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, according to a Los Angeles Times analysis. The long-term cost of the accident 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 Feb. 14, 2014, incident is also complicating cleanup programs at about a dozen current and former nuclear weapons sites across the U.S. Thousands of tons of radioactive waste that were headed for the dump are backed up in Idaho, Washington, New Mexico and elsewhere, state officials said in interviews.

Washington state officials were recently forced to accept delays in moving the equivalent of 24,000 drums of nuclear waste from their Hanford site to the New Mexico dump. The deal has further antagonized the relationship between the state and federal regulators.

“The federal government has an obligation to clean up the nuclear waste at Hanford,” Democratic Gov. Jay Inslee said in a statement. “I will continue to press them to honor their commitments to protect Washingtonians’ public health and our natural resources.”

Other states are no less insistent. The Energy Department has agreed to move the equivalent of nearly 200,000 drums from Idaho National Laboratory by 2018.

“Our expectation is that they will continue to meet the settlement agreement,” said Susan Burke, an oversight coordinator at the state’s Department of Environmental Quality.

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.

The dump was dug much like a conventional mine, with vertical shafts and a maze of horizontal drifts. It had operated problem-free for 15 years and was touted by the Energy Department as a major success until the explosion, which involved a drum of plutonium and americium waste that had been packaged at the Los Alamos National Laboratory.

The problem was traced to material — actual kitty litter — used to blot up liquids in sealed drums. Lab officials had decided to substitute an organic material for a mineral one. But the new material caused a complex chemical reaction that blew the lid off a drum, sending mounds of white, radioactive foam into the air and contaminating 35 percent of the underground area.

“There is no question the Energy Department has downplayed the significance of the accident,” said Don Hancock, who monitors the dump for the watchdog group Southwest Research and Information Center.

Though the error at the Los Alamos lab caused the explosion, 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.

Twenty-one workers on the surface received low doses of radiation that federal officials said were well within safety limits. No workers were in the mine when the drum blew.

Energy Department officials declined to be interviewed about the incident but agreed to respond to written questions. The dump is operated by Nuclear Waste Partnership, which is led by the Los Angeles-based engineering firm AECOM. The company declined to comment.

Federal officials have set an ambitious goal to reopen the site for at least limited waste processing by the end of this year, but full operations cannot resume until a new ventilation system is completed in about 2021.

The direct cost of the cleanup is now $640 million, based on a contract modification made last month with Nuclear Waste Partnership that increased the cost from $1.3 billion to nearly $2 billion.

The cost-plus contract leaves open the possibility of even higher costs as repairs continue. And it does not include the complete replacement of the contaminated ventilation system or any future costs of operating the mine longer than originally planned.

An Energy Department spokeswoman declined to address the cost issue but acknowledged that the dump would either have to stay open longer or find a way to handle more waste each year to make up for the shutdown. She said the contract modification gave the government the option to cut short the agreement with Nuclear Waste Partnership.