Hands-on maintenance for the target diagnostic system on top shielding of spallation neutron source at J-PARC

Naoe, Takashi   ; Saruta, Koichi   ; Teshigawara, Makoto   ; Kinoshita, Hidetaka   ; Wakui, Takashi  

At the pulsed spallation neutron source J-PARC, a liquid mercury target, three types of cryogenic liquid hydrogen moderators and a reflector are placed with a shielding inside a helium-filled vessel. In October, 2023, we accessed on the top surface of the shielding in the helium vessel, where 3 mm upper from target, and replaced a target diagnostic system for monitoring the beam induced acoustic vibration. To accomplish the hands-on-work safely, we took following safety measures: installation of a greenhouse with a local exhaust upon the helium vessel, define the upper limit of suspend work for dose rate (100 Sv/h) and tritium concentration in air (0.1 Bq/cm) in green house, wear air-line aspirator and anorak to prevent internal exposure, place fall prevention rope in addition to wear safety belt to eliminate the risk of fall from shield of 5m in height. The result showed that the tritium concentration in air in the greenhouse was successfully reduced to the background level after 4 hours of ventilation, and dose rate of work area was 40 Sv/h. Therefore, the workers completed hands-on-maintenance without any detectable internal exposure.