different models of commercial aircraft), or about whether the risk arises from many small accidents or a few large ones.

Safety of Normal Operations Although greater public concern surrounds reactor accidents, controversy surrounds the safety of the normal operations of nuclear power systems. Three separate areas of operation have figured in these controversies.

  1. The operation of reactors involves some discharge of radioactivity to the environment. Attempts have been made to implicate these discharges as significant causes of public morbidity.

  2. During the course of reactor operations, and specifically during maintenance periods, workers at nuclear power plants are exposed to radiation from contaminated equipment. It has been charged that this represents undue and uncompensated risk to the workers.

  3. At various stages in the nuclear fuel cycle, radioactive effluents are discharged. This is another potential source of public morbidity.

These points are discussed at greater length in chapter 9 of this report. The last point will be treated in the subsequent discussion of nuclear waste management and disposal. It is worth noting here that continued review has indicated that normal reactor discharges, within existing regulatory limits, are not significant causes of public morbidity. The evidence on which the first charge is based has not stood the test of scientific scrutiny. With regard to occupational exposures of workers in nuclear installations, on the other hand, it follows from the linear dose-response hypothesis that existing limits on occupational exposure to radiation present a marginal risk to workers of slightly increased risk of cancer. (This hypothesis is still in dispute, as discussed in detail in chapter 9.)

Reactor Accidents More than 98 percent of the radioactive atoms made in a reactor, and an even greater fraction of those that remain radioactive after a few seconds, are generated in the fuel. The two types of new radioactive atoms are fission products and actinides. Fission products are a congeries of nuclides of medium atomic mass number (66–172 amu, with the largest quantities found near mass numbers 95 and 140). Actinides are the elements beyond actinium, and include both natural and manufactured isotopes of thorium, protactinium, uranium, neptunium, plutonium, and heavier elements. The half-lives of fission products vary from fractions of a second to very large values, and different fission products become important at different times after they are formed. The artificial actinides of consequence have longer half-lives than the more important fission products, and correspondingly less radioactivity (for a given number of

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