uranium.
USEC, the energy company formed as a part of the US/Russian agreement to blend down surplus - it should
all be surplus - weapons grade HEU (Highly Enriched Uranium) for use in commercial nuclear power reactors.
It is part of the "Megatons to Megawatts" program, and has resulted in the dilution of 276 metric tons of bomb grade uranium to make over 8,000 tons of reactor fuel.
A typical nuclear reactor is loaded with about 100 metric tons of fuel, on which it can run for a period of close to two years. Thus the converted fuel is nearly enough to have run US nuclear reactors for about 80% of the entire US nuclear reactor fleet for about 2 years.
http://www.usec.com/v2001_02/HTML/Megatons_chronology.asp