K. Kitamura

High heat flux testing of a HIP bonded first wall panel with built-in circular cooling tubes

Abstract A HIP bonded DS-Cu/SS first wall (FW) with built-in circular cooling tubes was fabricated under the optimized HIP conditions. High heat flux testing of the panel was carried out on electron beam facility, JEBIS , at JAERI. The objective of this test is to examine the thermomechanical performance of the panel, including the integrity of the HIP bonded interfaces and also to examine the relation between the design fatigue curve and experimental results. Test conditions applied during these tests were 5.0–7.0 MW/m 2 in average, much higher than the ITER normal operation condition of 0.5 MW/m 2 , to accelerate the fatigue test with a repetition cycle up to 2500 cycles in total. High heat flux tests consisted of two test campaigns. Throughout these tests, no damages such as cracks were observed and no degradation in heat removal performance was also observed from temperature responses measured with thermocouples embedded within the panel. Thermomechanical integrity of the panel was confirmed within the parameter tests and the fatigue lifetime of the panel was found to be much longer than the design fatigue curve of this material, or even beyond the raw fatigue data.

Fatigue strength of tungsten-copper duplex structures for divertor plates

Abstract A tungsten-copper duplex structure is specified in a conceptual design of the Japan Fusion Experimental Reactor (FER). The evaluation of the fatigue and creep life of the interface region between tungsten and copper is essential for design of the divertor plate. Fatigue crack initiation life and crack propagation behavior at room temperature and 200°C were measured for fully-annealed OFHC copper and for tungsten-OFHC copper joints brazed with amorphous nickel-base filler metal. The debonding fatigue strength for the brazed joints was relatively high, but less than that of the copper. Fatigue crack growth rates in the braze layer was approximately similar to that of the copper. Fatigue lives were estimated for the divertor plate with small defects, and a method for analyzing the apparent K- values of interface cracks was presented.

Design and fabrication of forced-flow coils as an R&D program for Large Helical Device

Two forced-flow cooled NbTi superconducting coils (TOKI-TF, PF) have been designed and fabricated. The helical coil (TOKI-TF) is a 1/4-scale model of the Large Helical Device (LHD). It has a major radius of 0.9 m, a minor radius of 0.25 m, and a pitch number of 4. Nominal current and maximum field were designed to be 8 kA and 2.8 T, respectively. Another coil (TOKI-PF) was fabricated for the demonstration of LHD poloidal field coils. It consists of two double pancakes with an inner radius of 0.6 m and an outer radius of 0.82 m. The nominal current of 25 kA simulates that of LHD poloidal field coils. Cable-in-conduit-type conductors were used for the both coils. The test facility was also constructed with a vacuum vessel, a liquid nitrogen shield, 30-kA power leads, a heat exchanger, and cryogenic supports. Design concepts and details are presented.

Experimental and analytical studies on residual stress in the tungsten-copper duplex structure for a divertor application

Abstract Residual stresses that developed during cooling of the tungsten-copper duplex structure were measured by the strain gauge method and compared with those by thermoelastic-plastic analyses. Good agreement was obtained for both residual stress and displacement, even when the creep effect of the copper heat sink was neglected in the analyses. The residual stress on the tungsten top surface decreased with increase in the ratio of copper thickness ( t c ) to tungsten diameter ( D ). The effect of t c / D on the residual stress was large in the range of t c / D t c / D >1. The effective thickness of the plastic region in the copper heat sink was reduced in the same manner as the residual stress. The copper heat sink plastic developed first from the bonding interface and then from the center part of the bottom surface. The calculated edge stresses on the tungsten side surface were quite sensitive to the finite element mesh size near the interface edge, while stress on the tungsten top surface did not depend so much on the mesh size.

Cryogenic shear fracture tests of interlaminar organic insulation for a forced-flow superconducting coil

Since a forced-flow superconducting coil consists of metallic conductors and organic insulation with relatively low mechanical strength, a knowledge of mechanical behavior and properties of the interlaminar insulation is of great importance. Shear fracture tests of interlaminar organic insulation under biaxial stresses were carried out at low temperature, by using test specimens made of stainless steel pieces bonded with impregnated organic insulation (glass-kapton-glass). Inclination of adhesive surface was varied to get combinations of the debonding stresses and to construct the critical curve of the debonding fracture. The test results show that the critical curve approximately agreed with the Mohr-Coulomb criterion and that shear fracture resistance of the bonding interface enhanced with the increase of compressive stress value. >

Thermal fatigue tests of a W-Cu duplex structure for a divertor application

A tungsten-copper duplex structure has received consideration for use as a divertor plate in a conceptual design of the Fusion Experimental Reactor (FER). Thermal fatigue tests were performed on tungsten-copper brazed specimens using an argon-plasma jet as an energy source to heat the specimens. Two types of specimens were prepared: smooth and slit specimens at the braze region. The specimens were examined in detail through visual and non-destructive inspections after 100 cycles were carried out. Detectable change was not observed at the braze region of the specimens. From numerical analyses, a high shearing stress is expected to occur at the root of the slit, which is more than twice that for a smooth specimen. The results indicate that detailed techniques for detecting flaws lying especially in the edge region of the joining interface should be incorporated into the failure assessment of a duplex structure.

STRUCTURAL DESIGN AND ANALYSIS OF FORCED-FLOW SUPERCONDUCTING INNER VERTICAL COILS FOR THE LARGE HELICAL DEVICE

Abstract The Large Helical Device (LHD) is a next helical fusion experimental device for National Institute for Fusion Science (NIFS). A forced–flow cooling has been employed for the superconducting poloidal coils of the LHD, where high mechanical integrities are required to keep high magnetic field accuracy. Therefore, the coils must have enough structural stiffness to withstand the large electromagnetic forces and to suppress coil deformation within the allowable level. Then, 3-D structural analyses of the inner vertical poloidal (IV) coil have been performed to understand the mechanical behaviors of the conductor conduits and insulations and to assess their structural integrities, including overall analyses of the coil support system and a detailed analysis of the conductor conduit and insulation. The analytical results show that the IV coil has enough structural rigidities with maximum conduit stress of 260 MPa, which is sufficiently below the allowable value of the material. The results also indicate that the coil support structure can keep the maximum conductor deformation below ˜ 1.6mm which satisfies the required field accuracy of the LHD.

Optimization studies on interfacial mechanical strength in the graphite-copper bonded structure for a divertor application

Residual stresses in the interface region were evaluated for the graphite-copper bonded system to assess the mechanical strength of the bonding interface. The normal stress components of the residual stresses around the interface edge were compared for three types of bonded structures such as a net-type, monoblock-type and saddle-type ones and were calculated to be 44 MPa, 7 MPa and 13 MPa, respectively. Consequently, the saddle-type structure was found to be favorable in the views of its mechanical integrity, fabrication ease and maintenance. The residual stresses around the interface edge in the saddle-type structures with the wedge angles of 45/spl deg/ to 135/spl deg/ were also examined. As the results, an optimal bonded configuration of the graphite-copper saddle-type structure was found to have wedge angle of about 60/spl deg/ for the least value of residual stress.

Mechanical analysis and fabrication of the R&D forced-flow helical coil (TOKI-PF)

A forced-flow NbTi superconducting helical coil was fabricated as research and development of the helical coil for the Large Helical Device (LHD), corresponding to a 14 scale model of the LHD. A computer-controlled helical-winding machine employing the roller bending method with twisting capability was also developed. The maximum assembly error of the conductors decreased to about 0.7 mm by optimal control of the winding curvature and torsion of the conductors. Three-dimensional structural analysis of the helical coil and the support structure was carried out to assess the mechanical integrity of the coil and support structure and to understand their mechanical behavior under the electromagnetic force. Analytical results show that the coil support structure can keep the maximum deformation of the conductors less than .4 mm, which corresponds to the required field accuracy of 5 × 10−4.

Edge stresses in bonded armor system for divertor plate

To safely manage the high heat loads expected in the next fusion devices, efforts have been focused on the development of the actively cooled divertor plate which consists of graphite armor tiles bonded to copper structural members. Residual stresses in the interface region were evaluated for the graphite/copper bonded system. Eigenvalue analysis was performed to investigate the favorable free-edge geometry for the interface. Results for a joint having half-plane free-edge geometry indicated no stress singularity for theta /sub 1/ approximately 120 degrees , where theta /sub 1/ is the angle between the traction free surface of the graphite and the bonding interface. Thermal stresses in a joint having a conical bonding interface with inner and outer free edges were also examined for a sequence consisting of cooling from the brazing temperature and service with a pulsed heat load of 10 MW/m/sup 2/-5 s. The normal stress on the bonding interface calculated for graphite was tensile only in the neighborhood of the interface edges. This normal stress decreased but the high stress region expanded during heat loading. The Von Mises stress in the interface edge region decreased without apparent increase in plastic strain at two different times in the thermal cycle.<<ETX>>