Occupational Dose Evaluation in Support of the Development of the
October, 2012
Rare Element Resources, Inc. Bear Lodge Project
22
This is a reasonable assumption for the ore and pre-concentrate, but not necessarily for materials
that have undergone chemical processing (e.g., those in the Hydromet).
4.1.1
Dose Coefficients
Dose coefficients (DCs) of 1.2 x 10
-3
and 1.5 x 10
-3
mrem hr
-1
per pCi g
-1
for uranium-238 and
thorium-232, respectively, were used to estimate external doses from large sources of NORM
that would be indicative of an open pit surface mine (UNSCEAR, 2000). These DCs assume that
decay series radionuclides are in equilibrium.
Internationally accepted DCs of the ICRP (ICRP, 1994a) were used to estimate internal doses
from inhalation and ingestion of NORM in dust. Table 4.2 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.
Table 4.2 Worker Radiation Dose Coefficients for Inhalation and Ingestion Pathways
Worker Radiation Dose Coefficients
(mrem pCi
-1
)
a
Decay Series
Radionuclide
†
Inhalation
Ingestion
Uranium
Uranium-238
2.1x10
-2
1.6 x10
-4
Uranium-234
2.5 x10
-2
1.8 x10
-4
Thorium-230
2.7 x10
-2
7.8 x10
-4
Radium-226
8.1 x10
-3
1.0 x10
-3
Lead-210
4.1 x10
-3
2.5 x10
-3
Polonium-210
8.1 x10
-3
8.9 x10
-4
Sum of all uraniun-238 decay
series radionuclides except radon-
222
9.3 x10
-2
5.6 x10
-3
Thorium
Thorium-232
4.4 x10
-2
8.1 x10
-4
Radium-228
6.3 x10
-3
2.5 x10
-3
Actinium-228
4.4 x10
-5
1.6 x 10
-6
Thorium-228
1.2 x10
-1
1.3 x10
-4
Radium-224
8.9 x10
-3
2.4 x10
-4
Lead-212
1.2 x10
-4
2.2 x10
-5
Bismuth-212
1.4 x10
-4
9.6 x10
-7
Sum of all thorium-232 decay
series radionuclides except radon-
220
1.8 x10
-1
3.7 x10
-3
a
Worker radiation DCs from ICRP 68 (ICRP, 1994a) except for revised radium-226 inhalation coefficients
from ICRP 72 (ICRP, 1996). All are for least soluble form. Inhalation dose coefficients are for 5 micron particle size.
