Which statement best describes a quantitative risk assessment tool used after a PHA to calculate risk?

• A. QRA (Quantitative Risk Assessment) uses numerical probabilities and consequences to calculate risk.
• B. HAZOP uses numerical probabilities and consequences.
• C. What-If relies solely on qualitative judgments.
• D. Bow-tie analysis yields qualitative risk estimates.

Answer: (A)

When you want to put a number on risk after a PHA, you use a quantitative risk assessment. This approach explicitly uses numerical probabilities for initiating events and modeled consequences to compute risk, typically by combining how often something could happen with how bad the outcome would be. In practice, QRA builds scenarios, often with event trees or fault trees, to estimate frequencies of releases or accidents and then uses consequence models to translate those into outcomes like injuries or fatalities. The result is numerical risk metrics, such as an individual’s annual risk or overall population risk, that support comparisons and decisions on controls, siting, or emergency planning.

The other techniques are primarily qualitative or descriptive. HAZOP focuses on systematically identifying potential deviations and their consequences using guide words, and while it can inspire risk thinking, it doesn’t inherently produce numeric risk values. What-If is a brainstorming method that explores possible failure modes but is typically qualitative. Bow-tie analysis maps the cause-to-consequence pathways and barriers, often emphasizing structure and defenses rather than delivering numerical risk estimates.