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Albuquerque, N.M. • Los Alamos National Laboratory says it made mistakes in packing waste that has been linked to a radiation leak at the government's underground nuclear waste dump, but it remains unclear if the violations or its use of organic cat litter to absorb moisture played a role in the accident.

In a letter released by state regulators Thursday, lab officials told the New Mexico Environment Department that their internal probe of the handling of the toxic waste from decades of nuclear bomb building has uncovered several violations of its Hazardous Waste Facility Permit. The lab says it failed to follow proper procedures in making the switch from inorganic to organic litter and in its lack of follow up on waste that tests showed to be highly acidic.

The shortcomings were described as "unacceptable" by Principal Associate Lab Director Terry Wallace, according to an internal memo.

Teams of scientists and engineers are still trying to determine exactly what caused a barrel from Los Alamos to burst, and whether the switch in cat litter helped fuel what is suspected to have been some kind of reaction in the highly acidic waste that also contained lead.

Despite hundreds of experiments to date, investigators have been unable to create any reaction that would have caused the container to leak like it did on Feb. 14, sending radioactive particles into the air above the half-mile deep repository and contaminating 22 workers with low levels of radiation.

The accident has also indefinitely shuttered the mine, which is the nation's only permanent repository for plutonium-contaminated gloves, tools and clothing from the federal government's nuclear facilities.

According to the memo obtained by The Associated Press, Wallace told employees at a meeting Monday that the probe is focused on 16 barrels of highly acidic, nitrate-salt-bearing waste, including the drum that leaked at the Waste Isolation Pilot Plant. Ten of the other barrels are also underground at the mine in Carlsbad, New Mexico, while five are in temporary storage and under special monitoring at a private waste facility in Andrews, Texas.

Wallace is quoted in the memo as saying that a technical review "identified certain conditions that might potentially cause an exothermic reaction inside a drum. Among them are neutralized liquids, a low pH and the presence of metals."

"The low pH findings should have prompted a pause in work to ensure appropriate technical and regulatory reviews of next steps," Wallace said.

Wallace indicated that the lab's focus is now on correcting the processes to prevent any recurrence. "We need to get this right and set best practices for the entire Complex."

The state Environment Department said it was reviewing "these initial violations and plans to take appropriate actions once it concludes its independent review of the incidents at WIPP and LANL," using acronyms for the Waste Isolation Pilot Plant and the Los Alamos National Laboratory, respectively.

The state investigation is just one of many into the leak and an underground truck fire six days earlier at the U.S. Department of Energy facility.

Initial probes by federal regulators identified a host of management and safety shortcomings at the Waste Isolation Pilot Plant.