Release in a nuclear power plant

A hydrogen release occurred in a storage area of a nuclear power plant..
The storage consisted of three trailers made of 14 compressed gas cylinders each. The cylinders, mounted horizontally, had a nominal working pressure of 200 bar, and were all connected to the same line supplying hydrogen to the workshop.
The rupture of a faucet nut of one of the cylinders started the release. Since all the cylinders were connected to the same connector, the rupture induced the discharge of the all hydrogen stored: 2275 Nm3, without igniting.

The INITIATING cause of the accident the mechanical failure of a faucet nut on one of the cylinders, triggered by a stress corrosion cracking. The nut was made of brass.
The root or intermediate cause was a less than optimal design of the nut.

An inspection performed one year before, foreseen every three years, had not identified any abnormal situation .

A cause contributing to the aggravation of the consequence was that the whole storage content was connected without safeguards to one supply pipe. The report failed in identifying this aspects..

nut, faucet, local supply system, tube trailer

The gas released did not ignited.

Nothing is said on how the hydrogen leak was detected. The plant operator formed an emergency area prohibited to all vehicles and personnel. The leak was repaired by a fire brigade team member who closed all the valve of the affected trailers, protected by water jets.

The company providing the gas to the plant changed all the nuts on its cylinders and diffused the information to other subcontractors, insisting on the necessity of a regular visual inspection and of respecting the indicated tightening torque force.

The nuclear plant operator adopted the following corrective measures:
1) a procedure to control the tightness of all the connectors present in the hydrogen storage area;
2) an improved risk assessment;
3) a study to identify improvement needs on the trailer nuts and connections design: it resulted in an increase of the dimensions of the nuts, a modification of the metallurgy of the brass (composition, mechanical and thermal treatment, tightening without grease, etc.).