(adds detail, quotes from IAEA briefing)

By Mark Heinrich and Karin Strohecker

VIENNA, Aug 17 (Reuters) - The world's biggest nuclear plant performed safely during and after an earthquake in Japan but it may take more than a year to restart power production, U.N. inspectors said on Friday.

"It will take at least months, maybe a year or more. I would be surprised if it was less than a year," said Philippe Jamet, a senior International Atomic Energy Agency official.

Janet, who headed an IAEA team that examined the plant, said in-depth inspections of the Kashiwazaki-Kariwa reactor vessel and fuel elements had yet to be done.

If damage was discovered there, bringing the plant back on line could take even longer, he said.

The plant leaked low amounts of radiation -- below the maximum permitted under safety rules -- when a 6.8 magnitude quake struck on July 16, exceeding the worst seismic impact that the plant had been designed to withstand, according to a 39-page report issued at IAEA headquarters in Vienna.

"The installation behaved in a safe manner during and after the earthquake. In particular, the automatic shutdown performed successfully," the report said.

The plant will remain closed indefinitely for safety checks and the Japanese government has ordered other nuclear plant operators to undertake strict safety checks.

While safety-related systems and components of the plant, run by Tokyo Electric Power Co. (TEPCO) <9501.T>, seemed to be in surprisingly good condition, other key components had yet to be checked thoroughly, Jamet said.

"Another consideration is the possibility that long-term operation of components could be affected by hidden damage from this event," the report by his six-strong team said.

LONGER-TERM WEAKNESS?

Such components, in effect, could function well under normal conditions but not necessarily during a future accident, or safely over the longer term.

Non-safety related systems and components sustained serious damage from factors such as soil deformation and oil leakages, said the report.

"A re-evaluation of the seismic safety the Kashiwazika Kariwa nuclear power plant needs to be done with account taken of the lessons learned from the... earthquake and using updated criteria and methods."

Jamet said radioactive releases into the air and sea from the stricken plant were "extremely small" and the health and environmental impact was "practically negligible".

He also praised the cooperation and transparency of Japanese officials dealing with his mission on Aug. 6-10.

Japan's nuclear industry has been tarnished by cover-ups of accidents and fudged safety records. About one third of Japan's electricity grid is supplied by nuclear power, which has been central in Japanese efforts to combat global warming.

Japan initially told the IAEA it did not need help after the quake. Later, it said it would allow inspectors into the plant after local authorities expressed concern that talk of the plant's problems would damage tourism and fisheries.

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