dressing severe accidents. The USNRC also issued a policy statement on safety goals (USNRC, 1986).

By the mid-1980s, new computational models of severe accident phenomenology had been developed and subjected to peer review. Studies of reactor severe accidents and their public health consequences were being carried out throughout the 1980s in many countries with light-water reactor programs. Many conferences and symposia took place, and papers and reports were widely disseminated. In the United States, a major update of the WASH-1400 report was issued (USNRC, 1990). It evaluated severe accident risks at five nuclear plants.

Beginning in 1988, the U.S. nuclear industry performed assessments of severe accident vulnerabilities for each licensed nuclear power plant. These individual plant examinations were done for both internal and external event initiators and were essentially PRAs. The USNRC issued its perspectives documents starting in the late 1990s (USNRC, 1997a, 2002c) which summarized the plant vulnerabilities and proposed modifications for each plant.

At present, severe accident studies are continuing in most countries with light-water reactors. Many international conferences and symposia feature studies on refinement of tools and confirmation of theoretical models based on experiments. Most university programs on nuclear engineering cover severe accidents in their curricula, and the topic is covered in contemporary textbooks and monographs on reactor safety. Short courses on severe accidents are typically offered in conjunction with conferences on PRA. Severe accident management guidelines have been developed and refined based on insights from phenomenological studies.

The most recent risk study that uses current severe accident information is the State-of-the-Art Reactor Consequence Analyses Project (USNRC, 2013c,d). The USNRC is also performing a Level 3 risk analysis of a pressurized water reactor, which will be completed in the next few years.

beyond-design-basis events that have the potential to affect large geographic regions and multiple nuclear plants. These include earthquakes, tsunamis and other geographically extensive floods, and geomagnetic disturbances.

A design-basis event is a postulated event that a nuclear plant system, including its structures and components, must be designed and constructed to withstand without a loss of functions necessary to ensure public health and safety. Such events can include malfunctions of plant structures or components due to manufacturing defects or wear or failures caused by outside agents, for example, natural hazards. An event that is beyond design basis has characteristics that could challenge the design of plant structures



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