Risk Assessment in Regulation

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In assessing public safety and developing regulations for nuclear reactors and materials, the NRC traditionally used a deterministic approach by asking the following questions:

What can go wrong?
What are the consequences?

Now, the NRC uses a risk-informed and performance-based approach by also asking the following questions:

How likely is it that something will go wrong?
What performance is needed?

Risk is part of everyday life. For example, there is risk in driving to the store. Fortunately, however, we seldom have an accident while driving to the store and, when we do have an accident, the resultant injuries are usually not serious. Nonetheless, this example illustrates the two elements of risk — the likelihood (or probability) of an accident and the seriousness of the resultant injuries (or consequences). Combining the probability of an accident with its consequences gives us a measure of risk. For instance, the consequences of a large meteor striking your house would be devastating, but the risk is low because the probability of such an accident is very small.

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The NRC's Concept of Risk

The NRC's concept of risk combines the probability of an accident with the consequences of that accident. In other words, the NRC examines the following questions:

  • What can go wrong?
  • How likely is it?
  • What would be the consequences?

The NRC then uses risk information to reduce the probability of an accident and to mitigate its consequences. The following three activities illustrate this concept of risk: climbing Mt. Everest, skydiving, and riding a unicycle.

Climber on Mt. Everest

High Probability, High Consequence. An expedition to Mount Everest has a high probability of serious consequences, such as a fatal fall or frozen extremities. As a result, the overall risk is considered to be very high.


Low Probability, High Consequence. A skydiving accident, in which the parachute fails to open, can also have severe consequences (including fatality). However, the risk is acceptable to many people because using the proper safety precautions can adequately reduce the probability of an accident. As a result, the overall risk is considered to be moderate.

Man riding a unicycle.

High Probability, Low Consequence. A unicyclist has a relatively high probability of falling. However, the consequences of such an accident are relatively minor. The unicyclist usually lands on his or her feet or, at worst, takes a tumble. Thus, even though the probability of falling is high, the consequences are so minor that the overall risk is low.

The NRC uses Probabilistic Risk Assessment (PRA) to estimate risk by computing real numbers to determine what can go wrong, how likely is it, and what are its consequences.

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Information on NRC Risk Assessment