Around 6.15 am, a rupture disk at a chemical site opened on a grease cylinder of the medium return pressure system (MPR) on one of the lines in the polyethylene workshop that had been shut down for programmed maintenance; 1.4 t of ethylene was subsequently released into the atmosphere. The cloud dispersed at altitude upon leaving the stack.The shut-down procedure included the rinsing and purge of the reactor 3 times, each flushing operation being performed in 2 phases: the reactor is filled to 600 bar with the secondary compressor and depressurised via the MPR circuit. The rupture disk opens during the depressurisation of the 1st flushing operation. The line is cleaned, the blocked valves of the 2 MPR circuits are checked, the remaining greases are purged and the disk is replaced. The presence of a considerable greater quantity of grease in the MPR circuit than during normal operating regime, notably at the check valves, remains unexplained.A clogged valve on a neighbouring line had already led to the release of 3.2 t of ethylene on 07/21/05 (ARIA 30920). The design of these valves, which promotes blockage of the MPR circuits, should be studied in order to reduce the probability of grease accumulation at these locations. Their removal was also considered in late 2006, following a prior risk analysis owing to the role they play in installation safety (check valve).The installation of an additional preventive barrier on the MPR will be discussed within the scope of an additional danger study. Since proper cleaning of the installation has a certain impact on its safety, procedures must be formally drawn up regarding the nature and frequency of cleaning operations as well as the traceability of the controls performed. Performance indicators must be implemented to determine the efficiency of these cleaning operations. The causes can generate a significant quantity of grease and must be identified. The impact of the phases leading to significant pressure discontinuities must be analysed (shut-down, rinsing phase…).