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Thermal fatigue equipment to test joints of materials for high heat flux components
Fusion Engineering and Design 49 – 50 (2000) 377 – 382 www.elsevier.com/locate/fusengdes
Thermal fatigue equipment to test joints of materials for high heat flux components E. Visca *, S. Libera, A. Orsini, B. Riccardi, M. Sacchetti Associazione Euratom-ENEA sulla Fusione, C.R. Frascati, Via E. Fermi 45, 00044 Frascati, Rome, Italy
Abstract The activity, carried out in the framework of an ITER divertor task, was aimed at defining a suitable method in order to qualify junctions between armour materials and heat sink of plasma-facing components (PFCs) mock-ups. An equipment able to perform thermal fatigue testing by electrical heating and active water-cooling was constructed and a standard for the sample was defined. In this equipment, during operation cycles, two samples are heated by thermal contact up to a relevant temperature value (350°C) and then the water flow is switched on, thus producing fast cooling with time constants and gradients close to the real operating conditions. The equipment works with a test cycle of about 60 s and is suitable for continuous operation. A complete test consists of about 10 000 cycles. After the assembling, the equipment and the control software were optimized to obtain a good reliability. Preliminary tests on mock-ups with flat CFC tiles joined to copper heat sink were performed. Finite-elements calculations were carried out in order to estimate the value of the thermal stresses arising close to the joint under the transient conditions that are characteristic of this equipment. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermal fatigue; Plasma-facing component; Carbon fiber composite
1. Introduction The activity, performed in the framework of an ITER task, was aimed at defining an alternative method to qualify junctions between armour materials and heat sink of PFC mock-ups. A finite-elements calculation was carried out to evaluate the thermal stresses that can be expected with the proposed sample geometry and temperature cycling. The results of the calculation indi* Corresponding author. Tel.: + 39-06-94005581; fax: +3906-94005314. E-mail address: [email protected] (E. Visca).
cated that a cycling from 50 to 350°C induces thermo-mechanical stress values exceeding the elastic limit of the copper and that a lifetime between 5000 and 10 000 cycles can be estimated for a defected sample. After this confirmation, an equipment able to perform thermal fatigue testing by electrical heating and active water-cooling was designed and constructed and a standard sample was chosen. It must be noted that, the requirement of the equipment is not to impose the design heat flux through the joint in a steady state and then repeat it for n-times — instead it should apply thermomechanical stress by heating and cooling the
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 2 3 9 - 8
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E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
whole sample. In such a way, the actual thermal gradients usually applied by the e-beam facility during high heat flux tests cannot be reproduced but it is only possible to induce thermal stresses that are dependent on the material properties (a and E) mismatching used for the bonding of the heat sink and the armour materials. The finite-elements calculations that were carried out to estimate the value of the thermal stresses arising close to the joint under the transient conditions confirmed the validity of the approach proposed.
2. Technical specification The design maximum temperature was fixed at about 350°C in order to work close to the operational range of temperature between armour material and heat sink of the ITER divertor joint. In the equipment the samples are heated by thermal contact until the setting temperature is reached and then the water-cooling flow is switched on, thus producing fast cooling with time constants and gradients comparable with the design ones. The equipment works with a test cycle of about 60 s and is suitable for continuous and unattended operations; a complete fatigue test of about 10 000 cycles is foreseen. The minimum temperature is achieved by means of a cooling circuit that uses water from the distribution
network. It is chosen, test by test and, usually, it is near to 50°C. The design specifications of the equipment can be summarized as following: simultaneous testing of two samples; possibility to work in safe and unattended mode; automatic control of the electric heating; active water cooling of the samples by running water; sample and heater temperature data recording on personal computer; vacuum or nitrogen atmosphere; maximum temperature of the sample 350°C.
3. The sample geometry The requirements of the specimen are the following: as small as possible in order to keep its cost low; representative joined area; easy coupling with the cooling circuit; possibility to be instrumented in order to monitor the process. The coupling with the cooling circuit was chosen similar to that used at the JUDITH facility in Julich (Germany); it allows testing samples both with and without cooling tube. Fig. 1 shows a typical drawing of the sample proposed having a flat tile joined on a typical heat sink. The maximum sample length in this case is 40 mm. In case of sample cooling tubes (i.e. mono-block mockups) the maximum sample length is 32 mm. Using the proposed coupling, no flanges are needed also for this geometry.
4. Description of the facility
Fig. 1. Drawing of a typical sample.
The equipment (see Fig. 2) consists of a vacuum chamber and an electronic control unit. The vacuum chamber contains two air operated pliers that by alternative movement puts the four electric heaters in contact with the two samples and, after the setting temperature is reached, it moves away waiting for the cooling phase.
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5. The electrical heaters The heaters were manufactured by machining a block of OFHC copper. Five holes were drilled and reamed and the heater plugs were lubricated with a vacuum compatible lubricant before being inserted in the holes. Each heater set contains five heater plugs of 500 W each that can work up to 500°C. The temperature control is independent from the process and it is continuously monitored. Each specimen is heated by two heaters with a total power of 5000 W simultaneously applied to the two sides of the specimens. Fig. 2. Picture of the thermal fatigue equipment.
6. Process control The test cycle is of about 60 s depending on the testing range temperature and sample thermal capacity; a typical complete test will consist of 10 000 cycles. For a setting temperature of 300°C the four heater sets are heated up to 500°C. The installed electrical power is 2500 W for each set. The control and the data recording are performed by an automated PC data acquisition system that allows unattended and continuos operation with the possibility of monitoring the process by any remote network client. The system was tested in different conditions of temperature and atmosphere. It gave a good reliability up to a testing temperature of 300°C under nitrogen atmosphere. Higher testing temperatures improve the reliability of the heaters that are forced to work near to the heater plugs maximum temperature allowed. The cooling circuit is directly connected to the water distribution network. The recycling of the water by a loop could be compulsory in the case of presence of toxic armour material as Be. The water discharge is made at atmospheric pressure. In this way, the cooling circuit is at a low pressure (about 1 bar relative). The circuit flow rate measured is about 0.3 l/s; this flow rate can cool down the sample to the setting temperature in about 10 s.
The process parameters are controlled and recorded by a Personal Computer, in which a Data Acquisition board (DAQ) is installed. The signal conditioning devices (SCXI system by National Instruments) are used to make the input and output signals compatible with the DAQ board and to protect the DAQ board from signals that, reaching directly the board, could damage it. The conditioning is also used to amplify, filter and insulate the signals. The process control unit, together with the software developed with LabView language, is able to manage the following tasks: alternative moving of the mechanical parts by operating an air piston; heating and cooling phase changing control; data acquisition and visualization on the PC display of the specimens (two thermocouples for each sample) and of the heaters temperature (one thermocouples for each heater); counting of the cycles made by the specimens couple, of the total session number of cycles and the total number of cycles performed by the equipment; recording on files of the data readings during each cycle; monitoring of possible malfunctioning and execution of the necessary operations for a safe stop of the equipment;
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380
acting as a server for a network client to allow the remote monitoring of the system from another PC.
7. Preliminary tests In order to perform the preliminary tests some actively cooled samples were manufactured according to the drawings in Fig. 1. These mock-ups were manufactured by brazing a carbon fiber composite (CFC) tile (DUNLOP 678) on an OFHC copper heat sink. The brazing alloy was a copper-based alloy (TiCuSil by WESGO). The copper and the CFC temperatures were both monitored. In such a way, it is possible to determine when a detachment of the tile occurs. The graph in Fig. 3 reports a typical cycle. During the first campaign the cycling was sometime interrupted in order to perform some corrective actions on the equipment. The second test campaign instead was stopped on the cycle no. 7701 when the detachment of the tile in one of the mock-ups was detected. After this result, it was decided to continue the experimentation by testing specimens with imposed defects.
8. Thermo-mechanical analysis A finite element analysis was carried out by means of ABAQUS code aimed at simulating the experimental tests, by reproducing the temperature/time curve, and at evaluating the stress field induced by the thermal gradients in the samples tested. The OFHC copper properties, including the stress-strain curves, were supplied as function of the temperature; the CFC (Dunlop 672) properties assumed are listed in the Table 1. A non linear transient thermal analysis was carried out by means of a 2D model; the sample heating was simulated by imposing a constant temperature on the lateral face of the model; the heat sink conditions were considered by assuming water flowing with 3.8 m/s velocity and 16°C temperature. The temperature/time curve calculated reproduces with enough accuracy the experimental one. The thermal gradient thus estimated (see Fig. 4) were used to calculate the stress distribution. To calculate the thermal stress distribution, an elastic-plastic analysis was carried out by using a generalized plane stress model based on the same mesh used for thermal analysis. In order to have a realistic estimation of the thermal fatigue strain
Fig. 3. Typical cycle temperature recording for two samples Copper/CFC tile.
E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
381
Table 1 Dunlop 672 CFC properties ( parallel to fibres, Þ perpendicular to fibres, NA not available) Thermal conductivity (W/mK)
RT 1000°C
Density
Þ
243 97
45 20
1900 NA
Specific heat (J/kg per K)
716 1920
range, it was also necessary to take into account the residual stress induced by the brazing cycle. The specimens tested in the facility were produced by brazing CFC tiles on to pure copper by means of a silver-brazing alloy. The residual stresses were calculated by means of an adiabatic cooling to the room temperature from a ‘stress free’ temperature, (in the range of 600– 700°C), below which the brazing alloy start recovering its strength. After that, the thermal history calculated was used to find out the sample stress distribution during an entire thermal cycle of the test. The analysis results showed that, after the brazing process and at the end of heating phase, the sample copper part exhibits stress values that exceed the elastic limit. Fig. 5 shows the in plane shear and Mises stress after brazing; they evidence a significant concentration of residual stress in the copper part at the interface with CFC (max Mises 100 MPa; max shear 50 MPa). At the end of the heating phase, caused by the almost uniform heating, a certain stress relaxation took place with a resulting more complex distribution in the copper at the joint interface (see Fig. 6). The strain history reached shakedown conditions from the 5th cycle on; the estimated maximum equivalent strain range amplitude in the copper was 0.35% and it was located at the edge of brazing interface. By assuming the thermal fatigue governed by the OFHC copper and using the fatigue strength curve reported by Seki et al. [1], a lifetime between 5000 and 10 000 cycles can be estimated for the sample tested. Therefore, as a consequence of the cyclic thermal stress that occurs at the CFC-copper interface, the facility described,
Thermal expansion (1/K)
Young modulus (kg/m3)
Þ
0.4×10-6 1.9×10-6
9.6×10−6 30 11.7×10−6 NA
Þ 40 NA
can be used to perform tests to have indication about the thermal fatigue of brazed samples. In particular, the facility can be used to compare heat sink-armour samples manufactured by means of different joining parameters or technologies.
9. Conclusions The results obtained from the preliminary tests on CFC flat tile/copper mock-ups have shown that it is possible to induce thermo-mechanical stresses on the chosen geometry leading to a fatigue mechanism that can bring to the junction failure. The global coefficients of heat transfer calculated based on the data collected are about
Fig. 4. Temperature (°C) distribution after the heating phase
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E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
Fig. 5. Residual Mises (a) and shear (b) stress (MPa) distribution after brazing.
10 000 W/m2°C for the cooling phase and about 2000 W/m2°C for the heating phase. With these coefficients a cycle is concluded within 1 min. Considering that the equipment can work unattended, after seven days it reaches 10 000 cycles that are the design equipment target. We expect to obtain more information by testing samples with imposed defects, with a diverse geometry (i.e. monoblock mock-ups) and with
Fig. 6. Residual Mises (a) and shear (b) stress (MPa) distribution after the heating phase.
diverse armour materials, in order to characterize the equipment and the testing procedure.
References [1] M. Seki, et al., Fatigue strength of tungsten-copper duplex structures for divertor plates, J. Nucl. Mater. 155–157 (1988) 392.
Thermal fatigue equipment to test joints of materials for high heat flux components E. Visca *, S. Libera, A. Orsini, B. Riccardi, M. Sacchetti Associazione Euratom-ENEA sulla Fusione, C.R. Frascati, Via E. Fermi 45, 00044 Frascati, Rome, Italy
Abstract The activity, carried out in the framework of an ITER divertor task, was aimed at defining a suitable method in order to qualify junctions between armour materials and heat sink of plasma-facing components (PFCs) mock-ups. An equipment able to perform thermal fatigue testing by electrical heating and active water-cooling was constructed and a standard for the sample was defined. In this equipment, during operation cycles, two samples are heated by thermal contact up to a relevant temperature value (350°C) and then the water flow is switched on, thus producing fast cooling with time constants and gradients close to the real operating conditions. The equipment works with a test cycle of about 60 s and is suitable for continuous operation. A complete test consists of about 10 000 cycles. After the assembling, the equipment and the control software were optimized to obtain a good reliability. Preliminary tests on mock-ups with flat CFC tiles joined to copper heat sink were performed. Finite-elements calculations were carried out in order to estimate the value of the thermal stresses arising close to the joint under the transient conditions that are characteristic of this equipment. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Thermal fatigue; Plasma-facing component; Carbon fiber composite
1. Introduction The activity, performed in the framework of an ITER task, was aimed at defining an alternative method to qualify junctions between armour materials and heat sink of PFC mock-ups. A finite-elements calculation was carried out to evaluate the thermal stresses that can be expected with the proposed sample geometry and temperature cycling. The results of the calculation indi* Corresponding author. Tel.: + 39-06-94005581; fax: +3906-94005314. E-mail address: [email protected] (E. Visca).
cated that a cycling from 50 to 350°C induces thermo-mechanical stress values exceeding the elastic limit of the copper and that a lifetime between 5000 and 10 000 cycles can be estimated for a defected sample. After this confirmation, an equipment able to perform thermal fatigue testing by electrical heating and active water-cooling was designed and constructed and a standard sample was chosen. It must be noted that, the requirement of the equipment is not to impose the design heat flux through the joint in a steady state and then repeat it for n-times — instead it should apply thermomechanical stress by heating and cooling the
0920-3796/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 0 ) 0 0 2 3 9 - 8
378
E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
whole sample. In such a way, the actual thermal gradients usually applied by the e-beam facility during high heat flux tests cannot be reproduced but it is only possible to induce thermal stresses that are dependent on the material properties (a and E) mismatching used for the bonding of the heat sink and the armour materials. The finite-elements calculations that were carried out to estimate the value of the thermal stresses arising close to the joint under the transient conditions confirmed the validity of the approach proposed.
2. Technical specification The design maximum temperature was fixed at about 350°C in order to work close to the operational range of temperature between armour material and heat sink of the ITER divertor joint. In the equipment the samples are heated by thermal contact until the setting temperature is reached and then the water-cooling flow is switched on, thus producing fast cooling with time constants and gradients comparable with the design ones. The equipment works with a test cycle of about 60 s and is suitable for continuous and unattended operations; a complete fatigue test of about 10 000 cycles is foreseen. The minimum temperature is achieved by means of a cooling circuit that uses water from the distribution
network. It is chosen, test by test and, usually, it is near to 50°C. The design specifications of the equipment can be summarized as following: simultaneous testing of two samples; possibility to work in safe and unattended mode; automatic control of the electric heating; active water cooling of the samples by running water; sample and heater temperature data recording on personal computer; vacuum or nitrogen atmosphere; maximum temperature of the sample 350°C.
3. The sample geometry The requirements of the specimen are the following: as small as possible in order to keep its cost low; representative joined area; easy coupling with the cooling circuit; possibility to be instrumented in order to monitor the process. The coupling with the cooling circuit was chosen similar to that used at the JUDITH facility in Julich (Germany); it allows testing samples both with and without cooling tube. Fig. 1 shows a typical drawing of the sample proposed having a flat tile joined on a typical heat sink. The maximum sample length in this case is 40 mm. In case of sample cooling tubes (i.e. mono-block mockups) the maximum sample length is 32 mm. Using the proposed coupling, no flanges are needed also for this geometry.
4. Description of the facility
Fig. 1. Drawing of a typical sample.
The equipment (see Fig. 2) consists of a vacuum chamber and an electronic control unit. The vacuum chamber contains two air operated pliers that by alternative movement puts the four electric heaters in contact with the two samples and, after the setting temperature is reached, it moves away waiting for the cooling phase.
E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
379
5. The electrical heaters The heaters were manufactured by machining a block of OFHC copper. Five holes were drilled and reamed and the heater plugs were lubricated with a vacuum compatible lubricant before being inserted in the holes. Each heater set contains five heater plugs of 500 W each that can work up to 500°C. The temperature control is independent from the process and it is continuously monitored. Each specimen is heated by two heaters with a total power of 5000 W simultaneously applied to the two sides of the specimens. Fig. 2. Picture of the thermal fatigue equipment.
6. Process control The test cycle is of about 60 s depending on the testing range temperature and sample thermal capacity; a typical complete test will consist of 10 000 cycles. For a setting temperature of 300°C the four heater sets are heated up to 500°C. The installed electrical power is 2500 W for each set. The control and the data recording are performed by an automated PC data acquisition system that allows unattended and continuos operation with the possibility of monitoring the process by any remote network client. The system was tested in different conditions of temperature and atmosphere. It gave a good reliability up to a testing temperature of 300°C under nitrogen atmosphere. Higher testing temperatures improve the reliability of the heaters that are forced to work near to the heater plugs maximum temperature allowed. The cooling circuit is directly connected to the water distribution network. The recycling of the water by a loop could be compulsory in the case of presence of toxic armour material as Be. The water discharge is made at atmospheric pressure. In this way, the cooling circuit is at a low pressure (about 1 bar relative). The circuit flow rate measured is about 0.3 l/s; this flow rate can cool down the sample to the setting temperature in about 10 s.
The process parameters are controlled and recorded by a Personal Computer, in which a Data Acquisition board (DAQ) is installed. The signal conditioning devices (SCXI system by National Instruments) are used to make the input and output signals compatible with the DAQ board and to protect the DAQ board from signals that, reaching directly the board, could damage it. The conditioning is also used to amplify, filter and insulate the signals. The process control unit, together with the software developed with LabView language, is able to manage the following tasks: alternative moving of the mechanical parts by operating an air piston; heating and cooling phase changing control; data acquisition and visualization on the PC display of the specimens (two thermocouples for each sample) and of the heaters temperature (one thermocouples for each heater); counting of the cycles made by the specimens couple, of the total session number of cycles and the total number of cycles performed by the equipment; recording on files of the data readings during each cycle; monitoring of possible malfunctioning and execution of the necessary operations for a safe stop of the equipment;
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380
acting as a server for a network client to allow the remote monitoring of the system from another PC.
7. Preliminary tests In order to perform the preliminary tests some actively cooled samples were manufactured according to the drawings in Fig. 1. These mock-ups were manufactured by brazing a carbon fiber composite (CFC) tile (DUNLOP 678) on an OFHC copper heat sink. The brazing alloy was a copper-based alloy (TiCuSil by WESGO). The copper and the CFC temperatures were both monitored. In such a way, it is possible to determine when a detachment of the tile occurs. The graph in Fig. 3 reports a typical cycle. During the first campaign the cycling was sometime interrupted in order to perform some corrective actions on the equipment. The second test campaign instead was stopped on the cycle no. 7701 when the detachment of the tile in one of the mock-ups was detected. After this result, it was decided to continue the experimentation by testing specimens with imposed defects.
8. Thermo-mechanical analysis A finite element analysis was carried out by means of ABAQUS code aimed at simulating the experimental tests, by reproducing the temperature/time curve, and at evaluating the stress field induced by the thermal gradients in the samples tested. The OFHC copper properties, including the stress-strain curves, were supplied as function of the temperature; the CFC (Dunlop 672) properties assumed are listed in the Table 1. A non linear transient thermal analysis was carried out by means of a 2D model; the sample heating was simulated by imposing a constant temperature on the lateral face of the model; the heat sink conditions were considered by assuming water flowing with 3.8 m/s velocity and 16°C temperature. The temperature/time curve calculated reproduces with enough accuracy the experimental one. The thermal gradient thus estimated (see Fig. 4) were used to calculate the stress distribution. To calculate the thermal stress distribution, an elastic-plastic analysis was carried out by using a generalized plane stress model based on the same mesh used for thermal analysis. In order to have a realistic estimation of the thermal fatigue strain
Fig. 3. Typical cycle temperature recording for two samples Copper/CFC tile.
E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
381
Table 1 Dunlop 672 CFC properties ( parallel to fibres, Þ perpendicular to fibres, NA not available) Thermal conductivity (W/mK)
RT 1000°C
Density
Þ
243 97
45 20
1900 NA
Specific heat (J/kg per K)
716 1920
range, it was also necessary to take into account the residual stress induced by the brazing cycle. The specimens tested in the facility were produced by brazing CFC tiles on to pure copper by means of a silver-brazing alloy. The residual stresses were calculated by means of an adiabatic cooling to the room temperature from a ‘stress free’ temperature, (in the range of 600– 700°C), below which the brazing alloy start recovering its strength. After that, the thermal history calculated was used to find out the sample stress distribution during an entire thermal cycle of the test. The analysis results showed that, after the brazing process and at the end of heating phase, the sample copper part exhibits stress values that exceed the elastic limit. Fig. 5 shows the in plane shear and Mises stress after brazing; they evidence a significant concentration of residual stress in the copper part at the interface with CFC (max Mises 100 MPa; max shear 50 MPa). At the end of the heating phase, caused by the almost uniform heating, a certain stress relaxation took place with a resulting more complex distribution in the copper at the joint interface (see Fig. 6). The strain history reached shakedown conditions from the 5th cycle on; the estimated maximum equivalent strain range amplitude in the copper was 0.35% and it was located at the edge of brazing interface. By assuming the thermal fatigue governed by the OFHC copper and using the fatigue strength curve reported by Seki et al. [1], a lifetime between 5000 and 10 000 cycles can be estimated for the sample tested. Therefore, as a consequence of the cyclic thermal stress that occurs at the CFC-copper interface, the facility described,
Thermal expansion (1/K)
Young modulus (kg/m3)
Þ
0.4×10-6 1.9×10-6
9.6×10−6 30 11.7×10−6 NA
Þ 40 NA
can be used to perform tests to have indication about the thermal fatigue of brazed samples. In particular, the facility can be used to compare heat sink-armour samples manufactured by means of different joining parameters or technologies.
9. Conclusions The results obtained from the preliminary tests on CFC flat tile/copper mock-ups have shown that it is possible to induce thermo-mechanical stresses on the chosen geometry leading to a fatigue mechanism that can bring to the junction failure. The global coefficients of heat transfer calculated based on the data collected are about
Fig. 4. Temperature (°C) distribution after the heating phase
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E. Visca et al. / Fusion Engineering and Design 49–50 (2000) 377–382
Fig. 5. Residual Mises (a) and shear (b) stress (MPa) distribution after brazing.
10 000 W/m2°C for the cooling phase and about 2000 W/m2°C for the heating phase. With these coefficients a cycle is concluded within 1 min. Considering that the equipment can work unattended, after seven days it reaches 10 000 cycles that are the design equipment target. We expect to obtain more information by testing samples with imposed defects, with a diverse geometry (i.e. monoblock mock-ups) and with
Fig. 6. Residual Mises (a) and shear (b) stress (MPa) distribution after the heating phase.
diverse armour materials, in order to characterize the equipment and the testing procedure.
References [1] M. Seki, et al., Fatigue strength of tungsten-copper duplex structures for divertor plates, J. Nucl. Mater. 155–157 (1988) 392.