When a drum containing radioactive waste blew up in an underground nuclear dump in New Mexico two years ago, the Energy Department rushed to quell concerns in the Carlsbad desert community and quickly reported progress on resuming operations. The early federal statements gave no hint that the blast had caused massive long-term damage to the dump, a facility crucial to the nuclear weapons cleanup program that spans the nation, or that it would jeopardize the Energy Department's credibility in dealing with the tricky problem of radioactive waste. But the explosion ranks among the costliest nuclear accidents in U.S. history. The long-term cost of the mishap could top $2 billion, an amount roughly in the range of the cleanup after the 1979 partial meltdown at the Three Mile Island nuclear power plant in Pennsylvania. The dump, officially known as the Waste Isolation Pilot Plant, was designed to place waste from nuclear weapons production since World War II into ancient salt beds, which engineers say will collapse around the waste and permanently seal it. The equivalent of 277,000 drums of radioactive waste is headed to the dump, according to federal documents. It had operated problem-free for 15 years and was touted by the Energy Department as a major success until the explosion. Though [an] error at the Los Alamos lab caused the accident, a federal investigation found more than two dozen safety lapses at the dump. The dump's filtration system was supposed to prevent any radioactive releases, but it malfunctioned.