Explosion in the fuel cells compartment of a hydrogen bus

A fuel cells and hydrogen city bus was refuelling at a hydrogen refilling station. It had just terminated this operation and the engine was restarted, that an explosion occurred at the back of the vehicle, causing serious injuries to three persons. The explosion was located in the fuel cells compartment, and some source report that it occurred 11 seconds after the restart of the bus. The sources report also that there was no fire. However, the very fuzzy video shared by the sources reveals a considerable development of smoke, which let suspect that some kind of flash fire occurred.
One of the source reported that the incident was the occasion for another municipality to investigate the tightness of its fleet of 141 fuel cells buses, finding that approximately 20% of the buses were affected by small leaks, possibly caused by loose bolts connecting on the hydrogen fuel lines. The municipality fixed the mishaps and planned a regular inspection process.

The most plausible INITIATING CAUSE could be a small leak from some components in the fuel dells compartment during refuelling, which was able to create a explosive mixture and which ignited when the bus was turned on after refuelling.
The investigation by exprts could not identify any cause for the explosion.
From the finding of the later inspection to all the city fuel cell buses, regarding hydrogen small leaks, two ROOT CAUSE could be hypothetical advanced:
(i) One related to the lack of proper inspection, able to detect leaks causing hydrogen concentration in the fuel cell compartment above the safety values avoiding formation of explosive mixtures.
(ii) Another related to a lack of an effective risk assessment before designing the fuel cell compartment of the bus, creating the possibility of formation of explosive mixture there.

A fault code had appeared in the instrument dashboard. The bus did not showed anything abnormal during the journey. Nevertheless, it had ben decided to travel to the service centre for controls. Before doing this, however, the bus had to refuel. and was scheduled to travel to the service centre.

The bus was in service since two years.

The explosion seriously injured a gas station employee, who was struck in the face by debris. The driver and a mechanic, who were present at the scene, also suffered facial burns from the blast.

The fire brigade intervened. A thorough check of the whole refuelling station was performed, to check the possibility of an escalation. No presence of hydrogen was detected.
The city suspended all 18 hydrogen buses operating in the region.

(i) The ventilation in closed and semi-closed compartment of a hydrogen vehicle has to be designed to take into account possible hydrogen leak and to avoid formation of explosive mixture. To do so, proper accidental scenarios have to be considered during the risk assessment phase.
(ii) In this case, it is probable that the leak rate in the fuel cell compartment was larger than the one considered in the design accidental scenarios, allowing the formation of an explosive mixture.
(iii) Proper inspection should be able to detect and prevent too high leak rate.

[Note of the event validator: when this this evented was entered in HIAD (January 2025), the investigation was still ongoing and nothing is reported on the causes. The conclusions presented here have to be considered only a speculative exercise].

One of the city with a high number of the fuel cells bus of the type involved in this incident, t the let the buses been investigated by a team of experts. They found that they were affected by small leaks, possibly caused by loose bolts on the hydrogen fuel lines connections.
The connections were improved, after which the municipality planned an own regular inspection process, probably on top of the existing maintenance provided by the bus manufacturer.