K. Humer

Mechanical behavior of the ITER TF model coil ground insulation system after reactor irradiation

Abstract The mechanical properties of glass fiber reinforced plastics (GFRPs) suggested for the turn and ground insulation of the ITER toroidal field (TF) coils are subject to extensive investigations with respect to their design requirements at present. The insulation system used for the ITER TF model coil, manufactured by European industry, consists of a boron-free R-glass fiber reinforced tape, vacuum-pressure impregnated in a DGEBA epoxy system and partly interleaved with polyimide-foils (e.g. Kapton-H-foils). In order to assess the material performance under the actual operating conditions of ITER-FEAT, the system was irradiated in the TRIGA reactor (Vienna, Austria) to neutron fluences of 5×1021 and 1×1022 m−2 (E>0.1 MeV). The composite was screened at 77 K using static tensile, short-beam-shear (SBS) as well as double-lap-shear tests prior to and after irradiation. Furthermore, tension–tension fatigue measurements were done in order to simulate the pulsed ITER-FEAT operation. We observe that the mechanical strength and the fracture behavior of these GFRPs after irradiation are strongly influenced by the three factors: the winding direction of the tape, the quality of fabrication and the delamination process.

Novel radiation-resistant insulation systems for fusion magnets

Large, capital-intensive, superconducting or resistive magnets are essential components of most current and planned fusion devices. Magnets for these applications must be reliable, have a long mean-time-between-failure, and be able to be manufactured using cost-effective materials and fabrication processes. Electrical insulation is often the weak link in magnet design, due to insulation sensitivity to high radiation doses, embrittlement at cryogenic temperatures, and fabrication limitations. Improvements in electrical insulation can contribute to enhanced magnet system performance and achieve considerable cost reduction. For example, an insulator with improved radiation resistance would require less shielding, thus enabling the coil to be located closer to the radiation source, resulting in a lower field requirement for the coil, and thus reducing the conductor and structural needs for the magnet systems. In this manner, improvements in magnet insulator performance and processing can have a cascading effect on overall magnet system cost reductions. Composite Technology Development, Inc. has developed two new classes of insulation materials, an organic insulation system based on cyanate ester chemistry, and a ceramic insulation system that can be co-processed with the magnet. Both types of systems are suitable for the high radiation doses anticipated in Next-Step Option devices and future fusion reactors. This paper will describe the different material systems under current development, mechanical and electrical properties at cryogenic temperatures, and results of radiation exposure tests for these materials.

Radiation effects on insulators for superconducting fusion magnets

Abstract Fibre-reinforced plastics (FRPs) are candidate materials for the insulation of superconducting magnet coils in future fusion reactors. This paper reports on a test programme performed to assess the mechanical properties of these materials and to obtain information on the damage and fracture mechanisms. Different types of FRPs (epoxies and poly- and bismaleimides as resins; two- and three-dimensional E-, S- or T-glass fabrics as reinforcements) were irradiated at room temperature by 2 MeV electrons and 60 Co-gamma rays up to 1.8 × 10 8 Gy and by different reactor spectra up to a neutron fluence of 1 × 10 23 m −2 ( E > 0.1 MeV ) at room temperature, 80 K or 5 K. Mechanical tests in tension as well as in the intralaminar crack opening and shear mode were carried out on the irradiated samples at 77 K. After low temperature irradiation, half of the samples were subjected to a warm-up cycle to room temperature before testing at 77 K. Results on the influence of different radiation sources and annealing cycles on the mechanical properties of these materials will be discussed.

Influence of test geometry on tensile strength of fibre reinforced plastics at cryogenic temperatures

Abstract In view of future applications of fibre reinforced plastics as insulating materials for the windings of superconducting magnets in fusion reactors, the mechanical properties of these materials have to be tested under conditions which include the appropriate radiation environment expected at the magnet location. Since the established standards, e.g. for the measurement of the ultimate tensile strength, involve sample sizes which are far too large for testing under a radiation environment, scaling experiments have been made to investigate the influence of sample width and thickness on the ultimate tensile strength and the fracture strain at room temperature, 77 and 4.2 K. The data show that variation of sample width does not affect the results for ultimate tensile strength in a systematic way, and that a reduction of sample thickness leads to a slight reduction of the ultimate tensile strength (15−35% at room temperature, 5−15% at low temperatures). Fractographic investigations show no significant dependence of the fracture surfaces on the test geometry.

A study of short-beam-shear and double-lap-shear specimens of glass fabric/epoxy composites

Two experimental methods for determining the inter-laminar shear strength (ILSS), are compared: the short-beam-shear (SBS) and the double-lap-shear (DLS) test method. Specimens with a constant ply angle for all layers are considered. The experimental results show a significant difference (up to 50%) in the obtained ILSS. A finite element analysis shows that both test methods underestimate the real ILSS and demonstrate that the standardized ILSS evaluation procedures are more or less valid for the SBS test, but require modifications in the case of the DLS test. Acoustic emission measurements and numerical investigations were performed to determine the real ILSS from the DLS results. The real ILSS cannot be obtained from the SBS test without an extended analysis. It is, however, possible to determine bounds for the real ILSS from the SBS results.

Radiation hardness of newly developed ITER relevant insulation systems

Abstract The application of glass fiber reinforced plastics (GFRPs) as insulating materials for the superconducting (SC) magnet coils of the International Thermonuclear Experimental Reactor (ITER) fusion device imposes high demands on the material performance. Therefore, GFRPs with improved mechanical and electrical properties are of special interest. One of the most promising matrix materials for higher radiation resistance is the cyanate ester system. Two innovative insulation systems with boron-free S-glass fiber reinforcements and cyanate ester based matrix compositions were irradiated in the TRIGA reactor (Vienna, Austria) to neutron fluences of 1×1021, 1×1022 and 5×1022 m−2 (E>0.1 MeV). After irradiation, swelling and weight loss were measured prior to static and dynamic tensile as well as interlaminar shear testing at 77 K.

Low-temperature tensile strength of the ITER-TF model coil insulation system after reactor irradiation

Abstract The windings of the superconducting magnet coils for the ITER-FEAT fusion device are affected by high mechanical stresses at cryogenic temperatures and by a radiation environment, which impose certain constraints especially on the insulating materials. A glass fiber reinforced plastic (GFRP) laminate, which consists of Kapton/R-glass-fiber reinforcement tapes, vacuum-impregnated in a DGEBA epoxy system, was used for the European toroidal field model coil turn insulation of ITER. In order to assess its mechanical properties under the actual operating conditions of ITER-FEAT, cryogenic (77 K) static tensile tests and tension–tension fatigue measurements were done before and after irradiation to a fast neutron fluence of 1×10 22 m −2 ( E >0.1 MeV), i.e. the ITER-FEAT design fluence level. We find that the mechanical strength and the fracture behavior of this GFRP are strongly influenced by the winding direction of the tape and by the radiation induced delamination process. In addition, the composite swells by 3%, forming bubbles inside the laminate, and loses weight (1.4%) at the design fluence.

Mechanical properties and fracture behaviour of polyimide (SINTIMID) at cryogenic temperatures

Abstract In view of emerging applications in aeronautics, low temperature and nuclear fusion technology, the mechanical properties of plastics have to be tested both at cryogenic temperatures and under certain radiation environments. In the present paper measurements are reported of the elastic modulus, the ultimate tensile strength (UTS) and the failure strain of a polyimide (SINTIMID), which were carried out at room temperature, 77 and 4.2 K. In addition to this temperature dependence, the influence of sample size on the mechanical properties, as well as the fracture behaviour in mode I on cylindrical precracked samples with a circumferential notch, have been investigated. The results show an increase in both the elastic modulus and the UTS by 40 and 60%, respectively, on decreasing the temperature to 77 K, but no further change at lower temperatures. On the other hand, the failure strain decreases continuously (by ≈ 25%) down to 4.2 K, while the fracture toughness increases in a similar way (by ≈ 10%). No sample size dependence could be detected. These results will be discussed in conjunction with additional observations of the fracture behaviour made using optical and scanning electron microscopes.

Low-temperature interlaminar shear strength of reactor irradiated glass-fibre-reinforced laminates

Abstract Glass-fibre-reinforced plastics (GFRPs) are candidate insulating materials for superconducting magnet coils in future fusion reactors. Therefore, the influence of radiation damage (gamma and fast neutrons) especially on the interlaminar shear behaviour of these materials has to be investigated carefully. Different types of GFRP laminate (two-dimensional E- or S-glass fibre reinforcements, epoxy or polyimide resins) have been irradiated at room temperature in the TRIGA reactor (Vienna, Austria) and at 5 K in the FRM Munich (Garching, Germany) up to a neutron fluence of 5 × 10 22 m −2 ( E > 0.1 MeV ) prior to short-beam-shear (SBS) testing at 77 K. After low-temperature irradiation, half of the samples were subjected to a warm-up cycle to room temperature before testing at 77 K. Results on the influence of different radiation sources, irradiation temperatures and annealing cycles as well as the boron content of some laminates on the interlaminar shear strength (ILSS) are compared and discussed, together with microstructural observations made with a scanning electron microscope.

Characterization of reactor irradiated organic and inorganic hybrid insulation systems for fusion magnets

Because of their excellent electrical and mechanical material performance, organic and organic/inorganic hybrid materials are candidate insulation systems for fusion magnets. Wrappable inorganic insulation systems combine the high strength and modulus of a ceramic composite with the simple processing of an organic insulation system. Various types of organic as well as organic/inorganic hybrid material compositions containing a ceramic fiber fabric as well as a ceramic and an epoxy matrix were irradiated at ambient temperature in the TRIGA reactor (Vienna) up to neutron fluences of 1021, 1022 and 5×1022 m−2 (E>0.1 MeV). Tensile, short-beam-shear and double-lap-shear tests were performed at 77 K. The influence of reactor irradiation on the mechanical material performance will be presented for different compositions of these materials.