SF6 and water cold trap for TFTR deuterium-tritium operations

The Tokamak Fusion Test Reactor (TFTR) vacuum system is being upgraded to trap SF₆ gas which may leak from the neutral beam injectors during the planned deuterium-tritium experiments. This is necessary since SF₆ can, in 25% of a gram-mole, completely inactivate the platinum catalyst in the tritium cleanup system (TCS). Any SF₆ leaking into the injectors will initially be held on the liquid helium or liquid nitrogen cryopanels. During regens of the liquid helium panel, the deuterium and tritium will pass to the TCS, with the SF₆ being retained on the cryopanels. In the final phase of the accident recovery scenario, the cryopanels will be warmed to room temperature and the SF₆ collected by a liquid-nitrogen cooled cold trap. Design of the cold trap and measurements of the vapor pressure of SF₆ at 77 K as a function of surface coverage are presented. The SF₆ vapor pressure at 77 K, with only a few monolayers absorbed, was found to be <10^-7 torr. Modest pressures of ~10^-5 torr were measured after nearly 3×10⁵ torr-1 were deposited on a cold trap with an area of 0.1 m². In the final configuration, the cold trap will also be used to collect water which may have accidentally leaked into the vacuum system