An explosion occurred inside a chemical plant at the level of an electrolytic cell located in a chlorine production workshop, blasting the lid off the cell (vent function) and breaking a low-pressure chlorine gas pipeline. The ensuing quantity of chlorine discharged inside the workshop amounted to between 2 and 3 kg. The series of 132 electrolytic cells in the shop were shut down, followed by the entire production chain. The 2 technicians present were shocked by the accident yet only sustained slight injuries to their legs (irritations) due to electrolytic projections (pH = 3). The accident occurred as they were proceeding with a cell ‘vaccination’ procedure (improved seal on the orifice plate separating the two vapour spaces (i.e. chlorine and hydrogen) by means of injecting suspended silica and magnesium chloride or glass fibres) following a malfunction observed during routine controls. A reaction between H2 and Cl2 in the cell vapour space was responsible for this explosion. The migration of hydrogen towards the chlorine, producing an explosive mix, was due to excess pressure in the cathodic compartment. This phenomenon could have resulted from 3 situations: poor positioning of the soda stick, implying an overly high soda level inside the cell; obstruction of the soda outlet; and plugging of the H2 outlet. Moreover, the analysis of accident causes revealed the poor quality of the cell orifice plate: this plate was included in batches manufactured under atypical conditions (cross-linking temperature of 275°C instead of 355°C due to defective furnace regulation) that after the fact displayed a problem of premature ageing. The remedial measures adopted focused on replacing those cells whose orifice plates had been produced under the same temperature conditions; clarifying intervention guidelines relative to cell ‘vaccination’; and modifying hydrogen outlet pipes to facilitate controls and cleaning, thereby limiting the risk of plugging.