Hydrogen Explosion cont'd

DOE-HDBK-3010-94

7.0 Application Examples; Dissolving Operations Examples

The actual reaction rate will drop off from these values fairly quickly, but if the

values were sustained for a minute, the amount of hydrogen generated would range

from 0.6 to 2.2 l. The dissolving chamber and the slab solution vessel have 2 l and

14 l of gas space available while operating. For the slab vessel alone, the bounding

range of hydrogen estimated would yield concentrations of 4.2% and 16% for the

minimum and maximum cases respectively. Therefore, the attainment of explosive

conditions within the operating vessels themselves is a legitimate source of concern.

Given that an explosion in the vessels is considered feasible, the question arises as to

whether the explosion should be considered a deflagration or a detonation? The

vessels represent the main volume for hydrogen gas collection due to their size and

the placement of flame arrestors in the vent piping. Both vessels are vented

enclosures, as opposed to confined; there are no unique or highly energetic ignition

sources available; and the hydrogen generation rate and potential accumulations over

time are fundamentally limited. These factors indicate a detonation is not expected.

The hydraulic diameter for vessels such as the slab dissolver is obtained by

multiplying the ratio of cross-sectional area to perimeter by a factor of 4. When

dissolution is occurring, the controlling dimensions for hydraulic diameter are the

20 cm freeboard in the vessel and the 7.6 cm slab separation in which liquid is held.

The cross-sectional area is 152 cm2 and the perimeter is 55 cm, yielding a hydraulic

radius of 11 cm for a vessel with a length of 91 cm. The corresponding length to

hydraulic diameter ratio is therefore 8.3, which is below the minimum value of 10

necessary to consider transition to detonation (NFPA 68). The ratio for the

dissolving chamber is even less favorable.

The appropriate course for evaluating the accident source term is to consider the event

a deflagration. However, for the purposes of example, both deflagration and

detonation estimates will be made. For the case of a deflagration, the significant

venting area provided for the vessel in the form of an always open vent line and a

large, low-pressure blow-out plug is likely to prevent extensive explosion damage to

the surrounding environment. Between the vessel vent line and the blow-out plug, the

total available venting area is greater than the vapor space cross section, in

accordance with National Fire Protection Association guidelines (NFPA 68).

Accordingly, the vessel itself would be expected to survive. If it did, the release

would occur from the top of the vessel. This would correspond to the conditions of

depressurization of liquid containment via a failure above the liquid level at less than

50 psig previously discussed in the residue dissolution example of section 7.3.3. In

this case, however, little gas absorption in liquid would occur, and the associated

release could be essentially zero depending on the critical freeboard height.

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