The No. 1 and No. 2 reactors at Hamaoka, which has four nuclear power reactors, have been shut down after several water leaks were reported earlier this year and last year.

The No. 3 reactor was still in operation. The leak was found on a valve related to the water feed pump in the reactor, Fujimori said.

"The leak was found by a worker conducting an inspection... There was some seepage of water and a drop of water was seen falling once every 10 seconds," he said.

When asked whether any radiation had affected the environment or if any workers had been exposed, Fujimori said: "The leak was not of such a magnitude at all. No workers were exposed to radiation."

Chubu Electric reported the water leak to local municipalities when it was found last Wednesday, he said.

But since the leak was considered to be light, it did not publicise the incident until it updated a weekly status report on the Hamaoka nuclear power station on the station's website this week, Fujimori said.

Japan, reliant on nuclear energy for one-third of its power needs, has seen a number of accidents over the past decade that have undermined public support for its nuclear programme.