The Environmental Protection Agency on Wednesday licensed the federal government's $1.8 billion nuclear waste dump in the subterranean salt beds of southeastern New Mexico.

The long-awaited ruling means shipments of radioactive defense waste could begin rolling to the Waste Isolation Pilot Plant near Carlsbad by mid-June - unless halted by a court fight promised by environmentalists.One of the chief opponents - the Southwest Research and Information Center - said Tuesday it will sue both the EPA and the U.S. Department of Energy, which owns the repository, before the end of a 30-day period for final certification.

The repository has been planned for 24 years, and Congress authorized its development in 1979. The EPA Wednesday ruled the repository can safely contain the waste and that it complies with the agency's standards for radioactive waste disposal.

"We're confident that if this is litigated, we'll be successful," Richard Wilson, acting assistant administrator for the EPA's Office of Air and Radiation, said.

"At EPA, almost anything we do runs the risk of one side or the other bringing litigation," he said. "Our first responsibility is to the public to make sure our decisions are accurate."

The EPA's final certification runs 10,000 pages, includingocumentation, he said.

Energy Secretary Federico Pena also must sign off on the project. The Department of Energy promised a major announcement later Wednesday.

The EPA gave preliminary approval last October to the repository, dug out of the vast salt beds left by a sea that covered the area 250 million years ago.