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By Laura Zuckerman
March 10 (Reuters) - Four more workers have tested positive for exposure tied to an accidental release of radiation from an underground nuclear waste site in New Mexico, but tests have shown no further contamination in two sections of the site, officials said on Monday.
This brings to 17 the number of workers exposed to radiation at the Waste Isolation Pilot Project, but a U.S. Department of Energy spokesman in a statement characterized the level of exposure as "very low."
No workers were underground at the site in southeastern New Mexico when on Feb. 14 air sensors half a mile (800 meters) below surface in an ancient salt formation triggered an alarm, indicating excessive amounts of radioactive particles.
Thirteen workers working above ground when the accident happened were initially revealed last month to have tested positive for radiation exposure.
Another four workers who were at the site the day after the accident have tested positive for trace amounts of radiation, U.S. Department of Energy spokesman Bradley Bugger said in statement. None of the 17 employees is expected to experience any health effects, he said.
No workers were below ground when air sensors detected high levels of radiation and automatically switched to a filtration system designed to capture the vast majority of radioactive particles, which can harm humans if inhaled or ingested.
Probes sent over the weekend into a pair of shafts in the salt formation where nuclear waste is stored at the facility showed no detectable airborne radioactivity and instruments used in the test were not contaminated, officials said.
Since the Feb. 14 accident, this was the first testing of air below ground at the facility near Carlsbad which accepts equipment and clothing contaminated with radioisotopes like plutonium from U.S. nuclear labs and weapons sites.
Crews in recent days have used high-density foam to seal vents that released underground air to the surface, Bugger said. Monitoring shows no further radiation leakage from those vents, but officials are crafting methods to ensure against leakages in future caused by degradation of the foam over time, he said.
The Department of Energy and the contractor that runs the site, Nuclear Waste Partnership LLC, will conduct additional tests of air below ground before sending in investigators to determine the cause of the accident. (Reporting by Laura Zuckerman in Salmon, Idaho Editing by Alex Dobuzinskis and Eric Walsh)