- Email: [email protected]
Irradiation experiments of divertor pebbles
Fusion Engineering and Design 49 – 50 (2000) 223 – 228 www.elsevier.com/locate/fusengdes
Irradiation experiments of divertor pebbles M. Isobe a,*, K. Matsuhiro a, Y. Ueda a, M. Nishikawa a, J. Ohashi b a
Graduate School of Engineering, Osaka Uni6ersity, 2 -1 Yamada-Oka, Suita-shi, Osaka 565 -0871, Japan b Nuclear Fuel Industries, Ltd., Muramatsu 3135 -41, Tokai-Mura, lbaraki-Ken 319 -1112, Japan
Abstract A multi-layer coated pebble is the essential component of the pebble drop divertor system. For the test of the integrity of the multi-layer structure, two types of pebble were fabricated and irradiated by high flux beams. The one has a SiC kernel, and the other has a carbon kernel. Each type of pebble had a SiC tritium permeation barrier layer and a graphite plasma facing layer. These pebbles were irradiated by 6.7 MW/m2 of electron beam and 6.2×1021 D/m2 of high particle flux deuterium beam. After the irradiation, they were examined the failure of the coating layers and the sputtering yield of the surface layer. On the carbon kernel pebbles, no cracks and failures of the coating layers were detected after the irradiation. The test results of SiC kernel pebbles indicated there were some failures at the interface between a SiC layer and a surface graphite layer due to the fabrication process. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Divertor pebbles; Carbon kernel; SiC kernel
1. Introduction The pebble drop divertor is an advanced divertor system for high power density fusion reactors. In this system, a stream of free falling pebbles with multi-layer coating is used as a divertor wall. The multi-layer pebble consists of a spherical kernel, a surface plasma facing layer and some intermediate layers such as a tritium permeation barrier layer. The moving divertor wall concept using multi-layer pebbles has many favorite features for the fusion reactor, such as high heat load removal, low-Z plasma facing surface, continuous * Corresponding author. Tel.: + 81-6-68775111, ext. 3699; fax: + 81-6-68797867. E-mail address: [email protected] (M. Isobe).
replacement of eroded components, and fuel gas pumping function [1]. The performance and the operational limitations of the pebble drop divertor mainly depend on the irradiation characteristics of the multilayer pebbles. The maximum allowable heat loads of the pebble drop divertor is determined by the rise of surface temperature and the thermal stress induced by incident heat flux [2]. By comparing the fracture strength of kernel materials and the calculated stress on the surface of pebbles, it is shown that a pebble of 1–2 mm in diameter will not be damaged by the surface heat load of 30 Although the thermomechanical MW/m2. strength of the kernel determines the upper limit of the heat load, the failure of multi-layer structure by the irradiation also restricts the opera-
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tional conditions. The failure of the interface between layers reduces the thermal conductivity and causes the abnormal rise of surface temperature. A crack of tritium permeation barrier causes the loss of the feature of low bulk tritium retention. In this work, two types of multi-layer pebbles were fabricated by way of trial. The trial pebbles consisted of a kernel of carbon or SiC, a tritium permeation barrier layer of CVD SiC and a plasma facing layer of pyrolytic graphite. For the test of the integrity of the multi-layer structure in high heat flux and particle flux environment, the irradiation experiments were performed by electron beam and high particle flux deuterium beam. We examined the integrity of the plasma surface layer, the tritium permeation barrier layer and the interfaces between these layers. At the neutral bean irradiation experiment, the surface erosion was also measured. This paper describes the test results of the integrity of the multi-layer structure after irradiations and the erosion yield of plasma facing layer. The problems in the fabrication process of the multi-layer coating are also discussed.
2. Test pieces of multi-layer pebble For the experiments, two types of multi-layer pebbles were fabricated. The one had a SiC kernel, and the other had a carbon kernel. Each type
Fig. 1. The structure of the trial pieces of multi-layer pebble.
of pebble had an intermediate SiC layer as a tritium permeation barrier and a surface graphite layer as a plasma facing layer. About kernel materials, both SiC and carbon have high heat load resistance. The advantage of the SiC kernel is the highest heat load resistance, good mechanical strength and moderate heat capacity. Although the carbon kernel has small heat capacity, it is expected higher heat load resistance by combining with SiC layer, which works as a hard shell. The trial pebbles were fabricated in the high temperature gas cooled atomic reactor (HTGR) fuel plant at Nuclear Fuel Industries, Ltd. using the TRISO-coating technology for the HTGR fuel particles [3]. The TRISO-coating is produced on a kernel by chemical vapor deposition in a fluidized bed coater, which is a graphite tube with a coneshaped gas distributor at the bottom. At deposition temperature, reactants are put into the coater to produce a coating layer on the particles fluidized in the coater. Input gases for depositions of porous pyrolytic carbon, dense pyrolytic carbon and SiC are acetylne C2H2 and argon (Ar), propylene (C3H6) and argon, methyltrichlorosilane (CH3SiCl3) and hydrogen (H2), respectively. After the time to produce the desired thickness of the layer, the reactant gas is replaced by argon. Then, the coater and the coated particles are cooled down, and the coated particles are removed from the coater. The TRISO-coated HTGR fuel particle consists of UO2 kernel, porous pyrolytic carbon, inner dense pyrolytic carbon, SiC and outer dense pyrolytic carbon. These steps are repeated for the multi-layer coating process (Fig. 1). In our trial fabrication of divertor pebbles, the dimensions of the pebble were set to near the HTGR fuel particle to facilitate the selection of the process parameters, such as temperature, gas flow rate and processing time. The design parameters of the trial pebbles are shown in Table 1. Commercial SiC spheres of 1.1 mm in diameter and activated carbon spheres of 0.8 mm in diameter were used for the kernels. Two batches of the fabrication were performed for each type of pebbles. An amount of charged pebbles in the coater was about 1 kg per single batch of coating. On the activated carbon (porous graphite) kernels, addi-
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225
Table 1 Design parameters and the test results of trial pebbles Number
C-1 C-2 SiC-1 SiC-2
Kernel
Buffer
Tritium barrier
Plasma facing
Material
Diameter (mm)
Material
Thickness (mm)
Material
Thickness (mm)
Material
Thickness (mm)
C C SiC SiC
8079 40 8079 40 11249 63 11249 63
CVD-C CVD-C – –
7.799 1.20 7.799 1.20 – –
CVD-SiC CVD-SiC CVD-SiC CVD-SiC
7.19 9 1.25 15.66 9 2.17 57.74 96.52 109.28 95.36
CVD-C CVD-C CVD-C CVD-C
87.65 9 15.4 58.12 9 10.7 37.55 93.45 32.48 95.04
tional high density pyrolytic graphite layer was produced on the kernel in the same way as TRISO-coating. In the SiC coating on the SiC kernel, argon had to be mixed to the reactant gas to maintain the fluidization condition. The fabricated trial pebbles were tested their dimensions. Table 1 also shows the results. It showed we could not control the thickness of the coating layers in this trial fabrication. The process parameters are needed to be optimized for divertor pebbles.
3. Electron beam irradiation The trial pebbles were irradiated by high power electron beam for the test of the integrity of the multi-layer structure against the repetitive heat shocks. The schematic view of the experimental apparatus is shown in Fig. 2. An E-type electron beam gun (Anelva Co. Ltd.) which had the maximum power of 2 kW was used. The specimen was made of a 10 ×10 mm2 graphite plate, which was bonded dozens of multi-layer pebbles with a high temperature graphite adhesive. It was placed on the water cooled copper melting pod. The electron beam scanned over the surface of the specimen by given times and each pebble on the specimen received repetitive irradiation. During the irradiation, the surface temperature of the pebbles was measured by an infrared pyrometer. In this experiment, SiC-1 pebbles and C-2 pebbles were irradiated and tested. The electron beam power was 1.2 kW and 3 ×6 mm2 in spot size, so that the power density was 6.7 MW/m2. The
scanning period was 6 s, and the specimens were scanned for 30 and 100 times. The monitored surface temperature of the pebbles raised up to maximum value and downed to minimum periodically. The maximum and minimum temperature of the SiC kernel pebble and the carbon kernel pebbles were 1800/1200 K and 1800/1100 K, respectively. After the irradiation, the surface graphite layer was examined for the cracks and loss of the layer by the observation with SEM. The SiC layer was examined by X-ray microradiography and by the observation with SEM after burning off the surface graphite layer. To remove the surface layer, pebbles were heated at 900°C for 9 h in air. The burning tests can examine the failure of the SiC
Fig. 2. Experimental apparatus of electron beam irradiation.
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SiC layer of the carbon kernel pebbles was tested by methylene iodide (CH2I2) infusion. By the methylene iodide infusion followed by X-ray microradiography, through-coating failure can be detected since methylene iodide of 3.3× 103 kg/m3 in density infuses into the void volume of the porous pyrolytic carbon of the failed particle to increase the effective density the part, resulting in a decrease in X-ray transmissivity [4]. About the SiC kernel pebbles, the infusion cannot be monitored by X-ray micrography because the density of the kernel is similar to methylene iodide. From the test results of the carbon kernel pebbles, no failure was observed on both surface carbon layers and intermediate SiC layers. They also passed the burning tests and the methylene iodide infusion tests. However, most SiC kernel pebbles were observed the failure of the surface graphite layer. The SEM micrographs of SiC kernel pebble before and after irradiation are shown in Fig. 3. From the appearance of the surface before and after the irradiation, it is shown that the failure of the surface layer had existed before irradiation. About the SiC layer of the SiC kernel pebbles, no failure was observed from the surface observation after the removal of surface layer. Fig. 3. SEM micrographs of the surface of SiC kernel pebbles (a) before and (b) after the electron beam irradiation.
4. Neutral beam irradiation
Fig. 4. Experimental apparatus of deuterium beam irradiation.
layer of the carbon kernel pebbles because the kernel is burnt off together when there are cracks on the SiC layer. Furthermore, the integrity of the
From the experimental datum of the erosion yield of bulk graphite by low energy hydrogen isotope ions, the plasma facing layer of divertor pebbles will withstand hundreds of repetitive irradiation in divertor. However, the failure of multilayer coating structure may enhance the erosion. For example, the failure of interface between coating layers causes the abnormal rise of surface temperature and the evaporation of the surface layer. For the investigation of the effect to the particle impact erosion, the trial pebbles were irradiated by high flux deuterium beam. The schematic view of the experimental apparatus is shown in Fig. 4. A high flux neutral beam source which consists of a bucket type ion source and spherical extraction electrodes generates a pulse of high flux neutral beam at a focal point of the electrodes [5]. The specimen which was multi-
M. Isobe et al. / Fusion Engineering and Design 49–50 (2000) 223–228
227
Fig. 5. Temporal evolution of the surface temperature of the pebbles during high flux deuterium beam irradiations.
layer pebbles fixed on the graphite plate as same as that of electron beam experiments was placed at the focal point. The specimen was placed atilt 45 or 60°, and an oval mask made of molybdenum was placed in front of it to cover the base plate and the bonding area. The surface temperature of irradiated pebbles was monitored by a two color infrared pyrometer. The SiC-2 pebbles and C-1 pebbles were irradiated and tested, because they had a thicker surface layer than the others. In this experiment, the specimens were irradiated by 5 keV, D3 beam with particle flux of 6.2 ×1021 D/m2 s. The beam duration was varied 2 – 4 s to control the maximum temperature during irradiation. About 250–500 times of irradiation are performed, and total fluence was 1.2 – 2.3× 1024 D/m2. Fig. 5 shows the temporal evolution of the surface temperature during irradiation. Although the carbon kernel pebbles show the normal evolution, which is the temperature raised with the progress of the irradiation, the SiC kernel pebbles show abnormal behavior. The surface temperature signal of the SiC pebbles raised and downed quickly at the beginning of irradiation. Because the two color infrared pyrometer estimates the temperature from the ratio of the power of 1.35 and 1.55 mm infrared light, the temperature behavior of the SiC kernel pebbles can be explained as follows. At the beginning of irradiation, there were bright hot spots, which raised the temperature quickly, and the rest surface was still low temperature and emitted weak infrared light. So the pyrometer displayed the temperature of hot spots. Then, most of the surface was heated and
the radiation from there became larger than hot spots, and the pyrometer displayed the temperature of the normal surface. Therefore, the evolution of temperature signal apparently shows that some part of surface layer is defective and the reduction of thermal conduction occurred. The sputtering yield was measured by the weight loss method. Table 2 shows the results. To compare these results with the sputtering yield of bulk graphite, the temperature dependence of the chemical erosion must be considered. As shown in Fig. 5, the surface temperature was changed from 500 to 1200 K during irradiation and the averaged sputtering yield of bulk graphite over this range is about 0.6. So, it can be concluded that the abnormal erosion did not occur in this experiment.
5. Discussion and conclusions The results of irradiation tests are summarized at Table 3. The test results of the carbon kernel Table 2 Sputtering yield of plasma facing layer Fluence (D atoms per m2)
Loss (mg)
Erosion yield (Y)
1.3×1024 2.1×1024
310 420
0.060 0.068
SiC kernel u= 60° 1.2×1024 u =45° 2.3×1024
300 280
0.060 0.041
Carbon kernel u= 60° u =45°
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Table 3 Summary of integrity test after irradiation Carbon kernel
PFL n observation n n n n n n n n n n TPB n observationn n n n n n n n n n Methylene n iodine infusion n a b
SiC kernel
EB
NB
EB
NB
OK n n n n n OK n n n n n OK n n n n n
OK n n n n OK n n n n OK n n n n
a nnnn OK n n n n b nnnn
OK
a
b
Surface layer were flaked off before irradiation. Not applicable to SiC kernel pebbles.
Therefore, the problem may exist in the fabrication process. The essential factor in the TRISO coating process is the mode of fluidization of particles in a coater. As described in chapter 2, the weight of the kernel of a divertor pebble is different from that of a TRISO fuel particle. Especially, the weight of a SiC kernel (about 2.33 mg) is two times larger than that of a UO2 kernel (about 1.20 mg). This is the reason that the optimum fluidization mode could not be achieved at this trial fabrication. The insufficient control of process parameters also caused that the designed thickness of coating layers was not achieved on the carbon kernel pebbles. The optimization of the coating parameters for the fabrication of the divertor pebbles will be needed in the development of the pebble drop divertor. Fig. 6. Optical micrograph of the polished cross-section of interface-defective SiC kernel pebble.
References
pebbles show that the integrity of the multi-layer structure was maintained in the megawatts of electron beam and deuterium beam irradiation. As for the pebbles with SiC kernel, the surface observation before and after the electron beam irradiation and the temporal evolution of surface temperature during deuterium beam irradiation show the failure of coating due to the fabrication process. Fig. 6 shows the cross-section of an unirradiated SiC kernel pebble. The failure of the interface between SiC layer and a surface graphite layer is shown clearly. About the interface between a SiC layer and a surface graphite layer, the SiC kernel pebbles and the graphite kernel pebbles are identical in materials and coating technology.
[1] M. Isobe, Y. Ohtsuka, Y. Ueda, M. Nishikawa, Steady state wall pumping performance of pebble drop divertor, J. Nucl. Mat. 258 – 263 (1998) 745 – 749. [2] M. Isobe, K. Matsuhiro, Y. Ohtsuka, Y. Ueda, M. Nishikawa, Multilayer pebbles for application to new divertor system, Nuclear Fusion 40 (2000) 647 – 651. [3] K. Fukuda, T. Ogawa, K. Hayashi, S. Shiozawa, H. Tsuruta, I. Tanaka, N. Suzuki, S. Yoshimuta, M. Kaneko, Research and development of HTTR coated particle fuel, J. Nucl. Sci. Technol. 28 (1991) 570 – 581. [4] K. Minato, H. Kikuchi, K. Fukuda, N. Suzuki, H. Tomimoto, N. Kitamura, M. Kaneko, Internal flaws in the silicon carbide coating of fuel particles for high-temperature gascooled reactors, Nucl. Technol. 106 (1994) 342 – 349. [5] M. Nishikawa, Y. Ueda, S. Goto, Development of high current density neutral beam injector with a low energy for interaction of plasma facing materials, Fusion Eng. Des. 16 (1991) 351 – 356.
Irradiation experiments of divertor pebbles M. Isobe a,*, K. Matsuhiro a, Y. Ueda a, M. Nishikawa a, J. Ohashi b a
Graduate School of Engineering, Osaka Uni6ersity, 2 -1 Yamada-Oka, Suita-shi, Osaka 565 -0871, Japan b Nuclear Fuel Industries, Ltd., Muramatsu 3135 -41, Tokai-Mura, lbaraki-Ken 319 -1112, Japan
Abstract A multi-layer coated pebble is the essential component of the pebble drop divertor system. For the test of the integrity of the multi-layer structure, two types of pebble were fabricated and irradiated by high flux beams. The one has a SiC kernel, and the other has a carbon kernel. Each type of pebble had a SiC tritium permeation barrier layer and a graphite plasma facing layer. These pebbles were irradiated by 6.7 MW/m2 of electron beam and 6.2×1021 D/m2 of high particle flux deuterium beam. After the irradiation, they were examined the failure of the coating layers and the sputtering yield of the surface layer. On the carbon kernel pebbles, no cracks and failures of the coating layers were detected after the irradiation. The test results of SiC kernel pebbles indicated there were some failures at the interface between a SiC layer and a surface graphite layer due to the fabrication process. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Divertor pebbles; Carbon kernel; SiC kernel
1. Introduction The pebble drop divertor is an advanced divertor system for high power density fusion reactors. In this system, a stream of free falling pebbles with multi-layer coating is used as a divertor wall. The multi-layer pebble consists of a spherical kernel, a surface plasma facing layer and some intermediate layers such as a tritium permeation barrier layer. The moving divertor wall concept using multi-layer pebbles has many favorite features for the fusion reactor, such as high heat load removal, low-Z plasma facing surface, continuous * Corresponding author. Tel.: + 81-6-68775111, ext. 3699; fax: + 81-6-68797867. E-mail address: [email protected] (M. Isobe).
replacement of eroded components, and fuel gas pumping function [1]. The performance and the operational limitations of the pebble drop divertor mainly depend on the irradiation characteristics of the multilayer pebbles. The maximum allowable heat loads of the pebble drop divertor is determined by the rise of surface temperature and the thermal stress induced by incident heat flux [2]. By comparing the fracture strength of kernel materials and the calculated stress on the surface of pebbles, it is shown that a pebble of 1–2 mm in diameter will not be damaged by the surface heat load of 30 Although the thermomechanical MW/m2. strength of the kernel determines the upper limit of the heat load, the failure of multi-layer structure by the irradiation also restricts the opera-
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tional conditions. The failure of the interface between layers reduces the thermal conductivity and causes the abnormal rise of surface temperature. A crack of tritium permeation barrier causes the loss of the feature of low bulk tritium retention. In this work, two types of multi-layer pebbles were fabricated by way of trial. The trial pebbles consisted of a kernel of carbon or SiC, a tritium permeation barrier layer of CVD SiC and a plasma facing layer of pyrolytic graphite. For the test of the integrity of the multi-layer structure in high heat flux and particle flux environment, the irradiation experiments were performed by electron beam and high particle flux deuterium beam. We examined the integrity of the plasma surface layer, the tritium permeation barrier layer and the interfaces between these layers. At the neutral bean irradiation experiment, the surface erosion was also measured. This paper describes the test results of the integrity of the multi-layer structure after irradiations and the erosion yield of plasma facing layer. The problems in the fabrication process of the multi-layer coating are also discussed.
2. Test pieces of multi-layer pebble For the experiments, two types of multi-layer pebbles were fabricated. The one had a SiC kernel, and the other had a carbon kernel. Each type
Fig. 1. The structure of the trial pieces of multi-layer pebble.
of pebble had an intermediate SiC layer as a tritium permeation barrier and a surface graphite layer as a plasma facing layer. About kernel materials, both SiC and carbon have high heat load resistance. The advantage of the SiC kernel is the highest heat load resistance, good mechanical strength and moderate heat capacity. Although the carbon kernel has small heat capacity, it is expected higher heat load resistance by combining with SiC layer, which works as a hard shell. The trial pebbles were fabricated in the high temperature gas cooled atomic reactor (HTGR) fuel plant at Nuclear Fuel Industries, Ltd. using the TRISO-coating technology for the HTGR fuel particles [3]. The TRISO-coating is produced on a kernel by chemical vapor deposition in a fluidized bed coater, which is a graphite tube with a coneshaped gas distributor at the bottom. At deposition temperature, reactants are put into the coater to produce a coating layer on the particles fluidized in the coater. Input gases for depositions of porous pyrolytic carbon, dense pyrolytic carbon and SiC are acetylne C2H2 and argon (Ar), propylene (C3H6) and argon, methyltrichlorosilane (CH3SiCl3) and hydrogen (H2), respectively. After the time to produce the desired thickness of the layer, the reactant gas is replaced by argon. Then, the coater and the coated particles are cooled down, and the coated particles are removed from the coater. The TRISO-coated HTGR fuel particle consists of UO2 kernel, porous pyrolytic carbon, inner dense pyrolytic carbon, SiC and outer dense pyrolytic carbon. These steps are repeated for the multi-layer coating process (Fig. 1). In our trial fabrication of divertor pebbles, the dimensions of the pebble were set to near the HTGR fuel particle to facilitate the selection of the process parameters, such as temperature, gas flow rate and processing time. The design parameters of the trial pebbles are shown in Table 1. Commercial SiC spheres of 1.1 mm in diameter and activated carbon spheres of 0.8 mm in diameter were used for the kernels. Two batches of the fabrication were performed for each type of pebbles. An amount of charged pebbles in the coater was about 1 kg per single batch of coating. On the activated carbon (porous graphite) kernels, addi-
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225
Table 1 Design parameters and the test results of trial pebbles Number
C-1 C-2 SiC-1 SiC-2
Kernel
Buffer
Tritium barrier
Plasma facing
Material
Diameter (mm)
Material
Thickness (mm)
Material
Thickness (mm)
Material
Thickness (mm)
C C SiC SiC
8079 40 8079 40 11249 63 11249 63
CVD-C CVD-C – –
7.799 1.20 7.799 1.20 – –
CVD-SiC CVD-SiC CVD-SiC CVD-SiC
7.19 9 1.25 15.66 9 2.17 57.74 96.52 109.28 95.36
CVD-C CVD-C CVD-C CVD-C
87.65 9 15.4 58.12 9 10.7 37.55 93.45 32.48 95.04
tional high density pyrolytic graphite layer was produced on the kernel in the same way as TRISO-coating. In the SiC coating on the SiC kernel, argon had to be mixed to the reactant gas to maintain the fluidization condition. The fabricated trial pebbles were tested their dimensions. Table 1 also shows the results. It showed we could not control the thickness of the coating layers in this trial fabrication. The process parameters are needed to be optimized for divertor pebbles.
3. Electron beam irradiation The trial pebbles were irradiated by high power electron beam for the test of the integrity of the multi-layer structure against the repetitive heat shocks. The schematic view of the experimental apparatus is shown in Fig. 2. An E-type electron beam gun (Anelva Co. Ltd.) which had the maximum power of 2 kW was used. The specimen was made of a 10 ×10 mm2 graphite plate, which was bonded dozens of multi-layer pebbles with a high temperature graphite adhesive. It was placed on the water cooled copper melting pod. The electron beam scanned over the surface of the specimen by given times and each pebble on the specimen received repetitive irradiation. During the irradiation, the surface temperature of the pebbles was measured by an infrared pyrometer. In this experiment, SiC-1 pebbles and C-2 pebbles were irradiated and tested. The electron beam power was 1.2 kW and 3 ×6 mm2 in spot size, so that the power density was 6.7 MW/m2. The
scanning period was 6 s, and the specimens were scanned for 30 and 100 times. The monitored surface temperature of the pebbles raised up to maximum value and downed to minimum periodically. The maximum and minimum temperature of the SiC kernel pebble and the carbon kernel pebbles were 1800/1200 K and 1800/1100 K, respectively. After the irradiation, the surface graphite layer was examined for the cracks and loss of the layer by the observation with SEM. The SiC layer was examined by X-ray microradiography and by the observation with SEM after burning off the surface graphite layer. To remove the surface layer, pebbles were heated at 900°C for 9 h in air. The burning tests can examine the failure of the SiC
Fig. 2. Experimental apparatus of electron beam irradiation.
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SiC layer of the carbon kernel pebbles was tested by methylene iodide (CH2I2) infusion. By the methylene iodide infusion followed by X-ray microradiography, through-coating failure can be detected since methylene iodide of 3.3× 103 kg/m3 in density infuses into the void volume of the porous pyrolytic carbon of the failed particle to increase the effective density the part, resulting in a decrease in X-ray transmissivity [4]. About the SiC kernel pebbles, the infusion cannot be monitored by X-ray micrography because the density of the kernel is similar to methylene iodide. From the test results of the carbon kernel pebbles, no failure was observed on both surface carbon layers and intermediate SiC layers. They also passed the burning tests and the methylene iodide infusion tests. However, most SiC kernel pebbles were observed the failure of the surface graphite layer. The SEM micrographs of SiC kernel pebble before and after irradiation are shown in Fig. 3. From the appearance of the surface before and after the irradiation, it is shown that the failure of the surface layer had existed before irradiation. About the SiC layer of the SiC kernel pebbles, no failure was observed from the surface observation after the removal of surface layer. Fig. 3. SEM micrographs of the surface of SiC kernel pebbles (a) before and (b) after the electron beam irradiation.
4. Neutral beam irradiation
Fig. 4. Experimental apparatus of deuterium beam irradiation.
layer of the carbon kernel pebbles because the kernel is burnt off together when there are cracks on the SiC layer. Furthermore, the integrity of the
From the experimental datum of the erosion yield of bulk graphite by low energy hydrogen isotope ions, the plasma facing layer of divertor pebbles will withstand hundreds of repetitive irradiation in divertor. However, the failure of multilayer coating structure may enhance the erosion. For example, the failure of interface between coating layers causes the abnormal rise of surface temperature and the evaporation of the surface layer. For the investigation of the effect to the particle impact erosion, the trial pebbles were irradiated by high flux deuterium beam. The schematic view of the experimental apparatus is shown in Fig. 4. A high flux neutral beam source which consists of a bucket type ion source and spherical extraction electrodes generates a pulse of high flux neutral beam at a focal point of the electrodes [5]. The specimen which was multi-
M. Isobe et al. / Fusion Engineering and Design 49–50 (2000) 223–228
227
Fig. 5. Temporal evolution of the surface temperature of the pebbles during high flux deuterium beam irradiations.
layer pebbles fixed on the graphite plate as same as that of electron beam experiments was placed at the focal point. The specimen was placed atilt 45 or 60°, and an oval mask made of molybdenum was placed in front of it to cover the base plate and the bonding area. The surface temperature of irradiated pebbles was monitored by a two color infrared pyrometer. The SiC-2 pebbles and C-1 pebbles were irradiated and tested, because they had a thicker surface layer than the others. In this experiment, the specimens were irradiated by 5 keV, D3 beam with particle flux of 6.2 ×1021 D/m2 s. The beam duration was varied 2 – 4 s to control the maximum temperature during irradiation. About 250–500 times of irradiation are performed, and total fluence was 1.2 – 2.3× 1024 D/m2. Fig. 5 shows the temporal evolution of the surface temperature during irradiation. Although the carbon kernel pebbles show the normal evolution, which is the temperature raised with the progress of the irradiation, the SiC kernel pebbles show abnormal behavior. The surface temperature signal of the SiC pebbles raised and downed quickly at the beginning of irradiation. Because the two color infrared pyrometer estimates the temperature from the ratio of the power of 1.35 and 1.55 mm infrared light, the temperature behavior of the SiC kernel pebbles can be explained as follows. At the beginning of irradiation, there were bright hot spots, which raised the temperature quickly, and the rest surface was still low temperature and emitted weak infrared light. So the pyrometer displayed the temperature of hot spots. Then, most of the surface was heated and
the radiation from there became larger than hot spots, and the pyrometer displayed the temperature of the normal surface. Therefore, the evolution of temperature signal apparently shows that some part of surface layer is defective and the reduction of thermal conduction occurred. The sputtering yield was measured by the weight loss method. Table 2 shows the results. To compare these results with the sputtering yield of bulk graphite, the temperature dependence of the chemical erosion must be considered. As shown in Fig. 5, the surface temperature was changed from 500 to 1200 K during irradiation and the averaged sputtering yield of bulk graphite over this range is about 0.6. So, it can be concluded that the abnormal erosion did not occur in this experiment.
5. Discussion and conclusions The results of irradiation tests are summarized at Table 3. The test results of the carbon kernel Table 2 Sputtering yield of plasma facing layer Fluence (D atoms per m2)
Loss (mg)
Erosion yield (Y)
1.3×1024 2.1×1024
310 420
0.060 0.068
SiC kernel u= 60° 1.2×1024 u =45° 2.3×1024
300 280
0.060 0.041
Carbon kernel u= 60° u =45°
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Table 3 Summary of integrity test after irradiation Carbon kernel
PFL n observation n n n n n n n n n n TPB n observationn n n n n n n n n n Methylene n iodine infusion n a b
SiC kernel
EB
NB
EB
NB
OK n n n n n OK n n n n n OK n n n n n
OK n n n n OK n n n n OK n n n n
a nnnn OK n n n n b nnnn
OK
a
b
Surface layer were flaked off before irradiation. Not applicable to SiC kernel pebbles.
Therefore, the problem may exist in the fabrication process. The essential factor in the TRISO coating process is the mode of fluidization of particles in a coater. As described in chapter 2, the weight of the kernel of a divertor pebble is different from that of a TRISO fuel particle. Especially, the weight of a SiC kernel (about 2.33 mg) is two times larger than that of a UO2 kernel (about 1.20 mg). This is the reason that the optimum fluidization mode could not be achieved at this trial fabrication. The insufficient control of process parameters also caused that the designed thickness of coating layers was not achieved on the carbon kernel pebbles. The optimization of the coating parameters for the fabrication of the divertor pebbles will be needed in the development of the pebble drop divertor. Fig. 6. Optical micrograph of the polished cross-section of interface-defective SiC kernel pebble.
References
pebbles show that the integrity of the multi-layer structure was maintained in the megawatts of electron beam and deuterium beam irradiation. As for the pebbles with SiC kernel, the surface observation before and after the electron beam irradiation and the temporal evolution of surface temperature during deuterium beam irradiation show the failure of coating due to the fabrication process. Fig. 6 shows the cross-section of an unirradiated SiC kernel pebble. The failure of the interface between SiC layer and a surface graphite layer is shown clearly. About the interface between a SiC layer and a surface graphite layer, the SiC kernel pebbles and the graphite kernel pebbles are identical in materials and coating technology.
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