Sproat said some of the estimated cost increase comes from inflation. Also, Energy Department officials now expect the dump will hold more radioactive waste than the 77,000 tons initially approved by Congress.

A report with precise cost breakdowns will be released to Congress in the next several weeks, Sproat said.

Already, some 64,000 tons of radioactive spent fuel rods are stored at commercial reactor sites in 33 states, according to the Nuclear Energy Institute. Most of it is stored in vault-like pools while some has been moved into dry-cask storage, where Nevada lawmakers, who oppose Yucca Mountain, would like it to stay — or be moved into dry-cask storage on-site.

Sproat opposes that plan as impractical. He also objected to other interim storage options raised Tuesday by frustrated lawmakers, who reported hearing from constituents about the need for new energy sources.

Commercial nuclear power plants now produce some 20 percent of U.S. electricity, but concern about waste disposal has hampered the industry's growth.

Yucca Mountain was originally supposed to open in 1998 but has been beset by lawsuits and political and scientific controversies. The best-possible opening date is now 2020, Sproat told lawmakers at an Energy and Commerce subcommittee hearing.

Even that is contingent on a steady money stream, something that Senate Majority Leader Harry Reid, D-Nev., has blocked.

The Energy Department did succeed in submitting a required construction license application to the Nuclear Regulatory Commission last month. The commission has up to four years to decide whether to approve it — but that timeline, too, is dependent on congressionally approved budgets.

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E-mail: lee@desnews.com