Rain

- In order to minimise the impacts of micro-outages (due to electrical storms), the plant operator typically switched the power supply onto the four back-up electric power generating sets.

- In case of loss of electricity, the safety mode of the electrical power supply is triggered as a preventive measure.

Around fifty firefighters from Pas-de-Calais but also from the North were called to support the staff from the factory.
No evacuation or confinement plan for local residents was deemed necessary.

- The power supply was switched to the three available back-up electric power generating sets (at 10:20 pm).
- The two remaining generators proved insufficient to cope with the installation's power demand, which triggered the safety mode of the electrical power supply as a preventive measure due to loss of utility service (at 10:46 pm).
- The on-call maintenance electrician was called, as he was the only person actually certified to switch the power supply back onto the electricity grid (at 10:53 pm).
- The internal alarm was sounded, which in turn activated a crisis management unit and notified both on-call team and external first responders (at 11:01 pm).
- Pressure rose in one of the polymerisation reactors at the continuously-running station (first DC1 reactor, at 11:05 pm).
- In accordance with the plant's emergency procedure, gyro monitors were turned on, so as to contain vapours that may have been vented from reactor 1/DC2 line (at 11:15 pm).
- The site was connected to the grid network, though the units started up following a short lapse (at 11:18 pm).
- At 11:20, the disc of reactor 1 (DC2) burst at 5.8 bar, leading in turn to a discharge of 10 tonnes of polystyrene and another 3 tonnes of styrene into the atmosphere.
- A peripheral water curtain was activated to control the vapours (at 11:40).
- Both reactors from the workshop at the beginning of the polymerisation process were emptied into an emergency pit as a precautionary measure.
-The state of alert was lifted at 2 am.

27

1

No casualties were reported.

Following the request from the site, firefighters performed air quality checks, which were found to comply with standards (according to director of human resources of the company).

An emergency order was issued and the units were allowed to restart on 19 July.
The consequences of this accident remained confined to operating loses tied to installation shutdown.

Residential areas (e.g. housings, hotels)

Residents, not warned to evacuate the area, noticed a strong odour and respiratory discomfort.

Styrene concentrations around the pit and in 4 neighbouring municipalities had returned to zero at 12:25 am.

- A peripheral water curtain was activated to control the vapours.
- Both reactors from the workshop at the beginning of the polymerisation process were emptied into an emergency pit as a precautionary measure.

- The reactor's runaway response was due to the loss of utility services. Critical utilities that prevent the occurrence of runaway reactions must be ensured even during the impact of a natural hazard either by increasing the robustness of the utilities or by increasing redundancy.

- The design of the reactor had incorporated the possibility of runaway during steady-state operations. Following the accident, the focus was shifted on the runaway response during the start-up phases.

- The objective of the risk management measure was revised in order to limit disc rupture to thermal runaways during start-up, which entailed redesigning the disc. Once this modification had been completed, the process could be considered as intrinsically safe while operating in a steady state.

- The efficiency of the gyro monitors, along with the degree of polymerization (small quantity of released Styrene) and the confinement of the aqueous discharges within a basin, all helped limit the consequences to operating losses for the installation.

- Since the technician had not completed the manoeuvre to contain pressure surge, the emergency vent on the CMP unit's line 2 reactors had not opened early on during the incident, as indicated in the procedure. Preparedness for staff should be improved on procedures to mitigate/prevent accidents due to runaway reactions.

-Lack of communication to the public.

A protocol change regarding power supply management was carried out, involving:
- the principle of constant redundancy (even during thunderstorm events);
- verification of the availability and operability of critical safety item, CSI, (tied to a thermal runaway) should utilities be down;
- efficient lightning protection.

1. Immediate measures
- modification to the emergency shutdown procedure on the continuous line (open vent valve, and start-up of gyro monitors upon initiation of the emergency shutdown procedure);
- change of protocol regarding power supply during thunderstorms.

2. Medium-term measures
- technical-economic study to ensure continuous line operations under safer conditions;
- a risk analysis focusing on lightning.