Management of Commercially Generated Radioactive Waste - U.S. ...

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nuclides radium-226 ( 226 Ra) and radon-222 ( 222 Rn) as well as 31 other radionuclides.
Other radionuclides decay directly to stable nuclides. The most significant of the primo-
radial radionuclides in terms of dose is potassium-40 ( 4 0 K). Aside from a small contribu-
tion to dose by rubidium-87 ( 87 Rb), the remainder of the primordial radionuclides, including
plutonium-244 (244Pu), occur in extremely small amounts and make no significant contribution
to dose. Doses resulting from these primordial radionuclides are discussed below.
External Gamma Radiation. The significant contributors to dose to people from outside
of their bodies are 40K and the decay products of the 238U and 23 2 Th series. The principal
determinant of outdoor terrestrial radiation at a given location is the soil concentration
of natural radionuclides. In addition to soil composition, the radiation outdoors varies
depending on the moisture content of the soil, the presence and amount of snow cover, and
on the radionuclide concentration in the atmosphere which itself is quite variable.
Indoors, the level of radiation is modified by the degree of shielding provided by the
building materials against the outdoor radiation, and the amount of radiation originating
from radionuclides in the building materials. Variations in outdoor radiation will be par-
tially reflected indoors and, in addition, the contribution from radon decay products will
depend on the room air ventilation rate. Each of these factors can play an important role
in determining the exposure received by the population.
The overall population-weighted dose rate in the United States from external terres-
trial radiation is estimated to be 28 mrem/yr. Moreover, variability in external terres-
trial radiation is larger than that for other natural sources of human exposure. This
variation in dose rate is characterized by nominal external terrestrial dose rates to the
whole body of 15, 30, and 55 mrem/yr for the Atlantic and Gulf Coastal Plains, for the
majority of the United States, and for an undetermined area along the Rocky Mountains,
respectively.
Internally Deposited Radionuclides. While all natural radionuclides may add to inter-
nal (inside the body) radiation doses, only a few are found to be significant contributors.
These include 3 H, 14C, 40 K, and 226 Ra and 228 Ra and their decay products. Within the United
States, all of these are relatively uniformly distributed so that their levels in foods and
water do not vary appreciably with geographic location. In the United States widespread
food processing and widespread transportation of foods and people have an additional
"averaging" effect on radionuclide contents of diets throughout all geographic areas.
The'average total internal whole-body dose rate of about 22 mrem/yr is dominated by
about 20 mrem/yr from 40K.(a) Dose rates to specific organs from internally deposited
radionuclides are about 30 mrem/yr to the gonads and other soft tissues, 60 mrem/yr to bone
(a) Potassium is an essential element in the body and is physiologically controlled, hence
variations in dietary composition will have little effect on body content or radiation
dose received. The same is largely true for the cosmogenic radionuclides 3 H and 14 C.