The Current state of off-gas processing system at MLF

Masuda, Shiho; Kai, Tetsuya   ; Harada, Masahide   ; Kinoshita, Hidetaka   ; Seki, Masakazu; Takagi, Motonori; Kasugai, Yoshimi  ; Haga, Katsuhiro  

Mercury is used as a target material for spallation neutron source at J-PARC. It is assumed that most of gaseous nuclides (tritium, noble gases and mercury vapor) among spallation products accumulate in the cover gas (helium) of the surge tank in the Mercury Circulation System (MCS). In the mercury target vessel replacement which is carried out every year, cover gas is transferred to the off-gas processing system before deinstallation of the vessel. It became clear from the previous operation that a certain amount of tritium and noble gases remain absorbed on the inner wall of the MCS. It is necessary to suppress release of these nuclides to the hot cell during vessel replacement. Therefore, the MCS was purged by helium several times to reduce the radioactivity concentration in the system. As a result, the purge was effective for the reduction of noble gas, but not for tritium. Therefore, at the time of replacement, air is drawn into the off-gas processing system via the MCS using a vacuum pump, thereby forming a air flow toward the inside (air-flow control) of the MCS at the opening to suppress the release of tritium out of the system. In this report, we will introduce the role of the off-gas processing system in mercury target vessel replacement and describe the efficacy to suppress the noble gas and tritium release, and future plan of system improvement.