Level 3 PRA Project

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Risk and Probabilistic Risk Assessment

According to the traditional definition, risk is the product of the likelihood and consequences of an adverse event. Probabilistic risk assessment (PRA) is a systematic analysis tool consisting of specific technical elements that provide both qualitative insights and a quantitative assessment of risk by addressing the following questions, commonly referred to as the "risk triplet": (1) What can go wrong? (2) How likely is it? and (3) What are the consequences? Modern PRAs have also incorporated uncertainty analyses to address a fourth question: How confident are we in our answers to these three questions? In this way, PRAs allow the identification, prioritization, and mitigation of significant contributors to risk to improve nuclear power plant safety.

PRAs for nuclear power plants can vary in scope, depending on their intended use. The scope of a PRA is defined by the degree of coverage of the following five factors: (1) radiological hazards, (2) population exposed to hazards, (3) plant operating states, (4) initiating event hazards, and (5) level of risk characterization.

The Importance of Level 3 PRA

a flow chart of Probabilistic Risk Assessment (PRA) levels - consisting of the following items flowing from left to right: A round flowchart symbol: Initiating Event, a square flowchart symbol: Level 1 PRA Plant System Respones (Accident Sequences), A round flowchart symbol: Accident Sequences Causing Core Damage, a square flowchart symbol: Level 2 PRA Core and Containment Response (Severe Accident Progression), a round flowchart symbol: Radioactive Release, a square flowchart symbol: Level 3 PRA Influence of External Factors: -Type of release -Weather -Population -Evacuation, and finally, a round flowchart symbol: Consequences: Illnesses or Land Contamination

PRAs for nuclear power plants can estimate risk measures at three different levels of characterization using sequential analyses in which the output from one level serves as a conditional input to the next.  Using event trees and fault trees, a Level 1 PRA models various plant and operator responses to initiating events that challenge plant operation to identify accident sequences that result in reactor core damage. The estimated frequencies for all core damage accident sequences are summed to calculate the total core damage frequency (CDF) for the analyzed plant.

A Level 2 PRA models and analyzes the progression of "severe accidents"—those Level 1 PRA accident sequences that result in reactor core damage—by considering how the reactor coolant and other relevant systems respond, as well as how the containment responds to the accident. This analysis is based on both the initial status of structures and systems and their ability to withstand the harsh accident environment. Once the system and containment response is characterized, the frequency, type, amount, timing, and energy content of the radioactivity released to the environment—also known as source term characteristics— can be determined.

A Level 3 PRA models the release and transport of radioactive material in a severe accident and estimates the health and economic impact in terms of different offsite consequence measures, for example: (1) early fatalities and injuries and latent cancer fatalities resulting from the radiation doses to the surrounding population, and (2) economic costs associated with evacuation, relocation, property loss, and decontamination. Offsite consequences are estimated based on the Level 2 PRA source term characteristics, and on several other factors affecting the transport and impact of the radioactive material, including meteorology, demographics, emergency response, and land use. Combining the results of the Level 1 and Level 2 PRAs with the results of this consequence analysis, only the Level 3 PRA estimates the integrated risk (likelihood times consequences) to the public for the analyzed nuclear power plant. In fact, only a Level 3 PRA can estimate the two high-level quantitative health objectives related to early and latent cancer fatality risks that the U.S. Nuclear Regulatory Commission (NRC) identified in a 1986 safety goal policy statement on determining what level of risk is acceptable to ensure adequate protection of public health and safety

Project Objectives

The full-scope site Level 3 PRA project includes the following objectives:

  • Develop a Level 3 PRA, generally based on current state-of-practice methods, tools, and data, that (1) reflects technical advances since completion of the NUREG-1150 studies, and (2) addresses scope considerations that were not previously considered (e.g., low power and shutdown, multi-unit risk, and spent fuel storage).
  • Extract new risk insights to enhance regulatory decision making and help focus limited agency resources on issues most directly related to the agency's mission to protect public health and safety.
  • Enhance PRA staff capability and expertise and improve documentation practices to make PRA information more accessible, retrievable, and understandable.
  • Obtain insight into the technical feasibility and cost of developing new Level 3 PRAs.

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