Los Alamos National Laboratory, Los Alamos, N.M., temporarily halted work last week at its Neutron Scattering Center because workers there were worried about mercury contamination.
"It was an internal decision to stop work until we could fully address workers concerns," lab spokesman John Gustafson said Thursday.
Laboratory managers announced the "stop work" order Tuesday after workers became concerned that mercury contamination in two experimental rooms were spreading to other areas.
"We have no indication that mercury contamination at the facility presents a risk to employees or the environment," said Geoff Greene, deputy division director of the Los Alamos Neutron Science Center. "But because concerns have been raised, it is prudent to suspend operations in (the two rooms) until we can address safety issues and provide complete information to our workers."
The lab has been cleaning and decontaminating the two rooms since October.
Mercury can slow fetal and child development and cause brain damage.
The neutron center uses a high-energy proton accelerator to generate neutrons -- one of the components of an atom -- for scientific and defense applications. Mercury is used to stop neutron beams during experiments.
On a few occasions, the toxic material has spilled onto the floor in two of the rooms. The lab said spilled mercury is cleaned up using a special vacuum, but some tiny beads can still be missed.
Mercury detectors in the rooms show levels of the material well below OSHA exposure limits.
Workers cleaning up the areas were wearing protective clothing and respirators. None of their equipment showed exposure to the mercury, and blood and urine tests had negative results.
Greene said operations in the experimental rooms would not resume until lab officials are certain mercury doesn't pose an unnecessary risk to workers.