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J ournal of Solid Mechanics and Materials Engineering Vol. 2, No. 1, 2008 166 Oxidation Damage Evaluation by Non-Destructive Method for Graphite Components in High Temperature Gas-Cooled Reactor * Taiju SHIBATA ** , Tatsuya TADA ** , Junya SUMITA ** and Kazuhiro SAWA ** **High Temperature Fuel & Material Group, Japan Atomic Energy Agency, 4002 Oarai-machi, Ibaraki-ken, 311-1393 Japan E-mail: [email protected] Abstract To develop non-destructive evaluation methods for oxidation damage on graphite components in High Temperature Gas-cooled Reactors (HTGRs), the applicability of ultrasonic wave and micro-indentation methods were investigated. Candidate graphites, IG-110 and IG-430, for core components of Very High Temperature Reactor (VHTR) were used in this study. These graphites were oxidized uniformly by air at 500 0 C. The following results were obtained from this study. (1) Ultrasonic wave velocities with 1 MHz can be expressed empirically by exponential formulas to burn-off, oxidation weight loss. (2) The porous condition of the oxidized graphite could be evaluated with wave propagation analysis with a wave-pore interaction model. It is important to consider the non-uniformity of oxidized porous condition. (3) Micro-indentation method is expected to determine the local oxidation damage. It is necessary to assess the variation of the test data. Key words: Oxidation, Graphite, HTGR, VHTR, Ultrasonic Wave, Micro Indentation 1. Introduction High Temperature Gas-cooled Reactor (HTGR) is attractive nuclear reactor to obtain high temperature helium gas about 950 0 C to the reactor outlet. It is possible to utilize this high temperature helium gas not only for power generation but also for process heat source of hydrogen production system. To obtain the high temperature gas, graphite materials are used for reactor internals because of their superior heat resistibility. The High Temperature Engineering Test Reactor (HTTR) of Japan Atomic Energy Agency (JAEA) is the first HTGR in Japan. It is a test reactor having the maximum thermal power of 30 MW. Figure 1 shows in-core structure of the HTTR which is composed of prismatic graphite blocks piled on core bottom structure (1,2) . Figure 2 shows the block type fuel element of the HTTR (1) . A fuel assembly consists of fuel rods and a hexagonal graphite block, 360 mm across flats and 580 mm in height. The primary coolant helium gas flows through gaps between the holes and fuel rods. The Very High Temperature Reactor (VHTR) is a promising candidate for the Generation IV nuclear energy system (3) . For R&Ds on the VHTR, international collaboration is carried out through the Generation IV international forum (3) . Since the in-core graphite components of the VHTR will be used at severer condition than that at the HTTR, it is important to confirm their structural integrity during the lifetime. The *Received 20 Aug., 2007 (No. 07-0367) [DOI: 10.1299/jmmp.2.166]

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Page 1: Journal of Solid Mechanics and Materials Engineering

Journal of Solid Mechanicsand Materials

Engineering

Vol. 2, No. 1, 2008

166

Oxidation Damage Evaluation by Non-Destructive Method for Graphite

Components in High Temperature Gas-Cooled Reactor*

Taiju SHIBATA**, Tatsuya TADA**, Junya SUMITA** and Kazuhiro SAWA** **High Temperature Fuel & Material Group, Japan Atomic Energy Agency,

4002 Oarai-machi, Ibaraki-ken, 311-1393 Japan E-mail: [email protected]

Abstract To develop non-destructive evaluation methods for oxidation damage on graphite components in High Temperature Gas-cooled Reactors (HTGRs), the applicability of ultrasonic wave and micro-indentation methods were investigated. Candidate graphites, IG-110 and IG-430, for core components of Very High Temperature Reactor (VHTR) were used in this study. These graphites were oxidized uniformly by air at 500 0C. The following results were obtained from this study. (1) Ultrasonic wave velocities with 1 MHz can be expressed empirically by exponential formulas to burn-off, oxidation weight loss. (2) The porous condition of the oxidized graphite could be evaluated with wave propagation analysis with a wave-pore interaction model. It is important to consider the non-uniformity of oxidized porous condition. (3) Micro-indentation method is expected to determine the local oxidation damage. It is necessary to assess the variation of the test data.

Key words: Oxidation, Graphite, HTGR, VHTR, Ultrasonic Wave, Micro Indentation

1. Introduction

High Temperature Gas-cooled Reactor (HTGR) is attractive nuclear reactor to obtain high temperature helium gas about 950 0C to the reactor outlet. It is possible to utilize this high temperature helium gas not only for power generation but also for process heat source of hydrogen production system. To obtain the high temperature gas, graphite materials are used for reactor internals because of their superior heat resistibility.

The High Temperature Engineering Test Reactor (HTTR) of Japan Atomic Energy Agency (JAEA) is the first HTGR in Japan. It is a test reactor having the maximum thermal power of 30 MW. Figure 1 shows in-core structure of the HTTR which is composed of prismatic graphite blocks piled on core bottom structure(1,2). Figure 2 shows the block type fuel element of the HTTR(1). A fuel assembly consists of fuel rods and a hexagonal graphite block, 360 mm across flats and 580 mm in height. The primary coolant helium gas flows through gaps between the holes and fuel rods.

The Very High Temperature Reactor (VHTR) is a promising candidate for the Generation IV nuclear energy system(3). For R&Ds on the VHTR, international collaboration is carried out through the Generation IV international forum (3). Since the in-core graphite components of the VHTR will be used at severer condition than that at the HTTR, it is important to confirm their structural integrity during the lifetime. The *Received 20 Aug., 2007 (No. 07-0367)

[DOI: 10.1299/jmmp.2.166]

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components are damaged by neutron irradiation at high temperatures. The graphite materials show creep behavior above 500 0C under neutron irradiation as well as irradiation-induced dimensional change. These effects cause stress in the components and it is a crucial factor to determine their lifetime. It is considered in the components design.

Fig. 1 In-core structure of the HTTR.

Fig. 2 Block type fuel element of the HTTR.

The graphite components are also gradually damaged through the reactor

operation by quite small amount of impurities in the coolant helium gas, i.e. oxidation damage. The oxidation-induced degradation of the mechanical properties of nuclear graphites was investigated in relation to oxidation weight loss, burn-off(4). The oxidation degrades the graphite components and the damage assessment is one of the key technologies for their lifetime extension(5,6). The damages could be checked periodically at in-service inspection (ISI) by using a TV camera and by surveillance graphite test specimens loaded in the core. However, these methods cannot directly detect the oxidation damage inside the graphite block. Although the oxidation damage in the components would

CR standpipe

RPV

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not be significant in the reactor normal operation due to the high purity of helium coolant, it is necessary to develop the measurement method to extend their lifetime. For this purpose, the non-destructive evaluation method should be developed to detect inner oxidation damage of the graphite components.

The authors are trying to develop the non-destructive evaluation technique for mechanical properties of graphite components by ultrasonic wave and micro-indentation methods(5,6). This paper describes the applicability of these methods to the graphite oxidation damage evaluation.

2. Experimental

2.1 Graphite specimens IG-110 and IG-430 graphites (Toyo Tanso Co.) are used in this study. They are

fine-grained isotropic graphites manufactured by the isostatic pressing method. IG-110 is made from petroleum coke and IG-430 from coal tar pitch coke. IG-110 is used for the in-core graphite components in the HTTR, and IG-430 is an advanced grade, i.e. higher strength than IG-110. The both are candidate graphite grades for the in-core graphite components of the VHTR. Table 1 summarizes typical material properties of IG-110 and IG-430. IG-430 has about 20-40% higher strength than IG-110. Small specimens with 10x15x20 mm were prepared for the micro-indentation test. Block type specimens with 100x200x290 mm were prepared to measure ultrasonic wave velocity.

Table 1 Typical material properties of IG-110 and IG-430 graphites.

It is known that surface oxidation of graphite occurs above about 800 0C and inner oxidation occurs below that temperature(7). For the present oxidation test, the specimens were heated in a furnace with inducing air at 500 0C to give uniform inner oxidation condition (8). The oxidation ratio, burn-off, was changed as a parameter from 0 to 2 % by varying the oxidation time. The burn-off B is determined by specimen weight W0 and W respectively before and after the oxidation as:

B = (W0-W)/W0 . (1) It corresponds to the reduction rate of bulk density. For example, at 2% of burn-off, bulk density of IG-110 sample changes to 1.74 Mg/m3.

The surface condition change of the graphite specimen was observed by a scanning electron microscope (SEM). The small specimens with 10x15x20 mm were used for the observation.

2.2 Ultrasonic wave measurement Figure 3 shows a test apparatus for the ultrasonic wave measurement. A probe was held to obtain constant contact force to the graphite specimen by a spring. Grease is used between the probe and graphite specimen. The velocity of longitudinal wave for the each graphite specimen was measured by a digital ultrasonic testing machine (USI-550, Krautkramer). Probes with 6 and 24 mm in diameter were used for the small and large specimens, respectively. The measurement was carried out by reflection method with frequency of 1MHz.

Material IG-110 IG-430Bulk density (Mg/m3) 1.78 1.82Tensile strength (MPa) 25.3 37.2Compressive strength (MPa) 76.8 90.2Young’s modulus (GPa) 8.3 10.8

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2.3 Micro-indentation test The micro-indentation behavior was measured by a testing machine of

SHIMADZU Co. shown in Fig. 4. The relationship between indentation load and indentation depth was continuously recorded during the test. A diamond spherical shape indenter with 0.5 mm in radius was used. The maximum indentation load was selected as a parameter, 10, 25 and 50N. Fig.3 Test apparatus for ultrasonic wave

characteristics. Probe is held to keep constant contact force to specimen.

3. Results and discussion

3.1 Oxidized conditions The graphite specimens were oxidized at the temperature of 500 0C. Sato et al. (4) reported that they obtained relatively uniform oxidized IG-110 specimens at 600 0C. Since the present study was carried out at much lower temperature than their study, our obtained specimens would be uniformly oxidized.

Fig. 5 Surface condition change of IG-110 by oxidation.

Fig. 6 Surface condition change of IG-430 by oxidation.

Specimen

Probe

spring

Indentation machine

Frame

Frame controller

Indentation machine controller

Fig. 4 Micro-indentation testing machine.

IG-110, B= 0 IG-110, B= 0.02

IG-430, B= 0 IG-430, B= 0.02

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Figures 5 and 6 present surface condition change by the oxidation of 500 0C for IG-110 and IG-430 specimens, respectively. The SEM images were obtained at the same area by checking the scratched mark shown in the upper right corner. For the un-oxidized specimens, 0% of burn-off, small pores can be observed. The surface condition became rough with the oxidation. Severe non-uniform oxidation condition was not observed up to 2% of burn-off.

3.2 Wave velocity change by oxidation Figure 7 shows the obtained data of ultrasonic wave velocity with the oxidation for IG-110 (a) and IG-430 (b). At the un-oxidized condition, the velocity in IG-430 is higher than that in IG-110 reflecting the differences of the raw materials and the bulk density shown in Table 1. The velocity in the each graphite decreased with burn-off. At 2% of burn-off, the velocity decreased to about 90% that of the un-oxidized condition for IG-110, and about 95% for IG-430.

Fig. 7 Measured wave velocity in relation to burn-off; (a) IG-110 and (b) IG-430.

Sato et al. proposed the following empirical formula to express the degradation of material properties by burn-off B.

)exp(/ 0 nBSS −= , (2) where S0 and S are the material properties before and after the oxidation respectively, and n is the degradation exponent. Although this equation was applied to the mechanical properties such as Young's modulus, strength and fracture toughness, we tried to use this formula to evaluate the wave velocity change by burn-off. Although it is possible to evaluate the Young's modulus by using the velocity, we tried to evaluate the velocity data directly to check the variation of the data. The evaluation equations were determined by Eq. (2) based on the present experimental data as follows;

)04.6exp(/ 0 Bvv −= , for IG-110, (3)

)64.3exp(/ 0 Bvv −= , for IG-430. (4) Figure 8 shows the evaluation curves and experimental data. For example, at the 2% of burn-off, evaluation value is obtained to put 0.02 for B in Eqs. (3) and (4). The evaluation curves show good agreement with the obtained data. It suggests that the burn-off condition could be evaluated by the reduction of the ultrasonic wave velocity in the both IG-110 and IG-430 graphites.

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Fig.8 Velocity reduction evaluation by empirical formula; (a) IG-110 and (b) IG-430.

3.3 Velocity analysis by wave propagation model The evaluation of the oxidation condition such as pore size and distribution is

important for the assessment of degradation of the graphite components. Takatsubo and Yamamoto proposed a wave propagation model in porous ceramics considering wave-pore interaction process(9). In this model, spherical pores with unique radius r0 are assumed to be located uniformly in the porous ceramics. The interaction probability of the wave and the pore can be calculated by the porous condition such as pore size and porosity. The wave propagation characteristics through the ceramics are, then, evaluated with cumulating the interaction probability for a large number of pores in the ceramics. The wave velocity in the porous ceramics V0 is evaluated as a normalized value by the velocity in the ideal polycrystals without pore Vi as follows(9):

{ }8/)2/(31/1/ 000 −+= απβφiVV , (5)

pc VV /0 =α , (6)

)2/(4 00 rLp πβ = , (7)

where Vc is the velocity of wave which creeps around the pore and Vp is the velocity of wave which does not interact with the pore. 4Lp is a perimeter of the pore and 0φ is the

porosity of the porous ceramics. The above equations are simple and the velocity in porous ceramics is generally described by using porous condition. Shibata and Ishihara investigated the applicability of this model to IG-110 graphite(10,11). By comparing the Young's modulus data up to 10% of burn-off and the analysis, the trend of the oxidation-induced degradation was generally traced by this model. However, in the normal operating condition, the oxidation damage would not be large due to the high purity of helium gas coolant. It is important to check the applicability of this model for the low burn-off damage evaluation. In this study, we tried to apply the above propagation model for the analysis on oxidized graphite at low burn-off up to 2 %. In our analysis, the velocity in oxidized graphite V is assumed to be expressed by using the Eq. (5). The change of the wave velocity before and after the oxidation is hence expressed as follows:

8/)2/(318/)2/(31/ 000

0 −+−+

=απβφαπβφVV , (8)

where αβ / expresses the value after the oxidation. We assumed the uniform pore growth model by oxidation as shown in Fig. 9. At the

un-oxidized condition with porosity of 0φ , the spherical pores having unique pore radius r0

are distributed uniformly with pitch p. After the uniform oxidation, it is assumed that the

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pores became large at the same position and their shape changed to the non-spherical one. The pore oxidation ratio is assumed as unique for the every pore and the pore size is expressed by the equivalent radius r. αβ / value in Eq. (8) is affected by oxidation-induced pore shape change. Takatsubo and Yamamoto proposed αβ / value in relation to porous condition of ceramics, and it was demonstrated by using several alumina samples(9). We tried to use this relationship to the IG-110 graphite considering its porous condition. Figure 10 shows the assumed αβ / value for oxidized IG-110 graphite as a function of porosity. The region above the porosity of 0.21 shows the oxidized condition of IG-110. αβ / value increases with the porosity by the oxidation. It means the pore shape changed to the non-spherical one.

Fig. 9 Pore growth model for uniform oxidation.

Fig. 10 αβ / value for IG-110 in relation to porosity.

Figure 11 shows analytical results using Eq. (8). The velocity is expressed as the

normalized value by the velocity in the un-oxidized condition. The analysis shows that the velocity decreases with increasing burn-off. The effect of the pore shape change αβ / by burn-off was also evaluated by multiplying factors 3 and 6 on the original αβ / as a parametric analysis. We can see that the larger factor, the severer the velocity change ratio V/V0 is. The relation between the velocity and burn-off obtained experimental data, Eq. (3), is also drawn in the figure. The analysis with the original αβ / gives higher velocity than the experimental line. The difference ia about 10 % corresponding to the velocity about 200 m/s. It is about two times larger than the variation of the obtained velocity data shown in Fig. 7(a). Although it seems small, it would not be negligible. Figure 11 also shows that the analysis with factor of 6 is in good agreement with the experimental line. Since the original

αβ / was demonstrated by sintered alumina samples, it would not be precisely applicable to oxidized IG-110 graphite. It is known that the main oxidation mechanism on the graphite at 500 0C is the gas diffusion process through pores(7). In this case, the oxidation would affect grain boundaries and would give non-uniform porous condition. It is necessary to modify the uniform oxidation assumption for wave propagation analysis in IG-110 at

equivalent radius: r0

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burn-off up to 2%. These issues will be investigated in the next study.

Fig. 11 Analytical results of velocity reduction by uniform oxidation for IG-110.

3.4 Micro-indentation test The graphite block has holes for fuel rods, as shown in Fig. 2, and has temperature distribution in the block. Since the oxidation damage is caused according to the temperature, it is important to develop a method to detect the local oxidation damage. The micro-indentation method is one of the candidates for this purpose. The relation between indentation load and depth at micro-indentation test is affected by the material properties. It would be hence possible to determine the oxidation damage(12). Figure 12 shows the typical test data of the relationship between the indentation load and indentation depth for IG-110 graphite with the maximum indentation load of 25 N. The maximum indentation depth at the maximum load of 25N decreases with increasing the oxidation damage burn-off. The strength of the graphite decreases with the burn-off. Figure 13 shows the surface conditions after the indentation test. At the oxidized graphite surface, 2% of burn-off, the circular imprint is quite clear comparing with that at the un-oxidized one. It suggests the degradation of graphite by oxidation. From these results, the micro-indentation test could be applicable to measure the oxidation damage. It should be noted here that the micro-indentation method may not be non-destructive method, precisely. However, the maximum depth shown in Fig. 12 is about 100 μm, and it would not give large flaw to cause severe damage on the component. It should be considered in the future.

Fig. 12 Typical micro-indentation load-depth characteristics for oxidized IG-110.

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Fig. 13 Surface condition after the micro-indentation test.

Figure 14 shows the maximum indentation depth for the each indentation load as

a function of burn-off. The indentation depth for the each graphite is normalized by the maximum depth for the each un-oxidized specimen. For the both graphites, the variation of the obtained data is not small. The graphite microstructure, such as filler, binder and pore, are affected by the oxidation. The oxidation at the small region which subjected by the indenter would not be uniform. It is a possible reason of the variation of the test data in Fig. 14. It is necessary to assess the variation by statistic method to specify the oxidation damage. It should be investigated in the next study.

Fig. 14 Normalized maximum indentation depth for burn-off condition; (a) IG-110 and (b) IG-430.

4. Conclusions

To develop non-destructive evaluation methods for the oxidation damage on the graphite components in the HTGRs, the applicability of the ultrasonic wave and micro-indentation methods were investigated. Although the oxidation damage would not be significant in reactor normal operation, it is important for the lifetime extension of the graphite components in the VHTR. The uniform oxidation conditions of IG-110 and IG-430 graphites were attained by the air oxidation at 500 0C. The following results were obtained. (1) The ultrasonic wave velocities in the graphite samples decreased with increasing the oxidation. They can be expressed empirically by the exponential formulas to the burn-off. (2) The porous condition of the oxidized graphite could be evaluated with wave propagation analysis with a wave-pore interaction model. The analysis on the oxidized IG-110 shows the slightly less velocity reduction than the experimental data. It implies the uniform oxidation assumption in the analysis would not be precisely applicable. The effects non-uniformity of oxidized pores would be important factors. (3) The micro-indentation behavior on the graphite samples was changed according to the

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oxidation-induced degradation. It is necessary to assess the variation of the test data by statistic method to specify the oxidation damage. (Present study is the result of ‘Research and development for advanced high temperature gas cooled reactor fuels and graphite components’ entrusted to the Japan Atomic Energy Agency by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).)

References

(1) S. Saito, et al., Design of High Temperature Engineering Test Reactor (HTTR), JAERI 1332(1994).

(2) M. Ishihara, J. Sumita, T. Shibata, T. Iyoku, T. Oku, Nuclear Engineering and Design, Vol. 233(2004)pp.251-260.

(3) W. Hoffelner, G. Hayner and P. Billot, International R&D Roadmap for Generation IV-VHTR Materials Research, Proc. ICAP’05, Seoul, Korea(2005), Paper5206.

(4) S. Sato, K. Hirakawa, A. Kurumada, S. Kimura, E. Yasuda, Nuclear Engineering and Design, Vol. 118(1990)pp.227-241

(5) K. Sawa, S. Ueta, T. Shibata, J. Sumita, J. Ohashi, D. Tochio, “Research and development plan for advanced high temperature gas cooled reactor fuels and graphite components (contract research),” JAERI-Tech 2005-024(2005) [in Japanese].

(6) T. Shibata, S. Hanawa, J. Sumita, T. Tada, K. Sawa, T. Iyoku, Non-destructive evaluation on mechanical properties of nuclear graphite with porous structure, Proc. GLOBAL2005, Tsukuba, Japan(2005), Paper360.

(7) for example, H. Imai, J. Nuclear Science and Technology, Vol. 22(1980)pp.769-775[in Japanese].

(8) T. Tada, T. Shibata, J. Sumita, K. Sawa, Development of oxidation damage evaluation method for HTGR graphite component by ultrasonic-wave propagation characteristics (No.1), JAEA-Research 2007-079(2007) [in Japanese].

(9) J. Takatsubo and S. Yamamoto, Transactions of the Japan Society of Mechanical Engineers, Series A, Vol. 60(1994) 2126-2131 and 2132-2137 [in Japanese].

(10) T. Shibata and M. Ishihara, “Grain/pore microstructure-based evaluation method for variation of mechanical property of graphite components in the HTGR,” Trans. SMiRT-16, Washington D.C., USA (2001) #1114.

(11) T. Shibata and M. Ishihara, Nuclear Engineering and Design, Vol. 203(2001)pp.133-141. (12) J. Sumita, T. Shibata, T. Tada, K. Sawa, Development of evaluation method of residual

stress for graphite component of HTGR by Micro-indentation method (No.1), JAEA-Research 2007-073 (2007) [in Japanese].