A 55-gallon drum of nuclear waste, buried in a salt shaft 2,150 feet under the New Mexico desert, violently erupted late on Feb. 14 and spewed mounds of radioactive white foam. The flowing mass, ... laced with plutonium, went airborne, traveled up a ventilation duct to the surface and delivered ... radiation doses to 21 workers. The accident contaminated the nation's only dump for nuclear weapons waste ... and gave the nation's elite ranks of nuclear chemists a mystery they still cannot unravel. Six months after the accident, the exact chemical reaction that caused the drum to burst is still not understood. Indeed, the Energy Department has been unable to precisely identify the chemical composition of the waste in the drum. The accident at the facility near Carlsbad, N.M., known as the Waste Isolation Pilot Plant, or WIPP, is likely to cause at least an 18-month shutdown and possibly a closure that could last several years. A preliminary Energy Department investigation found more than 30 safety lapses at the plant, including technical shortcomings and failures in the overall approach to safety. The accident raises tough questions about the Energy Department's ability to safely manage the nation's stockpiles of deadly nuclear waste. The accident was a horrific comedy of errors," said James Conca, a scientific advisor and expert on the WIPP. "This was the flagship of the Energy Department, the most successful program it had. The ramifications of this are going to be huge. Heads will roll."