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

20

( 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.

22

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

23

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

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)

28

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

30

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.

31

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

32

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，

33

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

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

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

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

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

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