Occupational Dose Evaluation in Support of the Development of the
October, 2012
Rare Element Resources, Inc. Bear Lodge Project
21
Inhalation of radon (both radon-222 and radon-220) and their short-lived decay
products is another potential pathway for internal dose at the BLP. Active
ventilation of process buildings and natural ventilation of the mine area will
control workplace work place radon and radon decay products to acceptable
levels.
Another potential pathway for internal doses --but less important than inhalation--
is the unintended ingestion of dust containing thorium and uranium and their
decay products. This pathway is typically controlled administratively, by using
workplace rules and best practices such as prohibiting smoking in the workplace
and providing a clean lunch and break area. Contamination in the process plants
will be controlled via good housekeeping practices, to minimize the migration of
NORM from the plant to clean areas such as lunch/break rooms.
4.1
Worker Dose Estimates
The workers at BLP will be exposed to the NORM in both the ore and other materials at
various work locations. The magnitude of the potential exposures will depend on the
concentrations of radionuclides in the materials being mined and/or processed.
Table 4.1 provides the activity-based concentrations used for this dose assessment.
Table 4.1 Thorium-232 and Uranium-238 Activity Concentrations
for Material Used in Dose Evaluation
Material
Thorium Content
(wt %)
Uranium Content
(wt %)
Thorium-232
Concentration
(pCi/g)
a
Uranium-238
Concentration
(pCi/g)
a
High grade oxide
ore
0.036
0.01
40
33
Pre-concentrate
0.21
0.016
230
50
a
Mass units were converted to activity units using specific activity of 1.1 x 10
-7
and 3.3 x 10-7 Ci/g for thorium-232 and uranium-238,
respectively.
The nominal average uranium and thorium concentrations in the high grade oxide ore (Table 2.1)
were used to estimate prospective external doses. The average uranium and thorium
concentrations for the pre-concentrate from the PUG process presented in Table 2.1 were used to
estimate prospective internal doses. These two types of material were used for the dose estimates
because they have the highest concentrations of radionuclides and, therefore, the associated dose
estimates are conservative for all processes. The iron hydroxide sludge has a higher radionuclide
concentration than the pre-concentrate, but when combined with the sulfate sludge, the tailings
have a lower radionuclide concentration. Similarly, the OXCA ore has higher radionuclide
content relative to the high grade ore, but it is expected that --on a volume basis-- more high
grade ore will be processed and for this dose evaluation better represents actual conditions. The
thorium and uranium series decay products were also assumed to be in radioactive equilibrium.
