I.1.1 Dose from Airborne Noble Gases and Fission Products
Estimates of nuclide-specific ground-level activity concentrations in air at a particular direction and distance and annual and quarterly doses can be calculated as a function of distance and direction using accepted air dispersion methodologies that account for radioactive decay and plume depletion during transport, release height, and average annual (or longer) meteorology (wind speed, direction, atmospheric stability) as well as site-specific features such as terrain features. The organ dose resulting from immersion in air containing radioactive gases (sometimes referred to as a radioactive plume) at any location can then be calculated fairly accurately for each of the specific nuclides released and their specific gamma and beta emissions (Federal Guidance Report 12 [USEPA, 1993]).
The exposure rate from immersion in a plume of noble gases varies significantly with the composition of the gaseous cloud versus distance. The exposure rate from the various radioactive gases varies significantly due to large differences in the energies of their respective radiation emissions. As shown in Table I.1, the effective dose factors for short-lived emitters such as krypton-87 and 88 are significantly higher than that for longer-lived xenon-133, which comprises most of the airborne effluents from currently operating nuclear plants. However, because of the shorter half-lives of these radioisotopes, their relative contribution to doses to persons living farther downwind will be somewhat less than the relative effective dose factors shown in Table I.1.
I.1.2 Doses from Deposited Radionuclides
Calculations of external exposure and organ doses from particulate radioactive materials deposited on the ground are based on the same transport model used for estimating noble gas concentrations downwind and models for calculating dry and wet deposition and the dose rate per unit
TABLE I.1 Exposure Rate Dose Conversion Factors
|Nuclide||Half-life||Effective Dose Factor(Sv Bq-1 s m-3)|
|Kr-87||76 min||4.0 * 10-14|
|Kr-88||2.8 h||9.7 * 10-14|
|Xe-133||5.2 d||1.3 * 10-15|
|Xe-135||9.1 h||1.1 * 10-14|
|Xe-135m||15 min||1.9 * 10-14|
|Xe-138||14 min||5.5 * 10-14|
|SOURCE: Effective dose factors from Federal Guidance Report 12.|