Y-12 nuclear weapons plant in Oak Ridge, Tenn

An undated artist rendering shows the planned Highly Enriched Uranium Materials Facility at the Y-12 nuclear weapons plant in Oak Ridge, Tenn. Site preparation has begun on the $250 million fortress-like building that will accommodate Y-12's entire stockpile of bomb-grade uranium, now stored in six locations. Critics worry the new design is above ground. Earlier designs had it at least partially buried. Y-12 is the country's main repository of the radioactive material.