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.

The State railroad commission was called and notified of the leak at 12:55 am on November 13, 2018.
A plan was developed to isolate the line while minimising emissions from the plant reformer. An additional isolation valve on the affected pipeline the valve station was installed in order to isolate it from another hydrogen line.
Hydro-excavation was attempted to uncover the leak location but, due to soil instability, the depth of the excavation (over 25 foot), and its proximity to an interstate highway, the decision was made to abandon this activity and backfill the excavation.
A company expert in pipeline inspection and integrity was contracted to smart pig the affected pipeline section. Results from the in line inspection pig identified 5 areas of concern. All locations were at the bottom of the pipe and at low-lying areas. No property damage occurred due to the incident.
1700 thousand standard cubic feet went lost during the leak (unintentional release)

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.).