The Nuclear Energy Technology Center at Sandia performs risk assessments for all types of nuclear reactors, including commercial power plants and government-owned facilities. Our customers include the Nuclear Regulatory Commission and the Department of Energy. Sandia has developed state-of-the-art techniques to evaluate all phases of risk, from accident initiation through consequence assessment. All plant operating modes from full power to refueling can be considered using these techniques. Our risk assessments are supported by a sound technical base of experiments and computer models that address plant behavior for a variety of severe accident conditions.
Reactor risk assessments are often performed in four parts. These parts are described below.
Accident Sequence AnalysisAll types of accident initiators can and should be considered in a comprehensive risk assessment. These include equipment failures and human errors within the plant and external initiators, such as earthquakes and tornadoes. Sandia has developed methods for treating all such initiators in a consistent fashion so that results can be compared in a meaningful way. Event-tree and fault-tree techniques are used to delineate possible accident sequences and determine important failure combinations. Common-cause failures receive particular attention in these studies. The output from the accident sequence analysis is a set of accident sequences and their frequencies of occurrence.
Accident Progression and Source Term AnalysisGiven that a core damage accident has occurred, the accident progression analysis determines the subsequent plant behavior. This behavior includes a variety of core meltdown processes and containment responses. Advanced event tree techniques, backed up by analysis and experiments, are used to delineate the possibilities. Of particular interest is the mode and timing of containment failure (if any) relative to releases of fuel from the reactor vessel. The source term analysis uses the results of the accident progression analysis and determines the nature of any fission product releases, including the quantity of material, the release timing, and the energy associated with the release.
Consequence AnalysisOnce the source terms are known, a variety of consequences can be estimated, including both health and economic effects. The analyses can consider weather variations, along with emergency response measures such as evacuation.
Risk and Uncertainty AnalysisOnce the three previous analyses are complete, the results can be combined into an overall risk calculation. That is, the likelihoods of different accident scenarios are combined with their consequences. After point estimate calculations are complete, uncertainty and sensitivity analyses are performed to characterize the robustness of the results. The results can then be used by decision makers to identify issues that are most important to risk and to guide risk management decisions. Comparisons with safety goals can be made and a variety of cost-benefit studies can be performed.
Future Work
The risk analysis process described above is the most comprehensive of its type in the world. These techniques are now being adapted to other types of problems for the Department of Energy and other customers. The following capabilities are being provided to customers in these studies:
Decision analysis
Risk management
Hardware and software reliability
Human factors and human reliability
Accident simulation
Consequence modeling
Cost-benefit studies
Uncertainty analysis
Assessment of risks from natural disasters
Equipment response to abnormal environments
For further information, contact:
Allen L. Camp
Sandia National Laboratories, MS-0747
Albuquerque, NM 87185-0747
Phone: (505) 844-5960
e-mail: alcamp@sandia.gov
Submitted October 1996 Layout design by Wanda Mar.
Submitted October 1996