Plant-operator Tokyo Electric Power Co. said it was unclear how the mist, used to lower temperatures near a quake-proof building, became contaminated.
Initial tests found the workers had exposure of 10 becquerels per square centimeter above the neck, and were ordered to receive full-body scans to determine the exposure to their internal organs.
A radiation monitor sounded an alarm earlier Monday after it detected high concentrations of radioactivity. The mister was shut off and workers were prohibited from using tap water, which shares a source with the contaminated mist.
Just last week, TEPCO was forced to admit radioactive water was leaking from the facility and possibly contaminating the nearby Pacific ocean water.