Hidetaka Kinoshita

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.

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.

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.