WASHINGTON -- The Department of Energy is still failing to do a good job correcting mistakes in its nuclear waste program, a problem that could affect safety and delay the proposed Yucca Mountain repository even more if it is not fixed, investigators said in a report issued Friday.

Since DOE put a new corrections program in place in October 2003, workers have reported more than 5,600 potential problems with data, software and computer models for repository designs.

But auditors found more than half of the most significant potential problems were not addressed in a timely manner, and mistakes continued to be repeated. More than 100 possible problems that should have been handled through formal corrective action were not managed properly, they added.

The 20-page report released by the Energy Department's inspector general underscored DOE's continuing struggle to manage details of the complex undertaking.

DOE had asked the inspector general to audit its corrections program, which was put in place after it was found that earlier systems for fixing deficiencies were not working.

DOE accepted the latest audit, and "initiated an aggressive plan of action to improve the program," according to the report.

"This review shows the commitment that this department has toward improving the management and oversight of the Yucca Mountain Project," DOE spokesman Craig Stevens said in a statement. "This comprehensive report will help us make certain that our processes and procedures are sound as we move forward."

Bob Loux, a repository critic and director of the Nevada Agency for Nuclear Projects, said the latest audit spotlighted the same DOE problems as before with quality assurance.

"We have been seeing and hearing about these kinds of things for some time," Loux said. "If anything, the IG is understating the effect these problems have. In other nuclear facilities these things have resulted in cancellations."

The Energy Department has set a June 2008 goal to send the Nuclear Regulatory Commission a comprehensive application to license a repository where 77,000 tons of highly radioactive spent fuel would be handled and stored within Yucca Mountain, 100 miles northwest of Las Vegas.

Following an eight-month audit, inspectors said they were told by Yucca managers that some corrections "proved to be more complicated than anticipated." In some cases, corrections were delayed for budget reasons.

"As a result, potential conditions that could affect the ongoing design and analysis work may go unresolved," auditors said. Problems could delay NRC licensing to begin repository construction, they said.

DOE missed its original 1998 deadline to open a Yucca repository, and also abandoned a 2010 startup date. Its new target for repository operations is March 2017.