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Kogawa, Hiroyuki; Futakawa, Masatoshi; Haga, Katsuhiro; Tsuzuki, Takayuki*; Murai, Tetsuro*
JAEA-Technology 2022-023, 128 Pages, 2022/11
JAEA-Technology-2022-023.pdf:9.0MB
In a mercury target of the J-PARC (Japan Proton Accelerator Research Complex), pulsed proton beams repeatedly bombard the flowing mercury which is confined in a stainless-steel vessel (target vessel). Cavitation damage caused by the propagation of the pressure waves is a factor of the life of the target vessel. As a measure to reduce damages, we developed a bubbler to inject the gas microbubbles into the flowing mercury, which can reduce the pressure waves. To operate the mercury target vessel stably with the 1 MW high-intensity proton beams, further reduction of the damage is required. The bubbler setting position should be closer to the beam window to increase the bubble population, which could enhance the reduction effect on the pressure waves and damage. However, the space at the beam window of the target vessel is restricted. The bubbler design and setting position as well as the vane design for the mercury flowing pattern are optimized by means of a machine learning technique to get more suitable bubble distribution, increasing in bubble population and optimizing bubble size nearby the beam window of the target vessel. The results of CFD analyses performed with 1000 cases were used for machine learning. Since the flow rate of mercury affects the temperature of the target vessel, this was used for the constraint condition. As a result, we found a design of mercury target vessel that can increase the bubble population by ca. 20% higher than the current design.
Haga, Katsuhiro
Kasokuki, 18(4), p.210 - 216, 2022/01
The pulsed spallation neutron source driven by a high-power accelerator is one of the most powerful apparatus to provide high intensity and high quality neutrons with narrow pulse width for conducting cutting-edge researches in several domains of materials and life science. In this system, proton beams of several kW to MW order extracted from the high power accelerator is injected into a target, which is heavy metal, to generate vast amount of neutrons via the spallation reactions with the target nuclei, and slows down these neutrons to thermal to cold neutrons with a moderator and a reflector. Resultant neutron beams are then supplied to a suit of the state-of-the-art experimental devices. In this paper, mechanism to produce neutron beams and outline of the spallation neutron source, engineering design of a target system such as a mercury target, and technical topics to solve the pitting damage problem of the target vessel which is caused by the pressure wave of up to 40MPa at maximum generated in the mercury by the pulsed proton beam injection are reviewed by referring mainly to the mercury target system of the pulsed spallation neutron source at J-PARC.
Takada, Hiroshi
JAEA-Conf 2017-001, p.51 - 56, 2018/01
A pulsed spallation neutron source of Japan Proton Accelerator Research Complex (J-PARC) is aimed at promoting a variety of cutting-edge materials researches at state-of-the-art neutron instruments with neutrons generated by a 3-GeV proton beam with a power of 1-MW at a repetition rate of 25 Hz. In 2015, for the first time it received 1-MW equivalent proton beam pulse, and the beam power for user program was ramped up to 500 kW. The moderator system of the neutron source was optimized to use (1) 100% para-hydrogen for increasing pulse peak intensity with decreasing pulse tail, (2) cylindrical shape with 14 cm diam. 
12 cm long for providing high intensity neutrons to wide neutron extraction angles of 50.8 degrees, (3) neutron absorber made from Ag-In-Cd alloy to make pulse widths narrower and pulse tails lower. As a result, it gives highest intensity pulsed neutrons per incident proton in the world. Towards the goal to achieve the target operation at 1-MW for 5000 h in a year, efforts to mitigate cavitation damages at the target vessel front with injecting gas micro-bubbles into the mercury target are under way. Also, improvement of structural target vessel design is an urgent issue since there was failure twice at the water shroud of the mercury target due to the thermal stress during operating periods at 500 kW in 2015.
Kogawa, Hiroyuki
Jikken Rikigaku, 5(1), P. 64, 2005/03
no abstracts in English
Haga, Katsuhiro; Naoe, Takashi; Wakui, Takashi; Kogawa, Hiroyuki; Kinoshita, Hidetaka; Futakawa, Masatoshi
no journal, ,
For the mercury target of a pulsed spallation neutron source of J-PARC, cavitation damage of the target vessel wall which is caused by the pressure wave in mercury induced by high power pulsed proton beam of 1 MW is the crucial issue. So far, the analytical and experimental studies and the operational experiences of SNS suggest that the rapid mercury flow can mitigate the cavitation damages. In order to include this effect into the target design of J-PARC, we adopted doubled-walled structure to the beam window of the target vessel. The mercury flow channel with a narrow gap of 2 mm was made by adding an inner wall to just inside of the beam window. In order to investigate the mercury flow distribution and flow field, numerical simulations were carried out using the conventional code, ANSYS FLUENT. While the mercury velocity outside of the narrow channel was 1.2 m/s, the mercury velocity in the narrow channel increased to almost 4 m/s, which was promising to suppress the cavitation damages. The effect of the inner wall failure of the narrow channel on the mercury flow was also evaluated. The round hole was created on the inner wall at the center of the beam window. The simulation results and the water experimental results showed that the mercury flow velocity in the narrow channel was almost the same with the case without a hole if the hole diameter is around 10 mm.
Haga, Katsuhiro; Naoe, Takashi; Wakui, Takashi; Kogawa, Hiroyuki; Futakawa, Masatoshi; Kai, Tetsuya; Kinoshita, Hidetaka; Takada, Hiroshi
no journal, ,
The pressure waves are generated in the mercury target of J-PARC by the injection of high power pulsed proton beams and induce cavitation damages on the mercury target wall. The damages are the serious threat factor for the lifetime of the target vessel. After the beam operation of 200 kW corresponding to the accumulated power of 470 MWh, damages with the maximum depth of 0.25 mm were found on the inner surface of the mercury target, and the counter measure to mitigate the damage was recognized to be important. The technique to inject micro-bubbles into the mercury target vessel which has been developed to mitigate the cavitation damages was applied to the next target vessel and the beam operation in the range of 200 kW to 300 kW was continued to 2050 MWh. A specimen was cut out from the target vessel and inspected visually by a remote video camera. No conspicuous damages were found on the specimen. This fact demonstrates the efficacy of the micro-bubble injection to mitigate the cavitation damages. The newly installed target vessel has double wall structure at the beam window as the additional technique for the cavitation damage mitigation. The rapid mercury flow in the narrow channel made by the double wall prevent the cavitation bubble from growing and moderate the severity of the cavitation energy. The efficacy of the double wall structure will be investigated by cutting out the specimen from the target vessel in the future.
Kawamura, Shunsuke; Naoe, Takashi; Ikeda, Tsubasa; Tanaka, Nobuatsu*; Futakawa, Masatoshi
no journal, ,
A target vessel enclosing mercury made of stainless steel is used for the J-PARC spallation neutron source. It is severely damaged by the pressure-wave-induced cavitation with injecting intense proton beam. The front end of the target vessel has a double-walled structure with a narrow channel was adopted to the vessel for expecting to reduce cavitation damage. Effect of cavitation damage mitigation in narrow channel has been experimentally demonstrated. However, damage mitigation mechanism is not clarified yet. As a first step of studies to understand the mechanism of cavitation damage mitigation in narrow channel, growth and collapse behaviors of the spark-induced cavitation bubbles under flow condition were observed by using a high-speed video camera. Furthermore, the wall vibration by cavitation bubble collapse was measured by parametrically changing the flow velocity. The experimental results showed that the ejection angle of the microjet ejected by bubble collapsing leaned towards flowing direction as the flow velocity increases. The wall vibration was reduced with increasing flow velocity.
Kogawa, Hiroyuki; Kawashima, Hiroyuki; Ariyoshi, Gen; Wakui, Takashi; Saruta, Koichi; Naoe, Takashi; Haga, Katsuhiro; Futakawa, Masatoshi; Soyama, Hitoshi*; Kuji, Chieko*; et al.
no journal, ,
In a mercury target system of the J-PARC, an operation injecting microbubbles of helium gas into mercury is carried out to reduce the pressure waves that cause cavitation damage. It was confirmed the damage was mitigated by increasing the injection amount of gas bubbles, while the damage considered to be caused by impact pressure from the gas bubbles was observed. To improve durability, it is necessary to find the optimum bubble condition, and those are also important to evaluate the radiation damage of the vessel material and to develop a diagnosis technology. In this report, as the first report of the series, the outline of the development to improve the durability will be reported with the damage observation results.
Kawashima, Hiroyuki; Kogawa, Hiroyuki; Futakawa, Masatoshi; Soyama, Hitoshi*; Kuji, Chieko*; Tanaka, Nobuatsu*
no journal, ,
In the J-PARC mercury target, helium gas bubbles are injected to mitigate cavitation damage. However, it was recently shown that this injected gas bubbles may cause the damage. In this study, we focused on the local impact force generated when the bubbles collapse, as the first step to propose the optimum condition of the injected gas bubbles. Bubble behavior generated by the underwater spark discharge method near a wall surface was taken by the high-speed camera. At the same time, the vibration on the wall surface due to local impact force was measured with a laser Doppler vibrometer. In addition, bubble behavior was calculated by Keller-Miksis equation. Relationship between local impact force and bubble behavior will be discussed.
Ariyoshi, Gen; Ito, Kei*; Kogawa, Hiroyuki; Futakawa, Masatoshi
no journal, ,
Cavitation damage caused by pressure waves is one of the important issues which threaten the integrity of the mercury spallation target vessel in J-PARC. To mitigate the damage, technology using mercury-helium two-phase flow has been developed. Although effective bubble radius for absorption/attenuation of the waves is evaluated as less than 0.1 mm, actual bubble radius might be different from the evaluated one due to microbubble coalescence phenomena. Therefore, the purpose of present study is to clarify and predict the bubble radius distribution in the target. To achieve that, visualization of microbubble coalescence phenomena was performed by using air-water two-phase flow as a model flow. Obtained experimental results and numerical prediction code presently developed will be explained.
