Hydrogen release and ignition at the gas storage station of a nuclear power plant.

The accident took place when replacing a pallet of empty hydrogen cylinders with a new full one. The pallet in service (almost empty) was not disconnected from the gas supply line. When the operator in charge of handling (fork lift) begun to remove the pallet in service from its position, he teared off the hose connecting it to the pressure relief system. The pressurised hydrogen in the pipes and in the pallet escaped and ignited.

The intervention of the teams from the plant and external emergency services extinguished the fire in 1h45.

The INITIATING cause is a wrong application of the procedure of replacing gas cylinders.
There are however several contributing causes (uncontrolled access to the storage area, non respecting of the ATEX distance for welding work, abnormally high frequency to replace the gas pallets, etc.).
The root cause lies therefore in the lack of adaptation of the safety operation to the temporary different working conditions.

hydrogen cylinders pallet

A leak of hydrogen at the alternator required a temporary increased rate of hydrogen supply. Therefore the pallets had to be changed much more frequently.
Other maintenance work (including welding) was ongoing, and as a consequence the pallet storage location was open. The external company worker coming to change the pallet was able to access the storage without asking someone from the plant.

DESCRIPTION OF THE FACILITY
The storage consisted in 5 hydrogen pallets of 10 cylinders each of and 2 nitrogen pallets with 10 each. All the cylinders were of 50 l with 200 bar compressed gas.

One worker was injured because he fell when flying the location. The other due to a post-event shock.

The ASN (National Safety Authority) performed an investigation and concluded that:
1- Many procedures have been disregarded, including communications of the ongoing operations and the temporary changes
2- A reduction of the number of workers in teams, without correspondingly assessment of the consequences
3- A lack of related risk assessment
4- ATEX-compatible tools were not used in the zones (Forklift and welding material were inside the ATEX zone 2).