This paper shows that under certain conditions, adiabatic tests which are carried out in closed test containments do not reveal the "worst case" in terms of adiabatic temperature rise. If some of the products of the reaction driving the run-away are gases, the temperature of the reaction mass will increase more in open systems, where these gases can escape. This is because the residual mass which has to be heated by the reaction energy decreases. The effect depends on the kinetics, the heat capacity of the solids and the gases and on the mass fraction of non-condensable gases. The effect is particularly high for self-reactive substances and explosives. The paper presents the thermodynamic relations, which allow the identification of cases where open systems would be more conservative together with illustrative simulations.
