Takashi Naoe

Pitting Damage Formation up to over 10 Million Cycles Off-line Test by MIMTM

A liquid-mercury target system for the MW-scale target is being developed in the world. The pitting damage induced by pressure wave propagation gets to be one of critical issues to estimate the life of the target structure with mercury and to evaluate its structural integrity. The off-line test on the pitting damage at high cycles over 10 millions was carried out using a novel device, the MIMTM which drives electromagnetically to impose pulse pressure into the mercury. It was found from the pitting damage data obtained by the MIMTM and comparison with classical vibratory hone tests that the pitting damage can be characterized in two steps, an incubation period that can extend to 106 cycles in 20% cold worked 316SS and 107 cycles in surface hardening treated one and steady state erosion where mass loss scales with the number of cycles to approximately the 1.27 power for mercury. The length of the incubation period is primarily a function of the material and the intensity of the pressure. This observation provides a simple model for estimating lifetime for different materials and beam power.

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (II) : Bubbling Effect to Reduce Pressure Wave

Liquid mercury was suggested to be used as target material for high-power pulsed spallation neutron sources. In order to realize the high-power target, however, the pressure wave is a critical issue, which is caused by the thermal shock in mercury and causes cavitation at the moment when highly intense proton beams bombard mercury. R&D on pressure wave mitigation technologies is carried out for Japan Spallation Neutron Source (JSNS; 1MW/25 Hz). Microbubble injection into the mercury is one of prospective technologies to mitigate the pressure wave. The microbubble effect was experimentally investigated from the viewpoint of pitting damage due to the cavitation in the mercury loop with an electro-magnetic impact testing machine (MIMTM) and numerically examined from the viewpoint of bubble dynamics. In the present study, we confirmed that the microbubble injection is very effective to reduce pitting damage and the amplitude of negative pressure, which causes explosive growth of cavitation bubble.

Damage Diagnostic of Localized Impact Erosion by Measuring Acoustic Vibration

High power spallation targets for neutron sources are being developed in the world. Mercury target will be installed at the material and life science facility in J-PARC, which will promote innovative science. The mercury target is subject to the pressure wave caused by the proton bombarding mercury. The pressure wave propagation induces the cavitation in mercury that imposes localized impact erosion damage on the target vessel. The impact erosion is a critical issue to decide the lifetime of the target. The electric Magnetic IMpact Testing Machine, MIMTM, was developed to produce the localized impact erosion damage and evaluate the damage formation. Acoustic vibration measurement was carried out to investigate the correlation between the erosion damage and the damage potential derived from acoustic vibration. It was confirmed that the damage potential related with acoustic vibration is useful to predict the damage due to the localized impact erosion and to diagnose the structural integrity.

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (III)—Consideration of the Effect of Microbubbles on Pressure Wave Propagation through a Water Test—

The effects of microbubbles dispersed in a liquid on a high-rising-rate pressure wave were experimentally investigated with water. Intense, high-rising-rate pressure waves with a rise time of about 1.5 ms were produced by a spark discharge in water, and gas microbubbles were produced by two different bubble generators. Particular attention was focused on the attenuation effect of microbubbles on propagating pressure waves. The dependence of the attenuation effect on the radius and void fraction of the microbubbles was carefully examined. It was found that when the microbubbles are sufficiently small (e.g., about 50 μm in peak radius), the amplitude of wall vibration induced by the spark-induced pressure wave is dramatically decreased with an increase in void fraction. The present study provides strong experimental evidence that microbubbles can act as a strong absorber for high-rising-rate pressure waves as recently predicted numerically.

Lifetime Estimation of Microbubble in Mercury

Liquid mercury is selected to be the target ma-terial to produce neutron by spallation reaction taking intoaccount advantages of neutron yield and heat removal dueto self-circulation. At the moment when highly intense pro-ton beams are injected into the mercury at 25Hz, pressurewaves will be induced by rapid thermal expansion in mercu-ry. Negative pressure will be generated through pressurewave propagation and cause cavitation, which imposes cav-itation erosion on the target vessel wall.

Study on the evaluation of erosion damage by using laser ultrasonic integrated with a wavelet analysis technique

Spallation targets are the key components of accelerator driven systems (ADSs) that are being developed in the world. Erosion damages on the target vessels are anticipated. To prevent accidents occurrence due to erosion of spallation target vessel, the damage evaluation technique is desirable. The excited vibration of LBE target vessel will be monitored remotely to establish the technique. In this study, the basic researches were carried out through experiments and numerical simulations to investigate the interaction between ultrasonic waves and damage to understand the correlation between structural vibration and damage degree. Specimens with distributed erosion damage was irradiated by laser shots, and the vibration was detected by a laser vibrometer subsequently. A technique, Wavelet Differential Analysis (WDA), was developed to quantitatively and clearly indicate the differences caused by damage in the vibration signals. The results illustrated that the developed technique is sensitive to erosion damage with small size and is capable of quantitatively evaluating erosion damage. It is expected that the developed techniques can be applied to monitor the real spallation targets in the future.

Development of microbubble generator for suppression of pressure waves in mercury target of spallation source

A MW-class mercury target for the spallation neutron source is subjected to the pressure waves and cavitation erosion induced by high-intense pulsed-proton beam bombardment. Helium-gas microbubbles injection into mercury is one of the effective techniques to suppress the pressure waves. The microbubble injection technique was developed. The selection test of bubble generators indicated that the bubble generator utilizing swirl flow of liquid (swirl-type bubble-generator) will be suitable from the viewpoint of the produced bubble size. However, when single swirl-type bubble-generator was used in flowing mercury, swirl flow of mercury remains at downstream of the generator. The remaining swirl flow causes the coalescence of bubbles which results in ineffective suppression of pressure waves. To solve this concern, a multi-swirl type bubble-generator, which consists of several single swirl-type bubble-generators arraying in the plane perpendicular to mercury flow direction, was invented. The multi-swirl type bubble-generator was tested in mercury and the geometry was optimized to generate small bubble with low flow resistance based on the test results. It is estimated to generate the microbubbles of 65 μm in radius under the operational condition of the Japanese Spallation Neutron Source mercury target, which is the sufficient size to suppress the pressure waves.

Very High Cycle Fatigue in Pulsed High Power Spallation Neutron Source

Very high cycle fatigue degradation of type 316L austenitic stainless steel, which is used as the structural material of neutron spallation sources under intensive neutron irradiation environment, is investigated by using an ultrasonic fatigue testing machine. The strain rate imposed on the structure of neutron spallation source is almost equivalent to that produced in the testing machine. The temperature on the surface was controlled by the air-cooling. The effect of strain rate on the fatigue strength is recognized to increase the fatigue limit.

Pitting Damage and Residual Stress Induced by Cavitation Erosion on Mercury Target Vessel

Cavitation damage that might be imposed on the mercury target vessel of a pulsed spallation neutron source was evaluated from the standpoints of residual stress and plastic deformation. The residual stress distribution and plastic region of a type-316L austenitic plate, called 316SS, and that with 20% cold rolling, called 316CW, were measured using an X-ray diffraction technique. As a result of the peak width distribution in each specimen with the cavitation damage, the plastic region in the 316SS was deeper than that in the 316CW. In addition, the internal compressive residual stress of the 316SS was higher than that of the 316CW. The distributions of plastic strain and residual stress affect the crack propagation from the bottom of the pits. Taking into account the energy balance in each specimen subjected to the cavitation damage based on the distributions of plastic strain and residual stress, the difference in the fatigue limit degradation between 316SS and 316CW was explained.

Microbubble Formation at a Nozzle in Liquid Mercury

A liquid mercury target for MW class pulsed neutron sources is being developed in the Japan Atomic Energy Agency (JAEA). Cavitation will be induced by pressure waves that are caused by highly intense proton beam injection into the mercury target. Microbubbles 50 to 200 mm in diameter injected into the mercury target are plausibly effective for mitigating cavitation. The mitigation is dependent on the conditions of the injected bubble size and population. It is, therefore, important to understand bubble formation behavior in mercury in order to develop a microbubble injection method. Computational fluid dynamics (CFD) simulations were carried out under various mercury and gas flow rates to investigate the bubble formation behavior in mercury. Moreover, bubbles in stagnant mercury were visualized with X-ray to observe the formation behavior of bubbles at a micro-gas nozzle and compared with the simulation results. It was found that high surface tension makes the bubble grow around the outer surface of the nozzle under the stagnant condition and makes it larger until its effect decreases in the flow. In addition, the bubble diameter under the stagnant condition increases with increasing contact angle.