Probabilistic Risk Assessment
Risk assessment asks what can go wrong, how likely it is, and how bad it would be, expressed as probabilities rather than worst-case assertions.
The three questions
Probabilistic risk assessment, or PRA, formalizes three questions: what sequences of events can lead to harm, how probable each sequence is, and what its consequences are. Combining them gives a quantified picture of risk that can be compared, ranked, and reduced deliberately.
How it is built
- Identify initiating events, the faults that could start a sequence.
- Map how systems respond, using event and fault trees.
- Assign probabilities to each branch from data and models.
- Compute the consequence of each end state and combine.
Why probabilistic
A purely worst-case analysis can be so conservative that it hides which risks actually matter. Assigning probabilities shows where risk truly concentrates, so mitigation goes to the sequences that dominate rather than being spread evenly. This connects directly to sensitivity analysis.
Data and uncertainty
Branch probabilities come from component data, operating experience, and models, all uncertain. PRA carries that uncertainty through to the final risk numbers, so the result is a distribution, not a single figure presented as fact.
For Kronos
For the Hyperion breeder, PRA covers plasma events, magnet faults, cooling failures, and tritium release paths, drawing on disruption and inventory models. The lower neutron fraction of the D-3He burner changes its risk profile, analyzed on its own terms.
Role in licensing
PRA is central evidence in safety and licensing, and it is only credible because its inputs are reproducible and its uncertainty is stated honestly.