Cleanup workers have finished processing the remaining waste at a U.S. Department of Energy facility where four radioactive waste drums ruptured last year.

The four 55-gallon drums at the Accelerated Retrieval Project V facility, containing a mix of fluids and solvents that came from nuclear weapons production during the Cold War at the Rocky Flats Plant near Denver, ruptured in April 2018. 

No one was at the facility when the rupture happened and contamination was confined to the building. The incident led to a temporary shutdown of the facility as well as a temporary suspension of shipments to the Waste Isolation Pilot Plant near Carlsbad, N.M. According to an investigation and report cleanup contractor Fluor Idaho released in October, the rupture happened because a reactive metal with radionuclides heated up after being exposed to the air for the first time in almost 40 years.

After the accident, Fluor processed an additional 37 waste drums at the facility. Fluor announced Tuesday that all the remaining waste at the ARP V had been safely processed.

Now that the waste at ARP V is processed, Fluor will now plan to close the facility and focus on waste processing at the nearby ARP VII. There are 2,800 drums of radioactive sludge there, also containing decades-old waste from Rocky Flats, and Fluor expects to have it all processed and repackaged in late 2020. The waste there has all been exhumed already, said Fluor Idaho spokesman Erik Simpson.

Fluor said in a news release that ARP V's air filtration system, including reverse airflow and high-efficiency particulate air filtration, ensured the waste was contained there, and ARP VII has the same features.

"All safety protocols and lessons from the 2018 incident will be implemented at ARP VII as workers there safely process drums and ensure waste is compliant for out-of-state disposal," the news release said.

Workers at ARP VII recently finished reducing the size of some large, contaminated legacy waste boxes and debris at that site and repackaging them for shipment to permanent disposal facilities, according to the news release. Fluor also plans to move sludge waste that is at the Advanced Mixed Waste Treatment Project now — another cleanup facility that DOE announced in December it would close later this year as it is wrapping up its mission processing some old transuranic waste — to ARP VII, Simpson said.

Reporter Nathan Brown can be reached at 208-542-6757. Follow him on Twitter: @NateBrownNews.