A deflagration occurred at a chemical plant during a synthesis operation involving sodium tri-acetoxy-borohydride inside a 5,300-litre reactor. Around 5:30 pm, the technician observed that the acetic acid intake pipe was blocked. After opening the manhole with a wooden pole, the shop foreman dropped the stalactite of solidified acetic acid in the reactor: the resulting blast severely burned the foremans face.The event was caused by ignition of the hydrogen produced from the dropped quantity of acetic acid. The maximum amount of acetic acid introduced into the reactor was estimated by the operator at 3 kg, and it led to the formation of approx. 100 grams of hydrogen in this 5,300-liter reactor. The accident had no impact either on the environment or on other shop installations.