Inside a uranium materials transformation plant that was also producing fluorinated products, a leak estimated at 3 kg of fluorine (F2) occurred on one of the two stations filling bottles with a gas mixture containing fluorine (10% and 20%) as well as pressurised nitrogen (N2).

Around 10:30 am, the technician launched the simultaneous packaging of 4 frames, 2 on station A and 2 on station B. The frames had already been internally pressurised at 50 bar and needed to reach 105 bar. He had previously prepared this set-up (with the right F2/N2 ratio) by means of compressing on backup station C, which had since been pressurised to 70 bar. At noon, the technician stopped the simultaneous filling of both stations, as the pressure inside the 4 frames reached the 90-bar level. Only the filling of station A was continued. At 12:11 pm, the frame pressure of station A equalled 105 bar. Once the filling cycle had been completed, the automatic switch took place, allowing for the filling of station B frames, whose pressure remained at 90 bar. The technician then disconnected the frames from station A in order to replace them and continue with the production programme. Towards this aim, he opened the gate that had been confining the packaging installation. At 12:21 pm, pressure on the station B frames rose to 105 bar. The automated mechanism thus transferred the compressed F2/N2 mix onto station A. The technician however did not have sufficient time to replace the station A frames, and the F2/N2 mix was rejected at the 10-bar exterior pressure by the packaging brackets.

By 12:30, the internal safety teams had shut down the compressor, thereby stopping all discharge. Treated by the onsite medical unit, 3 employees, who had experienced a slight malaise, were able to return to work a few hours later, though as a precautionary measure remained under medical supervision for another 24 hours.

The incident was caused by the technician’s erroneous assessment of the installation by taking into account the internal pressure of station B frames at 50 bar instead of 90 bar. This mistaken value actually resulted from the simultaneous filling of stations A and B, as this operation had not been prohibited by the facility’s management system. Station B was idled and the HAZOP hazard analysis is updated in order to redefine safe operating conditions.