Radioactive waste also plays a significant role. Uranium tailings are waste by-product materials left over from the rough processing of uranium-bearing ore. They are not significantly radioactive. Mill tailings are sometimes referred to as 11(e)2 wastes, from the section of the Atomic Energy Act of 1946 that defines them. Uranium mill tailings typically also contain chemically hazardous heavy metal such as lead and arsenic. Vast mounds of uranium mill tailings are left at many old mining sites, especially in Colorado, New Mexico, and Utah. Although mill tailings are not very radioactive, they have long half-lives. Mill tailings often contain radium, thorium and
Uranium-238 about five times better as a gamma ray shield than lead, so a shield with the same effectiveness can be packed into a thinner layer. DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storage systems to store radioactive waste. The opposite of enriching is downblending. Surplus highly enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low enriched uranium suitable for use in commercial nuclear fuel. U from depleted uranium and natural uranium is also used with recycled Pu-239 from nuclear weapons