WASH-1400 considered the course of events which might arise during a serious accident at a (then) large modern Light water reactor. It estimated the radiological consequences of these events, and the probability of their occurrence, using a fault tree/event tree approach. This technique is called Probabilistic Risk Assessment (PRA). The report concluded that the risks to the individual posed by nuclear power stations were acceptably small, compared with other tolerable risks. Specifically, the report concluded, using the methods and resources and knowledge available at the time, that the probability of a complete core meltdown is about 1 in 20,000 per reactor per year.
The study was peer-reviewed by the 'Lewis Committee' in 1977, which broadly endorsed the methodology as the best available, but cautioned that the risk figures were subject to large uncertainty.
The methods used were comparatively simple and overly-pessimistic by today's standards, and based on early understanding of key phenomenology. Following a period of intensive (and expensive) research and discussion, inspired in part by Three Mile Island, WASH-1400 was replaced in due course first by NUREG-1150 and now by a new study being performed by the Nuclear Regulatory Commission called the State-of-the-Art Reactor Consequence Analyses (SOARCA) Specific Studies were also made of two plants at Zion and Indian Point - the so called Z/IP Study.
The PRA methodology became generally followed as part of the safety-assessment of all modern nuclear power plants. In the 1990s, all US nuclear power plants submitted PRAs to the NRC under the Individual Plant Examination program , and five of these were the basis for the 1991 NUREG-1150.
WASH-1400 is now NUREG-75/014.