At the OCI Nitrogen ammonia plant, located at the Chemelot site in Geleen, The Netherlands, a project has been initiated to monitor major accident processes at the site. This contribution answers the question whether indicators can be derived from the barrier system status to provide information about the development and likelihood of the major accident processes in the ammonia production process. The accident processes are visualized as scenarios in bowties. This research focuses on the status of the preventive barriers on the so called ‘left-hand side’ of the bowtie, before a hazard becomes uncontrollable. Both the quality – expressed in reliability/availability and effectiveness – and the activation of the barrier system give an indication of the development of the accident scenarios and the likelihood of the central event. This likelihood is calculated as a loss of risk reduction compared to the original design. The calculation gives in an indicator called “preventive barrier indicator”, which should initiate further action. Based on an example, it is demonstrated which actions should be taken and their urgency.