Hydrogen Detonation

DOE-HDBK-6004-99

Hydrogen Detonation

A hydrogen detonation is a potential hazard which may occur as part of a design basis accident

(typical probability > 10-6 per year). If it does occur and the vacuum vessel is a confinement or

containment barrier, then the required integrity of the barrier must be maintained during and after this

event, although the non-safety-related functions of the vacuum vessel (such as ability to maintain high

vacuum) can be compromised. If the vacuum vessel is not a confinement or containment barrier and

a hydrogen detonation is credible, it must be shown that no failure of a vacuum vessel component due

to this event can degrade the function of an adjacent safety-class system or item.

To determine if a potential hydrogen detonation can occur in a design basis accident, it is necessary

to evaluate the likelihood of having the three ingredients for detonation at the same time: hydrogen

and oxygen in the appropriate mixtures and an ignition source (NUREG/CR-4961). Generally, direct

initiation of hydrogen-air mixtures is possible with about 1 gram of high explosive (NUREG/CR-

4961) (this is equivalent to about 4 kJ of energy). Since the plasma typically contains much higher

levels of stored energy, it should be assumed that a point ignition source is always present during

normal operations and wall conditioning. The factors determining the likelihood of a detonation are

then the availability of hydrogen isotopes and air. Hydrogen isotopes are present in the solid matrix

of the plasma facing components at substantial levels. This is not ordinarily available for combustion

or detonation although a portion (including tritium) may be released if a detonation occurs. If hot

plasma facing components or the vacuum vessel are cooled with water, a leak could result in the

generation of hydrogen from water (steam)-Be (or C or W) reactions (Smolik 92, Smolik 91). The

precise amount of hydrogen generated depends on the first wall material and temperature and the size

and duration of the water leak but typical conditions in a D-T fusion plasma can generate sufficient

quantities of hydrogen for a detonation. Air (oxygen) also has to be present for a detonation. If air

is adjacent to the vacuum vessel, the in-leakage of air is possible due to the same event which

generated the hydrogen. For example, Be-steam reactions from a water leak during wall conditioning

can result in internal pressures of several bar or more (NET 93), which may be beyond the design

value of the vacuum vessel. This air source can be eliminated in the device design by incorporating

an inert gas volume in the region between the vacuum vessel and its ducts, and the next confinement

barrier. To determine the probability of a hydrogen detonation, a conservative analysis of the above

factors must be performed for a particular design. The likelihood of an in-vessel loss-of-coolant

accident cannot be generally excluded given performance of such actively-cooled systems to date and

the anticipated service conditions in a D-T fusion vacuum vessel.

To preclude a hydrogen detonation for consideration as a design basis accident, it will typically be

necessary to demonstrate a low event probability by:

1. Material selection in the plasma facing components or the fluids used for active in-vessel

component cooling, or

2. Use of an inert gas boundary as discussed above.