The immediate cause of the main explosion was the rupture of the 20 inch by-pass assembly between reactors 4 and 6. 40 to 60 tons of cyclohexane escaped and were ignited 30 seconds after by the reforming tower forming a cloud two hundred meters in diameter and 100 meters in height. Before this explosion, on 27 March, a vertical crack in reactor 5 had been discovered leading to a cyclohexane leak. A serious problem was identified, so this reactor was removed and replaced by a simple by-pass between reactors 4 and 6.