Junji Ohmori

Thermal shock tests of tungsten by H+-beam bombardment

Cyclic thermal tests were performed on tungsten using the 120 keV Neutral Beam Test Stand at IPP, Nagoya University. A maximum heat flux of 8.8 kW/cm 2 was first applied for 56.7 ms to large samples for a total of 5 cycles. Many surface microcracks were observed after two thermal shocks, but only a few active cracks grew further in a direction almost perpendicular to the heated surface with an average growth rate of about 0.2 mm/cycle. Thermal shocks of 50.4 ms were next applied with an average heat flux of 8.2 kW/cm 2 for a total of 23 cycles. Active cracks grew in the plane lying about 3 mm below the heated surface. No visible cracks were produced in one of the four samples, but cracking occurred after cutting the sample into four identical blocks. The thermal shock cracking was restricted in the region where grain growth had occurred.

Thermal shock resistances of aln and sic for first wall applications

Thermal shock tests were performed on hot pressed AlN and SiC using hydrogen beams as an energy source to heat large samples rapidly. The single-shot-resistance to thermal shock spalling was examined under three combinations of heat flux density and pulse duration time: 8.8. kW/cm 2 - 56.7 ms, 2.5 kW/cm 2 - 100 ms, and 4.7 kW/cm 2 - 100 ms, respectively. SiC survived a thermal shock with 2.5 kW/cm 2 - 100 ms, but failed under the other two heating conditions. AlN was spalled even by a thermal shock with 2.5 kW/cm 2 - 100 ms. The experimental observations on the tests with a thermal shock of 100 msec imply that the thermal shock cracking has occurred in the heating process rather than in the cooling process and that the cracks have not been initiated on the heated surface.

Heat load tests of superconducting magnets vibrated electromagnetically for the Maglev train

Abstract Superconducting magnets on Maglev trains vibrate due to harmonic ripples of electromagnetic flux generated by ground coils. Heat load caused by vibration in the magnet amounted to several tens of watts in the electromagnetic vibration test. This was mainly because a.c. loss was induced in the helium vessel housing the superconducting coil, due to relative vibration between the aluminium thermal shield and the coil. The heat load caused by vibration should be strictly restricted to less than 4W due to limited cryogenic refrigeration capacity. The heat load was tested using electromagnetic flux ripples for a superconducting magnet model of one coil which corresponds to 1 4 of an actual magnet. The flux ripples simulated the 6th harmonic of the actual ground levitation coil. Some ideas to reduce the heat load were tried for the magnet model, such as applying high resistance thermal radiation shielding, increasing rigidity of the vacuum vessel, and using high purity copper plating on the helium vessel. These ideas proved effective, and the maximum heat load due to vibration was held to less than 4 W per magnet for the one coil magnet model.

Analysis and experimental results on ingress of coolant event in vacuum vessel

Experiments on the ingress of coolant event (ICE) in the vacuum vessel of a fusion reactor have been carried out in the Japan Atomic Energy Research Institute (JAERI) as one of the research and development tasks for the International Thermonuclear Experimental Reactor (ITER) to obtain the thermofluid data for validation of safety analysis codes. The ICE experiment is numerically analyzed using the transient reactor analysis code (TRAC) which is one of the codes preparing for the safety analysis of the ITER. The TRAC has been modified so as to analyze the ICE phenomena in the vacuum vessel of a fusion reactor. Several ICE experiments have been carried out as benchmark tests for the safety analysis codes. We have analyzed those experiments by using the TRAC, and considered the difference between the analysis and experimental results. Analysis results of the temperature in the vacuum vessel show a tendency completely different from the experimental result. It is clarified that the present TRAC has not been verified on the scattering behavior of water droplets.

Characteristics of heating in a mechanically vibrated superconducting coil

Abstract It is revealed that much heat is generated not only by the eddy current due to the relative displacement between conducting elements in a magnet, but also by the mechanical vibrating deformation of a superconducting coil [Suzuki, E., Cryog. Eng. , 1994, 29 , 495–503]. We measured the characteristics of heating in the mechanically vibrating superconducting coil in resonance using an oil servo actuator. As a result of these experiments, the following facts are made clear: (i) the heat generated in a vibrating superconducting coil is larger in the twisting configuration than in the bending one under the vibrating mode; (ii) the characteristics of the increment in heating in the vibrating coil under the energizing and the de-energizing state are almost the same. We cite as a factor in heating phenomena of the mechanical excitation the frictional heat between the fasteners and a superconducting coil. The size of the displacements in these frictional parts are supposed to be of the order of several micrometres. We intend to make further analyses of the heating phenomena.

Mechanical behavior on residual stress in a tungsten-copper duplex structure after cyclic heat loads for a divertor application

Abstract Cyclic thermal tests have been carried out on the tungsten-copper duplex structures using an electron-beam test facility, with a particular emphasis on the redistribution of the residual stress after the cyclic heat loads of 14 MW/m 2 –2 s. The residual stresses on the tungsten surfaces were measured by the strain gauge method and compared with those by thermoelasto–plastic analysis. Good agreement was approximately obtained for both their stress magnitude and distribution. Both radial and hoop stress components σ R and σ θ on the tungsten top surface slightly increased after first heat cycle and then decreased with the increase of the number of heat load cycles. Axial stress component σ z on the tungsten side surface also behaved in the same manner as stresses on the top surface, while hoop stress component increased with the increase of the number of heat cycles. Relaxation of residual stress was induced with the elastic-plastic deformation of copper heat sink by cyclic heat loads.

Estimation on interfacial mechanical strength in the W/Cu bonded structure for high heat flux component

Cyclic thermal tests and thermoelastic-plastic structure analyses have been performed on tungsten-copper duplex structures to investigate mechanical behavior on residual stress after cyclic heat loads of 14 MW/m/sup 2/-2 sec. Good agreement was obtained for the residual stresses on both the tungsten top surface and side surfaces. Both the radial and hoop residual stress components /spl sigma//sub K//spl sigma//sub /spl theta// of the tungsten top surface decreased with the increase of the number of heat load cycles. Axial stress component /spl sigma//sub x/ on the tungsten side surface was also reduced with the increase of the number of heat cycles. Relaxation of the residual stress occurred by the elastic-plastic deformation of copper heat sink. It was found that cyclic thermal loads lead to the reduction of residual stress in the tungsten-copper duplex structure and it is favorable in view of the structural integrity of bonded structure.

The Electromagnetic Analysis for MAGLEV Train under Short-circuit of the Armature Coil

Abstract In order to ensure the reliability of MAGLEV train, it is necessary to analyze the electromagnetic phenomena on normal and abnormal operations. This report describes the electromagnetic analysis of the abnormal operations, and the comparison between the analysis and experiments. The wire-segments-model analysis makes it clear that the largest electromagnetic force acts to the superconducting magnet (SCM) when an armature coil is short-circuited. On the other hand, the finite element analysis shows the magnetic field is almost shielded by the aluminum plate of vacuum vessel, and only part of lateral force directly acts to the SC coil. These analytical results show good agreement with the experimental results.

HIGH HEAT FLUX TESTS OF GRAPHITES FOR LIMITER APPLICATIONS

Thermal shock tests have been performed on graphites (DFP 3–2 and ETP-10) using the 120 KeV Neutral Beam Test Stand at IPP, Nagoya University. Examples are given of the observed damages such as thermal shock cracking, localized erosion and formation of graphite particles isolated from the bulk material. Thermal analyses which include the sublimation effects have also been performed to better understand the experimental results on the weight loss. The experimental and analytical results indicate that the selection of graphitization temperature is of great importance in a design of a graphite limiter.