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The effect of neutron irradiation on the reactor graphite corrosion kinetics
Journal of Nuclear Materials 189 (1992) 333-342 North-Holland
The effect of neutron irradiation corrosion kinetics
on the reactor graphite
F.F. Zherdev, I.E. Komissarov, B.A. Gurovich and V.M. Markushev Russiiskii Nauchnii Tsentr, Kurchatovskii Institut, Ploshad Kurchatoua, 123182 Moscow, Russia Received 7 January 1992; accepted 15 April 1992
Investigations were carried out on the corrosion kinetics of initial and irradiated reflector and matrix graphites of several grades in helium flow containing various types of oxidants, The parameters of the sample irradiation were: the neutron fluence F = 1.5~ 1O21-2.O~ 10” cmm2 (E > 0.18 MeV) and the irradiation temperature T = 350-1100°C. The investigations were made in the temperature interval T < 1300°C over the concentration range of water vapor admixed to helium of 100-600 vpm, oxygen 350-6000 vpm and carbon dioxide 400-6000 vpm at an absolute total pressure of 1.32 bar. A qualitative change of the corrosion kinetics was found with the increase of the irradiation fluence of the samples. The increase in corrosion rate was explained by fracture of the graphite intercrystallite boundaries under irradiation and by increase in relative density of monocrystal surfaces opening into pores, normal to the basal plane.
1. Introduction Since the early days of the nuclear industry research labs of the countries building nuclear reactors have been engaged in experimental research concerning a structural material widely used in reactor designs - the reactor graphite. One of the directions of this research is the graphite corrosion stability to chemically active impurities in the environment. Corrosion of graphite affects its mechanical properties and causes adverse transfer of reaction products to the heat exchangers, where this may cause carbonization of structural materials. This necessitates the imposition of certain criteria for the selection of reactor graphite grades and coolant purification system capacities. Possible accident scenarios involving the injection of large quantities of oxidizing agents into the hot core should be specially considered [l]. Recently the designs of thermonuclear reactors have also met with similar problems [2]. The most extensive research of graphite corrosion had been performed up to the mid-80s in support of HTGR programs. Due to the well known events this research is now restricted to a number of research laboratories in Japan, Russia and Germany, while thermonuclear reactor related research is centered in the USA. Elsevier Science Publishers B.V.
The most widely used method for out-of-pile experimental investigation of the corrosion kinetics is continuous or stage-by-stage measurement of the weight change of graphite specimens placed in a corrosive environment with a controlled chemical composition [3,4,6]. Corrosion annealing at fixed temperature yields the dependence of physical properties of graphite on the burnup, and the experiment usually ends in the investigation of the porosity distribution over the cross section of the specimen [4,7-91. The raw experimental data are processed to obtain the temperature dependence of the kinetic constant of the reaction or the reaction gas-exchange constant [Xl]. The corrosion processes in the reactor evolve in the field of intense gamma radiation. To account for that there are attempts to simulate this in laboratory scale experiments by irradiation of the specimens and the corrosive media with gamma sources f4,5]. There are also studies of the effect of the presence and concentration of various graphite impurities on its corrosion kinetics (see references in ref. [61X In-pile investigations of graphite corrosion usually employ full-scale spherical fuel elements irradiated in ampoules or loop facilities [l&12]. In the loop experiments the coolant composition is controlled in the loop inlet and outlet.
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F.F. Zherdeo et al. / Corrosion kinetics of reactor graphites
Relatively little experimental data have been accumulated on the dependence of the corrosion reaction kinetics on the neutron fluence and the graphite irradiation temperature 13,121. The results available, regretfully only for a narrow range of irradiation parameters, indicate an intensification of the process with the neutron fluence incident on the graphite. The aim of the present work was a more detailed study of this phenomenon.
2. Experimental 2.1. Equipment A method of thermal gravimetry was used to study the corrosion reaction kinetics. It involved the measurement of the rate of weight decrease of a sample, heated up to a certain stationary temperature and blown round by a helium flow with a certain constant reagent concentration. A the~ogra~meter with vertical muffle and microbalance with a sensitivity of 4 X
10S7 g were used in the experiments. A feed control unit for muffle heaters made it possible to maintain a constant (with an accuracy of 0.5 K) gas temperature in the muffle operating zone. The temperature was monitored by a thermocouple the thermojunction of which was located inside the muffle at a distance of l-2 mm from the sample. Helium was used as carrier gas. It was additionally purified at the tank outlet by passing it under a pressure of 150 bar through a trap with a molecular sieve, which was placed in a Dewar flask with liquid nitrogen. The carrier gas was moistened by passing a portion of the flow in a bypass above a distilled water surface. The reagent gases were introduced into the helium flow through a thin capillary tube made of steel. The input of the capillary tube was held at constant pressure. The helium humidity was controlled by hygrometers located in the discharging branch of the inlet gas line and in the outlet line of the thermogravimeter operating muffle. Part of the gas through the inlet and outlet lines was directed towards the chromatograph, which had a light gas sensitivity of about 50 vpm. The
Table 1 Selected physical and mechanical properties of the graphite grades investigated RBMK
B-15
GRP-2
GR-1
SG
30PG
Filler
Calcined coke KNPS
Binder
High temperature coal tar pitch (2 impregnations)
Heat treatment temperature CC) Density (g/cm3) Elastic moduh a (GPa) Electrical resisitivity a t&G, m) Thermal conductivity * (20°C) (W/m K) Linear thermal expansion coefficient a (400°C) (lO-‘j K-l) Crystallite size b (nm) &I L, Porosity fcm3/g) Diameter of the majority of pores (p,rn)
2400-2600 I.72 6.5/5.0 10.3/14.8
2400 1.67 11.5/7.x 9.6,‘11.3
2600 1.83 12.0/8.5 8.0/10.8
2800 1.70 7.5/7.7 15.0/14.0
1800 1.84 10.5/11.4 13.5/12.8
1800 1.87 9.5/11.5 14.0/12.2
103/89
89/77
160/100
86/90
80 /90
60,‘70
3.2/4.9
4.3/4.7
4.8/6.1
6.8/6.7
5.7/5.4
5.4/4.8
100 20 0.11
80 20 0.1
>lOO 27 0.09
65.5 12.5
0.12
> 100 17 0.066
>I00 27 0.065
18-25
20
10
10
0.51
1.38
Composition on the 30PG grade base of uncalcined graphite coke Semi temperature coal tar pitch
a Values listed before the slash are for specimens cut in the direction parallel to the molding axis; those after the slash for the normal direction. b Linear dimensions of the area of coherent X-ray scattering in the (a) and (a> axes of the graphite monocrystal.
F.F. Zherdeo et al. / Corrosionkineticsofreactor graphites
Fig. 1. ~erimental facility diagram. 1 - the~ogravimeter balance, 2 - chromatograph, 3 - compressed gas reagent bottle, 4 - compressed helium bottle, 5 - cryogenic high pressure trap of impurities in helium, 6 - moistener, 7 capillary tube, 8 - gas sampler switch, 9 - hygrometer, 10 “CAh4AC” crate, 11 - PC, 12 - specimen, 13 - gas mixer.
diagram of the experimental installation is presented in fig. 1. Gravimetric data acquisition, processing and storage facility were made up of an IBM-compatible computer and hardware was made in the “CAMAC” standard. The system provided several channels for data a~uisition: the channels for the current value of the studied specimen weight; the gas temperature in the thermogravimeter muffle operating zone; the inlet and the outlet carrier gas humidities. Besides, the system provided means to control the process of chromatography, to read out a chromatograph signal and to analyze chromatograms, the final result being the values for the admixed gas concentration in helium at the inlet and outlet of the reaction muffle.
33.5
used as contact ring material in these reactors. A contact ring serves to provide a thermal contact between a moderator block of the core and a pressure tube. GR-1 graphite is a promising new material. It is planned to be used as the bottom reflector material for the VG-400 reactor. SG and 30PG graphites are variants of the VG-400 reactor spherical fuel element matrix material. The following experimental method of studying the kinetics of graphite corrosion was employed. The specimen studied was suspended from a beam in the thermogravimeter muffle by a platinum wire. After loading the specimen the muffle and gas pipes were repeatedly evacuated and filled with helium. Then the gas lines and the muffle were purged with purified helium for about 15 h. The temperature in the operating muffle was then increased to the maximum value planned for the experiment, and a specific concentration of reagent was set at the inlet of the muffle. The maximum temperature of corrosion annealing was attained when the growth of mass loss rate was saturated with further increase of temperature. This was caused by complete depletion of the reagent in the carrier gas in the course of its passage along the sample. To ensure valid results of the kinetic constant calculations from the corrosion annealing results, the data obtained in the temperature range of the above mentioned transient regime are needed. After setting a constant rate of decrease in the specimen weight the temperature was lowered by 20 K and the cycle of data acquisition was repeated. The stepwise decrease of the specimen temperature was performed until the reaction stopped and the specimen began to gain weight due to sorption of water (in the case of water vapour) or until the reaction rate slowed down to a level of 5 x lo-’ mg/s. The statistical error in determining the reaction rate did not exceed 1% in most cases. Chromatography of the gas at the reaction muffle inlet and outlet was performed at every stationary temperature.
2.3. Data processing 2.2. Experimental procedure Table 1 presents some of the physical and mechanical characteristics of the graphite grades used in the experiment. The B-15 and RBMK are similar materials used in the cores of RBMK and RBMK-type reactors as reflector and moderator blocks. GRP2 graphite is
The gravimetric muffle containing a specimen is a cylindrical axial-asymmetrical system. In a mathematical description of the diffusion-~netic problem it is convenient to present this system as a finite cylinder with nonuniformly distributed properties (diffusion coefficient of a reactive impurity, a diffusant sink power).
336
F. F. Zherdec et al. / Corrosion kinetics of reactor graphites
The system is described by the boundary lem:
ac
a
auc
+zDz-r-kc, Osziz,, C(r,
value prob-
OsrsR,,
z, 0) =O,
C( r, 0, t) = Cc,,
ac = 0, ar r=0, R, ac 0, az z=zg= D(r,z)
U(r, z) =
(1) OsrsR,,
Z,SZIZ,,
r>Rsr
osz
UOS, u
01
k(r, z) k, = i 0,
z,
=zo-z2=
z*>zrz(J,
r>R,,
z1 IL
OlZ
Z2
OsrsR,,
z,~z~z,,
r>Rs,
0 IZ
IZ,,
z,>z1zo,
z2 - z1 4 ’
where the meaning of the symbols is as follows: c: the reactive impurity concentration; the initial impurity concentration in the co: plane z = 0, (corresponds to the reagent inlet concentration in the experiment); the diffusion coefficient of the impurity in D12: helium at a given temperature; the graphite open porosity; P: linear velocity of carrier gas upstream and U,: downstream from the sample; linear velocity of carrier gas near the samIJos: ple; the inside radius of the reaction muffle; Ro R,: the specimen radius; the total length of the active area of the zo: muffle considered in the calculation; the length of the active area of the muffle z,: upstream of the sample considered in the calculation; (z. -z,): the length of the active area of the muffle downstream of the sample considered in the calculation; (z2 - zi): the specimen length: k: the diffusant sink power.
The calculation of the impurity diffusion coefficients in helium was carried out using the Wilkie-Lee modification of the Chapman-Enskog equation [13]. The problem as formulated in (1) had a number of physical simplification: a linear dependence of the diffusion coefficients in gas and graphite was introduced; there was no allowance for the geometric and time dependencies of the diffusion coefficient and sink power, caused by the change in the structure of pores and their surfaces with burnup. However, they are insignificant here, because the total specimen burnup during annealing periods did not exceed 1% as a rule. A homogeneous distribution of the carrier gas linear velocity along the system radius outside of the specimen was assumed. The search for the optimum values of the kinetic constants of the corrosion reactions was performed by means of optimization of the mass loss rate temperature dependence, obtained in a separate corrosion annealing, in the framework of different hypothesis on the process chemistry. The simplest case (CO, interaction with unirradiated graphite) assumed a first order reaction. In the case of graphite-to-oxygen interaction in the high temperature range, it was necessary to allow for the secondary reaction of carbon interaction with a primary reaction product, CO,. The search for the optimum values of the kinetic constants of the reaction was performed using the method of “ravines”, a variant of the gradient method which is employed when the target function has multiple extremes.
3. Results As the experimental results indicate, the character of the graphite corrosion kinetics depends greatly on the irradiation fluence. The following specific features can be noted as basic: - At high irradiation fluences the mass loss rate that naturally decreased with reduction of temperature started to grow again from about 800 to 840°C in the case of graphite-to-water vapour interaction and from about 860 to 940°C in the case of graphite-to-carbon dioxide interaction. The minimum and the maximum of the kinetic curves are then moved into the direction of higher temperatures with the increase of the reagent concentration. In this case, the reaction rate values are attained close to or exceeding the level corresponding to an active surface reaction on the same specimen at high temperature; (see fig. 2-5). In the case of the highest water vapour concentration a complete depletion of the reagent in the flow of the carrier gas seems
F. F. Zherdeu et al. / Cmosion kineticsof reactor graphites
337
10
IO i
%lO
E
-5’
-;
"E
4 ?
F ;
$2 10 10
-5
-71
D o -
v
I0 Y.5
400 600
8.5 10
/T,
v.p.m. v.p.m.
9.5
l/K
10.5
104/T,
Fig. 2. Temperature dependence of the oxidation reaction rate of the irradiated B-15 grade graphite by water vapour. Irradiation fluence: 1.3 X 10” cms2. Irradiation temperature: 650°C.
11.5
1: 5
l/K
Fig. 4. Temperature dependence of the oxidation reaction rate of the irradiated GRF2 grade graphite by water vapour. Irradiation fluence: 5.9 x 102’ cm-‘. Irradiation temperature: 720°C.
-3
10 -
.4_
10 ”
s
“E 910
F
--5_
P
‘O _;k----9.5 8.5
10.5
11.5
104/'T,l/K
Fig. 3. Temperature dependence of the oxidation reaction rate of the irradiated B-15 grade graphite by water vapour. Irradiation fluence: 9.4 x 102’ cm-‘. Irradiation temperature: 650°C.
10
--6_
IO
--76.C
I
8.0
12.0
10.0 1Q4/T,
1L 0
f/K
Fig. 5. Temperature dependence of the oxidation reaction rate of the irradiated RBMK grade graphite by carbon dioxide. Irradiation fluence: 2.0X 1O22cmW2. Irradiation temperature: 500%
338
F.F. Zherdev et al. / Corrosion kinetics of reactor graphites
10.5 104/T,
l/K
Fig. 6. Temperature dependence of the oxidation reaction rate of the GR-1 grade unirradiated and irradiated graphite samples by water vapour (the low temperature region).
11.5
104/T, l/K Fig. 7. Temperature
dependence of the oxidation reaction rate of the B-15 grade unirradiated and irradiated graphite samples by water vapour (500 vpm).
to occur in the region of low temperature inversion. At reduced concentrations of the reagent the effect is not so marked. This testifies to an order of reaction higher than one. - In the case of high water vapour concentrations
in helium when the temperature of the specimen irradiated to a fluence F > 10” cm-’ drops from 540 to 52O”C, the reaction ceases completely. During this stage (reduction of T by 20 K) the rate of mass loss, which is initially comparable to that at 125O”C, is replaced by a mass increase of the specimen due to sorption of water (see fig. 2); - In the case of graphite-to-water vapour and graphite-to-CO, interactions over the range of minimum fluences of the graphite irradiation the corrosion reaction proceeds more slowly than for the un-irradiated graphite (fig. 6-8, 10, 11). In the case of graphiteto-oxygen interactions this effect is weak or absent; when the sample was subjected to an insignificant corrosion annealing producing a burnup of * l%, its corrosion reaction after irradiation is less intense that that of a sample irradiated without prior annealing (fig. 12). The reason for the inverse behaviour of the reaction rate seems to be its inhibition by the products carbon monoxide or hydrogen. The possibility of this phe-
I
8.0
4 10
10.0 /T.
12.0
l/K
Fig. 8. Temperature dependence of the oxidation reaction rate of the RBMK grade unirradiated and irradiated graphite samples by carbon dioxide (2200 vpm). Irradiation temperature: 500°C.
339
F, F. Zherdev et al. / Corrosion kinetics of reactor graphites
s 10 10
10
o 2.3 10m l/cm*, T=400”C D unirradiated sample
8.0
10.0
14.0
12.0 104/T,
16.C
l/K
Fig. 9. Temperature dependence of the oxidation reaction rate of the RBMK grade unirradiated and irradiated graphite samples by oxygen (6200 vpm). Irradiation temperature: 500°C.
104/T,
l/K Fig. 11. Temperature dependence of the oxidation reaction rate of the 30PG grade unit-radiated and irradiated graphite samples by water vapour.
0 2.3 IO= l/cm’, T=400 ‘C \ b unirradiated sample
IO’/T, 10
,‘T,
l/K
Fig. 10. Temperature dependence of the oxidation reaction rate of the SG grade unit-radiated and irradiated graphite samples by water vapour.
l/K
Fig. 12. Temperature dependence of the oxidation reaction rate of the irradiated B-15 grade graphite samples by water vapour (different experimental procedures before successive cycles of irradiation). Irradiation temperature: 650°C.
340
F.F. Zherdev et al. / Corrosion kinetics of reactor graphites
nomenon is confined by the Langmuir-Hinshelwood law of acting surface. Corroborations can be results of the gas chromatography at the outlet of the reaction muffle in the case when the reaction of graphite-towater vapour interaction is investigated. They indicate that in the early annealing stage at T= 640°C the outlet gas does not contain carbon monoxide in a detectable concentration; at the same time at T= 84o”C, where the reaction rate passes a minimum and reaches the former level, there is a considerable carbon monoxide impurity in the gas. From this one can assume that the reaction is slowed down in the low temperature region between 750 and 800°C because of the sorption of its products (most likely carbon monoxide) in graphite. A sharp drop of the rate of the reaction is aggravated at T < 550-600°C by water sorption on reactionbeneficial sections of the graphite structure, which is evidenced by the specimen weight growth with further decrease of temperature. In the case of the interaction of graphite with CO, no such manifest effect is observed. At temperatures above 1000°C the reaction rate growth is retarded due to the reagent depletion in the carrier gas. We believe that the grounds are rather improbable to consider catalytic effects of platinum on the corrosion reaction of irradiated graphite. The platinum support has a contact area with the graphite specimen less than 0.5 mm*, compared with the specimen surface area of 400 mm’. If the increase of the mass loss rate of irradiated graphite was indeed caused by the catalytic effect of platinum in the contact area, the specimen would have dropped free from the support within first few hours of corrosion annealing. In the end, there would have been traces of intense corrosion. None of these effects were observed in experiment.
4. Discussion Some investigations of the changes of the linear dimensions of graphite specimens after irradiation at temperatures above 500°C showed that the rate of the radiation-induced dimensional changes grew proportionally to the average crystallite dimensions [14]. Transmission electron microscopy (TEM) examination of pyrographites and reactor graphites demonstrated in all cases a qualitative correlation between the radiation defect buildup (dimensions, density) and the magnitude of radiation-induced dimensional changes. The investigation of graphite specimens of different graphite
grades showed that the dimension and the density of the produced radiation defects varied with the change of the mean crystallite size from one graphite grade to another and depended on the crystallite size in a given specimen. This effect is especially apparent for specimens with small crystallite sizes, which are typical of reactor graphites and high irradiation temperatures in the order of 1000°C. Thus, graphite irradiations at about 500°C will cause internal stresses due to different rates of radiation-induced dimensional changes of ctystallites of different dimensions. Platonov et al. [15] cite spontaneous failure under irradiation of specimens not only of reactor graphites, but also of pyrographites. TEM examination of the RBMK and B-15 graphite grades showed that at a fluence slightly higher than the fluence corresponding to the maximum volumetric shrinkage of the graphite , the process of pore closure is complemented by cracking of filler-to-binder grain boundaries. This cracking occurs in a stage-wise manner. In the first stage, cracking of the filler crystallites (that have large dimensions) occurs along the base planes, spreading from the filler-to-binder boundary. This cracking is caused by the fact that the filler crystallites come into contact with those of the binder at the boundary; the latter have a greater growth rate in the (c) direction due to their smaller dimensions. Later in the process, splitting of the filler crystallites along the base planes is due to their dimensions in the (c) direction which are less than those of the binder crystallites along the base planes. At this stage the areas immediately adjacent to the filler-to-binder boundaries are made up of multiple pairs of filler and binder crystallites some lo-15 nm thick, separated by cracks. The crack surfaces correspond to the base planes of the hexagonal grid of the crystallites. Small intergranular stresses, appearing in graphite under irradiation are sufficient to split such pairs, due to their small cross-section. A massive splitting of pairs of filler and binder crystallites is observed with further increase of the neutron fluence on the specimen. This results in the formation of quasi-macroscopic cracks along the fillerto-binder boundaries; the process leaves exposed the crystallite surfaces which correspond to the {lOO]planes of the hexagonal graphite lattice. The reactivity of the monocrystal on these faces is known to be at least 20 times higher than that on the faces which correspond to the basal planes of the hexagonal graphite lattice
WA. Our investigations indicate that the irradiation of graphite to low fluences leads to a decreased rate of
F. F. Zherdev et al. / Corrosion kinetics of reactor graphites
10
10 i
“E
r
P 10
10
)
8.0
9.0 d/T,
10.0
11.0
1;
l/K
Fig. 13. Temperature dependence of the oxidation reaction rate of the unirradiated GRP2 grade and irradiated graphite samples by water vapour.
corrosion throughout the temperature range under study (fig. 8). At fluences close to those causing the maximum volumetric shrinkage of the material (in the case of B-15 and RBMK graphites at an irradiation temperature of 500°C this fluence is (1.0-1.5) x 10” cmd2) as well as under subsequent irradiation, the rates of corrosion reactions throughout the temperature range under study are significantly higher than in the case of unirradiated graphite. In the region of the maximum of the low temperature reaction rate inversion this difference amounts to three orders of magnitude. With the increase of the irradiation temperature these effects start to appear at lower fluences. Fig. 13 presents the temperature dependence of the reaction rates of oxidation of unirradiated and irradiated GRP2 graphite by water vapour. At an irradiation temperature of 400°C a fluence of 2.3 X 1021 cmp2 corresponds to the initial phase of volumetric shrinkage of the material and the reaction rate increase is relatively insignificant. A fluence of 5.9 x 1021 cmm2 at an irradiation temperature of 720°C corresponds to nearly the maximum of the volumetric shrinkage and to an increase of the reaction rate in the low temperature region of more
341
than two orders of magnitude. This suggests the reason for an increase of the corrosion rate with an increase of the degree of initial structure damage in B-15 graphite under irradiation. Corrosion annealing of graphite performed in the kinetic region of reaction results in an increase of micropore dimensions. This results in an increase of the fluence corresponding to the maximum volumetric shrinkage due to a retarded micropore closure. Thus, a graphite subjected to preliminary corrosion annealing should, in effect, demonstrate a higher corrosion stability after irradiation than a graphite not subjected to preliminary treatment (fig. 12). The above considerations are valid only for the region of low fluences. GR-1 graphite irradiated above 800°C promptly enters a stage of volumetric swelling. The inversed behaviour of the reaction rate-temperature dependence is observed in all of the irradiated specimens (fig. 6). Unlike in various other grades of graphite investigated this is observed in unirradiated material as well. The presence of the inversion in the unirradiated condition could possibly be attributed to specific features of the GR-1 graphite due to use of uncalcined coke as a binding material in the process of manufacture. Such technology causes a relatively higher abundance of binder-to-filler boundaries compared to the B-15 graphite. The effect of irradiation on the corrosion rate of the material should then become apparent at lower fluences. The plot in fig. 6 presents the temperature dependence of the corrosion reaction constant for the low temperature, kinetic region of the reaction. The results show that the reaction intensity passes through a minimum with increasing fluence.
5. Conclusions Investigations of the corrosion kinetics of initial and irradiated graphite of various grades revealed an effect of sharp intensification of oxidation reactions as the fluence approaches values corresponding to the maximum shrinkage of the graphite for the given temperature. In this case an inverse growth of the reaction rate with decrease of the irradiation temperature is observed if there is an interaction of graphite with water vapor or carbon dioxide. At the maximum inversion the reaction rate reaches values characteristic of temperatures higher by a few hundred degrees. It was discovered that an insignificant corrosion annealing prior to irradiation results in a less intensive effect of reaction rate growth with irradiation. The irradiation of graphite to low fluences corresponding to the stage of volumet-
342
F.F. Zherdeo et al. / Corrosion kinetics of reactor graphites
ric shrinkage results in a suppression of the corrosion reaction intensity. Results obtained from electron microscopy examinations permitted to explain the effect by the changes of the graphite microstructure under irradiation which causes closure of microporosity and reduction of crystallite surface area exposed to oxidizer. At further increase of the fluence, there is a growth of the relative
density of crystallite surfaces exposed to the oxidizer and correspondingly to the (100) planes of the monocrystal graphite lattice, where the reaction is much more intensive.
References Dl K. Kugeler, P. Schreiner and Ch. Epping, Kerntechnik 53 (1988) 155. El B.J. Merrill and M.H. O’Brien, 13th IEEE Symp. on Fusion Engineering, Knoxville, 1989, eds. M.S. Lubbell, M.B. Nestor and S.F. Vaughan, Oak Ridge National Laboratory, vol. 2, p. 785. [31 H. Iami, S. Nomura, T. Kurosawa, K. Fujii and Y. Sasaki, JAERI-M-9153 (1980). [41 H. Imai, K. Fujii, S. Nomura, T. Kurosawa and Y. Sasaki, JAERI-M-8848 (1980). [51 H. Imai, K. Fujii, S. Nomura, T. Kurosawa and Y. Sasaki, JAERI-M-9166 (1980).
161 T. Hino, M. Hashiba, K. Akimoto and T. Yamashina, J. Nucl. Sci. Technol. (Tokyo) 28 (1991) 20. [71 K.-J. Loenil3en, Jiil-2159 (1986). Bl W. Katscher, H.-K. Hinssen, R. Moormann, W. Delle and E. Wallura, High Temp.-High Press. 13 (1981) 275. [91 H.K. Hinssen, W. Katscher and K.-J. LoeniOen, Jiil-spez578 (1990). DOI ES. Golovina, High-temperature Burning and Gasification of Carbon (Energoatomizdat, Moscow, 1983). [ill N.I. Tihonov, A.A. Khrulev, V.A. Pavlov, V.I. Vasiljev, A.N. Fedoseenkov and S.H. Bashkirtsev, Sov. Voprosy atomnoj nauki i tochniki, Serija jadernaja tekhnika i tekhnologija, issue 2 (1990) p. 81. WI 0. Yu. Gerasimov, V.A. Kuzmichev, F.F. Zherdev, A.F. Prokhorov, V.I. Romanov, A.F. Chabak and P.A. Platono?, Sov. Voprosy atomnoj nauki i techniki, Serija jadernaja tekhnika i tekhnologija, issue 2 (1988) p. 23. [I31 R.C. Reid, J.M. Prausnitz and T.K. Sherwood, The properties of gases and Liquids (Himija, Leningrad, 1982) p. 471. 1141 P.A. Platonov, B.A. Gurovich, Ja.1. Shtrombakh and V.I. Karpukhin, Proc. Int. Conf. on Radiation Materials Science, AIushta, USSR, 22-25 May 1990 (KhFT’I, Kharkov, 1990) vol. 3, p. 132. [151 P.A. Platonov, Ya.1. Shtrombakh, V.I. Karpukhin, Yu.S. Virgiljev, O.K. Chugunov and E.I. Trofimchuk, Nuclear Hydrogen Energy and Technology (Energoatomizdat, Moscow, 1984) vol. 6, p. 77. [161 G.R. Hennig, Proc. 5th Conf. on Carbon, Pennsylvania, 1961 (Pergamon, New York, London, 1962) vol. 1, p. 143.
The effect of neutron irradiation corrosion kinetics
on the reactor graphite
F.F. Zherdev, I.E. Komissarov, B.A. Gurovich and V.M. Markushev Russiiskii Nauchnii Tsentr, Kurchatovskii Institut, Ploshad Kurchatoua, 123182 Moscow, Russia Received 7 January 1992; accepted 15 April 1992
Investigations were carried out on the corrosion kinetics of initial and irradiated reflector and matrix graphites of several grades in helium flow containing various types of oxidants, The parameters of the sample irradiation were: the neutron fluence F = 1.5~ 1O21-2.O~ 10” cmm2 (E > 0.18 MeV) and the irradiation temperature T = 350-1100°C. The investigations were made in the temperature interval T < 1300°C over the concentration range of water vapor admixed to helium of 100-600 vpm, oxygen 350-6000 vpm and carbon dioxide 400-6000 vpm at an absolute total pressure of 1.32 bar. A qualitative change of the corrosion kinetics was found with the increase of the irradiation fluence of the samples. The increase in corrosion rate was explained by fracture of the graphite intercrystallite boundaries under irradiation and by increase in relative density of monocrystal surfaces opening into pores, normal to the basal plane.
1. Introduction Since the early days of the nuclear industry research labs of the countries building nuclear reactors have been engaged in experimental research concerning a structural material widely used in reactor designs - the reactor graphite. One of the directions of this research is the graphite corrosion stability to chemically active impurities in the environment. Corrosion of graphite affects its mechanical properties and causes adverse transfer of reaction products to the heat exchangers, where this may cause carbonization of structural materials. This necessitates the imposition of certain criteria for the selection of reactor graphite grades and coolant purification system capacities. Possible accident scenarios involving the injection of large quantities of oxidizing agents into the hot core should be specially considered [l]. Recently the designs of thermonuclear reactors have also met with similar problems [2]. The most extensive research of graphite corrosion had been performed up to the mid-80s in support of HTGR programs. Due to the well known events this research is now restricted to a number of research laboratories in Japan, Russia and Germany, while thermonuclear reactor related research is centered in the USA. Elsevier Science Publishers B.V.
The most widely used method for out-of-pile experimental investigation of the corrosion kinetics is continuous or stage-by-stage measurement of the weight change of graphite specimens placed in a corrosive environment with a controlled chemical composition [3,4,6]. Corrosion annealing at fixed temperature yields the dependence of physical properties of graphite on the burnup, and the experiment usually ends in the investigation of the porosity distribution over the cross section of the specimen [4,7-91. The raw experimental data are processed to obtain the temperature dependence of the kinetic constant of the reaction or the reaction gas-exchange constant [Xl]. The corrosion processes in the reactor evolve in the field of intense gamma radiation. To account for that there are attempts to simulate this in laboratory scale experiments by irradiation of the specimens and the corrosive media with gamma sources f4,5]. There are also studies of the effect of the presence and concentration of various graphite impurities on its corrosion kinetics (see references in ref. [61X In-pile investigations of graphite corrosion usually employ full-scale spherical fuel elements irradiated in ampoules or loop facilities [l&12]. In the loop experiments the coolant composition is controlled in the loop inlet and outlet.
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F.F. Zherdeo et al. / Corrosion kinetics of reactor graphites
Relatively little experimental data have been accumulated on the dependence of the corrosion reaction kinetics on the neutron fluence and the graphite irradiation temperature 13,121. The results available, regretfully only for a narrow range of irradiation parameters, indicate an intensification of the process with the neutron fluence incident on the graphite. The aim of the present work was a more detailed study of this phenomenon.
2. Experimental 2.1. Equipment A method of thermal gravimetry was used to study the corrosion reaction kinetics. It involved the measurement of the rate of weight decrease of a sample, heated up to a certain stationary temperature and blown round by a helium flow with a certain constant reagent concentration. A the~ogra~meter with vertical muffle and microbalance with a sensitivity of 4 X
10S7 g were used in the experiments. A feed control unit for muffle heaters made it possible to maintain a constant (with an accuracy of 0.5 K) gas temperature in the muffle operating zone. The temperature was monitored by a thermocouple the thermojunction of which was located inside the muffle at a distance of l-2 mm from the sample. Helium was used as carrier gas. It was additionally purified at the tank outlet by passing it under a pressure of 150 bar through a trap with a molecular sieve, which was placed in a Dewar flask with liquid nitrogen. The carrier gas was moistened by passing a portion of the flow in a bypass above a distilled water surface. The reagent gases were introduced into the helium flow through a thin capillary tube made of steel. The input of the capillary tube was held at constant pressure. The helium humidity was controlled by hygrometers located in the discharging branch of the inlet gas line and in the outlet line of the thermogravimeter operating muffle. Part of the gas through the inlet and outlet lines was directed towards the chromatograph, which had a light gas sensitivity of about 50 vpm. The
Table 1 Selected physical and mechanical properties of the graphite grades investigated RBMK
B-15
GRP-2
GR-1
SG
30PG
Filler
Calcined coke KNPS
Binder
High temperature coal tar pitch (2 impregnations)
Heat treatment temperature CC) Density (g/cm3) Elastic moduh a (GPa) Electrical resisitivity a t&G, m) Thermal conductivity * (20°C) (W/m K) Linear thermal expansion coefficient a (400°C) (lO-‘j K-l) Crystallite size b (nm) &I L, Porosity fcm3/g) Diameter of the majority of pores (p,rn)
2400-2600 I.72 6.5/5.0 10.3/14.8
2400 1.67 11.5/7.x 9.6,‘11.3
2600 1.83 12.0/8.5 8.0/10.8
2800 1.70 7.5/7.7 15.0/14.0
1800 1.84 10.5/11.4 13.5/12.8
1800 1.87 9.5/11.5 14.0/12.2
103/89
89/77
160/100
86/90
80 /90
60,‘70
3.2/4.9
4.3/4.7
4.8/6.1
6.8/6.7
5.7/5.4
5.4/4.8
100 20 0.11
80 20 0.1
>lOO 27 0.09
65.5 12.5
0.12
> 100 17 0.066
>I00 27 0.065
18-25
20
10
10
0.51
1.38
Composition on the 30PG grade base of uncalcined graphite coke Semi temperature coal tar pitch
a Values listed before the slash are for specimens cut in the direction parallel to the molding axis; those after the slash for the normal direction. b Linear dimensions of the area of coherent X-ray scattering in the (a) and (a> axes of the graphite monocrystal.
F.F. Zherdeo et al. / Corrosionkineticsofreactor graphites
Fig. 1. ~erimental facility diagram. 1 - the~ogravimeter balance, 2 - chromatograph, 3 - compressed gas reagent bottle, 4 - compressed helium bottle, 5 - cryogenic high pressure trap of impurities in helium, 6 - moistener, 7 capillary tube, 8 - gas sampler switch, 9 - hygrometer, 10 “CAh4AC” crate, 11 - PC, 12 - specimen, 13 - gas mixer.
diagram of the experimental installation is presented in fig. 1. Gravimetric data acquisition, processing and storage facility were made up of an IBM-compatible computer and hardware was made in the “CAMAC” standard. The system provided several channels for data a~uisition: the channels for the current value of the studied specimen weight; the gas temperature in the thermogravimeter muffle operating zone; the inlet and the outlet carrier gas humidities. Besides, the system provided means to control the process of chromatography, to read out a chromatograph signal and to analyze chromatograms, the final result being the values for the admixed gas concentration in helium at the inlet and outlet of the reaction muffle.
33.5
used as contact ring material in these reactors. A contact ring serves to provide a thermal contact between a moderator block of the core and a pressure tube. GR-1 graphite is a promising new material. It is planned to be used as the bottom reflector material for the VG-400 reactor. SG and 30PG graphites are variants of the VG-400 reactor spherical fuel element matrix material. The following experimental method of studying the kinetics of graphite corrosion was employed. The specimen studied was suspended from a beam in the thermogravimeter muffle by a platinum wire. After loading the specimen the muffle and gas pipes were repeatedly evacuated and filled with helium. Then the gas lines and the muffle were purged with purified helium for about 15 h. The temperature in the operating muffle was then increased to the maximum value planned for the experiment, and a specific concentration of reagent was set at the inlet of the muffle. The maximum temperature of corrosion annealing was attained when the growth of mass loss rate was saturated with further increase of temperature. This was caused by complete depletion of the reagent in the carrier gas in the course of its passage along the sample. To ensure valid results of the kinetic constant calculations from the corrosion annealing results, the data obtained in the temperature range of the above mentioned transient regime are needed. After setting a constant rate of decrease in the specimen weight the temperature was lowered by 20 K and the cycle of data acquisition was repeated. The stepwise decrease of the specimen temperature was performed until the reaction stopped and the specimen began to gain weight due to sorption of water (in the case of water vapour) or until the reaction rate slowed down to a level of 5 x lo-’ mg/s. The statistical error in determining the reaction rate did not exceed 1% in most cases. Chromatography of the gas at the reaction muffle inlet and outlet was performed at every stationary temperature.
2.3. Data processing 2.2. Experimental procedure Table 1 presents some of the physical and mechanical characteristics of the graphite grades used in the experiment. The B-15 and RBMK are similar materials used in the cores of RBMK and RBMK-type reactors as reflector and moderator blocks. GRP2 graphite is
The gravimetric muffle containing a specimen is a cylindrical axial-asymmetrical system. In a mathematical description of the diffusion-~netic problem it is convenient to present this system as a finite cylinder with nonuniformly distributed properties (diffusion coefficient of a reactive impurity, a diffusant sink power).
336
F. F. Zherdec et al. / Corrosion kinetics of reactor graphites
The system is described by the boundary lem:
ac
a
auc
+zDz-r-kc, Osziz,, C(r,
value prob-
OsrsR,,
z, 0) =O,
C( r, 0, t) = Cc,,
ac = 0, ar r=0, R, ac 0, az z=zg= D(r,z)
U(r, z) =
(1) OsrsR,,
Z,SZIZ,,
r>Rsr
osz
UOS, u
01
k(r, z) k, = i 0,
z,
=zo-z2=
z*>zrz(J,
r>R,,
z1 IL
OlZ
Z2
OsrsR,,
z,~z~z,,
r>Rs,
0 IZ
IZ,,
z,>z1zo,
z2 - z1 4 ’
where the meaning of the symbols is as follows: c: the reactive impurity concentration; the initial impurity concentration in the co: plane z = 0, (corresponds to the reagent inlet concentration in the experiment); the diffusion coefficient of the impurity in D12: helium at a given temperature; the graphite open porosity; P: linear velocity of carrier gas upstream and U,: downstream from the sample; linear velocity of carrier gas near the samIJos: ple; the inside radius of the reaction muffle; Ro R,: the specimen radius; the total length of the active area of the zo: muffle considered in the calculation; the length of the active area of the muffle z,: upstream of the sample considered in the calculation; (z. -z,): the length of the active area of the muffle downstream of the sample considered in the calculation; (z2 - zi): the specimen length: k: the diffusant sink power.
The calculation of the impurity diffusion coefficients in helium was carried out using the Wilkie-Lee modification of the Chapman-Enskog equation [13]. The problem as formulated in (1) had a number of physical simplification: a linear dependence of the diffusion coefficients in gas and graphite was introduced; there was no allowance for the geometric and time dependencies of the diffusion coefficient and sink power, caused by the change in the structure of pores and their surfaces with burnup. However, they are insignificant here, because the total specimen burnup during annealing periods did not exceed 1% as a rule. A homogeneous distribution of the carrier gas linear velocity along the system radius outside of the specimen was assumed. The search for the optimum values of the kinetic constants of the corrosion reactions was performed by means of optimization of the mass loss rate temperature dependence, obtained in a separate corrosion annealing, in the framework of different hypothesis on the process chemistry. The simplest case (CO, interaction with unirradiated graphite) assumed a first order reaction. In the case of graphite-to-oxygen interaction in the high temperature range, it was necessary to allow for the secondary reaction of carbon interaction with a primary reaction product, CO,. The search for the optimum values of the kinetic constants of the reaction was performed using the method of “ravines”, a variant of the gradient method which is employed when the target function has multiple extremes.
3. Results As the experimental results indicate, the character of the graphite corrosion kinetics depends greatly on the irradiation fluence. The following specific features can be noted as basic: - At high irradiation fluences the mass loss rate that naturally decreased with reduction of temperature started to grow again from about 800 to 840°C in the case of graphite-to-water vapour interaction and from about 860 to 940°C in the case of graphite-to-carbon dioxide interaction. The minimum and the maximum of the kinetic curves are then moved into the direction of higher temperatures with the increase of the reagent concentration. In this case, the reaction rate values are attained close to or exceeding the level corresponding to an active surface reaction on the same specimen at high temperature; (see fig. 2-5). In the case of the highest water vapour concentration a complete depletion of the reagent in the flow of the carrier gas seems
F. F. Zherdeu et al. / Cmosion kineticsof reactor graphites
337
10
IO i
%lO
E
-5’
-;
"E
4 ?
F ;
$2 10 10
-5
-71
D o -
v
I0 Y.5
400 600
8.5 10
/T,
v.p.m. v.p.m.
9.5
l/K
10.5
104/T,
Fig. 2. Temperature dependence of the oxidation reaction rate of the irradiated B-15 grade graphite by water vapour. Irradiation fluence: 1.3 X 10” cms2. Irradiation temperature: 650°C.
11.5
1: 5
l/K
Fig. 4. Temperature dependence of the oxidation reaction rate of the irradiated GRF2 grade graphite by water vapour. Irradiation fluence: 5.9 x 102’ cm-‘. Irradiation temperature: 720°C.
-3
10 -
.4_
10 ”
s
“E 910
F
--5_
P
‘O _;k----9.5 8.5
10.5
11.5
104/'T,l/K
Fig. 3. Temperature dependence of the oxidation reaction rate of the irradiated B-15 grade graphite by water vapour. Irradiation fluence: 9.4 x 102’ cm-‘. Irradiation temperature: 650°C.
10
--6_
IO
--76.C
I
8.0
12.0
10.0 1Q4/T,
1L 0
f/K
Fig. 5. Temperature dependence of the oxidation reaction rate of the irradiated RBMK grade graphite by carbon dioxide. Irradiation fluence: 2.0X 1O22cmW2. Irradiation temperature: 500%
338
F.F. Zherdev et al. / Corrosion kinetics of reactor graphites
10.5 104/T,
l/K
Fig. 6. Temperature dependence of the oxidation reaction rate of the GR-1 grade unirradiated and irradiated graphite samples by water vapour (the low temperature region).
11.5
104/T, l/K Fig. 7. Temperature
dependence of the oxidation reaction rate of the B-15 grade unirradiated and irradiated graphite samples by water vapour (500 vpm).
to occur in the region of low temperature inversion. At reduced concentrations of the reagent the effect is not so marked. This testifies to an order of reaction higher than one. - In the case of high water vapour concentrations
in helium when the temperature of the specimen irradiated to a fluence F > 10” cm-’ drops from 540 to 52O”C, the reaction ceases completely. During this stage (reduction of T by 20 K) the rate of mass loss, which is initially comparable to that at 125O”C, is replaced by a mass increase of the specimen due to sorption of water (see fig. 2); - In the case of graphite-to-water vapour and graphite-to-CO, interactions over the range of minimum fluences of the graphite irradiation the corrosion reaction proceeds more slowly than for the un-irradiated graphite (fig. 6-8, 10, 11). In the case of graphiteto-oxygen interactions this effect is weak or absent; when the sample was subjected to an insignificant corrosion annealing producing a burnup of * l%, its corrosion reaction after irradiation is less intense that that of a sample irradiated without prior annealing (fig. 12). The reason for the inverse behaviour of the reaction rate seems to be its inhibition by the products carbon monoxide or hydrogen. The possibility of this phe-
I
8.0
4 10
10.0 /T.
12.0
l/K
Fig. 8. Temperature dependence of the oxidation reaction rate of the RBMK grade unirradiated and irradiated graphite samples by carbon dioxide (2200 vpm). Irradiation temperature: 500°C.
339
F, F. Zherdev et al. / Corrosion kinetics of reactor graphites
s 10 10
10
o 2.3 10m l/cm*, T=400”C D unirradiated sample
8.0
10.0
14.0
12.0 104/T,
16.C
l/K
Fig. 9. Temperature dependence of the oxidation reaction rate of the RBMK grade unirradiated and irradiated graphite samples by oxygen (6200 vpm). Irradiation temperature: 500°C.
104/T,
l/K Fig. 11. Temperature dependence of the oxidation reaction rate of the 30PG grade unit-radiated and irradiated graphite samples by water vapour.
0 2.3 IO= l/cm’, T=400 ‘C \ b unirradiated sample
IO’/T, 10
,‘T,
l/K
Fig. 10. Temperature dependence of the oxidation reaction rate of the SG grade unit-radiated and irradiated graphite samples by water vapour.
l/K
Fig. 12. Temperature dependence of the oxidation reaction rate of the irradiated B-15 grade graphite samples by water vapour (different experimental procedures before successive cycles of irradiation). Irradiation temperature: 650°C.
340
F.F. Zherdev et al. / Corrosion kinetics of reactor graphites
nomenon is confined by the Langmuir-Hinshelwood law of acting surface. Corroborations can be results of the gas chromatography at the outlet of the reaction muffle in the case when the reaction of graphite-towater vapour interaction is investigated. They indicate that in the early annealing stage at T= 640°C the outlet gas does not contain carbon monoxide in a detectable concentration; at the same time at T= 84o”C, where the reaction rate passes a minimum and reaches the former level, there is a considerable carbon monoxide impurity in the gas. From this one can assume that the reaction is slowed down in the low temperature region between 750 and 800°C because of the sorption of its products (most likely carbon monoxide) in graphite. A sharp drop of the rate of the reaction is aggravated at T < 550-600°C by water sorption on reactionbeneficial sections of the graphite structure, which is evidenced by the specimen weight growth with further decrease of temperature. In the case of the interaction of graphite with CO, no such manifest effect is observed. At temperatures above 1000°C the reaction rate growth is retarded due to the reagent depletion in the carrier gas. We believe that the grounds are rather improbable to consider catalytic effects of platinum on the corrosion reaction of irradiated graphite. The platinum support has a contact area with the graphite specimen less than 0.5 mm*, compared with the specimen surface area of 400 mm’. If the increase of the mass loss rate of irradiated graphite was indeed caused by the catalytic effect of platinum in the contact area, the specimen would have dropped free from the support within first few hours of corrosion annealing. In the end, there would have been traces of intense corrosion. None of these effects were observed in experiment.
4. Discussion Some investigations of the changes of the linear dimensions of graphite specimens after irradiation at temperatures above 500°C showed that the rate of the radiation-induced dimensional changes grew proportionally to the average crystallite dimensions [14]. Transmission electron microscopy (TEM) examination of pyrographites and reactor graphites demonstrated in all cases a qualitative correlation between the radiation defect buildup (dimensions, density) and the magnitude of radiation-induced dimensional changes. The investigation of graphite specimens of different graphite
grades showed that the dimension and the density of the produced radiation defects varied with the change of the mean crystallite size from one graphite grade to another and depended on the crystallite size in a given specimen. This effect is especially apparent for specimens with small crystallite sizes, which are typical of reactor graphites and high irradiation temperatures in the order of 1000°C. Thus, graphite irradiations at about 500°C will cause internal stresses due to different rates of radiation-induced dimensional changes of ctystallites of different dimensions. Platonov et al. [15] cite spontaneous failure under irradiation of specimens not only of reactor graphites, but also of pyrographites. TEM examination of the RBMK and B-15 graphite grades showed that at a fluence slightly higher than the fluence corresponding to the maximum volumetric shrinkage of the graphite , the process of pore closure is complemented by cracking of filler-to-binder grain boundaries. This cracking occurs in a stage-wise manner. In the first stage, cracking of the filler crystallites (that have large dimensions) occurs along the base planes, spreading from the filler-to-binder boundary. This cracking is caused by the fact that the filler crystallites come into contact with those of the binder at the boundary; the latter have a greater growth rate in the (c) direction due to their smaller dimensions. Later in the process, splitting of the filler crystallites along the base planes is due to their dimensions in the (c) direction which are less than those of the binder crystallites along the base planes. At this stage the areas immediately adjacent to the filler-to-binder boundaries are made up of multiple pairs of filler and binder crystallites some lo-15 nm thick, separated by cracks. The crack surfaces correspond to the base planes of the hexagonal grid of the crystallites. Small intergranular stresses, appearing in graphite under irradiation are sufficient to split such pairs, due to their small cross-section. A massive splitting of pairs of filler and binder crystallites is observed with further increase of the neutron fluence on the specimen. This results in the formation of quasi-macroscopic cracks along the fillerto-binder boundaries; the process leaves exposed the crystallite surfaces which correspond to the {lOO]planes of the hexagonal graphite lattice. The reactivity of the monocrystal on these faces is known to be at least 20 times higher than that on the faces which correspond to the basal planes of the hexagonal graphite lattice
WA. Our investigations indicate that the irradiation of graphite to low fluences leads to a decreased rate of
F. F. Zherdev et al. / Corrosion kinetics of reactor graphites
10
10 i
“E
r
P 10
10
)
8.0
9.0 d/T,
10.0
11.0
1;
l/K
Fig. 13. Temperature dependence of the oxidation reaction rate of the unirradiated GRP2 grade and irradiated graphite samples by water vapour.
corrosion throughout the temperature range under study (fig. 8). At fluences close to those causing the maximum volumetric shrinkage of the material (in the case of B-15 and RBMK graphites at an irradiation temperature of 500°C this fluence is (1.0-1.5) x 10” cmd2) as well as under subsequent irradiation, the rates of corrosion reactions throughout the temperature range under study are significantly higher than in the case of unirradiated graphite. In the region of the maximum of the low temperature reaction rate inversion this difference amounts to three orders of magnitude. With the increase of the irradiation temperature these effects start to appear at lower fluences. Fig. 13 presents the temperature dependence of the reaction rates of oxidation of unirradiated and irradiated GRP2 graphite by water vapour. At an irradiation temperature of 400°C a fluence of 2.3 X 1021 cmp2 corresponds to the initial phase of volumetric shrinkage of the material and the reaction rate increase is relatively insignificant. A fluence of 5.9 x 1021 cmm2 at an irradiation temperature of 720°C corresponds to nearly the maximum of the volumetric shrinkage and to an increase of the reaction rate in the low temperature region of more
341
than two orders of magnitude. This suggests the reason for an increase of the corrosion rate with an increase of the degree of initial structure damage in B-15 graphite under irradiation. Corrosion annealing of graphite performed in the kinetic region of reaction results in an increase of micropore dimensions. This results in an increase of the fluence corresponding to the maximum volumetric shrinkage due to a retarded micropore closure. Thus, a graphite subjected to preliminary corrosion annealing should, in effect, demonstrate a higher corrosion stability after irradiation than a graphite not subjected to preliminary treatment (fig. 12). The above considerations are valid only for the region of low fluences. GR-1 graphite irradiated above 800°C promptly enters a stage of volumetric swelling. The inversed behaviour of the reaction rate-temperature dependence is observed in all of the irradiated specimens (fig. 6). Unlike in various other grades of graphite investigated this is observed in unirradiated material as well. The presence of the inversion in the unirradiated condition could possibly be attributed to specific features of the GR-1 graphite due to use of uncalcined coke as a binding material in the process of manufacture. Such technology causes a relatively higher abundance of binder-to-filler boundaries compared to the B-15 graphite. The effect of irradiation on the corrosion rate of the material should then become apparent at lower fluences. The plot in fig. 6 presents the temperature dependence of the corrosion reaction constant for the low temperature, kinetic region of the reaction. The results show that the reaction intensity passes through a minimum with increasing fluence.
5. Conclusions Investigations of the corrosion kinetics of initial and irradiated graphite of various grades revealed an effect of sharp intensification of oxidation reactions as the fluence approaches values corresponding to the maximum shrinkage of the graphite for the given temperature. In this case an inverse growth of the reaction rate with decrease of the irradiation temperature is observed if there is an interaction of graphite with water vapor or carbon dioxide. At the maximum inversion the reaction rate reaches values characteristic of temperatures higher by a few hundred degrees. It was discovered that an insignificant corrosion annealing prior to irradiation results in a less intensive effect of reaction rate growth with irradiation. The irradiation of graphite to low fluences corresponding to the stage of volumet-
342
F.F. Zherdeo et al. / Corrosion kinetics of reactor graphites
ric shrinkage results in a suppression of the corrosion reaction intensity. Results obtained from electron microscopy examinations permitted to explain the effect by the changes of the graphite microstructure under irradiation which causes closure of microporosity and reduction of crystallite surface area exposed to oxidizer. At further increase of the fluence, there is a growth of the relative
density of crystallite surfaces exposed to the oxidizer and correspondingly to the (100) planes of the monocrystal graphite lattice, where the reaction is much more intensive.
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161 T. Hino, M. Hashiba, K. Akimoto and T. Yamashina, J. Nucl. Sci. Technol. (Tokyo) 28 (1991) 20. [71 K.-J. Loenil3en, Jiil-2159 (1986). Bl W. Katscher, H.-K. Hinssen, R. Moormann, W. Delle and E. Wallura, High Temp.-High Press. 13 (1981) 275. [91 H.K. Hinssen, W. Katscher and K.-J. LoeniOen, Jiil-spez578 (1990). DOI ES. Golovina, High-temperature Burning and Gasification of Carbon (Energoatomizdat, Moscow, 1983). [ill N.I. Tihonov, A.A. Khrulev, V.A. Pavlov, V.I. Vasiljev, A.N. Fedoseenkov and S.H. Bashkirtsev, Sov. Voprosy atomnoj nauki i tochniki, Serija jadernaja tekhnika i tekhnologija, issue 2 (1990) p. 81. WI 0. Yu. Gerasimov, V.A. Kuzmichev, F.F. Zherdev, A.F. Prokhorov, V.I. Romanov, A.F. Chabak and P.A. Platono?, Sov. Voprosy atomnoj nauki i techniki, Serija jadernaja tekhnika i tekhnologija, issue 2 (1988) p. 23. [I31 R.C. Reid, J.M. Prausnitz and T.K. Sherwood, The properties of gases and Liquids (Himija, Leningrad, 1982) p. 471. 1141 P.A. Platonov, B.A. Gurovich, Ja.1. Shtrombakh and V.I. Karpukhin, Proc. Int. Conf. on Radiation Materials Science, AIushta, USSR, 22-25 May 1990 (KhFT’I, Kharkov, 1990) vol. 3, p. 132. [151 P.A. Platonov, Ya.1. Shtrombakh, V.I. Karpukhin, Yu.S. Virgiljev, O.K. Chugunov and E.I. Trofimchuk, Nuclear Hydrogen Energy and Technology (Energoatomizdat, Moscow, 1984) vol. 6, p. 77. [161 G.R. Hennig, Proc. 5th Conf. on Carbon, Pennsylvania, 1961 (Pergamon, New York, London, 1962) vol. 1, p. 143.