Workers at Los Alamos National Laboratory have finished treating more than two dozen drums of waste containing some of the same ingredients that caused a 2014 radiation release at the federal government’s underground repository, the U.S. Energy Department announced Tuesday.

Agency and lab officials in an online newsletter called the treatment of the 27 drums a milestone for the northern New Mexico lab as it works to clean up contamination.

The containers of nitrate salt waste had not previously been treated, so crews over the last three months mixed the waste with water and an inert material to neutralize the contents’ reactive characteristics.

In the case of the container that ruptured in 2014, that waste had been inappropriately treated before being shipped to southern New Mexico’s Waste Isolation Pilot Plant. It was mixed with organic cat litter to absorb moisture, resulting in a chemical reaction and the radiation release that forced the repository to close for nearly three years.

The work at Los Alamos, the once-secret federal installation that helped develop the atomic bomb, comes after another recent project in which the lab had to remove, treat and repackage the waste in 60 drums that included the cat litter so they could be declared safe and ready for shipment to the repository.

The repository was forced to close in February 2014 after the improperly treated drum of waste from Los Alamos breached. A costly recovery followed and the incident highlighted safety and security concerns at both the lab and repository as watchdog groups and independent investigators highlighted oversight and management problems.

Shipments to the repository resumed last year after lengthy readiness reviews, policy changes and more worker training. The repository now receives an average of five or six shipments a week.

The Waste Isolation Pilot Plant is the nation’s only place for disposing of tons of Cold War-era waste generated over years of bomb-making and nuclear weapons research. That includes gloves, clothing, tools and other debris contaminated by plutonium and other radioactive elements.

The repository was carved out of an ancient salt formation in the 1980s about a half-mile (0.8 kilometers) below the desert, with the idea that shifting salt would eventually entomb the waste.

After the 2014 radiation release, limited ventilation underground due to contamination issues slowed disposal operations as well as mining and maintenance work. Workers who enter contaminated areas now have to wear protective clothing and carry monitoring equipment.

Changes in the schedule have been made to allow for mining and disposal work to be done on separate shifts. Officials said this ensures enough airflow for worker safety.

“Safety will always be the priority,” said Tammy Reynolds, deputy project manager and chief operating officer with Nuclear Waste Partnership, the contractor that runs the repository.

Federal officials and the contractor are still working on a new ventilation system that would provide pathways for filtered and unfiltered air but it could be 2022 before that’s complete. The current system operates only in a filtered mode, resulting in lower flows.