When a chemical accident occurs, the release depends on the status of the substance(s) in either the damaged process unit or carrier, i.e. the accident dynamics differs in presence of a compressed gas, a refrigerated liquefied gas, a compressed liquefied gas, or a subcooled liquid. In addition, the release properties (e.g., flow rate, liquid and vapor fractions, temperature, and momentum) may vary dynamically due to the modification of the conditions inside the damaged process unit and/or to the intervention of either the operator or the control system. This manuscript describes the features that an accident simulator should implement to account for the release variability. In particular, the paper focuses on the release of liquids, their spreading, and the formation of pools and pool-fires. The recommended features include the capability to simulate the automatic switch of the liquid pool from spreading to shrinking, its vanishing, the release in a bund, its ignition, the evaluation of the pool-fire geometrical dimensions and the heat flux radiated to the surrounding equipment and field operators. By implementing these features in an accident simulator, it is possible to get a more realistic picture of the accident dynamics, and of its magnitude and consequences. An accident simulator having the aforementioned features can be linked to a dynamic process simulator to evaluate the dynamics of the plant operating conditions during the accident unfolding, for both risk assessment and operators' training purposes. These features have been implemented in a simulator program, which is based on CPU-efficient models and algorithms, with the scope of demonstrating the feasibility of the discussed approach and its advantages. The importance of the aforementioned features is illustrated by means of some dedicated case-studies. (c) 2011 Elsevier Ltd. All rights reserved.