Neutron target for high-intensity operation at J-PARC MLF

Haga, Katsuhiro; Naoe, Takashi; Kogawa, Hiroyuki; Wakui, Takashi; Kinoshita, Hidetaka; Harada, Masahide

Proceedings of 16th International Particle Accelerator Conference (IPAC25) (Internet), p.3245 - 3249, 2025/06

https://doi.org/10.18429/JACoW-IPAC2025-FRXD1

In April 2024, the beam power at MLF attained 950 kW for the first time for long term user operation, and the beam power at the 3 GeV rapid cycle synchrotron (RCS) outlet was raised to 1 MW. This accomplishment means that the goal of the stable operation of the neutron source with 1 MW was almost achieved at last, and it's time to go on to the new stage of the neutron source R&D. There are two major challenges for the mercury target in the next stage. One is to attain the long-term operation of a mercury target. The service life of the target vessel is primarily determined by cavitation damage that occurs on the inner surface due to the injection of high-intensity pulsed proton beams. Until now, the vessel has been replaced annually to inspect the extent of the damage. However, based on the damage data obtained during 1 MW high-power operation, it has been determined that the vessel can withstand long-term operation for more than two years. Therefore, a new target vessel, which was replaced in 2024, is scheduled to be used for an extended period through 2027. Furthermore, since there are plans to increase the pulse intensity of the RCS in the future, it will be necessary to develop more effective pitting damage suppression techniques and new target vessels that can withstand even stronger proton beam pulses. In this presentation, the present status of the neutron source of MLF and future operation plans will be shown.

Damage on a solid-liquid interface induced by the dynamical behavior of injected gas bubbles in flowing mercury

Kogawa, Hiroyuki; Wakui, Takashi; Futakawa, Masatoshi

Fluids (Internet), 10(1), p.3_1 - 3_15, 2025/01

https://doi.org/10.3390/fluids10010003

Microbubbles have been applied in various fields. In mercury targets of spallation neutron sources, where cavitation damage is a crucial issue for the life estimation, microbubbles are injected into the mercury to absorb the thermal expansion of the mercury caused by the pulsed proton beam injection and reduce the macroscopic pressure waves, and result in reducing the damage. Recently, when the proton beam power was increased and the amount of injected gas bubbles was increased, unique damage morphologies were observed on solid-liquid interface. Detailed observation and numerical analyses revealed that the microscopic pressure emitted from the gas bubbles contracting is sufficient to form pit damage; i.e. the directions of streak-like defects which are formed by connecting pit damage coincides with the direction of gas bubble trajectories, and the distances between pits was understandable taking the natural period of gas bubble vibration into account. This indicates that the gas microbubbles to reduce the macroscopic pressure waves have potential to be inceptions of the cavitation damage due to the microscopic pressure emitted from gas bubbles. To completely mitigate the damage, we have to consider the two effects of injecting gas bubbles; reducing macroscopic pressure waves and microscopic pressure due to bubble dynamics.

Technical note for the cavitation damage inspection for interior surface of the mercury target vessel, 2; Damage depth measurement for cavitation erosion

Naoe, Takashi; Wakui, Takashi; Kinoshita, Hidetaka; Kogawa, Hiroyuki; Teshigawara, Makoto; Haga, Katsuhiro

JAEA-Technology 2023-022, 81 Pages, 2024/01

JAEA-Technology-2023-022.pdf:9.87MB

In the liquid mercury target system for the pulsed spallation neutron source of Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC), pressure waves that is generated by the high-energy proton beam injection simultaneously with the spallation reaction, resulting severe cavitation erosion damage on the interior surface of the mercury target vessel. Because the bubble of pressure wave-induced cavitation collapsing near the interior surface of the mercury target vessel with applying the large amplitude of localized impact on the surface. Since the wall thickness of the beam entrance portion of the target vessel is designed to be 3 mm to reduce thermal stress due to the internal heating, the erosion damage has the possibility to cause the vessel fatigue failure and mercury leakage originated from erosion pits during operation. To reduce the erosion damage by cavitation, a technique of gas microbubble injection into the mercury for pressure wave mitigation, and double-walled structure of the beam window of the target vessel has been applied. A specimen was cut from the beam window of the used mercury target vessel in order to investigate the effect of the damage mitigation technologies on the vessel, and to reflect the consideration of operation condition for the next target. We have observed cavitation damage on interior surface of the used mercury target vessel by cutting out the disk shape specimens. Damage morphology and depth of damaged surface were evaluated and correlation between the damage depth and operational condition was examined. The result showed that the erosion damage by cavitation is extremely reduced by injecting gas microbubbles and the damage not formed inside narrow channel of the double-walled structure for relatively high-power operated target vessels.

Spattering behavior induced by cavitation bubble collapsing nearby gas-liquid interface

Kawashima, Hiroyuki; Kurosawa, Takuma*; Teshigawara, Makoto; Naoe, Takashi; Tanaka, Nobuatsu*; Futakawa, Masatoshi; Kogawa, Hiroyuki

Advanced Experimental Mechanics, 7, p.43 - 48, 2022/08

https://doi.org/10.11395/aem.7.0_43

Laser cutting is expected for reducing the volume of radioactive components by taking account of radioactive levels. The scattering of radioactive dust such as fume and spatter, which is generated throughout the laser cutting process, gets to be a major issue for practical usage in cutting highly radioactive substances. One of driving forces of the spatter is likely to be attributed to the cavitation bubble collapsing caused by rapid thermal loading due to laser irradiation. In order to elucidate the mechanism of spattering, the technique of spark discharge in water was availed to generate the cavitation bubble. The growth and collapse behaviors influenced by the interaction between a free surface and bubbles resulting the spattering phenomena are observed by using an ultra-high-speed video camera, and the numerical simulation was carried out as well.

The Multiaxial creep-fatigue failure mechanism of Mod. 9Cr-1Mo steel under non-proportional loading; Effect of strain energy on failure lives

Ogawa, Fumio*; Nakayama, Yuta*; Hiyoshi, Noritake*; Hashidate, Ryuta; Wakai, Takashi; Ito, Takamoto*

Transactions of the Indian National Academy of Engineering (Internet), 7(2), p.549 - 564, 2022/06

https://doi.org/10.1007/s41403-021-00279-y

The strain energy-based life evaluation method of Mod. 9Cr-1Mo steel under non-proportional multiaxial creep-fatigue loading is proposed. Inelastic strain energy densities were calculated as the areas inside the hysteresis loops. The effect of mean-stress has been experimentally considered and the relationship between inelastic strain energy densities and creep-fatigue lives was investigated. It was found from the investigation of hysteresis loops, the decrease in maximum stress leads to prolonged failure life, while stress relaxation during strain holding causes strength reduction. The correction method of inelastic strain energy density was proposed considering the effect of maximum stress in hysteresis loop and minimum stress during strain holding, and strain energy densities for uniaxial and non-proportional multiaxial loading were obtained. Based on these results, the mechanisms governing creep-fatigue lives under non-proportional multiaxial loading have been discussed.

Separation/removal of steel surface coating film by laser cleaning

Yamane, Ikumi; Takahashi, Nobuo; Sawayama, Kengo; Nishiwaki, Hiroki; Matsumoto, Takashi; Ogawa, Jumpei; Nomura, Mitsuo; Arima, Tatsumi*

JAEA-Technology 2021-038, 18 Pages, 2022/02

JAEA-Technology-2021-038.pdf:1.61MB

We have dismantled uranium enrichment facilities in Ningyo-toge Environmental Engineering Center since their operation finished in 2001, and the total amount of metallic wastes is estimated to be about 130 thousand tons. Eighty percent of them can be disposed as nonradioactive waste (NR), but there are some steel parts possibly uranium-contaminated. We need removing painted surface of such steels and radiologically surveying to dispose them as NRs. Though painted surfaces have been conventionally removed through hand working with grinders, this manual work requires installation of green house, protective clothing, and full-face mask, in order to prevent dispersion and inhalation of airborne dusts. We desire further developments of surface cleaning techniques to reduce time, cost, workload, and secondary waste generation caused by excessive grinding. Therefore, in this study, we focused on the laser cleaning technology used for the separation and removal of paint films at construction sites. In order to improve the coating separation and removal technology for NR objects, we evaluated the coating separation and removal performance of NR steel surface by laser cleaning system, observed the coating scattering behavior by high-speed camera and investigated the coating recovery method, evaluated the laser separation and removal performance of steel surface powder, and thermodynamically evaluated the uranium compounds on steel surface. We additionally evaluated the feasibility of laser cleaning techniques in our works basing on these results, and discussed future work plans for further developments of laser cleaning techniques.

Benchmark analysis by Beremin model and GTN model in CAF subcommittee

Hirota, Takatoshi*; Nagoshi, Yasuto*; Hojo, Kiminobu*; Okada, Hiroshi*; Takahashi, Akiyuki*; Katsuyama, Jinya; Ueda, Takashi*; Ogawa, Takuya*; Yashirodai, Kenji*; Ohata, Mitsuru*; et al.

Proceedings of ASME 2021 Pressure Vessels and Piping Conference (PVP 2021) (Internet), 9 Pages, 2021/07

New design of high power mercury target vessel of J-PARC

Wakui, Takashi; Wakai, Eiichi; Kogawa, Hiroyuki; Naoe, Takashi; Hanano, Kohei*; Haga, Katsuhiro; Shimada, Tsubasa*; Kanomata, Kenichi*

Materials Science Forum, 1024, p.145 - 150, 2021/03

https://doi.org/10.4028/www.scientific.net/MSF.1024.145

To realize a high beam power operation at the J-PARC, a mercury target vessel covered with water shroud was developed. In the first step, to realize an operation at 500 kW, the basic structure of the initial design was followed and the connection method between the mercury vessel and the water shroud was changed. Additionally, the operation at a beam power of 500 kW was realized in approximately eight months. In the second step, to realize the operation at 1 MW, the new structure in which only rear ends of vessels were connected was investigated. Cooling of the mercury vessel is used to reduce thermal stress and thick vessels of the water shroud are used to increase stiffness for the internal pressure; therefore, it was adopted. The stress in each vessel was lower than the allowable stress based on the pressure vessel code criteria prescribed in the Japan Industrial Standard, and confirmation was obtained that the operation with a beam power of 1 MW could be conducted.

Pressure wave induced sound measurement for diagnosing the operation status of the J-PARC pulsed spallation neutron source

Naoe, Takashi; Kogawa, Hiroyuki; Wakui, Takashi; Teshigawara, Makoto; Haga, Katsuhiro; Futakawa, Masatoshi

Nuclear Instruments and Methods in Physics Research A, 982, p.164566_1 - 164566_6, 2020/12

https://doi.org/10.1016/j.nima.2020.164566

A liquid mercury target for the spallation neutron source is installed in the J-PARC. The liquid mercury is enclosed with the multi-walled stainless steel vessel. At the time of highly intense proton beams hits the target at a repetition rate of 25 Hz, pressure waves, that causes cavitation erosion, are generated owing the rapidly thermal expansion of mercury. We have installed the target diagnostic system consisting of a laser Doppler vibrometer (LDV) and a dynamic microphone to remotely investigate the structural integrity of the target under high-radiation environment. In this study, aiming to understand correlation between the acoustic vibration and the operation conditions such as the proton beam power and beam profile, proton beam induced acoustic vibration was measured by parametrically changing the target operation conditions. The result showed that the sound is well correlated with the operation conditions.

Mitigation of cavitation damage in J-PARC mercury target vessel

Naoe, Takashi; Kinoshita, Hidetaka; Kogawa, Hiroyuki; Wakui, Takashi; Wakai, Eiichi; Haga, Katsuhiro; Takada, Hiroshi

JPS Conference Proceedings (Internet), 28, p.081004_1 - 081004_6, 2020/02

https://doi.org/10.7566/JPSCP.28.081004

The beam window of the mercury target vessel in J-PARC is severely damaged by the cavitation. The cavitation damage is a crucial factor to limit lifetime of the target because it increases with the beam power. Therefore, mitigating cavitation damage is an important issue to operate the target stably for long time at 1 MW. At J-PARC, to mitigate the cavitation damage: gas microbubbles are injected into mercury for suppressing pressure waves, and double-walled structure with a narrow channel of 2 mm in width to form high-speed mercury flow (4m/s) has been adopted. After operation, the beam window was cut to inspect the effect of the cavitation damage mitigation on inner wall. We optimized cutting conditions through the cold cutting tests, succeeding in cutting the target No.2 (without damage mitigation technologies) smoothly in 2017, and target No.8 with damage mitigation technologies. In the workshop, progress of cavitation damage observation for the target vessel will be presented.