Mitigation technologies for damage induced by pressure waves in high-power mercury spallation neutron sources, 1; Material surface improvement

Naoe, Takashi   ; Futakawa, Masatoshi  ; Shobu, Takahisa  ; Wakui, Takashi  ; Kogawa, Hiroyuki  ; Takeuchi, Hirotsugu*; Kawai, Masayoshi*

Liquid-mercury target systems for MW-class spallation neutron sources are being developed in the world. At the moment the proton beam hits the target, pressure waves are generated in the mercury because of the abrupt heat deposition. The pressure waves interact with the target vessel leading to negative pressure that may cause cavitation along the vessel wall. Localized impacts by micro-jets and/or shock waves which are caused by cavitation bubble collapse impose pitting damage on the vessel wall. Bubble collapse behavior was observed by using a high-speed video camera, as well as simulated numerically. Localized impact due to cavitation bubble collapse was quantitatively estimated through comparison between numerical simulation and experiment. A novel surface treatment technique which consists of carburizing and nitriding processes was developed and the treatment condition was optimized to mitigate the pitting damage due to localized impacts.