Wiley Handbook of Science and Technology for Homeland Security, 4 Volume Set

PROBABILISTIC RISK ASSESSMENT (PRA)

GEORGE E. APOSTOLAKIS

Engineering Systems Division and Department of Nuclear Science and Engineering, Massachusetts Institute of Technology Cambridge, Massachusetts

1 INTRODUCTION

Probabilistic risk assessment (PRA) is a scenario-based analytical methodology that has been developed to manage the risks of complex technological systems such as nuclear power plants (NPPs) [1], chemical agent disposal facilities [2], and space systems (e.g. the International Space Station (ISS) [3]). It is also called probabilistic safety assessment (PSA) and quantitative risk assessment (QRA). In general terms, PRA answers the following three questions [4]: What can go wrong? What are the consequences? How likely is it? The principal PRA results are the probabilities of various consequences of accidents, the identification of the most likely scenarios (event sequences) that may lead to these consequences, as well as the most important (from a risk perspective) structures, subsystems, and components. This information is very valuable to operators, designers, and regulators because the responsible parties can focus resources on what is really important to the safe operation of the system.

For a given system, PRA answers the above three questions by proceeding as follows:

1. A set of undesirable end states is defined.
2. For each end state, a set of disturbances to normal operation is defined, which, if uncontained or unmitigated, can lead to this end state. These are called ...