The S2S assessment tool for unexpected reactors aims to identify how reactive chemical hazards can exist in process units or plant equipment outside the reactors. The assessment tool can also be used as a training tool since it is based on simple concepts, combining the following items:•A reaction triangle that describes the factors necessary for triggering a chemical reaction. The factors considered are temperature, concentration and residence time.•Process streams with physical and chemical properties that can be mapped on the reaction triangle.•Equipment or unit operations, which determine how process streams are positioned and moved in the reaction triangle. The design of the plant should ensure that these moves are safe. Equipment will have failure modes and can exhibit side effects that cause a shift in the reaction triangle that enables a reaction and may pose a hazard.•The tool is presented as a computer program that generates discrete questions to identify potential hazards and explains the likely mechanisms. It is applicable to batch, semi-batch and continuous processes, and it assumes that the user has previously identified/characterized the reaction hazards through literature studies, calculations, experiments or other S2S assessment methods. The tool can be found at the S2S web portal http://www.s-2-s.org/.