ITER will use in its fuel cycle much larger quantities of hydrogen isotopes than previous fusion experiments. Most of the hydrogen isotopes are processed in the Tritium Plant for the purpose of fuel recovery, purification and storage before reinjection into the torus. There are many industrial plants and laboratories which process hydrogen in quantities comparable to ITER and measures against hydrogen explosion have already been established and applied widely. For ITER, however, the additional radiological hazard of tritium and activation products is an extra safety concern. It is therefore necessary to both ensure adequate measures are included in the design to prevent explosions and assess hypothetical consequences to demonstrate the safety margins of ITER.

The main focus of the paper is on quantitative assessment of hydrogen and tritium hazards of the tritium plant. Standard industrial safety guidelines for ventilation requirements to prevent the accumulation of leaking hydrogen to a flammable mixture are reviewed and applied to the present ITER design taking into account the additional radiological hazard caused by tritium.

The scope covers the tritium plant and its confinement systems such as glove box detritiation systems, room atmosphere detritiation systems, vent detritiation systems, etc.

Basic questions about the amounts and peak concentrations of locally accumulated hydrogen in the vicinity of leaks or the possibility of build up of hydrogen pockets in thermal and gravity fields are addressed. To estimate the local amount of hydrogen in the flammable range of a hydrogen/air mixture, several diffusion and mixing models such as the spreading of gas clouds or gas jets and pool boiling of liquid hydrogen are discussed. Accumulation of hydrogen in ventilated and stagnant atmospheres by gravity and thermal fields are investigated, using models for buoyant plume rise and separation of gas mixtures in gravity fields.

As an example, the amount and rate of hydrogen released during the accidental de-pressurization of the cryogenic distillation column into the secondary confinement is modeled. The corresponding pressurization of the secondary confinement followed by its postulated additional failure with release to room atmosphere is described by estimating the amount and concentration of hydrogen in the room. The potential for damage to the building is assessed.

It is concluded that features to prevent damaging hydrogen explosions are incorporated into the tritium plant and the building design. Furthermore, a preliminary assessment of hypothetical accidents shows that the damage to other structures will be limited and that the ITER safety goals can be met.