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KASHIWAZAKI, Japan - More than a day after a powerful earthquake shook northern Japan, officials revealed Tuesday that a nuclear plant suffered a long list of problems including the leakage of radioactive water, an outbreak of fires and burst pipes.

The malfunctions at the Kashiwazaki power plant -- and the delays in acknowledging them -- are likely to feed concerns about the safety of Japan's 55 nuclear reactors, which supply 30 percent of the quake-prone country's electricity and have suffered a long string of accidents and cover-ups.

Tokyo Electric Power Co. said a total of 50 cases of malfunctioning and trouble had been found at the Kashiwazaki-Kariwa nuclear plant since Monday's magnitude 6.6 quake, which killed at least nine people and left 13,000 homeless.

The company said they were still inspecting the plant, which shut down automatically after the quake, and further problems could emerge.

Still, TEPCO spokesman Kensuke Takeuchi called the instances discovered so far "minor troubles" and said they posed no threat to people or the environment.

In five of the reactors, major exhaust pipes were knocked out of place and TEPCO was investigating whether they had leaked radioactive materials, the statement said.

TEPCO also said 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.

The company also said a small amount of radioactive materials cobalt-60 and chromium-51 had been emitted into the atmosphere from an exhaust stack. Monday's quake also initially caused a small fire at an electrical transformer in the sprawling plant.

Japan's nuclear power plants, which have suffered a string of accidents and cover-ups amid deep concerns they are vulnerable in earthquakes.

The Kariwa nuclear power plant, the world's largest in terms of power output capacity, stands near the epicenter of Monday's magnitude 6.6 quake.

Monday's quake initially triggered a small fire at an electrical transformer in the sprawling plant. But it was announced 12 hours later that the temblor also caused a leak of water containing radioactive material.

Later Tuesday, TEPCO said a small amount of radioactive materials cobalt-60 and chromium-51 had been emitted into the atmosphere from an exhaust stack, but posed no danger to the environment. It was unclear if that leak was caused by the quake.

Japanese Prime Minister Shinzo Abe criticized the delay in notifying the public.

"They raised the alert too late. I have sent stern instructions that such alerts must be raised seriously and swiftly," Abe told reporters in Tokyo. "Those involved should repent their actions."

Masanori Hamada, a professor of earthquake engineering at Tokyo's Waseda University, said the quake showed the government should push to increase the quake-resistance standards of its reactors.

"It's unthinkable that water leaks and fire could be triggered so easily," said Hamada. "TEPCO must provide a full explanation to the public."

The plant in Kashiwazaki-Kariwa, 135 miles northwest of Tokyo, eclipsed a nuclear power station in Canada's Ontario as the world's largest power station when it added its seventh reactor in 1997.

The Japanese plant, which generates 8.2 million kilowatts of electricity, has been plagued with mishaps. In 2001, a radioactive leak was found in the turbine room of one of the reactors.

The plant's safety record and its proximity to a fault line prompted nearby residents to file a series of lawsuits claiming that the government had failed to conduct sufficient safety reviews when it approved the plant's construction in the 1970s.

In 2005, a Tokyo court threw out a lawsuit filed by 33 residents, saying there was no error in the government safety reviews. The court also denied that the seismic fault under the plant could result in an accident during a major quake.

Nearly 13,000 people packed into evacuation centers such as schools and other secure buildings in the quake zone 160 miles northwest of Tokyo, the Fire and Disaster Management Agency said.

Victims were largely concerned with securing enough food, water and shelter for the night, but some said the threat of a devastating nuclear accident was always at the back of their minds.

"Whenever there is an earthquake, the first thing we worry about is the nuclear plant. I worry about whether there will be a fire or something. We have no information, it's really frightening," said Kiyokazu Tsunajima, who spent the first night sleeping in his car, afraid an aftershock might collapse his damaged house.

The Defense Ministry dispatched 450 soldiers to the devastated area to clear rubble, search for any survivors under collapsed buildings and provide food, water and toilet facilities. People formed long lines to fill bottles with fresh water.

About 50,000 homes were without water and 35,000 were without gas as of Tuesday morning, local official Mitsugu Abe said. About 27,000 households were without power.

The Japanese Meteorological Agency put the initial quake's magnitude at 6.8, while the U.S. Geological Survey said it was 6.6. The quake, which hit the region at 10:13 a.m., was centered off the coast of Niigata, 160 miles northwest of Tokyo.

The area was plagued by a series of aftershocks, though there were no immediate reports of additional damage or injuries from the aftershocks.

Near midnight, Japan's Meteorological Agency said a 6.6-magnitude quake hit off the west coast, shaking wide areas of Japan, but it was unrelated to the Niigata quake to the north and there were no immediate reports of damage.