Photo/Illutration The No. 3 reactor, front, at the Mihama nuclear plant in Mihama, Fukui Prefecture (Asahi Shimbun file photo)

Kansai Electric Power Co. on Aug. 3 announced a delay in the planned imminent restart of a reactor at its Mihama nuclear plant in Fukui Prefecture due to a leak of radioactive water.

The utility said the problem was discovered at the No. 3 reactor’s reactor auxiliary building during a regular inspection on Aug. 1.

Kansai Electric estimated that around 7 tons of contaminated water leaked in an area close to equipment designed to prevent water used to cool the reactor from spilling into the environment, officials said.

However, the company said the leak was contained in the auxiliary building and that no workers were exposed to radiation.

The Osaka-based utility said it did not anticipate any damage to the surrounding environment.

The incident occurred as the plant operator was gearing up to start commercial distribution of power at the reactor from Aug. 12.

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.

The No. 3 reactor, with a capacity of 0.82 gigawatt, first went into operation in 1976.

It 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 last October 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 nuclear disaster.

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.

It currently operates two reactors in Fukui Prefecture: the No. 3 and No. 4 reactors at its Oi nuclear plant.

The company plans to bring a total of five reactors, including the Mihama reactor, back online by the year-end.

The central government is pushing for nine nuclear reactors to be up and running nationwide to avoid a power crunch this winter.