The loss of control of a reaction caused a fire in a pharmaceutical plant caused a reaction runaway and the rupture of a 6 m reactor. A cloud containing sodium borohydride and hydrogen was emitted. The production manager evacuated the installation. When the cloud reached electrical equipment, an explosion occurred. The cooling circuit was damaged. The thermal oil leaked and ignited. The fire spread to a large part of the plant. Firefighters intervened. The water of extinction was confined in basins. Storage tanks that were not affected by the fire were emptied. During the accident, 30 t of oil burned. 50% of the installations were destroyed, representing a damage of 12 M .The runaway of the reaction was due to a change in the reaction temperature. In order to increase reaction efficiency, it had been lowered from 50 C to 20 C. The accident occurred during the first production under these conditions. No risk analysis of this operative change had been made beforehand. The standard procedures of the group to which the plant belongs did not take into account the risk of runaway exothermic reactions.