The paper deals with the thermal safety and the runaway risk evaluation for semi-batch reactors (SBR). The main contributions consist in: Construction of a new boundary diagram which describes the reaction temperature vs. target temperature and maximum allowable temperature in order to define the critical conditions for the thermal runaway; Identification of the most important governing parameters; Evaluation of the risk of accident by a probabilistic description of the prior cooling problem, the appropriate combination of the critical conditions and the assessment of occurrence probability of the thermal runaway accident. A heterogeneous liquid-liquid reacting system is considered as a case study. A sensitivity analysis is then performed in order to investigate the influence of cooling temperature on the maximum reaction temperature. Monte Carlo simulations are also used in order to calculate the probability of accidents. (C) 2016 Elsevier Ltd. All rights reserved.