The process of extracting uranium from ore involves crushing, grinding, leaching, separating the leach liquor from the ore, then recovering the uranium from the leach liquor. As a result, residual long-lived nuclides from the uranium decay series concentrate in the sand-like waste product (i.e., tailings), of which the most prominent constituents are radium-226 (Ra-226) and thorium-230 (Th-230). The passage of the Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978, established the regulatory framework, under which the U.S. Environmental Protection Agency (EPA) was charged with developing standards for the cleanup and disposal of tailings at 24 designated inactive uranium processing sites located in 10 states. Title 40, Code of Federal Regulations, Subpart B, Part 192.12 (40 CFR 192.12) requires that the concentration of Ra-226 in land averaged over any area of 100 square meters shall not exceed the background level by more than 5 picocuries per gram (pCi/g), averaged over the first 15 centimeters (cm) of soil below the surface, and 15 pCi/g, averaged over 15-cm-thick layers of soils more than 15 cm below the surface. However, Th-230 is not specifically addressed by the EPA in 40 CFR 192.12, which naturally decays with a half-life of 77,000 years to form Ra-226. Consequently, the cleanup of the initial Ra-226 contamination according to the standards will not necessarily mitigate against the eventual ingrowth of residual Ra-226 with time, due to the radioactive decay of residual Th-230. Therefore, to direct the excavation of residual Th-230, four generic protocols are being used at Uranium Mill Tailings Remedial Action (UMTRA) Project sites, as follows: 1) Determining the allowable remaining concentration of Th-230 in surface and subsurface soils; 2) Encountering Th-230 contamination in the unsaturated subsurface soil; 3) Encountering Th-230 contamination in the saturated zone; and 4) Verification sampling. The four generic protocols, developed in conjunction with the supplemental standards provision, ensure protection of the general public by reducing exposures to levels that are As Low As Reasonably Achievable (ALARA), while considering practical measures necessary to excavate Th-230 under conditions encountered at the UMTRA Project site.
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