Reports of an additional radioactive leak at an earthquake-damaged Japanese nuclear power plant emerged Thursday after the government closed the facility until extensive safety checks are completed.

Kashiwazaki Kariwa nuclear plant suffered burst pipes, water leaks and radioactive waste spillage when it was hit by Monday's earthquake.
(Kyodo News/Associated Press)

Japan's nuclear watchdog detected an additional radioactive leak from the nuclear power plant damaged in this week's earthquake, according to Kyodo News agency.

The report said the leak came from an exhaust vent in the Kashiwazaki-Kariwa plant in northwest Japan, but that the leak did not threaten the environment or public health.

The Nuclear and Industrial Safety Agency said it found radioactive iodine had leaked from the sprawling plant after Monday's quake.

The Kashiwazaki-Kariwa nuclear plant, the world's largest, was ordered shut indefinitely on Wednesday after the operator revealed some 50 problems at the seven-reactor complex.

The government will wait until an earthquake resistance study is completed and safety can be verified before allowing operations at the plant to resume, which could take as long as a year, Japan's Nikkei newspaper reported.

The problems included a small fire at an electrical transformer in the plant, a leak of water containing radioactive material and the tipping over of drums filled with low-level radioactive waste.

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.

The latest report comes after the company running the plant acknowledged the leak of radioactive water was 50 per cent larger than first reported.

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.

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.

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.

With files from the Associated Press