Even in the absence of depraved terrorist threats, nuclear plants have been designed to respond safely to postulated accidents. Redundant safety features are built into plants to trigger safe shutdown and containment of possible accidents. The defined accidents range from minor leakage and operator errors to a complete loss of coolant from the reactor. Post-accident scenarios are postulated by experts in reactor and containment thermal-hydraulics, and all other sciences embraced by nuclear power plant design. The probability of failure is determined for all engineered safety systems. Then analytical and experimental programs are employed to predict the long term post-accident thermal-hydraulic state of a plant and its effect on the environment. The postulated accidents and safety system responses include effects resulting from mechanical damage and component malfunctions, such as pipe ruptures and the failure of pumps and valves. The initiating causes can be material failure, human error, and environmental effects from earthquakes, floods, and other severe acts of nature. It is prudent to build on an already established safety and accident technology to include the effects of external, planned attacks on a nuclear plant. This process includes “matching wits” with the minds of those who plot evil, and reinforcing protective security barriers where potential vulnerabilities are detected. Hard questions to ask and answer are, “What are the greatest potential security threats to a nuclear power plant? What possible human activity could make them happen? How can they be prevented?” Reactor and containment thermal-hydraulics contributes significantly to answering these questions.

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