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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.
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.
Haga, Katsuhiro; Kogawa, Hiroyuki; Wakui, Takashi; Naoe, Takashi; Wakai, Eiichi; Takada, Hiroshi
no journal, ,
At the Japan Spallation Neutron Source (JSNS), there was trouble twice at the water shroud of the mercury target due to the thermal stress during operating periods with proton beam power at 500 kW in 2015. The target vessel, made from 316L stainless steel, has a triple-walled structure consisting of mercury vessel, inner and outer water shrouds, where the interstitial space between the mercury vessel and the water shroud is filled with helium gas. For the first trouble, the leak was to the outside of the shroud and the location was specified at a welded portion around a bolt connection. For the second trouble, the leak was detected in the helium layer. Results of FEM simulations suggested that the location was susceptible to fatigue cracking due to the cyclic thermal stress induced by beam trips. The cause of the troubles was attributed to the complicated structural design in which incompleteness remained at welding. The next target vessel design was improved to reduce welding lines ca. 30% and bolt connection as much as possible.
Matsuda, Hiroki; Meigo, Shinichiro; Iwamoto, Hiroki
no journal, ,
no abstracts in English
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.
Wakui, Takashi; Saito, Shigeru; Wakai, Eiichi; Sakai, Tomoki*; Mori, Kotaro*; Futakawa, Masatoshi
no journal, ,
One of dominant factors to determine the lifetime of the mercury target in J-PARC is the radiation damage. Authors suggested the tensile properties evaluation technique from numerical tensile tests using material properties estimated from inverse analyses on indentation tests. The technique was applied to ion-irradiated materials, and the validity of the technique was investigated by comparing the result with results of the PIE on the targets of SNS. By conducting indentation tests on samples cut out from used targets, it is expected that the residual lifetime estimation can be conducted considering various effects; fatigue, temperature, LME, etc. superimposed on the radiation damage from evaluating hardness and tensile properties obtained by the technique. The technique and comparison results 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.
Murata, Atsushi*; Saruta, Koichi; Wakui, Takashi; Li, Y.*; Tsutsui, Kihei*; Futakawa, Masatoshi
no journal, ,
Vibration and sound have been used in diagnosis of mechanical structures in a variety of industries. Under high radiation environments, however, conventional electric sensors such as an accelerometer and a microphone are not suitable especially for in-situ diagnostic systems in nuclear reactors, spallation neutron source systems, etc. since electric signals become degraded by background radiation noise. In this study, the applicability of the optical sound measurement using an laser Doppler vibrometer was investigated through the comparison between fundamental experimental results and 3D FEM analyses (LS-DYNA) for the purpose of developing an in-situ diagnostic system of a mercury target.
