Joint European Torus (JET) has been planning a tritium operation sequence for which some of the safety analyses need to be repeated. In this framework, three different scenarios of Loss of Vacuum Accident (LoVA) require inventory release assessment. The starting event for all investigated scenarios is a local failure of the vacuum system integrity that results in an initial ingress of the surrounding air and a later release of the system inventory.
The heat transfer between the hot torus walls and the inflowing air determines the extent of the later release when the air heats up and the pressure in the torus increases above the ambient pressure. This thermal effect was significantly underestimated in the initial safety study.
A transient CFD analysis was conducted for one of the cases with a nominal failure size of 50 mm in diameter. The torus is initially under ultra-high vacuum (below 1.0e-8 mbar) with a uniform temperature of the inventory and the walls of 320°C. The numerical simulation was conducted for a time interval of 60 s. From the start, the simulation timestep has to be kept very small due to a large pressure difference between the ambient and the torus interior that initiates a hypersonic jet.

During the simulation, the heat transfer from the wall to the air flow was monitored. The obtained heat transfer coefficient was later used in a MELCOR model of the JET torus to calculate the extent and behaviour of the released inventory.