Liquid hydrogen tank explosion

A rupture disc opened on a 76 m3 (20,000-gallon) liquid hydrogen tank, causing the vent stack to release cold gaseous hydrogen. The gas ignited causing a fire on the vent stack's outlet.
Emergency responders intervened by removing the remaining hydrogen but left a small volume which could not be removed manually. During this operation, the vacuum system lost its heat isolation capacity due to a (mechanical?) failure.
This triggered a rapid evaporation of the residual liquid hydrogen with consequent increase of the tank internal pressure. In answering to this, the emergency responders sprayed the tank with water and directed a stream onto the fire exiting the vent stack. The water jet was pointed directly into the open vent stack, thus interacting with hydrogen vapours from the residual hydrogen (between −253 °C and −241 °C). This caused the water to freeze in the vent stack. This ice blocked the stack, which was the only exit for the cold hydrogen gas. Eventually, all residual hydrogen in the tank warmed up, causing the tank to over-pressurize and rupture with an explosion known as a BLEVE (Boiling Liquid Expanding Vapour Explosion).

The INITIATING CAUSE was the opening of the burst disc. It is unclear if this was triggered by a material failure or by an increase of the tank internal pressure beyond the design pressure of the disc.
The incident escalated to an explosion (BLEVE) due to wrong decisions/actions of the emergency responders, to which probably contributed the lack of emergency procedures of the plant, up to a lack of proper risk assessment. These consideration point at a shortcoming in the safety management as ROOT CAUSE.

burst disc, vent stack, tank

One of the source mentions that the tank was equipped with "pressure relief devices, rupture discs, and normal venting of hydrogen". it is unclear what the latter maybe, probably manual valves.
It is also unclear why the burst disc opened in first instance. The reports do not mention either why the tank lost its thermal isolation capability during the emptying operations, which hare also not detailed.

Minot injury, some property damage and temporary closure of the facility.

When intervening against the ignited hydrogen from the tank venting stack, the emergency responders emptied the tank, leaving however a small quantity which could not be removed manually.
When intervening against the loss of thermal isolation and increased evaporation, they caused an escalation of the incident, by blocking the vent with frozen waters, which eventually brought to the explosion.

This case showed that water as emergency means to stop fire has to be used with caution in case of LH2, due to the possibility of water freezing and consequent obstruction of the available gas release devices. On the other side, in cases involving fire of compressed hydrogen, water jet were effective to cool down the tanks and avoid that they reach overpressure. Therefore, emergency teams have to be informed in detail on the nature of the hydrogen storage affected by an incident.

(i) An additional secondary backup vent stack was added to liquid hydrogen tanks. This secondary stack is designed to be used only if needed in the event the main vent stack became plugged with ice, as it happened in the incident. The main vent stack remained the primary means of venting all relief devices. The secondary vent stack would only be used if the main vent stack failed.
(ii) A sign was placed on all liquid hydrogen tanks, indicating that no water is to be put on the vent stack.