Preliminary Public Dose Evaluation in Support of the Development of the
November 2012
Rare Element Resources Bear Lodge Project
28
4.3
Dose Conversion Factors
The potential annual dose to members of the public from the inhalation of radon-220, radon-222
and their associated short lived decay products was estimated in part by using dose conversion
factors and decay products fraction of equilibrium values (indoors) contained in Annex E of
Sources and Effects of Ionizing Radiation- Volume II
(UNSCEAR,2006). AERMOD can output
radon concentrations in units of pCi per liter (L). Therefore, it is helpful to develop a dose
conversation factor (DCF) for this concentration unit to estimate dose at the receptor locations.
Doses from radon were assumed to occurring indoors (i.e. average indoor equilibrium factors
were used). The radon dose conversion factors were calculated using the following equation:
=
Where:
DFC
Rn
= the DCF for estimated radon-222 and radon-220
concentrations (mrem pCi
-1
L)
f
rn
= the fraction of equilibrium for radon decay products (0.4 for radon-222
decay products and 0.02 for radon-220 decay products).
DCF
EEC
= DCF equilibrium-equivalent concentration [EEC (0.0226 and 0.148
mrem pCi
-1
L hr
-1
for radon-222 and radon-220 respectively)]
T
= exposure duration (8,760 h yr
-1
)
The above equation and assumptions provides DFC
Rn
s of 79.2 and 25.9 mrem pCi
-1
L for radon-
222 and radon-220, respectively. Annual doses from inhalation of radon and associated decay
products were estimated by multiplying the predicted radon concentration by the respective
DCF.
Internationally-accepted Dose Coefficients (DCs) of the ICRP (ICRP, 1994a) were used to
estimate internal doses from inhalation of NORM in dust. Table 4.5 lists the DCs. This
evaluation omits the small contributions to dose from the uranium-235 decay series. The DCs for
each radionuclide in the thorium and uranium decay series were summed, resulting in two DCs:
one each for the thorium and uranium decay series. As in the external dose estimate,
radionuclides in each decay series were assumed to be in equilibrium.
