Robustness is a system property that reflects its resistance to the initiation of hazardous events and their progression beyond defined limit state thresholds. Under scoped perturbations a robust system should not display disproportionate respones or cascading failures. Three levels of robustness are identified: Operational (R1), Survival (R2) and Reserve (R3) that span responses in state space ranging from the safe operating limit(SOL) up to system failure at the ALS limit. Systems engineering methods can be used to define system boundaries then map sub-systems and their interactions. HAZIDS can be used to identify critical failure modes for engineering analysis. Risk methods can be used to assess the acceptability of the robustness measure.

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