A Risk Analysis of the Release of Liquid Hydrogen in Road Tunnels: The Effects of Mechanical Ventilation Combined With Geometric and Traffic Characteristics

The transportation of liquid hydrogen (LH₂) via road tankers could prove to be the most cost-effective short-term option for long-distance delivery. However, there are significant risks, particularly in confined spaces like road tunnels. An accidental release of LH₂ in these structures is likely to create a flammable hydrogen cloud, the explosion of which generates overpressures whose magnitude depends on several mutually dependent variables, including geometry, traffic, and ventilation. Nevertheless, the combined effect of the above-mentioned variables on user safety in the event of an accidental leakage and explosion of LH₂ from a road tanker in a tunnel has yet to be investigated in detail. This study develops 3D CFD models of both the release and explosion of LH₂ to address this issue, along with a comprehensive parametric analysis that considers different tunnel lengths, negative and positive longitudinal slopes, traffic volumes, and ventilation types (i.e., natural or longitudinal mechanical). The CFD code used was preliminarily calibrated against experimental literature tests. Subsequently, a risk analysis was carried out using the CFD results in terms of overpressures, which, combined with a probit function, made it possible to estimate the number of potential fatalities. Consequently, a probability matrix of the risk of having a given number (N) of fatalities was built as a function of the tunnel length, ventilation type (i.e., natural or mechanical), longitudinal slope, and traffic volume. The results revealed the benefits of positive gradients as well as of implementing a longitudinal mechanical ventilation system. In contrast, longer tunnels increase the probability of having a given number of fatalities. This study might serve as a reference for tunnel operators in the choice of mitigation measures and/or traffic control strategies to limit the negative consequences of the release of liquid hydrogen in road tunnels.