In pharmaceutical and fine chemical industries, fast and strongly exothermic reactions are often carried out in semibatch reactors (SBRs) to better control the heat evolution by the feeding rate. In fact, for such processes, a thermal runaway event may be triggered whenever the rate of heat removal becomes lower than the rate of heat production. Such a dangerous phenomenon consists in an uncontrolled reactor temperature increase that, occurring in practically adiabatic conditions, can trigger secondary undesired exothermic reactions or worse, decompositions of the whole reacting mixture with consequent reactor pressurization due to uncontrollable gases formation. In this work, dedicated software has been developed and used to simulate a cooling system breakdown in an industrial SBR where the nitration of 4-Chlorobenzotrifluoride is carried out. The mathematical model is able to simulate both reactor temperature and pressure vs. time profiles thanks to a complete description of both the desired reaction and the unwanted reacting mixture decomposition kinetics. Different accidental scenarios have been simulated, showing both the wide different consequences that can arise from the same initiating event and, therefore, the usefulness of a complete simulation of the hypothesized accidental scenario in the frame of a Quantitative Risk Analysis. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.