Officials are investigating whether there were any radioactive leaks, a day after the quake spilled radioactive water into the sea.
While the quake had triggered a leak of water with radioactive material on Monday at world's largest nuclear power plant at Kashiwazaki city, near the epicenter, officials said it caused no harm to the environment.
On Tuesday, however, officials were investigating a possible second leak, saying about 100 drums containing low-level nuclear waste fell at the plant during the quake and were found a day later, some of the lids open.
Tokyo Electric Power Co., which runs the plant, was still trying to determine whether any hazardous material had spilled.
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