Bayesian network and probability boxes to analyze uncertainty in unavailability assessment

In this article the problem of uncertainty in assessing unavailability of Safety Instrumented Systems (SIS) is treated. The characteristic parameters of the SIS including Common Cause Failure (CCF) factors are replaced by probability densities families (p-boxes) allowing in reliability experts to express their uncertainty on the statement of values probabilities. We show how the imprecision is modeled and propagated in a Bayesian Networks which induces uncertainty about the failure probability on demand of the SIS and its Safety Integrity Level (SIL). We will demonstrate how the uncertainty on some characteristic parameters values causes significant variations on the level risk.