Graph theory is a reliable tool for vulnerability analysis of interdependent systems with complex cascading effects . Vulnerability analysis of CPI systems subject to domino effects has drawn much attention . In this context, regional vulnerability was defined as the capability of CPI system to escalate a primary accident to higher level accidents . Khakzad and Reniers adopted a directed graph to model cascading effects. Thus, the efficacy of a set of graph metrics and measurements was examined in both unit and plant-wide vulnerability analysis of process plants.