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

D.2
employed here except that the plume width is determined by sector boundaries rather than by
a Gaussian concentration gradient. The contribution of gamma radiation to total-body dose
was estimated by calculating the tissue dose at 5 cm depth. An occupancy factor may be used
to account for the fraction of the year a person is exposed to the cloud. Also a shielding
factor may be employed to correct for any shielding by buildings or structures between the
recipient and the cloud.
D.1.1.3 Inhalation Dose
The air concentrations, derived as described above, were used along with the ventila-
tion rate and dose factors to estimate the dose through the inhalation of radionuclides
dispersed in the air.
The ventilation rate is the volume of air taken in by an individual per unit time. A
value of 0.23 &/sec was used in this study (ICRP 1959).
The inhalation dose factor is given in units of rem/yr per Ci/yr intake and is depen-
dent on the complex transport, retention, and elimination of radionuclides through the
respiratory and gastrointestinal tracts. The model of the respiratory tract adopted by the
Task Group on Lung Dynamics forms the general basis for the calculation of this dose factor
(ICRP 1966). The computer code used for the calculations was DACRIN (Houston et al. 1974).
D.1.1.4 Ground Contamination Dose
Radionuclides from the air may settle on the ground, where they can accumulate during
the time of the release. These can be a source of radiation for an individual or population
groups.
This dose is determined using the 1) air concentration, 2) deposition "velocity" of the
radionuclides traveling to the surface from the air, 3) an exponential expression which
accounts for the accumulation of the radionuclide on the ground over a certain time period,
4) a dose factor, and 5) an occupancy factor.
The deposition "velocity" given in terms of m/sec is highly dependent on surface rough-
ness, wind speed, and particle size. Based on many experimental studies, values of
0.001 m/sec for particles and 0.01 m/sec for iodine gas were selected for use in this report
(Slade 1969).
The time over which the radionuclides accumulate in the soil is dependent on the life-
time of the facility releasing the material. In this study a value of 30 years is used,
which is considered to be about the average lifetime of a nuclear facility.
The dose factor for the dose from ground irradiation is calculated by assuming that a
receptor is 1 m above a large, nearly uniform, thin sheet of contamination (Soldat 1971,
Fletcher and Dotson 1971). A factor of 0.5 to account for dose reduction due to ground
surface roughness is also included in dose factors. These dose factors have units of rem/hr
per pCi/m 2 of surface.