By Laura Zuckerman

(Reuters) - A nuclear waste repository in New Mexico was ordered by the state on Tuesday to craft a plan to hasten the sealing off of underground vaults where drums of toxic, plutonium-tainted refuse from Los Alamos National Laboratory may have caused a radiation release.

The directive by state Environment Secretary Ryan Flynn said the drums, buried half a mile below ground at the Waste Isolation Pilot Plant near the town of Carlsbad, "may present an imminent and substantial endangerment to health or the environment".

The repository, the only facility of its kind in the United States, has been shut since Feb. 14, when unsafe radiation levels were detected there.

The leak of radiation, a small amount of which escaped to the surface and exposed 22 workers at the plant, ranks as the facility's worst accident and one of the few blemishes on its safety record since it opened in 1999.

Flynn ordered managers of the U.S. Energy Department facility to submit schedules by May 30 for the "expedited" closure of two disposal chambers that collectively contain 368 containers of improperly packaged waste from Los Alamos.

Plant operators believe nitrate salts and organic kitty litter used to absorb liquids in drums of hazardous debris from Los Alamos caused a chemical reaction that breached the waste containers and released radiation from them.

Flynn on Monday ordered Los Alamos, 300 miles across the state northwest of Santa Fe, to submit a plan by Wednesday afternoon spelling out how it would isolate 57 similar drums still at the lab complex to lessen safety hazards there.

Los Alamos has placed drums with the same mix of nitrate salts and kitty litter in a special protective dome and is monitoring them for any rise in temperature after subjecting them to additional packaging, lab officials said in an email to Reuters on Tuesday.

Another 25 drums with the same materials were shipped between April 1 and May 1 to a commercial nuclear waste facility in Texas for temporary storage. 


(Reporting by Laura Zuckerman from Salmon, Idaho; Editing by Steve Gorman and Richard Borsuk)