Homes > About NRC
> How We Regulate
> Risk Assessment
> Probabilistic
Risk Assessment (PRA)
Probabilistic Risk Assessment (PRA)
On this page:
Definition of PRA
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. Thus, PRA provides insights into the strengths
and weaknesses of the design and operation of a nuclear power plant.
For the type of nuclear plant currently operating in the United States,
a PRA can estimate three levels of risk.
- A Level 1 PRA estimates the frequency
of accidents that cause damage to the nuclear reactor core. This is commonly
called core damage frequency (CDF).
- A Level 2 PRA, which starts with the
Level 1 core damage accidents, estimates the frequency of accidents that
release radioactivity from the nuclear power plant.
- A Level 3 PRA, which starts with the
Level 2 radioactivity release accidents, estimates the consequences in
terms of injury to the public and damage to the environment.
Level 1 PRA
A Level 1 PRA models the various plant responses to an event that challenges
plant operation. The plant response paths are called accident sequences.
A challenge to plant operation is called an initiating event. There
are numerous accident sequences for a given initiating event. The various
accident sequences result from whether plant systems operate properly or
fail and what actions operators take. Some accident sequences will result
in a safe recovery and some will result in reactor core damage. The accident
sequences are graphically represented with event
trees. Each event in the event tree (called a top
event) generally depicts a system that is needed to respond to
the initiating event. An analysis is performed for each top event in the
event tree. This analysis is graphically represented with a fault
tree.
The frequency for each core damage accident sequence is estimated, and the
frequencies for all core damage sequences are summed to calculate the total
core damage frequency. In that way, the Level 1 PRA provides the first measure
of risk -- core damage frequency -- which is the input to the Level 2 PRA.
Level 2 PRA
A Level 2 PRA models the plant's response to the Level 1 PRA accident sequences
that resulted in reactor core damage. Such core damage sequences are typically
referred to as severe accidents. Toward that
end, a Level 2 PRA analyzes the progression of an accident by considering
how the containment structures and systems respond to the accident, which
varies based on the initial status of the structure or system and its ability
to withstand the harsh accident environment. Thus, a Level 2 PRA must consider
the key phenomena that affect accident progression. The following are two
of the countless examples that illustrate the phenomena that must be considered:
- Do tubes in the steam generator rupture?
- Is the reactor core debris in a coolable configuration?
Once the containment response is characterized, the analyst can determine
the amount and type of radioactivity released from the containment. Thus,
the Level 2 PRA estimates the second measure of risk -- radioactivity release
-- which is the input to the Level 3 PRA.
Level 3 PRA
A Level 3 PRA is often called a consequence analysis. Consequences
result from the radioactive material released in a severe accident.
A Level 3 PRA estimates those consequences in the following terms:
- Health effects (such as short-term injuries or long-term cancers) resulting
from the radiation doses to the population around the plant
- Land contamination resulting from radioactive material released in the
accident
Consequences are estimated based on the characteristics of the radioactivity
release calculated by the Level 2 PRA. Those consequences depend on several
factors. For example, health effects depend on the population in the plant
vicinity, evacuation conditions, and the path of the radioactive plume. The
plume, in turn, is affected by wind speed and direction, as well as
rainfall or snowfall. Similarly, land contamination depends on the characteristics
of the radioactivity release and how the land surrounding the plant is used.
The Level 3 PRA estimates the final measure of risk by combining the consequences
with their respective frequencies. (What can go wrong, how likely is it,
and what are the consequences?) For instance, a Level 3 PRA might estimate
that an accident would create one chance in a million that a person living
near the plant would experience radiation exposure equivalent to a chest
x-ray, and one chance in a billion that some people would develop cancer
over the next 50 years.
To request additional information, Contact
Us About Risk-Informed, Performance-Based Regulation.
|
