Fire during LH2 transfer

A LH2 trailer was delivering at a customer location. The LH2 transfer had just been terminated and the driver was ready to depressurise the trailer to remove the hose.
When he started to open the vent valve, the relief system diverter valve came apart. The escaping hydrogen caught fire. The vacuum caps located on the top, rear of the trailer (above the cold end of the trailer) and the top front of the trailer were dislodged and fire proceeded to come out of the openings.

The local fire-fighters arrived on scene in less than 10 minutes, followed by the emergency medical service. The company sent a safety specialist to the scene.
The fire-fighters were instructed to use unmanned hoses and to keep water on the customer stationary storage vessel, the LH2 trailer and the tractor, and to wait for fire in the trailer to burn itself out. The following morning, approximately 15 hours after the start of the emergency, flames were no longer evident. One hour later, the company safety specialist on the scene and the fire department declared the scene secure.

The investigation concluded that:
(1) Ambient air entered the system by diffusion and convection, in quantity enough to start combustion.
(2) This ambient air condensed and froze as it approached the trailer's inner vessel.
(3) The solid frozen air was trapped in the piping connected to the vessel nozzle.
(4) When the driver turned the 2" vent valve open to a high discharge rate to depressurize the trailer, the solid frozen air was disturbed and sufficient friction caused an ignition of the hydrogen and air in the piping connected to the nozzle.
(5) A detonation occurred in the horizontal piping causing a facture at the nozzle

INITIATING CAUSE: loss of confinement causing air to access the liquid hydrogen vessel.
Several details of the incident remain unexplained. For example, where the location and the modalities for air to access the vessel, or why the vacuum caps were involved in the fire.
ROOT CAUSE: unknown, although a material failure causing such consequences should have been considered in the safety design (risk assessment). This hints at some shortcoming in the technical mitigating measures of the system.

hose, connection, vent valve

The drive had just terminated the LH2 transfer from the trailer to the stationary tank.

Both drivers were evaluated on site and taken by ambulance to local hospitals for further evaluation.
Approximately 1600 kg of LH2 were lost.

The local fire-fighters arrived on scene in less than 10 minutes, followed by the emergency medical service.
The company was in phone contact with the teams on the site, until their safety specialist arrived at the scene. The fire-fighters were instructed to use unmanned hoses and to keep water on the customer stationary storage vessel, the LH2 trailer and the tractor, and to wait for fire in the trailer to burn itself out. The also succeed to reach the location of an to activate the emergency shut-off button of the site

The following morning, approximately 15 hours after the start of the emergency, flames were no longer evident. One hour later, the company safety specialist on the scene and the fire department declared the scene secure.