In a chemical plant, one of the valve fastening nuts broke on the outlet of a chlorine cylinder; 1 litre of liquefied gas spilled from a connection bracket in the confined storage room. The plant was evacuated. The containment installation was working normally and prevented a toxic gas release into the atmosphere. The inspection authorities for classified facilities was informed after a delay and requested a detailed report on the causes and circumstances behind the incident, in addition to the adoption of measures designed to reduce the probability of repeat occurrence. According to the operator, an inappropriate valve for the cylinder outlet flange, along with excessive tightening of the nut directly involved, resulted in this accident. The presence of a spur serving to centre the valve on the cylinder, whose outlet had been modified, could be cited as a cause. The spur had been cut back until 2004, but went uncut following a recent valve replacement campaign: this also influenced the nut rupture.