By Laura Zuckerman

(Reuters) - Managers mishandled a radiation leak at a New Mexico nuclear waste dump in which 21 workers were exposed to airborne radioactive particles due in part to substandard equipment and safety systems, a U.S. investigator said on Wednesday.

But the contamination from the underground salt mine in the Chihuahuan Desert - where radioactive waste from U.S. nuclear labs and weapons facilities is deposited - was unlikely to have harmed the workers' health, inspectors said.

Ted Wyka, chairman of a federal accident review board, said improperly placed or inoperative air monitors, a substandard ventilation system and mismanagement contributed to the February 14 leak of radioisotopes including plutonium.

The preliminary findings by Wyka and other officials assembled by the U.S. Energy Department, which oversees the plant, came during a public meeting in Carlsbad, New Mexico on Wednesday.

Also on Wednesday, a three-member inspection team surveyed a waste disposal room where the accident likely happened and found contamination but no signs of damage. It was the first time inspectors in protective gear had accessed the site.

No employees were below ground when an air-monitoring system detected contamination of the mine. Subsequent testing of employees showed 21 had inhaled or ingested radioactive particles in amounts unlikely to harm their health, officials said.

The contamination found in surface air did not pose a risk to the general public or the environment, they said.

Wyka said "confusion and delays" after the leak caused managers to initially fail to report the emergency to state and federal regulators as required.

Ineffective emergency drills and officials who "failed to believe initial indications of the release" added to an atmosphere of inadequate management and oversight, Wyka said.

Bob McQuinn, new president of Nuclear Waste Partnership, a site contractor, said upgrades were underway.

(Reporting by Laura Zuckerman in Salmon, Idaho; Editing by Eric M. Johnson, John Stonestreet)