The world’s biggest nuclear power plant, which was hit by a magnitude 6.8 earthquake, was not designed to withstand tremors of such force and might have been built directly above an active faultline, the operating company admitted on Wednesday. The revelations could trigger fresh concerns about the wisdom of building reactors in one of the world’s most active earthquake zones. More than 50 nuclear plants provide about 30 per cent of Japan’s electricity.

Tokyo Electric power (Tepco) on Wednesday apologised for under-estimating the amount of radioactive water that had leaked from the Kashiwazaki-Kariwa nuclear plant into the ocean following the earthquake, which struck Niigata province on Monday morning. It said the mistake was due to a calculation error.

Even the higher amount – about 50 per cent above its original estimate – posed no environmental risk, it said. Nuclear safety officials also found low levels of radioactivity in the air near ventilation stacks.The nuclear regulator and the city of Kashiwazaki both said the plant would not be able to restart until officials could prove it was safe. Hiroshi Aida, mayor of Kashiwazaki, said it would be "difficult to restart the plant in its current state".

Akira Fukushima, deputy director-general for safety at the trade ministry’s nuclear safety agency, said there had been 50 accidents and malfunctions at the plant after the earthquake. Tepco had this year assured local residents that the plant was not built in an active earthquake zone.