The Environmental Protection Agency has granted the Energy Department conditional permission to store radioactive and hazardous waste in a salt cavern 2,150 feet underground in New Mexico.

EPA's decision to grant the first variance for such wastes represents a modest but significant victory for the Energy Department, which has suffered one setback after another in its attempts to restart the nation's nuclear weapons manufacturing complex and clean up the dangerous wastes left by decades of violating environmental laws.

Energy Secretary James D. Watkins told EPA Administrator William K. Reilly in a "Dear Bill" letter last December that opening the New Mexico facility known as the Waste Isolation Pilot Plant, or WIPP, "is my top priority."

But the Energy Department is far from full operation of WIPP.

EPA authorized storage there for a five-year test period of 8,500 drums of waste, just 1 percent of the projected capacity and not nearly enough to ease the nuclear waste crisis hindering weapon-making operations at such facilities as Rocky Flats, near Denver, and the Idaho National Engineering Laboratory.

In addition, the Energy Department needs approval from Congress to take title to the 10,000-acre site, about 25 miles from Carlsbad, from the Interior Department's Bureau of Land Management. Sen. Pete V. Domenici (R-N.M.) has said he will try to block the transfer until he is satisfied that shipments of the plutonium-contaminated wastes pose no safety threat to state residents.

Under federal law, EPA cannot authorize land disposal of untreated hazardous waste until the organization producing the waste can prove it will not "migrate" beyond the disposal site boundary.

EPA's conditional approval of WIPP is likely to be challenged in court by the Natural Resources Defense Council and other organizations that say the weapons plant wastes are potentially explosive and likely to migrate.

Last May, NRDC made public an internal EPA memorandum suggesting that flammable gases leaking from the drums of mixed hazardous and nuclear waste could explode. At that time, no explosions of waste drums were known to have occurred and the Energy Department said none was likely because the wastes it was handling do not include flammable organic compounds such as acetone.

But NRDC scientists said this week that they have since uncovered two possible cases of explosions of similar wastes, one at the Lawrence Livermore National Laboratory in California in 1984, and one at Argonne National Laboratory in Illinois in 1976.

"I don't know definitively if the drums that exploded contained the kind of waste that would be stored at WIPP," said NRDC scientist Jim Werner, a former Lawrence Livermore employee.

But he said evidence indicates that they did, including the fact that investigators used an "alpha scanner" searching for emission of alpha radiation.

"You only do that when you're looking for plutonium," Werner said.