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JAEA Reports

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.

JAEA Reports

Technical note for the cavitation damage inspection for interior surface of the mercury target vessel, 1; Development of specimen cutting machine for remote handling

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

JAEA-Technology 2022-018, 43 Pages, 2022/08

JAEA-Technology-2022-018.pdf:7.84MB

In the liquid mercury target system for the pulsed spallation neutron source of Materials and Life science experimental Facility (MLF) at the Japan in the Japan Proton Accelerator Research Complex (J-PARC), cavitation that is generated by the high-energy proton beam-induced pressure waves, resulting severe erosion damage on the interior surface of the mercury target vessel. The erosion damage is increased with increasing the proton beam power, and has the possibility to cause the leakage of mercury by the penetrated damage and/or the fatigue failure originated from erosion pits during operation. To achieve the long term stable operation under high-power proton beam, the mitigation technologies for cavitation erosion consisting of surface modification on the vessel interior surface, helium gas microbubble injection, double-walled beam window structure has been applied. The damage on interior surface of the vessel is never observed during the beam operation. Therefore, after the target operation term ends, we have cut out specimen from the target nose of the target vessel to inspect damaged surface in detail for verification of the cavitation damage mitigation technologies and lifetime estimation. We have developed the techniques of specimen cutting out by remote handling under high-radiation environment. Cutting method was gradually updated based on experience in actual cutting for the used target vessel. In this report, techniques of specimen cutting out for the beam entrance portion of the target vessel in high-radiation environment and overview of the results of specimen cutting from actual target vessels are described.

Journal Articles

Development of the high-power spallation neutron target of J-PARC

Haga, Katsuhiro; Kogawa, Hiroyuki; Naoe, Takashi; Wakui, Takashi; Wakai, Eiichi; Futakawa, Masatoshi

Proceedings of 19th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-19) (Internet), 13 Pages, 2022/03

The cross-flow type target was developed as the basic design of mercury target in J-PARC, and the design has been improved to realize the MW-class pulsed spallation neutron source. When the high-power and short-pulsed proton beam is injected into the mercury target, pressure waves are generated in mercury by rapid heat generation. The pressure waves induce the cavitation damages on the target vessel. Two countermeasures were adopted, namely, the injection of microbubbles into mercury and the double walled structure at the beam window. The bubble generator was installed in the target vessel to absorb the volume inflation of mercury and mitigate the pressure waves. Also, the double walled target vessel was designed to suppress the cavitation damage by the large velocity gradient of rapid mercury flow in the narrow channel of double wall. Finally, we could attain 1 MW beam operation with the duration time of 36.5 hours in 2020, and achieved the long term stable operation with 740 kW from April in 2021. This report shows the technical development of the high-power mercury target vessel in view of thermal hydraulics to attain 1 MW operation.

Journal Articles

Effect of gas microbubble injection and narrow channel structure on cavitation damage in mercury target vessel

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

Materials Science Forum, 1024, p.111 - 120, 2021/03

The mercury target vessel for the at the J-PARC neutron source is severely damaged by the cavitation caused by proton beam-induced pressure waves in mercury. To mitigate the cavitation damage, we adopted a double-walled structure with a narrow channel for the mercury at the beam window of the vessel. In addition, gas microbubbles were injected into the mercury to suppress the pressure waves. The front end of the vessel was cut out to inspect the effect of the damage mitigation technologies on the interior surface. The results showed that the double-walled target facing the mercury with gas microbubbles operating at 1812 MWh for an average power of 434 kW had equivalent damage to the single-walled target without microbubbles operating 1048 MWh for average power of 181 kW. The erosion depth due to cavitation in the narrow channel was clearly smaller than it was on the wall facing the bubbling mercury

Journal Articles

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

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.

Journal Articles

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

 Times Cited Count:1 Percentile:26.45(Instruments & Instrumentation)

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.

Journal Articles

Conceptual design of an abnormality sign determination system for the general control system of the Materials and Life Science Experimental Facility at J-PARC

Sakai, Kenji; Oi, Motoki; Teshigawara, Makoto; Naoe, Takashi; Haga, Katsuhiro; Watanabe, Akihiko*

Journal of Neutron Research, 22(2-3), p.337 - 343, 2020/10

For operating a spallation neutron source and a muon target safely and efficiently, a general control system (GCS) operates within Materials and Life Science Experimental Facility (MLF). GCS administers operation and interlock processes of many instruments under various operation status. Since the first beam injection in 2008, it has operated stably without any serious troubles for more than ten years. GCS has a data storage server storing operational data on status around target stations. It has functioned well to detect and investigate unusual situations by checking data in this server. For continuing stable operation of MLF in future, however, introduction of abnormality sign determination system (ASDS) will be necessary for picking up potential abnormalities of target stations caused by radiation damages, time-related deterioration and so on. It will judge abnormalities from slight state transitions of target stations based on analysis with various operational data throughout proton beams, target stations, and secondary beams during long-term operations. This report mentions present status of GCS, conceptual design of ASDS, and installation of an integral data storage server which can deal with various data for ASDS integrally.

Journal Articles

Water leakage due to the welding defect and improvement to reach 1-MW beam operation in the mercury target of J-PARC

Kogawa, Hiroyuki; Wakui, Takashi; Naoe, Takashi; Haga, Katsuhiro; Takada, Hiroshi; Futakawa, Masatoshi

Journal of Nuclear Science and Technology, 57(5), p.487 - 494, 2020/05

 Times Cited Count:1 Percentile:12.47(Nuclear Science & Technology)

Neutron flux per pulse reached world record at neutron source in the J-PARC. In the J-PARC, mercury target system is used as a spallation neutron source. A target vessel has the multi-walled protection system that comprises a mercury vessel enclosed with a double-walled water shroud. This is to prevent the leakage of the mercury outside the mercury vessel. The multi-walled structure needed to be complicated with a lot of welding lines. However, during the operation, we faced an unscheduled shutdown due to water leakage to the intermediate layer between the mercury vessel and water shroud. An investigation on the cause of the leakage was carried out. It is deduced that the leakage path was formed due to the crack propagation from welding defects that is caused by the complicated multi-walled structure. The crack propagation is attributed to the repeated stress by pressure waves generated in the mercury target. Based on the investigation results, the design was improved to remove the welding line on the complicated structure and to realize the stable operation with 1 MW proton beam.

Journal Articles

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

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 ($$sim$$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.

Journal Articles

Change in mechanical properties by high-cycle loading up to Gigacycle for 316L stainless steel

Naoe, Takashi; Harjo, S.; Kawasaki, Takuro; Xiong, Z.*; Futakawa, Masatoshi

JPS Conference Proceedings (Internet), 28, p.061009_1 - 061009_6, 2020/02

At the J-PARC, a mercury target vessel made of 316L SS suffers proton and neutron radiation environment. The target vessel also suffers cyclic impact stress caused by the proton beam-induced pressure waves. The vessel suffers higher than 4.5$$times$$10$$^8$$ cyclic loading during the expected service life of 5000 h. We have investigated fatigue strength 316L SS up to gigacycle in the previous studies. The cyclic hardening and softening behavior were observed. In this study, to evaluate the cyclic hardening/softening behavior, the dislocation densities of specimens were measured using the neutron diffraction method at the MLF BL-19. The result showed that the dislocation density of a 316L SS was increased with increasing the number of loading cycles. By contrast, in the case of cold-rolled 316L SS, annihilation and re-accumulation of dislocation by cyclic loading were observed. In the workshop, result of neutron diffraction measurement will be introduced with the progress of fatigue test.

Journal Articles

New design and fabrication technology applied in mercury target vessel #8 of J-PARC

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

JPS Conference Proceedings (Internet), 28, p.081002_1 - 081002_6, 2020/02

A mercury target vessel of J-PRAC is designed with a triple-walled structure consisting of the mercury vessel and a double-walled water shroud with internal and external vessels. During the beam operation at 500 kW in 2015, small water leakages from a water shroud of the mercury target vessel occurred twice. Design, fabrication and inspection processes were improved based on the lessons learned from the target failures. The total length of welding lines at the front of the mercury target vessel decreases drastically to approximately 55% by adopting monolithic structure cut out from a block of stainless steel by the wire-electrical discharge machining. Thorough testing of welds by radiographic testing and ultrasonic testing was conducted. The fabrication of the mercury target vessel #8 was finished on September 2017 and the beam operation using it started. Stable beam operation at 500 kW has been achieved and it could experience the maximum beam power of 1 MW during a beam test.

Journal Articles

Pulsed pressure induced cavitation erosion in mercury narrow channel under flowing conditions

Naoe, Takashi; Kogawa, Hiroyuki; Tanaka, Nobuatsu*; Futakawa, Masatoshi

Advanced Experimental Mechanics, 4, p.17 - 21, 2019/08

We have introduced the following two techniques to mitigate the pressure wave-induced cavitation damage in the mercury target. One is the gas microbubble injection into the flowing mercury, and the other is the double-walled structure with a narrow gap channel at the proton beam entrance portion of the mercury vessel. The latter is expected to mitigate the cavitation damage due to the high-speed liquid flow ($$sim$$ 4 m/s) and the narrow gap boundary (2 mm). To quantitatively investigate the effect of double-walled structure on cavitation damage, cavitation damage tests were conducted by parametrically changing mercury flow velocity and gap width of the channel wall. The results showed that the damage evaluated as a surface roughness was reduced by increasing the flow velocity. By contrast, the effect of gap width on cavitation damage was hardly observed under flowing conditions.

Journal Articles

Evaluation growing and collapsing behaviors of cavitation bubbles under flowing condition

Kawamura, Shunsuke; Naoe, Takashi; Ikeda, Tsubasa*; Tanaka, Nobuatsu*; Futakawa, Masatoshi

Advanced Experimental Mechanics, 4, p.33 - 37, 2019/08

A mercury enclosure vessel made of stainless steel is used as a spallation target in the pulsed spallation neutron source at J-PARC. It is severely damaged by the cavitation induced with pressure waves in association with the pulsed proton beam injection. A double-walled structure with a narrow mercury channel was adopted in the front end of the target vessel to reduce the cavitation damage. It has been experimentally demonstrated that the cavitation damage could be mitigated in the narrow channel but its mechanism has been unclarified yet. In this study, we investigated the cavitation from growing to collapsing through visualizing the spark-induced cavitation bubbles under flow field using a high-speed video camera. Furthermore, we measured the wall vibration due to the cavitation bubble collapse with changing flow velocity parametrically. It was found that the microjet collided perpendicular to the wall in the stagnant flow condition while it collided with an inclined angle from the perpendicular direction, suggesting that the collision pressure on the wall was reduced by flowing.

Journal Articles

Optimum temperature for HIP bonding invar alloy and stainless steel

Wakui, Takashi; Ishii, Hideaki*; Naoe, Takashi; Kogawa, Hiroyuki; Haga, Katsuhiro; Wakai, Eiichi; Takada, Hiroshi; Futakawa, Masatoshi

Materials Transactions, 60(6), p.1026 - 1033, 2019/06

 Times Cited Count:3 Percentile:18.28(Materials Science, Multidisciplinary)

The mercury target has large size as 1.3$$times$$1.3$$times$$2.5 m$$^{3}$$. In view of reducing the amount of wastes, we studied the structure so that the fore part could be separated. The flange is required to have high seal performance less than 1$$times$$10$$^{-6}$$ Pa m$$^{3}$$/s. Invar with low thermal expansion is a candidate. Due to its low stiffness, however, the flange may deform when it is fastened by bolts. Practically invar is reinforced with stainless steel where all interface between them has to be bonded completely with the HIP bonding. In this study, we made specimens at four temperatures and conducted tensile tests. The specimen bonded at 973 K had little diffusion layer, and so fractured at the interface. The tensile strength reduced with increasing the temperature, and the reduced amount was about 10% at 1473 K. The analyzed residual stresses near the interface increased by 50% at maximum. Then, we concluded that the optimum temperature was 1173 K.

Journal Articles

Development on laser cutting technique to suppress spatter particles aiming at disposal of radio-active waste

Naoe, Takashi; Teshigawara, Makoto; Futakawa, Masatoshi; Mizutani, Haruki; Muramatsu, Toshiharu; Yamada, Tomonori; Ushitsuka, Yuji*; Tanaka, Nobuatsu*; Yamasaki, Kazuhiko*

Proceedings of 8th International Congress on Laser Advanced Materials Processing (LAMP 2019) (Internet), 5 Pages, 2019/05

Laser cutting is one of the options in the disposal of radio-active waste, such as spallation neutron target vessel in J-PARC, etc. Due to unique characteristic of laser, such as non-contact system, it is more easily to provide remote-controlled system in comparison with conventional one, such as mechanical cutting machine, etc. However, a demerit of laser cutting is the sputter and fume caused by laser cutting, resulting in contamination with radio-active materials its surroundings. Recently it was developed that the spatter suppression technique by controlling laser beam profile in laser welding process. In order to apply this suppression technique to laser cutting, first of all, we attempted to observe the phenomenon at melting area during laser cutting using a high-speed video camera in order to make the physical model. The result showed that the appearance of fume and sputter were independently confirmed in the time evolution.

Journal Articles

Effect of artificial defects on the very high cycle fatigue behavior of 316L stainless steel

Xiong, Z.*; Naoe, Takashi; Futakawa, Masatoshi

Metals, 9(4), p.412_1 - 412_11, 2019/04

AA2019-0011.pdf:2.4MB

 Times Cited Count:8 Percentile:43.48(Materials Science, Multidisciplinary)

The effect of surface defects on the very high cycle fatigue (VHCF) behavior were investigated on the solution annealed (SA) and cold-rolled (CW) 316L. Surface defects were artificially created using indentation. VHCF test was conducted using an ultrasonic fatigue method. The results showed that the fatigue crack initiation was independent of the indent with the applied range of depth in this research. Furthermore, the critical depth of the indent was evaluated based on an empirical formula. In the case of SA, the VHCF strength was not affected when the indent depth was less than 40 $$mu$$m, which is consistent with the value obtained from the empirical formula. In the case of 20% CW, VHCF strength was not affected when the indent depth was less than 80 $$mu$$m. The results were much larger than the results obtained from the empirical formula and might have been caused by the plastic deformation, residual stress and probable deformation induced martensite transition around the indent.

Journal Articles

Numerical study on the potential of cavitation damage in a lead-bismuth eutectic spallation target

Wan, T.; Naoe, Takashi; Kogawa, Hiroyuki; Futakawa, Masatoshi; Obayashi, Hironari; Sasa, Toshinobu

Materials, 12(4), p.681_1 - 681_15, 2019/02

 Times Cited Count:2 Percentile:18.28(Chemistry, Physical)

Journal Articles

Ultrasonic inspection technique for weld part of mercury target vessel for spallation neutron source

Wakui, Takashi; Wakai, Eiichi; Naoe, Takashi; Kogawa, Hiroyuki; Haga, Katsuhiro; Takada, Hiroshi; Shintaku, Yohei*; Li, T.*; Kanomata, Kenichi*

Choompa Techno, 30(5), p.16 - 20, 2018/10

A mercury target vessel has been used for the spallation neutron source at J-PARC. It has a complicated multi-layered structure composed of a mercury target and a surrounding double-walled water shroud, which is assembled with thin plates (minimum thickness of 3 mm) by welding. Thus, welding inspection during the manufacturing process is important. We investigated the applicability of new ultrasonic inspections using specimens (thickness of 3 mm) with defects to improve the accuracy of welding inspection for the mercury target vessel. Immersion ultrasonic testing using a probe (frequency of 50 MHz) could detect a spherical defect with a diameter of 0.2 mm. The size was smaller than target value of 0.4 mm. The length of unwelded region estimated using the phased array ultrasonic testing corresponded with the actual length (0.8 - 1.5 mm).

Journal Articles

Effect of welding on gigacycle fatigue strength of austenitic stainless steels

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

Advanced Experimental Mechanics, 3, p.123 - 128, 2018/08

A mercury target vessel, composed of SUS316L, is used for the pulsed neutron source and is assembled via TIG welding. While in operation, the target vessel suffers ca. 10$$^9$$ loading cycles with a high strain rate of ca. 50 s$$^{-1}$$ because of the proton-beam-induced pressure waves in mercury. The gigacycle fatigue strength for solution annealed SUS316L stainless steels and its welded specimens were investigated through ultrasonic fatigue tests. The experimental results showed that an obvious fatigue limit was not observed at fewer than 10$$^9$$ cycles for the base metal. In the case of no weld defects observed via penetration tests, the fatigue strength of the removed-weld-bead specimen, in which the weld lines were arranged at the center of the specimen, appeared to be slightly higher than that of the base metal. By contrast, as-welded specimens with the weld bead intact showed apparent degradation of the fatigue strength owing to the stress concentration around the weld toe.

Journal Articles

Temperature measurement for in-situ crack monitoring under high-frequency loading

Naoe, Takashi; Xiong, Z.*; Futakawa, Masatoshi

Journal of Nuclear Materials, 506, p.12 - 18, 2018/08

BB2016-1012.pdf:0.95MB

 Times Cited Count:7 Percentile:58.44(Materials Science, Multidisciplinary)

A mercury target for neutron source (made of 316L SS) suffers not only proton and neutron radiation damage, but also cyclic impact stress caused by pressure waves. In the previous study, we carried out an ultrasonic fatigue test to investigate the gigacycle fatigue strength of 316L SS, concluding that specimen surface temperature rose abruptly more than 300$$^{circ}$$C just before failure. In this study, to clarify the mechanism of the temperature rise, we measured temperature distribution with a thermography during the fatigue test. The experimental results showed that the temperature rose locally only at the crack tip and the peak position moved with the crack propagation. We also carried out a nonlinear structural analysis by LS-DYNA to estimate the temperature rise with strain energy of elements. The analytical result showed that the heat due to plastic deformation at the crack tip is dominant for the temperature rise rather than the friction between crack surface.

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