Katsuhiro Haga

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.

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.

Experimental study on heat transfer and pressure drop in mercury flow system for spallation neutron source

In the design of MW-class spallation target system, using mercury to produce practical neutron applications, keeping the highest level of safety is vitally important. To establish the safety of spallation target system, it is essential to understand the thermal hydraulic properties of mercury. Through thermal hydraulic experiments using a mercury experimental loop, which flows at the rate of 1.2 m3/hr maximum, the following facts were experimentally confirmed. The wall friction factor was relatively larger than the Blasius correlation due to the effects of wall roughness. The heat transfer coefficients agreed well with the Subbotin correlation. Furthermore, for validation of the design analysis code, thermal hydraulic analyses were conducted by using the STAR-CD code under the same conditions as the experiments. Analytical results showed good agreement with the experimental results, using optimized turbulent Prandtl number and mesh size.

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.

Mitigation technologies for damage induced by pressure waves in high-power mercury spallation neutron sources (IV) – measurement of pressure wave response and microbubble effect on mitigation in mercury target at J-PARC –

ABSTRACT A mercury target system to produce neutron beams has been operated at the spallation neutron source in the Japan Proton Accelerator Research Complex (J-PARC). Pressure waves are generated in mercury by rapid heat generation due to bombardment by high-intensity short-pulse proton beams. The pressure waves not only cause cyclic stress but also induce the cavitation damage on the target vessel. Reduction of these pressure waves is important from the viewpoint of extending the lifetime of the target vessel in future power-up operations. The injection of microbubbles into mercury is effective for reducing pressure waves. Accordingly, a microbubble generator was installed in the mercury target vessel and an in situ diagnostic system that measures the displacement velocity of the target vessel induced by the pressure waves was also set up in J-PARC to investigate the effect of proton beam condition and the effect of the microbubbles. Consequently, we found that the peak displacement velocity of the target vessel decreased owing to microbubble injection. The ratios of the peaks obtained with bubble injection to that without bubble injection were 1/3 and 2/3 when the injected gas fractions were 0.4% and 0.1%, respectively.

Differences and Similarity in the Dynamic and Acoustic Properties of Gas Microbubbles in Liquid Mercury and Water

Differences and similarities in the dynamics of microbubbles in liquid mercury and water are clarified and summarized in order to evaluate the validity and usefulness of experiments with water as an alternative to experiments with mercury. Pressure-wave induced cavitation in liquid mercury is of particular concern in the high-power pulsed neutron sources working in Japan and the U.S. Toward suppressing the pressure waves and cavitation, injection of gas microbubbles into liquid mercury has been attempted. However, many difficulties arise in mercury experiments mainly because liquid mercury is an opaque liquid. Hence we and collaborators have performed water experiments as an alternative, in conjunction with mercury experiments. In this paper, we discussed how we should use the result with water and how we can make the water experiments meaningful. The non-dimensional numbers of bubbly liquids and bubbles’ rise velocity, coalescence frequency, and response to heat input were investigated theoretically for both mercury and water. A suggestion was made to ‘‘see through’’ bubble distribution in flowing mercury from the result of water study, and a notable similarity was found in the effect of bubbles to absorb thermal expansion of the liquids.

Distribution of Microbubble Sizes and Behavior of Large Bubbles in Mercury Flow in a Mockup Target Model of J-PARC

(2010). Distribution of Microbubble Sizes and Behavior of Large Bubbles in Mercury Flow in a Mockup Target Model of J-PARC. Journal of Nuclear Science and Technology: Vol. 47, No. 10, pp. 849-852.

Technical investigation on small water leakage incident occurrence in mercury target of J-PARC

ABSTRACT Japan Proton Accelerator Research Complex experienced failures of two mercury targets, which were Target #5 and #7, in 2015 when the facility was operating with a proton beam power of 500 kW. The failures involved coolant water leak from the water shroud. In this paper, we investigate the root cause of the Target #5 failure. The results of the visual inspections, mockup tests, and analytical evaluations suggested that the water leak was caused by the possible combination of two incidents. One was the diffusion bonding failure due to the large thermal stress induced by welding of the bolt head during the fabrication process, and the other was the thermal fatigue failure of the seal weld due to the repetitive beam shutdown during beam operation. Though the investigation into the root cause of the Target #7 failure is still going on, these target failures point to the importance of eliminating initial defects and the need to secure the rigidity and stability of welded structures. The next mercury target, Target #8, was fabricated with an improved design and fabrication process to reduce the possibility of similar failures. The beam operation of this mercury target is planned to be started in October 2017.

Off-gas processing system operations for mercury target vessel replacement at J-PARC

An off-gas processing system was installed in the J-PARC spallation neutron source to reduce radioactivity of xenon-127 and tritium contained in a helium cover gas in a surge tank of a mercury circulation system to obey the regulation by law. In addition to this role it has been utilized to a purging process before the target vessel replacement and an air-flow control procedure to minimize uncontrollable radioactivity release during the replacement. An standard and urgent model plans of the off-gas processing system operation were established indicating that 31 days were required at least to replace the target vessel.

Thermal Hydraulic Design of a Double-walled Mercury Target Vessel

To mitigate the cavitation damage of the mercury target vessel operating at the spallation neutron source of J-PARC, a double-walled structure for the target vessel was investigated and designed by numerical simulation. It was found that rapid mercury flow in the narrow channel at the beam window and sufficient cooling performance of the target wall were attained. Moreover, the rapid mercury flow might be maintained even in the case of an inner-wall fracture.