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Uranium Milling

Uranium milling and disposal of the resulting byproduct material waste by NRC licensees are regulated under 10 CFR Part 40, Appendix A. Uranium ore is extracted from the earth through a variety of mining techniques.

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Extraction Methods

Traditionally, uranium was extracted from open pits and deep shaft mines. In the past decade, alternative methods, such as solution extraction (in situ leach) in which solutions are injected into underground uranium deposits to dissolve uranium, have become more widely used. Uranium is extracted from ore at uranium mills and at in-situ leach facilities. (The NRC-licensed heap leach and ion-exchange facilities no longer operate.) Both extraction processes concentrate the uranium into the product known as "yellowcake" (U3O8) because of its color.

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Uranium Mills and Milling

A uranium mill is a chemical plant that extracts uranium from mined ore. At conventional mills, the ore arrives via truck and is crushed and leached. In most cases, sulfuric acid is the leaching agent, but alkaline leaching can also be done. The leaching agent not only extracts uranium from the ore but also several other constituents: vanadium, selenium, iron, lead, and arsenic. Conventional mills extract 90 to 95 percent of the uranium from the ore. Mills are typically in areas of low population density, and they process ores from mines within 50 kilometers (30 miles). Most mills in the United States are decommissioning. (See Uranium Recovery Sites Undergoing Decommissioning for more information.)

As defined in 10 CFR Part 40, uranium milling is any activity that produces byproduct material. Part 40 defines byproduct material as it is defined in Section 11e.(2) of the Atomic Energy Act: "...the tailings or wastes produced by the extraction or concentration of uranium or thorium from any ore processed primarily for its source material content" but adds "...including discrete surface wastes resulting from uranium solution extraction processes." In-situ leach facilities perform uranium milling under this definition.

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In-Situ Leach Milling Facilities

In-situ leach (ISL) milling facilities are one means of extracting uranium from underground. ISL facilities recover uranium from low grade ores that may not be economically recoverable by other methods. In this process, a leaching agent, such as oxygen with sodium carbonate, is injected through wells into the ore body to dissolve the uranium. The leach solution is pumped from there to the processing plant and ion exchange separates the uranium from the solution. After additional purification and drying, the yellowcake is placed in 55-gallon drums. About 12 such ISL facilities exist in the United States. Of these, four are licensed by the NRC, and the rest are licensed by Agreement States.

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Potential Industrial Hazards and Wastes

Potential Hazards: The NRC requirements for uranium mills control industrial hazards and address waste and decommissioning concerns. Because this uranium is not enriched, there is no criticality hazard and little fire or explosive hazard for it. The solvent extraction process does present a fire hazard, however. The primary hazards associated with milling operations are occupational hazards found in any metal milling operation that uses chemical extraction plus the chemical toxicity of the uranium itself.

Radiological hazards are low at these facilities as uranium has little penetrating radiation and only moderate non-penetrating radiation. The primary radiological hazard is due to the presence of radium in the uranium decay chains and the production of radon gas from the decay of radium and radon progeny (short-lived radon decay products).

Mill Tailings: The solid (sandy) waste from the conventional uranium milling process is called mill tailings. Uranium mill tailings, which contain most of the progeny of uranium, are a significant source of radon and radon progeny releases to the environment. The hazards from radon involve inhalation of radon progeny that may be deposited in the respiratory tract. Alpha radiation could be emitted into those tissues and can pose a cancer risk to those workers.

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Wednesday, September 26, 2007