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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.”

Ordinary uranium also proved to be much easier to work with than 233. But the government assembled a few bombs with the 233 version, and a research reactor in Tennessee briefly switched to it as fuel in 1968. But very little was used, so the material sat for decades in government laboratories and weapons plants.

Now, wary of the security risks posed by the stockpiles, the Energy Department is making plans to dispose of them at a cost estimated at $473 million. The department faces other disposal challenges, including how to handle tens of thousands of tons of spent fuel from civilian reactors, but uranium 233 is different, given that in the proper form it could easily be used to make a bomb.

Robert Alvarez, a senior scholar at the Institute for Policy Studies who was an Energy Department adviser during the Clinton administration, estimates that the government spent at least $5.5 billion, accounting for inflation, to produce the uranium 233. He contends that the government is poised to compound its original error in making the material by disposing of it in a way that is not secure.

The department’s plan is to take the uranium made at Indian Point, now stored in 403 stainless steel tubes at a plant in Oak Ridge, Tenn., and bury the containers at a low-level waste dump that consists of trenches that are up to 40 feet deep at the Nevada National Security Site, where nuclear weapons were tested until 1992.

Workers will dig narrow “slit trenches” at the bottom of the standard ones, descending another 8 to 10 feet.

The locations would not be disclosed, and anyone bent on retrieving the material would not know where the slits were within the trenches. The uranium 233 is in a ceramic form that binds to the metal of the tubes, which are welded shut, providing an extra level of protection, the department says.

Energy Department officials said that restoring 233 to a form that could be used in weapons was beyond the industrial capacity of most countries and that retrieving it would require heavy equipment. Still, the hardest part of making a uranium bomb has already been accomplished — separating the isotopes that split, and thus are bomb fuel, from the isotopes that do not.

Because the uranium 233 is mixed in with dangerously radioactive materials, it is “self-protecting” to an extent, said Charles D. Ferguson, the president of the Federation of American Scientists and an expert on the nonproliferation of nuclear weapons. Over time, he said, the radiation will diminish, and some of it has died away because the uranium 233 is 50 to 60 years old.

Yet Mr. Alvarez maintains that the disposal plan is insufficient. Shallow land burial “sets a bad precedent in terms of international safeguards,” he said.

In a recent research paper, he also argued that the material should be made useless for making bombs by diluting it with a plentiful form of uranium that will not sustain a nuclear reaction.

It is not clear what that would cost, but the Energy Department has rejected the additional expense as unnecessary.


Read more at: www.nytimes.com.