Welcome to A People's Atlas of Nuclear Colorado

To experience the full richness of the Atlas, please view on desktop.
Bill Gillette/Environmental Protection Agency, Mill tailings pile at the Uravan mill operated by Union Carbide, 1972, National Archives and Records Administration

Issue Brief

Uranium Milling Overview

After uranium ore is mined, it is milled. Milling extracts the uranium from the ore through chemical processes. In a conventional mill, ore is first crushed and then mixed with sulphuric acid, which removes uranium along with other heavy metals, molybdenum, vanadium, selenium, iron, lead, and arsenic. In the in-situ recovery process, chemicals are directly injected into the ground to dissolve uranium deposits without the need to excavate. Dissolved or leached uranium is concentrated, purified, and dried to become the final product: yellowcake. After milling, other companies buy the uranium and enrich it in order to use the material as nuclear fuel.

The milling process creates byproduct materials. This sandy waste is often referred to as tailings and contains heavy metals and radium. As the radium decays over thousands of years, the tailings produce a radioactive gas called radon. For decades, tailings were often piled at mill sites, where they were carried by wind and water to contaminate surrounding areas. The Atomic Energy Act and Energy Policy Act define byproduct material according to radiation level and from what material it was produced. These materials are strictly regulated by the U.S. Nuclear Regulatory Commission (NRC) because of the hazard they pose to public health and safety.

To secure uranium mill tailings, they are placed in piles lined, covered, and monitored for leaks for long-term storage or disposal. The NRC has established regulations for the management and storage of tailings. For example, the NRC established criteria for the tailings’ disposal, inspection of retention systems, long-term surveillance and control of the tailings impoundment, and eventual government ownership of the tailings site under an NRC general license.

Previously, mill tailings were used in some Western mining areas as building materials which presented extreme radon and radiation hazards to the communities. In response, the 1978 Uranium Mill Tailings Radiation Control Act (UMTRA) stopped the use of mill tailings in construction projects and led to a general cleanup of former uranium mill sites. This initial cleanup of former uranium mill sites in Colorado is complete; focus has shifted to “post-UMTRA" issues, which include managing the discovery of new uranium mill tailings.


Colorado Department of Public Health and Environment. "Uranium Mill Tailings Sites." Accessed July 31, 2020.

Spoon, Marianne. “How Uranium Mining Works.howstuffworks.com. Accessed July 31, 2020.

U.S. Environmental Protection Agency. "Radioactive Waste from Uranium Mining and Milling." Accessed July 31, 2020.

U.S. Nuclear Regulatory Commission. “Backgrounder on Uranium Mill Tailings.” July 7, 2020 [last updated]. Accessed July 31, 2020.

U.S. Nuclear Regulatory Commission. “Byproduct Material." July 6, 2020 [last updated]. Accessed July 31, 2020.

U.S. Nuclear Regulatory Commission. "Uranium Recovery (Extraction) Methods." March 29, 2018 [last updated]. Accessed August 6, 2020.

U.S. Nuclear Regulatory Commission. "Uranium Mill Tailings." August 3, 2017 [last updated]. Accessed July 31, 2020.

World Nuclear Association. "Nuclear Fuel Cycle Overview." May 2020 [last updated]. Accessed July 31, 2020.

Last Updated:


Continue on "Overburden"