This is not the first time unexplained, dangerous accidents have challenged U.S. government nuclear storage protocol and viability.
Back in 2014, for example, at the Waste Isolation Pilot Plant, "radioactive materials leaked from a damaged storage drum. Analysis of several accidents, by DOE, have shown lack of a "safety culture" at the facility," according to Wikipedia (photo shown).
According to the LA Times, in this latest incident, which happened in February of this year, the experts still have not determined whty the waste became unstable.
"A 55-gallon drum of nuclear waste buried 2150 ft. below the earth essentially "erupted," into a foaming mass, sending plutonium-laced airborne particles up a ventilation shaft and poisoning 21 workers.
The accident contaminated the nation's only dump for nuclear weapons waste — previously a focus of pride for the Energy Department — 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, a serious error in a handling process that requires careful documentation and approval of every substance packaged for a nuclear dump." (full article).