Hiroyuki Kogawa

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.

R & D on mercury target pitting issue

Abstract A technical issue in mercury spallation target development is pitting, which appears on the target vessel in conjunction with the pressure wave. Pitting has been found in off-beam line test by split Hopkinson pressure bar (SHPB) test as well as in the on-beam test of mercury target at WNR of LANSCE. In SHPB tests pressure in mercury was reduced from 80 to 40, 20 and 10 MPa. Specimens made of type 316 stainless steel were inspected before and after the impacting test at ×450 magnification. Results show that over 20 MPa pitting was generated. But at the lowest pressure in mercury, the number of pits was very limited and substantial damage was small. Substantial damage by pitting is characterized by holes where mass is removed from the wall. Depression itself may not be a substantial damage as long as it is not accompanied by holes.

Bubble dynamics in the thermal shock problem of the liquid metal target

The thermal shock stress in a mercury target vessel was analyzed. The target receives the incident proton beam at an energy of 1 MW with a pulse duration of 1 μs. A negative pressure of 61 MPa was generated following the dispersion of the compression field at 52 MPa which was generated by the proton beam injection. It is expected that cavitation may be caused by the negative pressure. In order to evaluate the cavitation behavior and the following material damage mechanism, a simulation study was carried out using the equation of motion based on bubble dynamics for a single bubble, and fundamental parameter analysis was carried out. It is found that a bubble has a volume expansion of more than 1000 times with a change of the pressure at the window of the target vessel. Consequently wave propagation will be affected. Theoretical consideration was given to the wave motion of propagation in a bubbly liquid. The equation of state in a bubbly liquid can be approximated by polynomials. The diameter of a bubble and the bubble volume fraction inherent in mercury can be estimated if the critical pressure, the sound velocity, and resonance frequency are measured by static and dynamic experiments.

Failure Probability Estimation of Multi-walled Vessels for Mercury Target

A liquid mercury target for MW-class pulse spallation neutron sources is being developed. Failure probability analyses on the mercury target that will be installed at the material and life science facility in the Japan Proton Accelerator Research Complex (J-PARC) were carried out taking account of the stress condition caused by various types of loading and material degradation due to neutron irradiation and pitting damages. The mercury target consists of multi-walled vessels; a mercury vessel and a safety hull, to prevent mercury leakage to the outside, i.e., the mercury vessel that is in contact with mercury is covered by the safety hull. The failure probability of the safety hull was estimated to be lower than 10−6 for the expected designed lifetime. On the other hand, the failure probability of the mercury vessel directly subjected to thermally shocked pressure waves in mercury increased with the operation time and the protonbeam power, and was estimated to be ca. 99.9% for the designed lifetime of the MW-class target.

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.

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.

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.

Inertia effect on thermal shock by laser beam shot

A stress wave generates and propagates in a beam window, a target container and solid target plates of the target system for neutron spallation source because of a thermal shock induced by an injection of a high-intensity proton beam. In this study, a thermal shock experiment was carried out using a ruby laser to investigate the behavior of the stress wave propagation. Numerical analyses on the stress waves induced by the thermal shock were carried out. It was confirmed that tensile stress waves were generated at the border of the heated area because of the inertia effect of the propagation of compressive stresses to outside of the heated area associated with the thermal shock. Subsequently the tensile stress waves concentrate at the center of the non-heated surface to become large. These trends are adequately supported by the numerical results.