Nuclear reactor safety, probability approach

The notion that methods of statistical analysis should be applied to reactor safety standards was first put forward by Siddall of Atomic Energy of Canada Ltd., Chalk River, Ontario in 1959 (57). This early paper is of interest because it invokes the notion of a balance between increased wealth of the community that may be expected to accrue from the advent of nuclear power on the credit side, and risks of injuries and deaths because of the hazards of the nuclear process on the other it goes on to suggest money costs (economic criteria) as the avenue through which to achieve such a balance. The details given in the paper are only generally relevant today, but some of the introductory sentences have a modern sound to them and are worth quoting as an introduction to the basic philosophy of the probability approach to reactor safety. The study of nuclear-reactor safety (i.e., in 1959, some 15 years ago in the life of an industry now only 20 years of age) is in an unsatisfactory state. Some aspects of the problem have received... 

In this paper, we discuss several categories of decay data which have contributed to low-energy nuclear physics, indicate some of the ways they are useful in solving problems in other areas and identify needs for further measurements. Illustrations include half-life and emission-probability data of actinide nuclides important for reactor technology and useful as reference standards for nuclear-data measurements. Decay data of highly neutron-rich fission-product nuclides are important in such diverse areas as astrophysics and reactor-safety research. Some of these data needs and experimental approaches suitable for satisfying them are presented. 

The Reactor Safety Study was prompted in part by a request from Senator John Pastore for a comprehensive assessment of reactor safety. The AEC s first response to this request was the WASH-1250 report entitled The Reactor Safety Study of Nuclear Power Reactors (Light Water-Cooled) and Related Facilities, which was published in final form in July 1973. However, WASH-1250 did not provide a probabilistic assessment of risk as requested in Senator Pastore s letter. At the time, relevant probabilistic estimates were quite limited in scope and/or highly subjective. For example, in a policy paper dated November 15, 1971, to the commissioners proposing an approach to the preparation of environmental reports, the regulatory staff estimated that the probability of accidents leading to substantial core meltdown was 10 per reactor-year. In retrospect, this was a highly optimistic estimate, but it typifies the degree to which meltdown accidents were considered "not credible."... 

DOE commissioned an independent safety, assessment (ISA) of the electric power systems for the Savannah River production reactors. This report (Reference 1) recommended that a graduated-risk- acteptance approach be used to determine the acceptability of the electric. power sy stem for restart. A. key point in this approach is that at one end of the risk spectrum there needs to be a very strong confidence that the systems can accommodate the postulated transients and accidents that could.occur in. the near. future. At the other end of the risk spectrum, posftulated accidents that are rare (that is extremely low probabilities) do not necessarily have to be resolved completely prior to restart. This approach is consi tent with that used by the NRC when considering the readiness of a licensed nuclear power plant to restart. The general restart criteria below are based upoh this graduated-risk-acceptance approach. 

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