Chubu Electric Power Co. said 29 workers were exposed to radiation after tainted water leaked at its Hamaoka nuclear plant in Shizuoka Prefecture.
No radiation reached the environment in the Tuesday incident, it said.
The workers were exposed to 0.05 millisievert of radiation or less, about the same as an X-ray and not enough to sicken them, company spokesman Atsuo Sawaki said.
Fifty-three liters of water containing 300 times more radiation than allowed by government regulations leaked from a storage tank for the No. 3 reactor at the plant in Omaezaki, said Toshimitsu Shibata, another spokesman. The utility is investigating the cause, he said. Operations at the 1,100-megawatt reactor weren't affected.
Water was being removed from the tank ahead of an inspection when an alarm sounded at 4:16 p.m., signaling leaks had occurred, according to a statement on Chubu Electric's Web site.
Twenty-one workers were contaminated at the time of the leak, while two more at the scene didn't show significant levels of radiation when tested later, Sawaki said. Eight other workers were affected Wednesday during cleanup efforts, he said.