Preliminary Public Dose Evaluation in Support of the Development of
November , 2012
Rare Element Resources, Inc. Bear Lodge Project
7
1.3.2
Units of Radioactivity and Dose
The U.S. has not consistently adopted the International System of Units for radioactivity or
radiation dose. The unit for radioactivity in the U.S. is the curie (Ci). The Ci is based on the
decay rate of 1 gram (g) of radium-226 and is equal to 3.7 x 10
10
radioactive decays per second
(s). The Ci is a large amount of radioactivity, consequently environmental levels of NORM are
often expressed in picocuries (pCi [1x10
-12
Ci]), which are twelve orders of magnitude less than
a Ci. The amount of radioactivity is directly related to the amount of the radionuclide and
inversely related to its half-life. The specific activity of a radionuclide relates the amount of
radioactivity per unit mass and can be expressed in units of Ci g
-1
. For example, the specific
activity of natural uranium is 7.1 x 10
-7
Ci g
-1
; therefore a concentration of 1 milligram (mg) per
kilogram (kg) of natural uranium in soil is equal to 710 pCi kg- of soil. Similarly, the specific
activity of natural thorium is 2.2 x 10
-7
Ci g
-1
; therefore 1 mg kg
-1
of natural thorium in soil is
equal to 220 pCi kg
-1
of soil. The activities of specific isotopes of uranium can be determined by
multiplying the radioactivity percent abundance in Table 1.1 by the activity of natural uranium.
For natural thorium, the activity is split equally between thorium-232 and thorium-228, provided
equilibrium (i.e., the radioactivity is the same) of the series radionuclides exists.
The U.S. unit for dose equivalent is the rem and is equal to the absorbed dose in rad (100 ergs g
-
1
) multiplied by a radiation weighting factor. The dose equivalent, hereafter referred to as “dose”,
is also often expressed as millirem (mrem [10
-3
rem]).
Note that the risk of detrimental stochastic effects resulting from a dose of radiation is assumed
to be proportional to the dose (International Commission on Radiological Protection [ICRP],
1990), although this has not been demonstrated at low doses typical of routine public exposures
to NORM.
1.3.3
Background Radiation Doses in the United States
Background radiation doses from natural sources, while variable, always occur to some degree.
Sources of background radiation dose include the following:
•
Cosmic radiation from the sun and other celestial sources,
•
Terrestrial radiation from radionuclides present in soils and rocks,
•
Radon and its short-lived decay products in indoor and outdoor air, and
•
Radionuclides in the body, acquired from the inhalation of air, and ingestion of food and
water.
The global average background dose rate from naturally occurring sources is approximately 240
mrem y
-1
: some places in the world have dose rates of up to 1000 mrem y
-1
(UNSCEAR, 2000).
