Hydrogen fire from a gas pipeline in a nuclear power plant

A hydrogen fire occurred in a nuclear power plant outside the controlled area.

The leaking point was on a pipe system of the distribution circuit of the gas network (6 bar nominal working pressure). The specific location was a joint on the fire-protection cupboard containing the hydrogen cylinders. The fire started during a repairing intervention on a cupboard nearby, performed by a sub-contractor using a hot surface welding equipment.

Internal emergency services isolated and depressurized the pipes, and extinguished the flame.

INITIATING CAUSE was the local leak from a hydrogen supply line and its ignition by welding works.

ROOT CAUSE, as reported by the ASN report referred to, is the lack of risk assessment (analysis, implementation) related the temporary modification of the hydrogen distribution system (a nitrogen line).

hydrogen pipeline, nuclear power plant

A repair and modification of the hydrogen storage and distribution system was ongoing.

No consequence

The national nuclear authority performed a 'reactive' visit and issued a report with legal recommendations

The ASN inspection issued recommendations and request of information aiming at clarifying/improving the following points:
1. The demonstration to have in place a fire control procedure, requires also the organisation of drills;
2. A new risk assessment has to be performed every time that a change occurs in the boundary condition under which the previous risk assessment was performed. This particular case regards the condition related to hydrogen fires, because instead of the 90 kg of hydrogen considered as max. site storage the real quantity was 250 kg;
3. This task assessment has also to re-evaluate the need and characteristics of ATEX zones, every time modifications, also temporary, have been made to the normal operative conditions. In this particular case, a welding plug was working in a zone very near to pressurised hydrogen systems. Also other systems, such as the forklift, was not ATEX-proof.
4. The conclusion from points 2 and 3 must be implemented, existing procedures must be adapted, and involved workers must receive specific safety awareness training. In this particular case, the hydrogen storage and distribution system was under reconstruction, and temporary solution were found to guarantee hydrogen delivery, for example by using a nitrogen line.
5. Maintenance operations and their frequency must be traceable (recorded and available), including tightness tests, and maintenance on flexible cables from the gas cylinders to the distribution line;
6. The quantity of the water used and the way how it has been collected/confined must be recorded. This is was not important in the present case, but in case of major fires, the water used may containing possibly toxic or other components which represent a risk for river and underground waters.