Accidental events that have occurred in chemical and nuclear plants show that the application of technical regulations on its own, even when extremely detailed, does not assure that adequate safety levels are reached and, above all, maintained in time. The paper illustrates, with a simple example, how a risk analysis can be considered as an essential tool for a correct plant design, above all when dealing with major risk installations. In particular the use of the Recursive Operability Analysis is shown, in the phase of accident identification, since it allows to directly draw the logic trees, both Fault and Event Trees for the quantification phase, thus assuring the congruence of the whole analysis.