Use of Probabilistic Safety Assessment for Infrastructure Risk Modeling Zdenko Šimić, Vladimir Mikuličić, Igor Vuković Faculty of Electrical Eng. and Computing, University of Zagreb Unska 3, 10000 Zagreb, Croatia Tel.: +385 1 6129-985, fax.: +385 1 6129-890 zdenko.simic@fer.hr Abstract. This article deals with use of Probabilistic Safety Assessment (PSA) model for infrastructure risk assessment. The first part of the paper provides background information leading to the selection of PSA as the preferred method for risk assessment. The article also discusses the initiating events, consequential events, and the consequences, dealing with scenarios and detailed list of various failure events, the associated failure modes and their cause/effect which could deal with various infrastructure interdependency risk. Example of event tree and fault- tree for selected infrastructure are discussed and presented. Emphases are given on the advantages and disadvantages of using PSA for infrastructure risk assessment. Main advantages are in the capability of PSA to deal with numerous scenarios and to provide comparative risk assessment for various infrastructure types and some interdependencies. PSA is proven methodology for combining available data with expert opinion to model complex systems risk accounting for numerous number of initiating events, systems reliability, human responses and common causes. Final results are both important for qualitative and quantitative usage in infrastructure safety optimization. This is valuable inside particular infrastructure type or between infrastructures for specific region or country. Example of nuclear power plant PSA model and interdependency on power infrastructure is presented. Keywords. Infrastructure, risk, PSA, event tree, fault tree 1. Introduction Most of the time key word is risk but driving issue is safety. Since safety is directly determined by risk, and it seems easier to analyze risk, we want to assess the risk. In order to know the risk we have several approaches to predict risk: testing, collecting and processing experience data, or modeling. Least preferable, and most uncertain, way of determining risk is with model assessment. Modeling is last option used only when there is not enough data from tests or experiences. Even when risk is assessed with model any experience and testing of some subsystems or elements is valuable modeling input. Important reason to find out what is the risk is to compare risk with what is considered acceptable. If estimated risk is greater than what is considered safe or target then system has to be improved or benefit from system has to be proved sufficient to balance higher risk and expense of risk reduction. Hear again testing, experience data or assessment is required for risk reduction. It is common problem for highly safe