WASHINGTON — At the dawn of the civilian nuclear age in the 1950s, one of the pressing questions was how to find enough fuel for reactors and bombs. The government and the private sector seized on a man-made substitute for natural uranium, producing about 3,400 pounds of an exotic and expensive material called uranium 233.

Today, the problem is how to safely get rid of it.

“We do consider this to be waste,” said David G. Huizenga, the senior administrator for environmental management at the Energy Department. “There’s no further need for it.”

Uranium 233 looked attractive because it could be made in a reactor from thorium, a cheap and abundant radioactive metal, and, almost magically, the reactor would produce more fuel than it consumed. Utilities manufactured some of it at the Indian Point I reactor in Westchester County, N.Y., which is now retired, and at reactors in Colorado, Illinois and Pennsylvania.

But in the end, ordinary uranium was cheaper, and 233 was not needed.

“Nuclear physicists weren’t geologists and didn’t understand the supply of uranium,” said Frank N. Von Hippel, a physicist and public policy specialist at Princeton. “It turned out there was more uranium than people thought and less nuclear power than people thought there would be.”