Probabilistic risk assessment (PRA) is the full quantitative risk model: build out every accident sequence, put a frequency on each one, and add them up. It is the nuclear power industry term, and in the US it is what the NRC means when it talks about risk-informed regulation. A PRA runs in three levels. Level 1 estimates core damage frequency (CDF), Level 2 estimates the frequency and size of a radioactive release, and Level 3 estimates public health and environmental consequences. Mechanically it is event tree analysis (ETA) for the accident sequences and fault tree analysis (FTA) for the systems inside them, the same structural pair the process industries use inside a quantitative risk analysis (QRA).
The name is what changes across borders. The IAEA and most European and Asian regulators say probabilistic safety assessment (PSA), and the UK Office for Nuclear Regulation (ONR) says probabilistic safety analysis, which also shortens to PSA. All of them use the same Level 1, 2, and 3 structure, so a Level 1 PSA and a Level 1 PRA are the same deliverable under different letterhead.
Deep dive: For the free NUREG failure rate data that US PRA work makes public, and how it maps onto SIF hardware, see Where Does the Data Come From? A Guide to Failure Rate Data Sources in Functional Safety.
Key Points
- Level 1 gives CDF, Level 2 gives release frequency, Level 3 gives offsite consequences, and each level feeds the next.
- The three-level structure is international, not a US convention. What changes outside the US is the name, not the method.
- A PRA analyzes the events it was handed, so it cannot surface a hazard nobody identified upstream.
Example
A Level 1 PRA takes loss of offsite power as an initiating event, branches through whether the emergency diesel generators start and whether operators restore power in time, and sums the frequency of every path that ends in core damage.
See Also: PSA, ETA, FTA, probabilistic
Cited Sources
- US Nuclear Regulatory Commission, Glossary of Risk-Related Terms in Support of Risk-Informed Decisionmaking (NUREG-2122)
- International Atomic Energy Agency, Development and Application of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants