- Email: [email protected]
Dielectric strength, swelling and weight loss of the ITER Toroidal Field Model Coil insulation after low temperature reactor irradiation
Cryogenics 40 (2000) 295±301
www.elsevier.com/locate/cryogenics
Dielectric strength, swelling and weight loss of the ITER Toroidal Field Model Coil insulation after low temperature reactor irradiation K. Humer a,*, H.W. Weber a, R. Hastik b, H. Hauser b, H. Gerstenberg c a
Atominstitut der Osterreichischen Universit aten, Stadionallee 2, A-1020 Wien, Austria b Institut f ur Werkstoe der Elektrotechnik, TU Wien, A-1040 Wien, Austria c TU M unchen, FRM Reaktorstation, D-85748 Garching, Germany Received 26 May 2000; accepted 10 July 2000
Abstract The insulation system for the Toroidal Field Model Coil of ITER is a ®ber reinforced plastic (FRP) laminate, which consists of a combined Kapton/R-glass±®ber reinforcement tape, vacuum-impregnated with an epoxy DGEBA system. Pure disk shaped laminates, FRP/stainless-steel sandwiches, and conductor insulation prototypes were irradiated at 5 K in a ®ssion reactor up to a fast neutron ¯uence of 1022 mÿ2 (E > 0:1 MeV) to investigate the radiation induced degradation of the dielectric strength of the insulation system. After warm-up to room temperature, swelling, weight loss, and the breakdown strength were measured at 77 K. The sandwich swells by 4% at a ¯uence of 5 1021 mÿ2 and by 9% at 1 1022 mÿ2 . The weight loss of the FRP is 2% at 1 1022 mÿ2 . The dielectric strength remained unchanged over the whole dose range. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Insulating material; Irradiation; Electrical properties; Swelling; Weight loss
1. Introduction Radiation eects on insulating materials for the windings of superconducting magnets in future fusion reactors, i.e., glass±®ber reinforced plastics, have been identi®ed as an area of concern for the long-term operation of such magnets [1±3]. In earlier works [4±6], we have simulated the operating conditions of the magnet insulation and addressed the in¯uence of dierent radiation spectra on the mechanical damage process of various types of ®ber reinforced plastics (FRPs). Since many years, potential insulating materials for ITER (International Thermonuclear Experimental Reactor) have also been investigated by the European and US Home Teams with respect to their electrical performance at cryogenic temperatures with and without reactor irradiation prior to testing [7±12]. This contribution reports on the assessment of the electrical properties (i.e. the dielectric strength and the breakdown voltage) of the conductor insulation system used for the Toroidal Field Model Coil (TFMC) of *
Corresponding author. Tel.: +43-1-58801-14156; fax: +43-158801-14199. E-mail address: [email protected] (K. Humer).
ITER, and to measurements of swelling and weight loss following low temperature reactor irradiation.
2. Sample manufacturing procedures The investigated insulation system for the TFMC of ITER is a laminate, which consists of a combined Kapton/R-glass±®ber reinforcement tape, vacuum-impregnated with an epoxy DGEBA system [13]. The test specimens were manufactured by Ansaldo, Genova, Italy. The manufacturing processes were as follows. 2.1. Pure disk shaped laminate The laminates from which the disks were cut, were obtained by wrapping the combined Kapton/glass-fabric tape (half-overlapped) of the same kind as that used for the TFMC, around a steel plate. The two longitudinal sides of the steel plate were well rounded, in order to enable safe wrapping. Before the application of the insulation, Mylar was wrapped around the plate for easy detachment of the laminate after the vacuum pressure impregnation (VPI) process. Two extra steel plates and a set of bolted beams were used to press the insulation
0011-2275/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 1 - 2 2 7 5 ( 0 0 ) 0 0 0 4 4 - 8
296
K. Humer et al. / Cryogenics 40 (2000) 295±301
during the impregnation process. The whole arrangement was then vacuum impregnated. After the VPI process, two insulation plates were obtained by cutting the insulation along the two longitudinal sides of the steel plate. The disk shaped laminates were obtained by laser cutting the 0.5 mm thick insulation plates. 2.2. Disk shaped laminate-stainless steel sandwich The manufacturing procedure of the insulation plates was the same as above. For the laminate-steel sandwich, the following fabrication steps were made. A 0.5 mm thick Te¯on sheet was used to ®x the steel disks on the insulation plate. The Te¯on sheets had 35 holes with a diameter of 8 mm to accommodate the steel disks. As for the pure disk shaped laminates, an outer frame was used to press the steel disks against the insulation, and further on, the insulation against the core steel plate. After the VPI process, the disk shaped insulation plated with the steel disks (sandwiches) were cut from the mould by laser cutting. 2.3. Conductor insulation prototype sample This specimen consists of two concentric stainless steel tubes and an insulation tube located between the steel tubes. In order to avoid excess voltages at the ends of the tubes, the sharp edges of the steel tubes were softened by ¯anging the inner and outer tube radially inwards and outwards, respectively, with suitably shaped tools. Four pieces of Te¯on ®llers were then manufactured and ®tted inside the inner steel tube to keep this space free from resin during the impregnation process. The Kapton-glass insulation tape was then wrapped onto the inner steel tube, and the Te¯on ®llers were used to de®ne the two boundaries of the whole tube. The outer steel tube was made by rolling and pressing a sheet of stainless steel onto the surface of the insulation tube. Finally, the whole arrangement was wrapped with a Tedlar (Te¯on-type) tape on the outside to enable the extraction of the specimen from the cured resin bath. After the impregnation process, the Te¯on ®ller and the Tedlar tape were removed. The ®nal sample geometries and speci®cations, which were chosen in view of the existing space limitations in the low temperature irradiation facility, can be seen in Fig. 1. Photographs of the specimen geometries are presented in Figs. 2 and 3.
Fig. 1. Insulator sample (a), insulator±metal sandwich sample (b), and conductor insulation prototype sample (c).
This reactor is operating at a c-dose rate of 2:8 106 Gyhÿ1 , a fast neutron ¯ux density of 2:9 1017 mÿ2 sÿ1 (E > 0:1 MeV), and a total neutron ¯ux density of 9:5 1017 mÿ2 sÿ1 , respectively. After irradiation, all samples were warmed-up to room temperature and stored for radioactive decay. For the electrical tests a specially designed device was manufactured to allow testing of both unirradiated and irradiated specimens in liquid nitrogen. The disk shaped laminates could be positioned easily and quickly
3. Irradiation and test procedures After initial measurements of the sample dimensions and weights, all irradiations were performed in the FRM I reactor (Garching, Germany) at 5 K to neutron ¯uences of 1021 , 5 ´ 1021 and 1022 mÿ2 (E > 0:1 MeV).
Fig. 2. Photograph of the disk shaped laminate and of the sandwich.
K. Humer et al. / Cryogenics 40 (2000) 295±301
Fig. 3. Photograph of the conductor insulation prototype sample (tube).
between the two electrodes. The device was placed into a polystyrene container ®lled with liquid nitrogen. As can be seen in Fig. 4, the stainless steel electrodes were ®xed onto brass rods, and further on, mounted into an insulating plastic frame. Two connecting leads were ®xed with screws on both sides of the rods leading to the high voltage power supply. The electrical tests of the disk shaped laminate and of the sandwich were carried out at room temperature and
Fig. 4. Measuring device for electrical tests.
297
at 77 K prior to and after low temperature reactor irradiation. The breakdown voltage of the test samples was evaluated according to the ``Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies'' (ASTM D149-97a); ``Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials under Direct-Voltage Stress''(ASTM D3755-97) with Appendix B: ``Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Electrical Insulating Materials under DC Voltage at Cryogenic Temperatures''.
4. Results 4.1. Swelling and weight loss of the irradiated samples After irradiation, the sample dimensions and weights were measured again and then analysed in order to evaluate the radiation induced swelling and weight loss of the specimens. The results are shown in Figs. 5 and 6, where the swelling and the weight loss of the samples are plotted as a function of the fast neutron ¯uence. The swelling of the laminate (Fig. 5) amounts to 1.5% at a fast neutron ¯uence of 1 1021 mÿ2 and to 2% at a fast neutron ¯uence of 5 1021 mÿ2 . A considerable increase of swelling (to 5%) is found at a ¯uence of 1 1022 mÿ2 . The swelling of the sandwich (FRP laminate on stainless steel) amounts to 1% at 1 ´ 1021 mÿ2 and increases continuously to 9% at a ¯uence of 1 1022 mÿ2 , whereas the swelling of the conductor insulation prototype sample (outer diameter) is almost negligible (0±0.5%, cf. Fig. 5). On the other hand, the weight loss of the laminate increases continuously to 2% at a ¯uence of 1 1022 mÿ2 (cf. Fig. 6). Each data point in Figs. 5 and 6 represents an average, calculated from
Fig. 5. Swelling of the laminate, the sandwich, and the conductor insulation prototype sample (tube) after reactor irradiation.
298
K. Humer et al. / Cryogenics 40 (2000) 295±301
Fig. 6. Weight loss of the laminate after reactor irradiation.
Fig. 7. Scaling result on irradiated disk shaped laminates.
4.3. Electrical tests on the irradiated samples ®ve to eight measurements (samples showing signi®cant deviations were rejected in the statistical calculations), except those for the conductor insulation prototype sample, where the average was calculated from only two measurements. In general, the measured eects on swelling and weight loss are in good agreement with literature data on similar laminates [9,10]. 4.2. Scaling experiments on unirradiated disk shaped samples Before the electrical tests on the irradiated samples were carried out, scaling experiments were made on approximately 0.5 mm thick pure laminates of the same material at room temperature (under oil environment) and at 77 K, in order to investigate the in¯uence of the diameter of the disk shaped samples as well as of the size of the stainless-steel electrodes on the measured dielectric strength. The three dierent con®gurations investigated are summarized in Table 1. The scaling results for both temperatures are shown in Fig. 7. As can be seen there, no in¯uence of the disk diameter on the dielectric strength was found, which con®rms the suitability of 12.5 mm diameter disk shaped laminates for a reliable assessment of the electrical properties (i.e. the breakdown voltage and the dielectric strength) under low temperature irradiation conditions. The dielectric strength is about 10% lower at 77 K.
The dielectric strength of the laminate and of the sandwich as a function of the neutron ¯uence is plotted in Fig. 8. No degradation of the dielectric strength was found for both the laminate and the sandwich over the whole dose range. Slightly higher values were found for the sandwich, but the overall eects are within the calculated standard deviations. The conductor insulation prototype sample (tube) was manufactured with a thickness of the insulations system, which is twice that of the disk shaped laminates (cf. Fig. 1). Unfortunately, the voltage needed for electrical breakdown through the laminate exceeded the maximum voltage generated by the high voltage power supply, and thus, breakdown could not be achieved on all of these tubes. Nevertheless, all conductor insulation prototype samples were found to withstand a voltage of at least 60 kV dc without electrical breakdown. 4.4. Fractography of irradiated samples after breakdown After the electrical measurements, fractographic investigations of the surface structure of the irradiated
Table 1 Sample-electrode con®gurations selected for the scaling tests
a b
Con®guration
Disk diameter (mm)
Electrode diameter (mm)
1 2 3
50 25 12.5
12.5a 6.4a 6.4 and 1.6b
Two electrodes opposing. Electrodes opposing.
Fig. 8. Dielectric strength of the laminate and of the sandwich as a function of the neutron ¯uence.
K. Humer et al. / Cryogenics 40 (2000) 295±301
laminates were made in an scanning electron microscope (SEM). Typical photographs are presented in Fig. 9, where the surfaces of the laminates are shown following reactor irradiation at 5 K to a fast neutron ¯uence of 1 1021 ; 5 1021 , and 1 1022 mÿ2 (E > 0:1 MeV), respectively, and following electrical breakdown at 77
299
K. For the lower dose level, small epoxy plates of the outer layer of the laminate are found to be delaminated partially from the compound after breakdown. At higher doses, parts of the outer epoxy layers delaminate completely from the compound and are pulled out. In addition, some inner layers were also partly destroyed.
Fig. 9. From top to bottom: SEM photographs of the surface of the laminate following reactor irradiation to neutron ¯uences of 1021 , 5 ´ 1021 and 1022 mÿ2 (E > 0:1 MeV) and electrical breakdown at 77 K.
300
K. Humer et al. / Cryogenics 40 (2000) 295±301
In general, higher dose levels lead to larger areas of destroyed material. 5. Discussion 5.1. Assessment of sample quality The specimens were subjected to an initial optical inspection. Firstly, all of the disk shaped laminates showed surfaces, which were not completely smooth. We noticed especially the formation of small grooves at the locations of the adhesives between the Kapton foils and the glass±®ber tapes, which led to a considerable decrease of the thickness of the laminate in these regions. Secondly, the Kapton-glass fabric tape was wrapped approximately half-overlapping within the laminate, and therefore, regions with one or two Kapton foils along the thickness were produced. Consequently, we noticed during the electrical tests, that the measured breakdown strength depended on the position of the electrodes on the sample surface. Although we tried to ®nd the optimal position of the electrodes for each sample (as far as this could be done considering the small sample dimensions), both eects lead to rather high standard deviations of the evaluated breakdown strengths (cf. Figs. 7 and 8). After irradiation, the complete set of specimens was subjected to an optical inspection again. The outer layers of some specimens irradiated to a ¯uence of 1022 mÿ2 (E > 0:1 MeV) were found to have delaminated completely from the compound. 5.2. Swelling eects Whenever FRPs are irradiated with fast neutrons, swelling in the through-thickness direction of the laminate is a well known eect, especially when the material is irradiated at cryogenic temperatures. In this case, hydrogen gas will be produced in all types of epoxy resins. During low temperature irradiation, free radicals of larger molecules are formed and displaced from the main chain of the macro-molecules of the plastic material. While keeping the material at low temperatures does not lead to signi®cant changes of the outer material dimensions, swelling takes place when the material is warmed up to higher temperatures (at least 100 K or room temperature). At this temperature, the hydrogen molecules diuse rapidly and tend to escape the solid material. In addition, some free radicals will combine into larger gas molecules and diuse slowly from the compound or will form some pores or enlarge already existing pores in the material. However, during warm-up of the prepreg laminate to room temperature, some of the gas will escape and some will remain within the pores of the composite. On the whole, these eects cause
swelling in the through-thickness direction of the laminate. Furthermore it is well known, that the eects of gas production and swelling are more pronounced in difunctional types of epoxy resins (bisphenol-A types, DGEBA, i.e., the investigated TFMC insulation system; bisphenol±F types) than in the multifunctional epoxies (TGDM) [10]. Consequently, swelling is an important parameter for the engineering and design criteria of superconducting magnet coils for fusion reactors. The speci®ed fast neutron ¯uence for the magnet insulation of ITER is 5 1021 mÿ2 (E > 0:1 MeV). For the TFMC insulation of ITER (cf. Fig. 5), we found at this dose level 2% for the pure laminate, and 4% for the laminate/steel sandwich, which is in good agreement with data from the ``US/ITER Insulation Irradiation Program'' [9,10]. With regard to our measurements on swelling for the sandwiches, we wish to point out that those parts of the sandwich consisting only of the pure laminate were also measured in the through-thickness direction. Within experimental accuracy, the data on swelling agree with those obtained on the pure disk shaped laminates. Therefore, we assume that the dierence in the swelling behavior between the pure disk shaped laminate and the sandwich may be related to the eects discussed above and to the laminate-stainless steel interface which could not be measured separately. A possible contamination with Te¯on during the fabrication process of the samples (cf. Section 2) may also be considered as an explanation of this dierence in swelling between the pure laminate and the sandwich structure. On the other hand, an optical investigation of the samples did not show any obvious eects for both types of samples, and therefore, did not lead to further insights. 6. Summary In this contribution, the electrical properties (i.e. the dielectric strength and the breakdown voltage) of the conductor insulation system used for the TFMC of ITER, have been assessed in conjunction with data on swelling and weight loss following low temperature reactor irradiation at 5 K up to a neutron ¯uence of 1022 mÿ2 (E > 0:1 MeV). The results may be summarized as follows: 1. Swelling. At a ¯uence of 5 1021 mÿ2 (E > 0:1 MeV), i.e., the ITER dose level, the data show half overlapping error bars between the pure laminate and the insulation/steel sandwich, the overall eects are within 4%. In the con®guration of the conductor insulation prototype sample, swelling is almost negligible. 2. Weight loss. Up to a ¯uence of 1022 mÿ2 (E > 0:1 MeV), the weight loss of the laminate increases continuously to 2%.
K. Humer et al. / Cryogenics 40 (2000) 295±301
3. Dielectric strength. At 77 K, this quantity amounts to about 70 kV mmÿ1 for the pure laminate. Slightly higher values were found for the laminate-steel sandwich. No degradation of the dielectric strength was found for both the laminate and the sandwich over the whole dose range. In summary it should be noted, that the typical insulation thickness for the TFMC of ITER [13] is in a range between 1.9 and 10 mm (e.g. TFMC conductor insulation thickness: 2.5 mm, pancake insulation: thickness 1.9 mm, and ground insulation: thickness 1.9 mm + 8 mm 10 mm). Considering the data on the electrical tests presented in this contribution, we conclude that the electrical performance of these materials exceeds by far the design limits of the ITER EDA criteria. Acknowledgements The authors are greatly indebted to Dr. R. Maix and Mr. H. Fillunger (EUHT) for their eorts with the sample procurement. The technical assistance of Mr. E. Tischler is also acknowledged. This work has been carried out within the Association EURATOM-OEAW. References [1] Brown BS. Radiation eects in superconducting fusion-magnet materials. J Nucl Mater 1981;97:1±14.
301
[2] Coltman Jr. RR. Organic insulators and the copper stabilizer for fusion reactor magnets. J Nucl Mater 1982;108&109:559±71. [3] Weber HW, Tschegg EK. Test program for mechanical strength measurements on ®ber reinforced plastics exposed to radiation environments. Adv Cryog Engrg 1990;36:863±9. [4] Humer K, Weber HW, Tschegg EK. Radiation eects on insulators for superconducting fusion magnets. Cryogenics 1995;35:871±82. [5] Humer K, Spieûberger SM, Weber HW, Tschegg EK, Gerstenberg H. Low temperature interlaminar shear strength of reactor irradiated glass ®ber reinforced laminates. Cryogenics 1996;36:611±7. [6] Humer K, Rosenkranz P, Weber HW, Fabian PE, Rice JA. Mechanical properties of the ITER CS Model Coil insulation under static and dynamic load after reactor irradiation. J Nucl Mater [in press]. [7] Schutz JB, Darr JB, Reed RP. Dielectric strength of candidate ITER insulation materials. Adv Cryog Engrg 1994;40:1059±66. [8] Schutz JB, Fabian PE, Hazelton CS, Bauer-McDaniel TS, Reed RP. Eects of cryogenic irradiation on electrical strength of candidate ITER insulation materials. Cryogenics 1995;35:759±62. [9] Reed RP, Fabian PE, Schutz JB. Development of US/ITER CS Model Coil turn insulation. Adv Cryog Engrg 1998;44A:175±82. [10] Reed RP, Fabian PE, Schutz JB. US ITER insulation irradiation program. Final Report, Cryogenic Materials, Inc. and Composite Technology Development, Inc., Boulder, CO, 31 August 1995. [11] Broadbent AJ, Crozier J, Smith KD, Street AJ, Wiatrzyk JM. Electrical breakdown strength results from the EU testing program for potential ITER insulation. Adv Cryog Engrg 1996;42:213±8. [12] Broadbent AJ, Crozier J, Smith KD, Street AJ, Wiatrzyk JM. Initial results from the testing of potential ITER insulation materials. Cryogenics 1995;35:701±3. [13] Maix R, et al. Manufacture of the ITER TF model coil. In: Proceedings of the 20th SOFT, 1998; Marseille. p. 833±36.
www.elsevier.com/locate/cryogenics
Dielectric strength, swelling and weight loss of the ITER Toroidal Field Model Coil insulation after low temperature reactor irradiation K. Humer a,*, H.W. Weber a, R. Hastik b, H. Hauser b, H. Gerstenberg c a
Atominstitut der Osterreichischen Universit aten, Stadionallee 2, A-1020 Wien, Austria b Institut f ur Werkstoe der Elektrotechnik, TU Wien, A-1040 Wien, Austria c TU M unchen, FRM Reaktorstation, D-85748 Garching, Germany Received 26 May 2000; accepted 10 July 2000
Abstract The insulation system for the Toroidal Field Model Coil of ITER is a ®ber reinforced plastic (FRP) laminate, which consists of a combined Kapton/R-glass±®ber reinforcement tape, vacuum-impregnated with an epoxy DGEBA system. Pure disk shaped laminates, FRP/stainless-steel sandwiches, and conductor insulation prototypes were irradiated at 5 K in a ®ssion reactor up to a fast neutron ¯uence of 1022 mÿ2 (E > 0:1 MeV) to investigate the radiation induced degradation of the dielectric strength of the insulation system. After warm-up to room temperature, swelling, weight loss, and the breakdown strength were measured at 77 K. The sandwich swells by 4% at a ¯uence of 5 1021 mÿ2 and by 9% at 1 1022 mÿ2 . The weight loss of the FRP is 2% at 1 1022 mÿ2 . The dielectric strength remained unchanged over the whole dose range. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Insulating material; Irradiation; Electrical properties; Swelling; Weight loss
1. Introduction Radiation eects on insulating materials for the windings of superconducting magnets in future fusion reactors, i.e., glass±®ber reinforced plastics, have been identi®ed as an area of concern for the long-term operation of such magnets [1±3]. In earlier works [4±6], we have simulated the operating conditions of the magnet insulation and addressed the in¯uence of dierent radiation spectra on the mechanical damage process of various types of ®ber reinforced plastics (FRPs). Since many years, potential insulating materials for ITER (International Thermonuclear Experimental Reactor) have also been investigated by the European and US Home Teams with respect to their electrical performance at cryogenic temperatures with and without reactor irradiation prior to testing [7±12]. This contribution reports on the assessment of the electrical properties (i.e. the dielectric strength and the breakdown voltage) of the conductor insulation system used for the Toroidal Field Model Coil (TFMC) of *
Corresponding author. Tel.: +43-1-58801-14156; fax: +43-158801-14199. E-mail address: [email protected] (K. Humer).
ITER, and to measurements of swelling and weight loss following low temperature reactor irradiation.
2. Sample manufacturing procedures The investigated insulation system for the TFMC of ITER is a laminate, which consists of a combined Kapton/R-glass±®ber reinforcement tape, vacuum-impregnated with an epoxy DGEBA system [13]. The test specimens were manufactured by Ansaldo, Genova, Italy. The manufacturing processes were as follows. 2.1. Pure disk shaped laminate The laminates from which the disks were cut, were obtained by wrapping the combined Kapton/glass-fabric tape (half-overlapped) of the same kind as that used for the TFMC, around a steel plate. The two longitudinal sides of the steel plate were well rounded, in order to enable safe wrapping. Before the application of the insulation, Mylar was wrapped around the plate for easy detachment of the laminate after the vacuum pressure impregnation (VPI) process. Two extra steel plates and a set of bolted beams were used to press the insulation
0011-2275/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 1 - 2 2 7 5 ( 0 0 ) 0 0 0 4 4 - 8
296
K. Humer et al. / Cryogenics 40 (2000) 295±301
during the impregnation process. The whole arrangement was then vacuum impregnated. After the VPI process, two insulation plates were obtained by cutting the insulation along the two longitudinal sides of the steel plate. The disk shaped laminates were obtained by laser cutting the 0.5 mm thick insulation plates. 2.2. Disk shaped laminate-stainless steel sandwich The manufacturing procedure of the insulation plates was the same as above. For the laminate-steel sandwich, the following fabrication steps were made. A 0.5 mm thick Te¯on sheet was used to ®x the steel disks on the insulation plate. The Te¯on sheets had 35 holes with a diameter of 8 mm to accommodate the steel disks. As for the pure disk shaped laminates, an outer frame was used to press the steel disks against the insulation, and further on, the insulation against the core steel plate. After the VPI process, the disk shaped insulation plated with the steel disks (sandwiches) were cut from the mould by laser cutting. 2.3. Conductor insulation prototype sample This specimen consists of two concentric stainless steel tubes and an insulation tube located between the steel tubes. In order to avoid excess voltages at the ends of the tubes, the sharp edges of the steel tubes were softened by ¯anging the inner and outer tube radially inwards and outwards, respectively, with suitably shaped tools. Four pieces of Te¯on ®llers were then manufactured and ®tted inside the inner steel tube to keep this space free from resin during the impregnation process. The Kapton-glass insulation tape was then wrapped onto the inner steel tube, and the Te¯on ®llers were used to de®ne the two boundaries of the whole tube. The outer steel tube was made by rolling and pressing a sheet of stainless steel onto the surface of the insulation tube. Finally, the whole arrangement was wrapped with a Tedlar (Te¯on-type) tape on the outside to enable the extraction of the specimen from the cured resin bath. After the impregnation process, the Te¯on ®ller and the Tedlar tape were removed. The ®nal sample geometries and speci®cations, which were chosen in view of the existing space limitations in the low temperature irradiation facility, can be seen in Fig. 1. Photographs of the specimen geometries are presented in Figs. 2 and 3.
Fig. 1. Insulator sample (a), insulator±metal sandwich sample (b), and conductor insulation prototype sample (c).
This reactor is operating at a c-dose rate of 2:8 106 Gyhÿ1 , a fast neutron ¯ux density of 2:9 1017 mÿ2 sÿ1 (E > 0:1 MeV), and a total neutron ¯ux density of 9:5 1017 mÿ2 sÿ1 , respectively. After irradiation, all samples were warmed-up to room temperature and stored for radioactive decay. For the electrical tests a specially designed device was manufactured to allow testing of both unirradiated and irradiated specimens in liquid nitrogen. The disk shaped laminates could be positioned easily and quickly
3. Irradiation and test procedures After initial measurements of the sample dimensions and weights, all irradiations were performed in the FRM I reactor (Garching, Germany) at 5 K to neutron ¯uences of 1021 , 5 ´ 1021 and 1022 mÿ2 (E > 0:1 MeV).
Fig. 2. Photograph of the disk shaped laminate and of the sandwich.
K. Humer et al. / Cryogenics 40 (2000) 295±301
Fig. 3. Photograph of the conductor insulation prototype sample (tube).
between the two electrodes. The device was placed into a polystyrene container ®lled with liquid nitrogen. As can be seen in Fig. 4, the stainless steel electrodes were ®xed onto brass rods, and further on, mounted into an insulating plastic frame. Two connecting leads were ®xed with screws on both sides of the rods leading to the high voltage power supply. The electrical tests of the disk shaped laminate and of the sandwich were carried out at room temperature and
Fig. 4. Measuring device for electrical tests.
297
at 77 K prior to and after low temperature reactor irradiation. The breakdown voltage of the test samples was evaluated according to the ``Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies'' (ASTM D149-97a); ``Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials under Direct-Voltage Stress''(ASTM D3755-97) with Appendix B: ``Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Electrical Insulating Materials under DC Voltage at Cryogenic Temperatures''.
4. Results 4.1. Swelling and weight loss of the irradiated samples After irradiation, the sample dimensions and weights were measured again and then analysed in order to evaluate the radiation induced swelling and weight loss of the specimens. The results are shown in Figs. 5 and 6, where the swelling and the weight loss of the samples are plotted as a function of the fast neutron ¯uence. The swelling of the laminate (Fig. 5) amounts to 1.5% at a fast neutron ¯uence of 1 1021 mÿ2 and to 2% at a fast neutron ¯uence of 5 1021 mÿ2 . A considerable increase of swelling (to 5%) is found at a ¯uence of 1 1022 mÿ2 . The swelling of the sandwich (FRP laminate on stainless steel) amounts to 1% at 1 ´ 1021 mÿ2 and increases continuously to 9% at a ¯uence of 1 1022 mÿ2 , whereas the swelling of the conductor insulation prototype sample (outer diameter) is almost negligible (0±0.5%, cf. Fig. 5). On the other hand, the weight loss of the laminate increases continuously to 2% at a ¯uence of 1 1022 mÿ2 (cf. Fig. 6). Each data point in Figs. 5 and 6 represents an average, calculated from
Fig. 5. Swelling of the laminate, the sandwich, and the conductor insulation prototype sample (tube) after reactor irradiation.
298
K. Humer et al. / Cryogenics 40 (2000) 295±301
Fig. 6. Weight loss of the laminate after reactor irradiation.
Fig. 7. Scaling result on irradiated disk shaped laminates.
4.3. Electrical tests on the irradiated samples ®ve to eight measurements (samples showing signi®cant deviations were rejected in the statistical calculations), except those for the conductor insulation prototype sample, where the average was calculated from only two measurements. In general, the measured eects on swelling and weight loss are in good agreement with literature data on similar laminates [9,10]. 4.2. Scaling experiments on unirradiated disk shaped samples Before the electrical tests on the irradiated samples were carried out, scaling experiments were made on approximately 0.5 mm thick pure laminates of the same material at room temperature (under oil environment) and at 77 K, in order to investigate the in¯uence of the diameter of the disk shaped samples as well as of the size of the stainless-steel electrodes on the measured dielectric strength. The three dierent con®gurations investigated are summarized in Table 1. The scaling results for both temperatures are shown in Fig. 7. As can be seen there, no in¯uence of the disk diameter on the dielectric strength was found, which con®rms the suitability of 12.5 mm diameter disk shaped laminates for a reliable assessment of the electrical properties (i.e. the breakdown voltage and the dielectric strength) under low temperature irradiation conditions. The dielectric strength is about 10% lower at 77 K.
The dielectric strength of the laminate and of the sandwich as a function of the neutron ¯uence is plotted in Fig. 8. No degradation of the dielectric strength was found for both the laminate and the sandwich over the whole dose range. Slightly higher values were found for the sandwich, but the overall eects are within the calculated standard deviations. The conductor insulation prototype sample (tube) was manufactured with a thickness of the insulations system, which is twice that of the disk shaped laminates (cf. Fig. 1). Unfortunately, the voltage needed for electrical breakdown through the laminate exceeded the maximum voltage generated by the high voltage power supply, and thus, breakdown could not be achieved on all of these tubes. Nevertheless, all conductor insulation prototype samples were found to withstand a voltage of at least 60 kV dc without electrical breakdown. 4.4. Fractography of irradiated samples after breakdown After the electrical measurements, fractographic investigations of the surface structure of the irradiated
Table 1 Sample-electrode con®gurations selected for the scaling tests
a b
Con®guration
Disk diameter (mm)
Electrode diameter (mm)
1 2 3
50 25 12.5
12.5a 6.4a 6.4 and 1.6b
Two electrodes opposing. Electrodes opposing.
Fig. 8. Dielectric strength of the laminate and of the sandwich as a function of the neutron ¯uence.
K. Humer et al. / Cryogenics 40 (2000) 295±301
laminates were made in an scanning electron microscope (SEM). Typical photographs are presented in Fig. 9, where the surfaces of the laminates are shown following reactor irradiation at 5 K to a fast neutron ¯uence of 1 1021 ; 5 1021 , and 1 1022 mÿ2 (E > 0:1 MeV), respectively, and following electrical breakdown at 77
299
K. For the lower dose level, small epoxy plates of the outer layer of the laminate are found to be delaminated partially from the compound after breakdown. At higher doses, parts of the outer epoxy layers delaminate completely from the compound and are pulled out. In addition, some inner layers were also partly destroyed.
Fig. 9. From top to bottom: SEM photographs of the surface of the laminate following reactor irradiation to neutron ¯uences of 1021 , 5 ´ 1021 and 1022 mÿ2 (E > 0:1 MeV) and electrical breakdown at 77 K.
300
K. Humer et al. / Cryogenics 40 (2000) 295±301
In general, higher dose levels lead to larger areas of destroyed material. 5. Discussion 5.1. Assessment of sample quality The specimens were subjected to an initial optical inspection. Firstly, all of the disk shaped laminates showed surfaces, which were not completely smooth. We noticed especially the formation of small grooves at the locations of the adhesives between the Kapton foils and the glass±®ber tapes, which led to a considerable decrease of the thickness of the laminate in these regions. Secondly, the Kapton-glass fabric tape was wrapped approximately half-overlapping within the laminate, and therefore, regions with one or two Kapton foils along the thickness were produced. Consequently, we noticed during the electrical tests, that the measured breakdown strength depended on the position of the electrodes on the sample surface. Although we tried to ®nd the optimal position of the electrodes for each sample (as far as this could be done considering the small sample dimensions), both eects lead to rather high standard deviations of the evaluated breakdown strengths (cf. Figs. 7 and 8). After irradiation, the complete set of specimens was subjected to an optical inspection again. The outer layers of some specimens irradiated to a ¯uence of 1022 mÿ2 (E > 0:1 MeV) were found to have delaminated completely from the compound. 5.2. Swelling eects Whenever FRPs are irradiated with fast neutrons, swelling in the through-thickness direction of the laminate is a well known eect, especially when the material is irradiated at cryogenic temperatures. In this case, hydrogen gas will be produced in all types of epoxy resins. During low temperature irradiation, free radicals of larger molecules are formed and displaced from the main chain of the macro-molecules of the plastic material. While keeping the material at low temperatures does not lead to signi®cant changes of the outer material dimensions, swelling takes place when the material is warmed up to higher temperatures (at least 100 K or room temperature). At this temperature, the hydrogen molecules diuse rapidly and tend to escape the solid material. In addition, some free radicals will combine into larger gas molecules and diuse slowly from the compound or will form some pores or enlarge already existing pores in the material. However, during warm-up of the prepreg laminate to room temperature, some of the gas will escape and some will remain within the pores of the composite. On the whole, these eects cause
swelling in the through-thickness direction of the laminate. Furthermore it is well known, that the eects of gas production and swelling are more pronounced in difunctional types of epoxy resins (bisphenol-A types, DGEBA, i.e., the investigated TFMC insulation system; bisphenol±F types) than in the multifunctional epoxies (TGDM) [10]. Consequently, swelling is an important parameter for the engineering and design criteria of superconducting magnet coils for fusion reactors. The speci®ed fast neutron ¯uence for the magnet insulation of ITER is 5 1021 mÿ2 (E > 0:1 MeV). For the TFMC insulation of ITER (cf. Fig. 5), we found at this dose level 2% for the pure laminate, and 4% for the laminate/steel sandwich, which is in good agreement with data from the ``US/ITER Insulation Irradiation Program'' [9,10]. With regard to our measurements on swelling for the sandwiches, we wish to point out that those parts of the sandwich consisting only of the pure laminate were also measured in the through-thickness direction. Within experimental accuracy, the data on swelling agree with those obtained on the pure disk shaped laminates. Therefore, we assume that the dierence in the swelling behavior between the pure disk shaped laminate and the sandwich may be related to the eects discussed above and to the laminate-stainless steel interface which could not be measured separately. A possible contamination with Te¯on during the fabrication process of the samples (cf. Section 2) may also be considered as an explanation of this dierence in swelling between the pure laminate and the sandwich structure. On the other hand, an optical investigation of the samples did not show any obvious eects for both types of samples, and therefore, did not lead to further insights. 6. Summary In this contribution, the electrical properties (i.e. the dielectric strength and the breakdown voltage) of the conductor insulation system used for the TFMC of ITER, have been assessed in conjunction with data on swelling and weight loss following low temperature reactor irradiation at 5 K up to a neutron ¯uence of 1022 mÿ2 (E > 0:1 MeV). The results may be summarized as follows: 1. Swelling. At a ¯uence of 5 1021 mÿ2 (E > 0:1 MeV), i.e., the ITER dose level, the data show half overlapping error bars between the pure laminate and the insulation/steel sandwich, the overall eects are within 4%. In the con®guration of the conductor insulation prototype sample, swelling is almost negligible. 2. Weight loss. Up to a ¯uence of 1022 mÿ2 (E > 0:1 MeV), the weight loss of the laminate increases continuously to 2%.
K. Humer et al. / Cryogenics 40 (2000) 295±301
3. Dielectric strength. At 77 K, this quantity amounts to about 70 kV mmÿ1 for the pure laminate. Slightly higher values were found for the laminate-steel sandwich. No degradation of the dielectric strength was found for both the laminate and the sandwich over the whole dose range. In summary it should be noted, that the typical insulation thickness for the TFMC of ITER [13] is in a range between 1.9 and 10 mm (e.g. TFMC conductor insulation thickness: 2.5 mm, pancake insulation: thickness 1.9 mm, and ground insulation: thickness 1.9 mm + 8 mm 10 mm). Considering the data on the electrical tests presented in this contribution, we conclude that the electrical performance of these materials exceeds by far the design limits of the ITER EDA criteria. Acknowledgements The authors are greatly indebted to Dr. R. Maix and Mr. H. Fillunger (EUHT) for their eorts with the sample procurement. The technical assistance of Mr. E. Tischler is also acknowledged. This work has been carried out within the Association EURATOM-OEAW. References [1] Brown BS. Radiation eects in superconducting fusion-magnet materials. J Nucl Mater 1981;97:1±14.
301
[2] Coltman Jr. RR. Organic insulators and the copper stabilizer for fusion reactor magnets. J Nucl Mater 1982;108&109:559±71. [3] Weber HW, Tschegg EK. Test program for mechanical strength measurements on ®ber reinforced plastics exposed to radiation environments. Adv Cryog Engrg 1990;36:863±9. [4] Humer K, Weber HW, Tschegg EK. Radiation eects on insulators for superconducting fusion magnets. Cryogenics 1995;35:871±82. [5] Humer K, Spieûberger SM, Weber HW, Tschegg EK, Gerstenberg H. Low temperature interlaminar shear strength of reactor irradiated glass ®ber reinforced laminates. Cryogenics 1996;36:611±7. [6] Humer K, Rosenkranz P, Weber HW, Fabian PE, Rice JA. Mechanical properties of the ITER CS Model Coil insulation under static and dynamic load after reactor irradiation. J Nucl Mater [in press]. [7] Schutz JB, Darr JB, Reed RP. Dielectric strength of candidate ITER insulation materials. Adv Cryog Engrg 1994;40:1059±66. [8] Schutz JB, Fabian PE, Hazelton CS, Bauer-McDaniel TS, Reed RP. Eects of cryogenic irradiation on electrical strength of candidate ITER insulation materials. Cryogenics 1995;35:759±62. [9] Reed RP, Fabian PE, Schutz JB. Development of US/ITER CS Model Coil turn insulation. Adv Cryog Engrg 1998;44A:175±82. [10] Reed RP, Fabian PE, Schutz JB. US ITER insulation irradiation program. Final Report, Cryogenic Materials, Inc. and Composite Technology Development, Inc., Boulder, CO, 31 August 1995. [11] Broadbent AJ, Crozier J, Smith KD, Street AJ, Wiatrzyk JM. Electrical breakdown strength results from the EU testing program for potential ITER insulation. Adv Cryog Engrg 1996;42:213±8. [12] Broadbent AJ, Crozier J, Smith KD, Street AJ, Wiatrzyk JM. Initial results from the testing of potential ITER insulation materials. Cryogenics 1995;35:701±3. [13] Maix R, et al. Manufacture of the ITER TF model coil. In: Proceedings of the 20th SOFT, 1998; Marseille. p. 833±36.