About 1.6 tons of radioactive coolant water poured Tuesday morning from a pump at the No. 3 reactor of the Ikata nuclear power plant in Ehime Prefecture, Shikoku Electric Power Co. said.
The power company said monitoring devices near the building that houses the reactor recorded a slight rise in environmental radiation.
Shikoku Electric said the radiation level is 0.5 percent of the amount that mandates operators to report to the national government.
No radiation was detected at 13 checkpoints in the larger area surrounding the reactor, it said.
Shikoku Electric said it became aware of the leak after an alarm went off just before 10 a.m. Engineers turned the pump off and the nuclear plant continued operations using backup equipment.
The coolant apparently leaked from the pump's roller bearing. Officials said they detected excessive heat coming from the bearing.
This is the second such incident in about 18 months.
In September 2002, about 230 ml of radioactive coolant water leaked from a pipe at Ikata nuclear plant's No. 1 reactor.