s 3. 47
The graphi te IG-11 produced by Toyo Tanso Co. L td .,Japan is served as
moderator and st ructure mater ia l in H T R - 1 0 . The graphi te is quasi- isotropic
f ine-grained nuclear graphi te wh ich manufactured f r om the petro leum
coke by rubber pressing [ 2 1 ] . The propert ies of graphi te are shown in Tab. 5.
Comparison w i t h IG-110 used in H i g h Temperature Engineering Test Reactor
( H T T R ) in Japan,the IG-11 has h igh thermal conduct iv i ty and smal l coeff icient
thermal expansion. B u t , the ash content of IG-11 is very higher than IG-110
about 100 t imes. The poros i ty is also higher than IG-110.
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Tab. 5 The properties of graphite IG-11
Proper ty U n i t Va lue
Densi ty g / c m 3 1. 76
Poros i ty % 20 Gra in size / im 20
An i so t ropy rat io 1 .04
Tensi le s t rength MPa 25. 40
Compression s t reng th MPa 76. 22
Bending s t reng th M P a 39. 44
Shore Hardness 55
Elast ic modulus / / GPa 9. 04
Elast ic modu lus丄 GPa 10. 16
T h e r m a l conduct iv i ty (20 °C) / / W / m K 144.43
T h e r m a l conduct iv i ty (20 ° C ) 丄 W / m K 147.07
Coeff ic ient of t he rma l expansion ( 2 0 。 -500。C) / / 1 0 - 6 / K 4. 08
Coeff ic ient of thermal expansion ( 2 0 ^ - 5 0 0 。 C ) 丄 10_ 6 /K 3. 9
A s h rate 1 0 - 6 479
Since the impur i t ies have a catalysis funct ion on graphi te ox ida t ion, the
contents of impur i t ies are also impor tan t to assess graphite oxidat ion. Tab. 6
l ists the contents of impur i t ies respectively.
Tab. 6 The contents of impurities in graphite IG-11
Impur i t i es Content ( 1 0 ~ 6 )
F 9 . 5 8 ‘
Ca 22. 32
M g 0. 99
A l - < 0 . 40
L i 0 . 06
Sm < 0 . 05
Gd < 0 . 05
Co 0. 20
Cr < 0 . 10
N i 8 . 3 1
Cd < 0 . 05
V 177. 2
Si 0 . 7 0
B - 2. 9
A s h 497
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7. Experimental apparatus and contents Our experiments are main ly s tudy the inf luence of temperature on graphi te
ox idat ion behaviours. The primary study of graphite oxidation behaviour under
different conditions was carried out by thermogravimetric analysis. The thermobalance
used is a TA2000C model f rom the M E T T L E R Company. Th is system al lows both
d i f ferent ia l calor imetr ic and thermograv imetr ic measurements to be taken
simultaneously on the same sample. The wo rk ing temperature range of th is
apparatus goes f r o m ambient temperature to 1 200 °C. Moreover , i t wou ld be
possible to couple this apparatus w i t h a mass spectrometer,which could provide
an on l ine analysis of gases produced dur ing the reaction.
Temperature is p r imary factor to inf luence the oxidat ion behaviours of
graphite. For s tudy ing the ox idat ion rate in d i f ferent ox idat ion regime,dif ferent
temperatures are selected. The oxid is ing gas is dry air ( H 2 0 < 2 X 10—6 ) w i t h
f l ow rate 20 m l / m i n . the oxid is ing environments are l isted in Tab. 7.
Tab. 7 Test condition at different temperatures
Tes t temperature Sample shape H e i g h t Diameter gas F l o w rate
C C ) ( m m ) ( m m ) ( m l / m i n )
400 cyl inder 10 10 dry air 20
500 cy l inder 10 10 d ry air 20
600 cyl inder 10 10 d ry air 20
700 cyl inder 10 10 dry air 20
800 cyl inder 10 10 dry air 20
900 cyl inder 10 10 d ry air 20
1 000 cyl inder 10 10 d ry air 20
1 100 cyl inder 10 10 dry air 20
1 200 cyl inder 10 10 d r y air 20
1 600 cyl inder 10 10 dry air 20
8, Operating procedure The thermograv imetr ic analyses w i t h thermobalance are operated by the
fo l l ow ing procedure :
( 1 ) We igh t ing the sample at ambient temperature
( 2 ) Placing the sample in the thermobalance
( 3 ) Hel ium or nitrogen sweeping the sample (min imum HPE: 〇 2 < 3 X 10—6,
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H 2 O < 3 X 1 0 — 6 ,C n U m < 1 5 X 1 0 " 6 ) at 150 °C,the f low rate Q = 2 0 m l / m i n
for 2 hours
(4 ) Increasing to nominal temperature
( 5 ) Gas in ject ion (a i r,He/a i r m ix tu re or H e / w a t e r m ix tu re ) for 4 hours at /
nominal f l ow rate
( 6 ) Shutdown of heating and cool ing s t i l l under c i rculat ion of the same test
gas w i t h f l ow rate Q=20 m l / m i n
( 7 ) When the temperature is lower than 50 °C,helium or n i t rogen sweeping
Q = 2 0 m l / m i n for 15 minutes ( m i n i m u m HPE: 〇 2 < 3 X 1 0 — 6 , H 2 0 < 3
X 1 0 - 6 , C n H m < 1 5 X 1 0 " 6 )
( 8 ) Removal of sample
Some data are needed to characterise the ox idat ion of graphite:
• The mass loss dur ing the test under nominal condit ions
• The var iat ion of density before /a f ter test
• Resistance to compression before/a f ter test.
The samples are machined f r o m di f ferent graphite block adopted in
H T R - 1 0 .
9, Experimental results of IG-11 Fig. 3 shows the ox idat ion quanti t ies of graphite IG-11 at d i f ferent
temperatures. The values in Fig. 3 are the mean of the two tests for each
temperature. The ox idat ion quanti t ies were calculated f r o m the weight decrease
of graphi te specimens and were normal ized by comparison w i t h the s tar t ing
ox idat ion weight of the specimens. I t was found that the oxidation amount increased
w i th temperature. A t low temperatures,between 400 °C and 500 °C,the oxidation
extent was very small, about 0. 042% for 400 °C and 0. 387% for 500 °C. The
oxidation quantity increased greatly at temperatures f rom 500 °C to 800 I t reached
23. 305% at 800 °C and then leveled off at about 850 °C. There was l i t t le change in
oxidation quantities between 850〜1 000 °C. The oxidation quantity at 1 000 °C was
only about 1 % greater than at 850 °C. A t ox idat ion temperatures above
1 000 °C, the extent of ox idat ion again increased. The max imal ox idat ion
quant i ty was 34. 362% at 1 200 °C. A t low temperatures, ox idat ion is contro l led
by the chemical react ion rate. The chemical reaction rate is very low at low
temperatures, so the ox idat ion amount is also very small. W i t h increasing
34
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oxidat ion temperature, the chemical reaction rate increases rap id ly,so the
relative oxidat ion quant i ty also increases sharply at low temperatures. Fig. 4
shows the var iat ion of mu l t i p l y ing factor M for di f ferent oxidat ion temperatures.
The mu l t ip ly ing factors are defined by fo l lowing equation:
M丁 = ( 8 )
T—100
where Mr is the mu l t ip l y ing factor when the temperature is T. QT is the
oxidat ion quant i ty at temperature T . Q T - I O O is the oxidat ion quant i ty at
temperature T-100 as a reference due to 100 °C gaps in the tests temperatures.
Fig. 4 shows that the mu l t i p l y ing factor M at both 500 °C and 600 °C is more
than 9. When t e m p e r a t u r e 〉 7 0 0 °C,the mu l t ip l y ing factor decreases sharply,
dropping to about 1 after 900 °C. Because the chemical reaction rate increases
rapidly w i t h temperature, the transfer of oxidizing gas gradual ly becomes an
impor tant factor for ox idat ion rate control . Though the transfer of oxidizing gas
also increases w i t h temperature, the transfer rate increases more s lowly than the
chemical reaction rate. A t h igh temperatures, the chemical reaction rate is
higher than the transfer rate of oxidizing gas. So oxidat ion rate is decided mainly
by the transfer rate of oxidizing gas,resulting in mu l t ip ly ing factors close to 1 at
h igh temperatures. Due to the transfer rate increasing w i t h the temperature? the
mu l t ip l y ing factor is more than 1.
40
,35
30
.0 400 600 800 1 000 1 200
Oxidation temperature (。C)
Fig. 3 Graphite oxidation extents at different oxidation temperatures
| 25
| 20
I 15
35
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Oxidation temperature (°C)
Fig. 4 Mul t ip ly ing factor at different oxidation temperatures
The variations of ox idat ion rate w i t h oxidat ion t ime are shown in Fig. 5.
400。C,700 °C and 900 °C were selected to represent d i f ferent contro l regimes.
The values of ox idat ion rates in Fig. 5 are the mean values of the two tests for
each temperature. For di f ferent regimes, the changes of oxidat ion rate w i t h t ime
express obvious differences. But for al l temperatures? the oxidat ion rate was
very l ow at the beginning of ox idat ion because there was l i t t le oxidant gas in the
oxidat ion chamber. The ox idat ion chamber was f i l led w i t h n i t rogen before
oxidat ion began. When oxidat ion began,the air was piped in and the n i t rogen
was gradual ly displaced by air. A t about 90 second,the oxidat ion rates began to
increase. Th is indicates the transfer of oxidizing gas f rom its in t roduct ion to the
t ime i t reaches the specimen surface takes about 90 second. A f t e r 90 seconds,
there was a max imum oxidat ion rate of about 130 second at 500 °C and 700。C •
The same phenomena is reported in other grades of nuclear graphi te [ 2 2 ] . A f t e r
300 second,the ox idat ion rate at 500 °C was independent of ox idat ion t ime. But
at 700。C,the oxidation rate increased gradually w i th oxidation time. A t 900。C,the
oxidat ion rate decreased s l igh t ly w i t h oxidat ion t ime.
The differences in oxidat ion rates are due to the change of the to ta l reaction
surface. I t has been established that graphite is a porous media. The reaction
between oxidizing gas and carbon atoms occurs at the wa l l of the pores. A t low
36
-
t(s)
Fig. 5 Variat ion of oxidation rate wi th oxidation time at different temperatures
temperatures, the oxidat ion rate is very low and there is no essential change in
graphite microstructure. So the to ta l reaction surface and the concentrat ion of
oxidizing gas are constant th roughout the course of oxidation. Th is means the
oxidat ion rate is independent of the oxidat ion t ime. As temperature increases,
the oxidat ion rates also increase,changing the microstructure of the graphite.
The closed pores are opened and micropores are converted to macropores or
mesopores to increase the reaction surface. The fresh addit ional surface leads to
an increase in oxidat ion rate. The reaction surface w i l l reach a max imum w i t h
increased oxidat ion. Loren Ful ler,et al. [ 2 3 ] found the max imum to be at ^
40% burn-of f . Su [ 2 4 ] reported the max imum at 20% — 30% burn-of f . The
variations of oxidat ion rate w i t h burn-of f are shown in Fig. 6(due to low burn-
of f at 500。C, the rate at 500。C is not included in Fig. 6 ) . Beyond the
maximum, the reaction surface w i l l decrease because the pore wal ls g row and
jo in each other. W i t h fu r ther increase of the oxidat ion temperature ? the
chemical reaction rate accelerates great ly. In the boundary layer control led
regime, the oxidat ion reactions are concentrated in a superficial layer. The
chemical reaction at the superf icial surface of graphite is so h igh that the most of
the oxidant is consumed there. Oxidat ion attacks at the superficial surface cause
a geometry change of graphite specimens w i thou t any damage to the specimen
inter ior . W i t h ox idat ion, the specimens? surface areas shr ink. When there is
37
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enough oxidant to cause an ox idat ion react ion, the oxidat ion rates are
proport ional to the surface area of specimens at h igh temperaturesC25] •
Therefore, the oxidat ion rate at h igh temperature decreases w i t h oxidat ion t ime
or burn-of f .
鬥 f- 700
900。C [1 u 0 5 10 15 20 25 30
Bum-off (%)
Fig. 6 Variat ion of oxidation rate wi th burn-off
The average oxidat ion rate by oxidat ion t ime are given as fo l lows !
C/3
| 0.015 g
2 反
14 000 (9 )
Rr is the average ox idat ion rate by oxidat ion t ime at temperature T,and R{ is
the oxidat ion rate at ox idat ion t ime i. The t ime range considered was 301 〜
14 300 second because before 300 second the oxidat ion rates were unstable due
to oxidizable material,and because at the end of oxidat ion there was disturbance
due to change of f low. The temperature dependence of Rf normalized to the
or ig inal weight of graphite samples is shown in Fig. 7. The var iat ion of ox idat ion
rate w i t h temperature is shown in three di f ferent ranges. A t 400 〜600 °C,the
oxidat ion rate increases rapid ly w i t h temperature. The f i t l ine has a 19. 071 of
slope. Using the Ar rhen ius relat ionship to describe the temperature dependence
38
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0.6 0.8 1 1.2 1.4 1.6
1 000/r(l/K)
Fig. 7 The temperature dependence of graphite oxidation rates
in different controlled regimes
The act ivat ion energy of graphi te ox idat ion has been studied by many
groups over the years,and some representative values of act ivat ion energy are
l isted in l i terature [ 23 ] . The act ivat ion energy of graphi te IG-110,
manufactured by pur i f y ing IG—11,is also given as 188 k j / m o l . The act ivat ion
energy of IG-11 is not as large as that of IG-110 ^s. The reason is that IG-11 has
high- level impur i t y . The typical value recommended in l i terature [ 2 7 ] is
170 k j / m o l for graphi te ox idat ion. Gerasimov has calculated the act ivat ion
energy of graphi te ox idat ion to be 172 kJ /mo l [ 2 8 ] . Between 600 〜800。C, the
increasing t rend slows and the slope of the f i t t i ng line is —8. 660 5. Us ing the
Ar rhen ius re la t ionship, the act ivat ion energy in this range is 72. 01 k j / m o l,
wh ich is almost half of the act ivat ion energy at 400 〜600 °C. A t temperatures
over 800。C,the ox idat ion rates levels of f . The act ivat ion energy in th is range is
of the ox idat ion ra te, the slope is —瓦 and the intercept is in A . E is the
activation energy,R is the universal gas constant and A is the pre-exponent ial
factor. Thus the act ivat ion energy is 158. 56 k j / m o l , wh ich is close to act ivat ion
energy 155 k j / m o l given by Kawakami [ 2 6 ] .
- -
1M
o
(s.g)/§ (o)^』UOIIBPIXO)UI
39
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very small. I t is noted that the ox idat ion rate has a certain increase at 1 200 °C,
as a resul t of large amounts of CO generated in the oxidat ion reaction. A t h igh
temperatures, the chemical react ion rate is very fast,but the concentrat ion of
oxidant at the graphi te surface is very low. A s there is not enough ox idan t ,
ox idat ion reactions w i t h oxygen main ly produce CO. M u c h CO is carried away
by carrier gas,but some CO reacts w i t h oxygen to produce C0 2 dur ing the
t ranspor t f r om the react ion surface to the free stream zone [26 ,29]. When the
ox idat ion temperature fur ther increases,the CO content i n the product m ix tu re
w i l l increase and the C0 2 content w i l l decrease.
For the present s tudy,a graphi te ox idat ion experiment was conducted at
1 500。C. Due to the h igh ox idat ion ra te, the ox idat ion t ime was only 900
second. The f l ow rate of d ry air remained 20 m l / m i n (a t room temperature) .
T h e var iat ion of ox idat ion rate w i t h t ime is shown in Fig. 8. The ox idat ion rate
began to increase as 0 2 increased, ar r iv ing at the graphi te surface at 105 second
and reaching a stable value at 650 second. The stable value for the ox idat ion rate
was about 0, 045 m g / s . For the f l ow rate of 20 m l / m i n at 25 °C, the f l ow of
oxygen was 1. 72X10—4 m o l / m i n . I f a l l oxygen was consumed to produce carbon
dioxide, the ox idat ion rate wou ld be 0. 034 m g / s,m u c h lower than the actual
ox idat ion rate. Th i s indicated h igh quanti t ies of CO were generated dur ing
ox idat ion reaction.
0,05
J 250 500 750 1 000
Ks)
Fig. 8 Graphi te oxidation at 1 500 °C
The var iat ion of act ivat ion energies w i t h burn-o f f is l isted in Tab. 8. Due to
-
small burn-o f f at low temperatures, the table only shows the act ivat ion energy
between 600 〜 8 0 0 。 C . The values of E/R and In A f rom the Ar rhen ius
relat ionship al l decrease w i t h the burn-o f f of graphite. The decrease of act ivat ion
energy arises f r o m the var iat ion of reaction rate due to the increase of the
reaction surface as previously described. The decrease of In A indicates that the
temperature dependence of the ox idat ion rate decreases w i t h burn-of f .
Tab. 8 The variation of In A and E/R with burn-off at 600 � 8 0 0 °C
Burn -o f f ( % ) InA E/R R2
0. 5 - 2 . 256 1 9. 474 5 0. 999 9
1 - 2 . 638 2 9. 028 1 1. 000 0
2 —3. 390 5 8. 196 9 0. 999 5
3 —3. 745 7 7. 797 2 0. 998 8
The graphi te ox idat ion tests were per formed in duplicate simultaneous
tests. The test data expressed some dispers ib i l i ty . Parameter j> was introduced
to ref lect the relat ive d ispers ib i l i ty for the two tests. The value of 令 was
calculated by the fo l l ow ing equation: J? — T?
^ = o , p X 1 0 0 % (10)
I < L - R K S
where Rt and Rs are the h igh and low values of test results at each temperature.
Tab. 9 gives value • at d i f ferent temperatures. I n the chemical reg ime, the
results express much higher d ispers ib i l i ty than in the in-pore d i f fus ion and
boundary layer contro l led regimes. I n the chemical regime, the chemical rate is
very s low and very sensitive to impur i t y . Some impur i t ies have elements that
catalyse carbon-oxygen reactions. These elements include Na,K,Ca,Cu, T i ,
Fe , M o , Cr,Co,Ni and V [ 2 6 , 3 0 , 3 1 ] . T h e inhomogeneous d is t r ibu t ion of impu r i t y
leads to the d ispers ib i l i ty of ox idat ion results. I n the in-pore d i f fus ion and
boundary layer contro l led regimes, the chemical reaction rate is accelerated due
to the increasing temperature. The mean ox idat ion rate at 700 °C is 1 750 t imes
higher than at 400。C • The to ta l impu r i t y content is not more than 500X 10 —6 •
The effect of impu r i t y on ox idat ion behaviours is local and is weakened by the
increase of the thermal reaction rate. When the ox idat ion rate reaches the
magnitude of 10—2 mg / s, t he difference due to inhomogeneous d is t r ibu t ion can 41
-
be ignored ( to Tab. 9 ) .
Tab. 9 Dispersibility of oxidation rate at different temperatures
Average ox ida t i on ra te by t ime T e m p e r a t u r e 於(%)
L a r g e ( m g / s ) Sma l l ( m g / s ) M e a n ( m g / s )
400 °C 7. 3 4 X 1 0 - 6 4. 0 7 X 1 0 一 6 5. 7 1 X 1 0 - 6 28. 65
500。C 5. 8 5 X 1 0 - 4 1. 3 2 X 1 0 —4 3. 5 8 X 1 0 — 4 63. 11
600 °C 5. 4 5 X 1 0 - 3 1. 7 4 X 1 0 - 3 3. 5 9 X 1 0 — 3 51. 69
700 °C 1. 0 9 X 1 0 —2 1. 0 7 X 1 0 - 2 1 . 0 8 X 1 0 - 2 0. 75
800。C 2. 3 0 X 1 0 - 2 2. 2 6 X 1 0 - 2 2. 2 8 X 1 0 - 2 1. 06
900 °C 2. 5 8 X 1 0 - 2 2. 5 1 X 1 0 - 2 2. 5 5 X 1 0 - 2 1. 49
1 000 °c 2. 7 8 X 1 0 - 2 2. 6 8 X 1 0 - 2 2. 7 3 X 1 0 - 2 1. 86
1 100 °c 2. 9 6 X 1 0 " 2 2. 7 8 X 1 0 - 2 2. 8 7 X 1 0 - 2 3. 02
1 200 °C 3. 4 6 X 1 0 - 2 3. 2 3 X 1 0 - 2 3. 3 5 X 1 C T 2 3. 51
10. Conclusions The temperature dependence of ox idat ion behaviours of nuclear graphite
IG-11 for the H T R - 1 0 were invest igated for the temperature range of 400 to
1 200。C • The main conclusions f r o m the invest igat ion were :
1) The ox idat ion quanti t ies at temperatures between 400〜500 °C are very
smal l , but the relat ive ox idat ion quant i ty increases sharply. A t 500 〜
800。C,the oxidation quantity increases greatly,and levels off at 850 °C. A t
temperatures over 1 000 °C, the ox idat ion quant i ty begins to increase
s l igh t ly again.
2) For d i f ferent contro l led regimes, the variat ions of ox idat ion rate over
t ime are d i f ferent . I n the chemical regime,due to low bu rn -o f f, t he
ox idat ion rate remains constant except that the beginning stage has a
max imum. I n the in-pore d i f fus ion contro l led regime, the ox idat ion rate
increases gradual ly over t ime after the beginning max imum rate. The
increase of the ox idat ion rate is ascribed to the increase of the react ion
surface and of effective d i f fus ion coeff icient. I n the boundary layer
contro l led reg ime, the ox idat ion rate decreases w i t h t ime due to the
decrease of the surface area.
3) The Ar rhen ius re lat ionship was used to describe the temperature
42
-
dependence of oxidat ion behaviour. A t 400〜600。C,the a'ctivation energy
is 158. 56 k j / m o l and intercept In A is 9. 168 1. A t 600 〜800。C, the
activation energy is 72. 01 k j / m o l and intercept In A is 2. 917 7. A t
temperatures〉800 °C,the activation energy is very low. The oxidat ion
rate has a certain increase at h igh temperatures as a result of CO
production.
4) A t 600〜800 °C,the activation energy and intercept both decrease w i t h
burn-of f . I t is expected that the activation energy has a m in imum w i t h
burn-of f .
5) A t low temperatures, test results have great dispersibi l i ty because the
oxidat ion behaviour at low temperatures is very sensitive to the
inhomogeneous d is t r ibut ion of impuri t ies.
Note This work was performed by Dr. Xiaowei Luo during his visi t in C E A
(Cadarache) f rom A p r i l , 2002 to A p r i l , 2003.
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45