HOUSTON, March 9 (Xinhua) -- Four more workers have tested positive for radiation following a leak at an underground nuclear waste plant in the U.S. state of New Mexico, bringing the total of affected workers to 21, plant operators said Sunday.
The leak, which happened early February at the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico, entered the first phase of its recovery process over the weekend, according to a statement published on the plant's website.
So far, 21 workers have tested positive for radiation, including 13 employees who were confirmed positive late February.
There has been no detectable contamination in their urine samples, which, according to the statement, indicates contamination was not inhaled into their lungs.
The WIPP stores waste that emits alpha and beta radiation, so the risk is of inhalation not penetration, it said.
"The levels of exposure are extremely low, and none of the employees is expected to experience any health effects from the exposures. The four most recent positive results were at a barely detectable level and reflect extremely low levels of exposure," the statement said.
Plant operators said new air testing conducted during the weekend showed no detectable radioactive contamination in the air at the site.
Tammy Reynolds, WIPP's recovery process manager, said they would do a final analysis of these samples before they send anyone down the underground facility, which "may occur as soon as the end of the week."
WIPP, the country's first deep underground nuclear repository, was shut down on Feb. 14 after an air sensor detected unusually high levels of radioactive particles on its underground levels. No workers were reportedly underground at that time and no injuries or damages were reported.
Initially, officials said radiation was believed to be confined on the underground levels and no contamination had been detected on surface levels, but an independent monitor center found radioactive isotopes in an air sensor a half mile (600 meters) from the site several days later.
State and federal authorities confirmed the reading and the U.S. Department of Energy sent a team to investigate.
The cause of the leak remains unknown. A truck fire was reported at the underground site on Feb. 5 and prompted evacuations, but officials said the fire was in a different part of the site and did not seem related to the leak.
The plant stores "transuranic waste" leftover from nuclear weapons research and testing from the nation's past defense activities, according to the Energy Department. The waste includes clothing, tools, rags and other debris contaminated with radioactive elements.