A leak of radioactive water from a Japanese nuclear power plant was 50 per cent larger than first reported, but remains below dangerous levels, the company that operates the plant said Wednesday.

Kashiwazaki-Kariwa, the world's biggest nuclear plant, was damaged Monday when a powerful earthquake shook the area, causing radioactive water to leak into the Sea of Japan. The plant has been shut down until its safety is assessed.

The Kashiwazaki Kariwa nuclear plant in Kashiwazaki, photgraphed from helicopter on Tuesday.
(Associated Press)

In a statement, Tokyo Electric Power Co. said it had initially misreported the amount of radiation in the 1,200 litres of water that leaked, but said it was still "one-billionth of Japan's legal limit."

"We made a mistake in calculating the amount that leaked into the ocean. We apologize and make correction," the statement said.

The International Atomic Energy Agency, the United Nations' nuclear watchdog, called on Japan to hold a thorough probe into the leak.

"It's clear that this earthquake, as TEPCO, the operating company, indicated, was stronger than what the reactor was designed for," IAEA head Mohamed ElBaradei told reporters in Kuala Lumpur on Wednesday.

"I would hope and I trust that Japan would be fully transparent in its investigation of that accident. The agency would be ready to join Japan through an international team in reviewing that accident and drawing the necessary lessons."

Visible damage

Monday's quake killed 10 people and injured dozens more. About 10,000 people were forced from their homes, hundreds of buildings were destroyed and about 40,000 homes are without water.

The tremor initially triggered a small fire at an electrical transformer in the sprawling plant. It was announced 12 hours later that the quake also caused a leak of water containing radioactive material.

The company also said a small amount of the radioactive materials cobalt-60 and chromium-51 had been emitted into the atmosphere from an exhaust stack.

Japanese Prime Minister Shinzo Abe has criticized the company for the delay in releasing information.

Reporters given a tour of the sprawling compound on Wednesday saw cracked roads, buckled sidewalks and toppled concrete fences. Teams of repair workers climbed over the three-storey transformer building.

Company apologizes

During an inspection of the damage the same day, Tokyo Electric Power Co. president Tsunehisa Katsumata declared the site a "mess" and apologized for "all the worry and trouble we have caused."

"It is hard to make everything go perfectly," he said. "We will conduct an investigation from the ground up. But I think fundamentally we have confirmed that our safety measures worked."

Katsumata, dressed in overalls, bowed in apology to the mayor of the nearby city of Kashiwazaki. Located about 220 kilometres northwest of Tokyo, the city is home to about 94,000 people.

Officials from the power company and Japan's Meteorological Agency, who are studying the quake's aftershocks, say a fault line may stretch beneath the grounds of the nuclear plant.

The onshore fault lines in the area have been mapped, but the offshore survey has not been completed.

Japan's coast guard said it would launch a study of the ocean floor off Kashiwazaki starting Friday to better map fault lines in the area and to determine how close any fault line is to the plant.

Japan sits atop four tectonic plates and is one of the world's most earthquake-prone countries.

With files from the Associated Press