Technology Status of Hydrogen Road Vehicles

To ease the introduction of a new technology, examining some of the worst-case accidents may be advisable,
and would help overcome a subjective public assessment.
The worst case for LH 2/LNG tanks is a burst releasing all mechanical, chemical, and thermal energy stored.
An experimental series of tests was divided in two parts to test the consequences:
1. LH 2 was abruptly released at 2.5 times the operating pressure using a cutting charge. This gave
information on the fundamental processes involved when LH 2 tanks burst, an obvious conclusion being
that the operating pressure should be kept as low as possible.
2. A fault tree analysis identified the potential real mechanisms for burst (pressure buildup, fire, mechanical
damage), and the consequences were measured.
The pressure buildup tests, such as could happen in a severe road accident, were simulated by destroying the
vacuum insulation and blocking the overflow/safety valves. The buildup gives an initial crack which at critical
length or growth rate can give catastrophic rupture, or which can give leak-before-break. With the minimum
regulatory wall thickness (2 mm) the pressure reached bursting value in 10 min, which was 15 times the
maximum permissible one. The effects of these tests have been already described in Pehr (1994, 1995). With
a 50% thinner wall, the initial crack allowed leak-before-break, effectively avoiding rupture.
The fire tests were made with ignited propane giving >900ºC temperatures over and near the tank surface; the
heat flow inward, heating and evaporating the hydrogen, was as high as 27 kW. The test rig meets the IAEA
requirements for fire tests.
Two tanks were subjected to the fire tests: one with inner and outer vessels of austenitic stainless steel, the
other with outer vessel of the same steel but the inner vessel of cold-tough aluminum alloy. The safety valves
were placed outside the fire zone to prevent premature leak, but the unburned hydrogen was