Minimal Cut Sets Evaluation as a Qualitative Method for Security System Vulnerability Analysis

Many radioactive source theft and loss incidents in the past were involving source in industrial and medical application. Radioactive source could be used as a radiological emitting device (RED) or a radiological dispersal device (RDD) for any malicious purpose. Security system is used to secure radioactive source from any sabotage or theft that could lead casualties or other damages. Assessment of security system is needed to evaluate its capability against a defined threat. In the case of very limited resources to do a rigorous quantitative assessment, minimal cut sets evaluation can be used as a qualitative method for vulnerability analysis of the system. This qualitative method was tested to assess a security system in a radiotherapy facility. Target identification, threat assessment, and facility characterization were conducted to have the insight of the case and the system in order to build the fault tree diagram of the system. Method of Obtaining Cut Sets (MOCUS) algorithm was applied to the fault tree diagram to get the minimal cut sets of the system failure. Cut set parameters were combined with the characteristics of effective security system to define some criteria for the minimal cut sets evaluation. Vulnerability of the system was determined by evaluating the parameters of obtained minimal cut sets to the defined criteria, with respect to the understanding of the facility, threat, and the target. This method also showed the significance of a component to the system failure. The evaluation of the facility case in this work showed that the security system was vulnerable and human employee was one of the crucial component of the system failure.