The effluent release and meteorological data collected by plant licensees and reported to the U.S. Nuclear Regulatory Commission (USNRC) are intended to demonstrate compliance with applicable USNRC regulations. These data were not intended to be used for dose reconstruction to support an epidemiologic study. The suitability of this information to support an epidemiologic study depends on the intended use of the dose reconstruction. For example, it might be necessary to obtain hourly or daily data on effluent releases and meteorological conditions at each facility to reconstruct doses to specific individuals living near those facilities. One the other hand, data that are averaged over longer time periods (weeks and months) might be sufficient to obtain rough estimates of annual doses to populations as a function of distance and direction from those facilities. Dose reconstruction is discussed in Chapter 3.


The operation of nuclear plants produces large quantities of radioactive materials (Appendix D). Quantities of radioactive materials are most readily expressed in terms of activity, defined as the rate of radioactive decay of that material. Activity is usually expressed in units of becquerels (abbreviated Bq; 1 Bq = 1 decay per second) or curies (abbreviated Ci; 1 Ci = 3.7 × 1010 [37 billion] decays per second).2 An operating nuclear reactor can contain on the order of 1014 Ci of activity excluding very-short-lived radionuclides (NCRP, 1987). Most of this activity is the result of fission of the reactor fuel (see Appendix D).

A small fraction3 of this activity is typically emitted to the environment each year as a result of normal plant operations. Radioactive effluents are released in airborne and liquid form. They originate from several sources within a nuclear plant:

  • Fission of residual uranium contained on the exterior of the fuel rods, referred to as tramp uranium.
  • Leaks from failed fuel rods.
  • Diffusion of radioactive gases through intact fuel rods.
  • Activation of materials in reactor cooling water.

2 These units are used interchangeably in this chapter, depending on the source of data. In ternational organizations generally use becquerels. Nuclear facility licensees and the regulator generally use curies.

3 As will be shown elsewhere in this chapter (see Figures 2.1 through 2.4), operating nuclear plants currently release a few curies to a few hundred curies of activity per year to the envi ronment. However, some plants emitted several hundred thousand curies of activity per year to the environment in the past.

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