In this work, we have analyzed the application of CFD (computational fluid dynamics) to simulate consequences of stirrer failures, which can lead to reactor thermal runaway. For a typical industrial batch reactor, several "CFD computational experiments" have been carried out to explore runaway and non-runaway situations. Then the results of the CFD simulations, supported by an on-line divergence criterion, have been used to detect a dangerous reactor behavior. It has been found out, that even for the reactor operated at potentially safe conditions a failure of the stirring system can lead to serious thermal runaways. A significance of a proper location of temperature probe for an early runaway detection has also been discussed. A modification of the applied divergence criterion to avoid false alarms has been proposed and tested. (c) 2007 Elsevier Ltd. All rights reserved.