Uranium

Mining and Milling Waste

Waste from the uranium industry occurs at all stages of the nuclear fuel cycle. From mining alone, there are four main types: waste rock from the actual mining; tailings from the ore processing (commonly called “milling”); wastewater; and industrial waste, such as the radioactively contaminated scrap produced when a uranium mill is closed down. Uranium ore occurs naturally as mineralization in sandstones and quartz pebble conglomerate rocks, as well as (to a lesser extent) in other types of mineral vein deposits. Mining takes place in 18 countries. It is most prominent in Australia, Brazil, Canada, Kazakhstan, Malawi, Namibia, Niger, Russia, South Africa, and the United States. The ore's uranium content, which [Page 957]is sourced mainly from open-pit or underground mines, is often 0.1–0.2 percent. Hence, to obtain a sufficient grade of ore, large amounts of it have to be mined. Initially, up until the 1960s, uranium mining predominantly occurred in open-pit mines from ore deposits located near the surface. For the following 20 years, underground mines were viable. After the world market decrease of uranium prices from the 1980s onward, however, most became inefficient.

Low-level waste from nuclear power plants Is problematic because of the toxicity of uranium and Its compounds, rather than radioactivity. The sludge contains heavy metals and contaminants such as arsenic and chemical reagents used during milling.

Uranium ore goes through several processes of conversion and enrichment to concentrate the radioactivity by 2–3 percent and thus become nuclear reactor–grade fuel or by 80 percent to become potent enough for use in atomic weapons. A series of crushing mills grind the crude uranium ore to the consistency of fine sand, to which chemical solvents are added for dissolving out the uranium. A mixture of uranium oxides, with a chemical formula equivalent to U308, emerges from the process. This mixture, known as “yellowcake,” makes up the raw material for all the subsequent processes that eventually lead to a chain reaction in a reactor core. Yellowcake contains 85 percent uranium by weight, but besides this, there remains a hundred-fold quantity of residual sand, called “tailings,” which also contain the radium that had accompanied the uranium. Per ton of ore, over 3,700 liters of liquid waste are also produced. This sludge is both chemically toxic and mildly radioactive.

Mill tailings, normally dumped as sludge in special piles or ponds, are generated in nearly the same quantities as those of the ore milled. At a grade of 0.1 percent uranium, 99.9 percent of the material mined is left over. Apart from the portion of the uranium removed, the sludge contains all the constituents of the ore. As long-lived decay products such as thorium-230 and radium-226 are not removed, the sludge contains 85 percent of the initial radioactivity of the ore. Due to technical limitations, all of the uranium present in the ore cannot be extracted. The sludge, therefore, also contains 5–10 percent of the uranium initially present in the ore.

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