The company said it had contained the leak and does not expect it to affect the environment, but added the incident would delay the planned restart of the unit, which had been offline for the addition of anti-terrorist measures.
The utility said the problem was discovered at Unit 3’s reactor auxiliary building during a regular inspection.
According to the Kyodo news agency, an alarm sounded in the power plant indicating a reduction in the amount of water supplied to the pump.
Mihama-3, a 780-MW pressurised water reactor unit, began commercial operation in 1976. Two other plants at the site were permanently shut down in 2015.
The incident occurred as the plant operator was gearing up to start commercial distribution of power at the reactor from 12 August.
Kansai Electric said it does not know when the reactor can resume operations as it needs to identify the cause of the leak and fix the problem.
PM Wants More Reactors OnlineMihama-3 was restarted in June 2021 after being taken offline in 2011 for regular maintenance, becoming the nation’s first reactor built more than 40 years ago to still be in service.
But operations were suspended in October 2021 in accordance with updated reactor safety regulations that require all utilities to construct an anti-terrorism facility. The measure was imposed in the aftermath of the 2011 Fukushima-Daiichi nuclear accident.
At the behest of the central government, Kansai Electric was working to move up the restart date by more than two months so as to prepare for a possible power supply shortage this winter.
Last month Japan’s prime minister Fumio Kishida said he wants to operate nine commercial nuclear power plants this winter to address a looming electricity shortage, increasing the number online from five.
The move will allow the country to secure about 10% of its electricity needs from nuclear energy, Mr Kishida said.