Preliminary Public Dose Evaluation in Support of the Development of
November , 2012
Rare Element Resources, Inc. Bear Lodge Project
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Starting with uranium-238, a series of 14 different radionuclides (including uranium-234) are
produced through radioactive decay until non-radioactive lead-206 is reached. This radioactive
decay chain is called the uranium-238 decay series and its radionuclides emit alpha, beta and
gamma radiation (Figure 1.3).
In terms of radiation dose to the public, one important member of the uranium-238 decay series
is radon-222 (radon). As a noble gas, radon is chemically inert and —if formed in near-surface
soil or rock from the decay of radium-226— is free to diffuse into the atmosphere. Radon (and
its short-lived decay products) can accumulate inside homes or other structures and in the United
States (US) is the largest contributor to background radiation dose from natural sources.
Generally, radon is not a concern in the outdoor environment because of atmospheric dispersion.
Buildings with industrial processes often have high ventilation rates compared to residential
houses, thereby mitigating the potential impacts of radon therein.
Natural thorium consists almost entirely of thorium-232 by mass. Like uranium-238, thorium-
232 is the first radionuclide in a decay series ending in non-radioactive lead-208. As shown in
Figure 1.4, there are 11 radionuclides in the series with decays resulting in emission of alpha,
beta and gamma radiation. Radon-220 (thoron), known historically as thoron because it is in the
thorium-232 decay series, is an isotope of radon that is released to the atmosphere from radium-
224 in near-surface soil and rock. With a half-life of 55 s, thoron decays rapidly and —coupled
with atmospheric diffusion—is generally not considered an environmental concern.
For the purposes of this report, the term radon includes radon-222 and radon-220.
