Hydrogen hazard in the tritium plant of the ITER fusion reactor

The ITER ventilation system adds air at the top and exhausts it from the bottom of the room to avoid rapid spreading of tritium into the ventilation system. The ITER strategy of double confinement and inertization keeps the probability of hydrogen explosion below design base values; however, for hypothetical hydrogen releases, the rising of hydrogen to the ceiling and spreading in a layer along the ceiling is modeled, to estimate the amount of flammable and detonable hydrogen. In the case of high energetic releases, jet dilution and entrainment of hydrogen from the layer by the air jets of the ventilation system as well as by the turbulence of the room atmosphere, limit the build up of flammable hydrogen to <200 mol. For slow releases e.g. continued pumping after line break, plume dilution and air turbulence are small and the amount of hydrogen build up is up to 750 mol D₂, but for a flow rate more than one order of magnitude higher than the typical ITER flow rate <0.25 mol/sec. Higher flows would lead to a concentration above the detonation limit. Deflagration of the 750 mol leads to an overpressure <0.3 bar, tolerated by the building. The amount of detonable hydrogen stays below 2 mol for the cases considered