Shortly before 3 p.m., two subcontractors from a plastics manufacturing plant were fastening a panel of insulation onto an autoclave when a nitrocellulose explosion occurred. They suffered slight burns on the face and arms, and were taken to the hospital by the emergency services.

The subcontractors were installing a stainless steel insulation panel using a portable electric screwdriver. The insulating wool was uncontaminated as it was new and stored in its packaging. Traces of dry nitrocellulose were found on the scaffolding around the autoclave and on the other insulating plates awaiting restoration. The explosion was probably initiated by a hot spot that appeared while the plate was being secured with screws (metal/metal friction).

The presence of nitrocellulose on the plates was caused by an overflow from the autoclaves containing nitrocellulose under water. These autoclaves were not equipped with overflow protection and the level of the autoclaves was being managed manually. Water containing nitrocellulose fines had trickled along the plates and into the recovery zones between the plates, the junction of which was not completely hermetic due to the degradation of the silicone joints over time (ten-year inspection of the autoclaves). Furthermore, the plates were not cleaned and re-wetted prior to reassembly.

The operator studied solutions to limit the risks of overflow and to guarantee proper sealing of the insulating plates over time (silicone not being a reliable solution). The work permit and the procedure relating to the removal/replacement of the insulation plates were modified to include the inspection and systematic cleaning of the components removed, whatever its intended destination. The operator inspected the condition of the insulation on the other autoclaves (joints between plates).