In some ways a gaseous hydrogen fuel leak is less dangerous than a leak of diesel fuel or gasoline. Leaking diesel fuel and gasoline can puddle and spread over a large area, and the puddles will persist because they evaporate slowly (Amerada Hess, 2001).
Gaseous hydrogen leaks tend to be vertical with only a relatively narrow area/volume in which a flammable mixture exists;9 the hydrogen quickly dissipates in open air to nonhazardous levels.
If designed properly, the most likely location of a major hydrogen leak from a vehicle will be through the PRD, which should vent away from the occupied area of the vehicle. PRDs are designed to vent the entire contents of a hydrogen tank in only a few minutes, after which there is no lingering risk of hydrogen fire or explosion if the release was in the open air. Large hydrogen leaks inside buildings are more dangerous unless the facility has been designed to evacuate the leaked gas and to minimize ignition sources at ceiling level.
Leaking liquid hydrogen can pool and spread, but will quickly evaporate as it is heated by the surrounding air. As it evaporates, the cloud of gaseous hydrogen formed over the spill may move horizontally as it rises and dissipates.
While diesel fuel and gasoline leaks are easily visible and accompanied by a strong characteristic smell, gaseous hydrogen leaks are invisible and odorless (see Table 5). The only indication of a gaseous hydrogen leak may be a whistling noise similar to escape of other high-pressure gases. A liquid hydrogen leak may be accompanied by an area of fog surrounding the leaking hydrogen and/or the formation of frost on the tank or lines in the vicinity of the leak, because the super cold hydrogen cools the surrounding air and causes water vapor to condense.
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