updated 9/26/2007 11:44:51 AM ET 2007-09-26T15:44:51

Engineers moved some planned structures at the Yucca Mountain nuclear waste dump after rock samples indicated a fault line unexpectedly ran beneath their original location, an Energy Department official said Monday.

Allen Benson, a spokesman for the U.S. Department of Energy in Las Vegas, said adjustments to the project were made in June.

"That's why we do studies, to come up with information to make the repositories safer," Benson said.

The department responded to a published report that cited a May 21 letter in which U.S. Geological Survey maps showed the Bow Ridge fault "may be farther east than projected." The Las Vegas Review-Journal said it obtained the documents last week.

Bob Loux, head of the Nevada Agency for Nuclear Projects and the state's chief anti-Yucca administrator, said he was not reassured by what he called "just-in-time engineering."

"This represents a complete lack of understanding about the site's characteristics," Loux said. "They've been out there for 25 years or longer. And they get surprises like this. This is basic geology, stuff they should have known all along."

A May 21 letter and U.S. Geological Survey maps show a fault beneath where officials hope to build concrete pads to store spent radioactive fuel canisters for cooling before they are entombed in tunnels inside the mountain, about 90 miles northwest of Las Vegas.

Core samples from 250 feet below the surface show the fault is hundreds of feet east of where scientists thought it was, and that it passes beneath the initial site for the storage pads, forcing their relocation, Benson said.

Recent rock core sampling operations have spurred a legal battle in federal court in Las Vegas, where the State Engineer Tracy Taylor has asked U.S. District Judge Roger Hunt to order the Energy Department to stop using Nevada water for cooling and lubricating drill rigs and creating mud to collect rock samples. Hunt denied the state request last week.

Congress in 2002 picked Yucca Mountain to become the nation's nuclear waste dump, with plans calling for entombing 77,000 tons of spent radioactive fuel hauled to Nevada from 39 states. But the plan has been delayed by legal challenges, money shortages, scientific controversies and political opposition. Planners now concede the dump won't open before 2017.

Project officials say they are continuing to develop repository design, construction and operating plans in preparation for applying next year for a license from the Nuclear Regulatory Commission.

To meet that self-imposed June 2008 deadline, project planners are taking a nontraditional approach to risk assessments, based on probabilities that earthquakes will occur.

A scientist for a project contractor told an oversight panel last week in Las Vegas that planners expect to know by February what could go wrong with surface facility equipment, and potential consequences. Michael Frank of Bechtel SAIC Co. called the task "a very large effort with a compressed schedule."

A panel consultant, seismologist Leon Reiter, who was at the meeting, said more than 10 faults within a 20-mile radius of Yucca Mountain could generate ground motion.

He said one fault, the Solitario Canyon just west of the planned repository, is capable of producing an earthquake with a magnitude of about 6.5.

Copyright 2007 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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