A safety yardstick for evaluating nuclear power reactors

Abstract

To communicate with the public, it would be helpful for the nuclear industry to have a simple quantitative measurement system to describe the meaning of all the qualitative terms being used. The Nuclear Regulatory Commission's Advanced Reactor Policy calls for additional safety margin to be provided between the current nuclear plant safety levels and the levels provided by any new design. To aid public acceptance, this “margin” should be measurable in terms the public can understand. Unquantified terms such as “passive” and “inherent” are not well defined as yet, and as many have pointed out, are often used incorrectly. Another area of confusion for the general public is the “probability” of an accident occurring. Three Mile Island and Chernobyl have diminished the meaning to the public of measuring safety based on accident probabilities. As far as the public is concerned, the nuclear industry has proven that severe accidents can occur, even the low probability ones. Radiation exposure to workers, neighbors, family members, and its impact on their future generations is the major public concern. Nevertheless, some tie between safety levels and the probability of an accident occurring is required for effective communication. The kinds of nuclear plant accidents that alarm the general public normally involve reactor failures that produce fission product releases from the fuel beyond normal operating levels. A news release reporting on any event involving the words “radiation” or “fission products” sets off alarms in the minds of the general public. This paper develops a relative measurement system that uses “reaction time factors” to produce quantitative values that can be used to define when the term “inherent” is valid, when “passive” is more appropriate, and when neither one should be used. A reaction time factor is defined as the reaction time available for preventing major fuel failure following bounding events. The current operating reactor types examined include the Light Water Reactors (LWR), the Chernobyl type (RBMK), the British Advanced Gas Reactor (AGR), and High Temperature Gas-Cooled Reactors (HTGR). Advanced reactor types examined include the Advanced Light Water Reactor (ALWR), the Advanced Liquid Metal Reactor (ALMR), and the Modular High Temperature Reactor (Modular-HTGR). Using “time” as a common yardstick measurement approach, reaction time factors for the above reactors range from seconds to ones approaching infinity. Using the criteria set forth in this paper, the terms “inherent safety” should be used only when the reactor concept's reaction time to fuel failure approaches an infinite time for events with probabilities of occurrence much less than 1 in 1,000,000 per year. The term “passive safety” should be used only when the concept's reaction time to fuel failure approaches an infinite time for events with probabilities of occurrence equal to or greater than one in 1,000,000 per year.