Workers at a nuclear waste plant in the U.S. state of New Mexico are preparing to enter the facility's underground dump for the first time since a radiation leak shut down the facility last month, U.S. media reported Monday. The U.S. Department of Energy said 35 workers have undergone training simulations at a Potash mine before entering the Waste Isolation Pilot Plant near Carlsbad, New Mexico, local TV KOB reported. The workers spent last week in training for various scenarios that could occur in the mine. They went through underground sessions using protective gear and air-breathing units, the report said. The underground dump was shuttered on Feb. 14 when air sensors detected unusually high levels of radioactive particles on its underground levels. It has been kept closed as reports came that small amount of radiation was detected both at the underground and surface levels. A total of 17 workers were confirmed positive for radiation, but the level of their exposure was "extremely low," according to the Energy Department. The cause of the radiation 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 repository stores waste leftover from nuclear weapons research and testing from the nation's past defense activities, according to the Energy Department website. The waste includes clothing, tools, rags and other debris contaminated with radioactive elements, largely plutonium.