In a plastics manufacturing plant, an incident occurred on reactor 20 (enamelled polymerisation reactor – operating pressure: 20 bar, volume: 40 m³). This reactor held at 0.9 bar was used as an “expansion tank” for the other reactors of the line awaiting drainage following a mechanical break downstream of the production line. It was used from 8.16 am for initial balancing. The technicians had reduced all water injections into this reactor. Its pressure gradually rose from 4 to 8 bar (at 3.23 pm). The erroneous injection of water into the reactor, which was not used as a polymerisation reactor, then caused a significant increase in pressure. On detection of high pressure, the reactor’s safety system opened the safety valve allowing introduction of the inhibitor (300 l at 100 bar). This introduction then caused the valve to open at 20 bar towards the torch system. The MVC discharge into this system and therefore the incident’s environmental impact was not substantial (< 5 kg), but the pressure in the reactor continued to build up, reaching 28 bar in 2 min, after the injection of inhibitor. Depressurisation by the RL/RLbis system (discharge system planned for a hydraulic pressure buildup in order to evacuate a liquid phase) was not active as the reactor was in HOLD position (reactor fallback position where load shedding is not managed). The reactor was therefore unable to empty in time. The operator carried out several corrective actions: change of all the joints on this reactor after drainage and change of the pressure relief valve, etc. That same day, an internal inspection visit was scheduled to determine whether the enamel had been affected (visual inspection). During this visit, the pressure reached remained below the reactor's test pressure. The reactor was then to be restarted only if no anomaly was detected. Given the pressure differences between the standby reactors and the R20, the latter, which was not operating, had been isolated too quickly after balancing the other reactors via the boiler manifold.