In this photograph released by Japan Coast Guard, black smoke rises from a burning electrical transformer near one of Kashiwazaki Kariwa Nuclear Plant's four reactors after a fire broke out, following a strong earthquake in Kashiwazaki, northwestern Japan, on Monday.

KASHIWAZAKI, Japan — A nuclear power plant near the epicenter of a powerful earthquake suffered a slew of problems, including spilled waste drums, leaked radioactive water, fires and burst pipes, the reactor's operator said today — more than 24 hours after the tremors struck northern Japan.

The problems 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 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.

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."

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.

People packed school gymnasiums and community centers in the city, camping out on traditional Japanese futon mattresses and fanning themselves from the muggy summer heat.

Thunderstorms and flooding were expected today throughout the quake zone, increasing the likelihood that the quake-softened, water-logged ground would give way.