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

D.9
D.2.2 Carbon-14
Most 14 C released to the atmosphere from nuclear facilities will be in the form of
carbon dioxide (C0 2 ), with possible traces of organic compounds released from certain
specific processes within the nuclear fuel cycle. After mixing with the existing CO 2 in
the atmosphere, the 14 C0 2 can either become incorporated directly in plant material or
washed out of the atmosphere onto land or water surfaces.
Most analyses of the long-term radiation doses to large population groups from 14C
include the following assumptions:
1. Carbon-14 is released to the atmosphere as CO 2 .
2. It mixes rapidly with all carbon in the world's atmosphere--6.2 x 1017 g
(320 ppm CO 2 ).
3. Mechanisms that remove carbon into less accessible sinks such as the deep ocean
or that dilute the 14C02 with increased CO 2 releases from future fossil-fuel
combustion can be ignored.
4. The specific activity (that is, activity of 14 C per unit weight of carbon) in
the tissues of man eventually equilibrates with that in the atmosphere.
More complicated models are possible. Machta (1973) developed a seven-compartment
model for C02, similar to the one discussed for 3 H. It was further modeled by the EPA
(Magno et al. 1974 and Fowler et al. 1976) for use in predicting radiation doses to large
populations from 14 C injected into the troposphere by the nuclear industry. The EPA model
was used only to predict the specific activity of 14 C in the troposphere including, how-
ever, modifications for the sinks mentioned in assumption 3. Assumption 4 was then used to
calculate dose to man. Fowler et al. (1976) included an estimate that 99% of man's 14C
intake is through food and only 1% is through inhalation.
Killough (1977) further modified the EPA seven-compartment model to incorporate newer
data on diffusive vertical transport of CO 2 in the deep ocean and the relationship between
the concentration of inorganic carbon in the ocean surface waters and the partial pressure
of dissolved CO 2 . The computer code developed by Killough to implement the resulting
model is documented in detail.
For purposes of the commercial waste management analysis, the conservative model out-
lined in assumptions 1 through 4 was adopted. This model was also adopted by the Nuclear
Regulatory Commission (NRC) in its testimony at the Allied General Nuclear Services (AGNS)
reprocessing plant license hearings (Eckerman 1974). By comparison the doses calculated
using this simple approach are about 25% higher than those calculated by EPA (Fowler et al.
1976), 50% higher than those estimated by Baker (1976), and nearly seven times higher than
those obtained by Killough (1977). The comparison with Killough is not, however, straight-
forward because of the assumptions of growing population and increasing CO 2 concentrations
used by that author.