Risk assessment methodology for extreme wind and missile effects on critical facilities

The TORMIS methodology has been applied to a number of probabilistic risk assessments of critical facilities in the continental United States. These analyses have centered on the estimation of tornado missile impact and damage risks to individual targets as well as to groups of targets at specific plants. A number of advancements and generalizations in the approach have recently been made. These include: (1) generalization of windfield options to include straight winds (WINMIS) and hurricanes (HURMIS); (2) generalization of the scoring to enable analysis of Boolean system expressions for damage probabilities on compound series and parallel safety trains; (3) generalization of the failure criteria to include wind pressure as well as missile impact; (4) generalization of the plant modeling capability to enable more detailed treatment of targets partially or fully enclosed by vulnerable cladding and to allow tracking of missiles inside such enclosures; and (5) incorporation of windspeed criteria for structural failure and subsequent production of potential missiles. This paper will present some of the basic theory and key results of recent TORMIS, WINMIS, and HURMIS applications. The influence of uncertainties in the estimation process and the data needed for plant-specific risk assessments will also be discussed