TOKYO — Japanese officials announced Sunday that the cooling system at a second nuclear reactor crippled by Japan’s devastating earthquake had failed completely, even as they took the extraordinary step of flooding a separate reactor with seawater in a last-ditch effort to avoid a nuclear meltdown. An explosion occurred at the Daiichi nuclear power plant in northern Japan after the earthquake.
The announcement Sunday compounded what was already the worst nuclear accident in Japan’s history — and perhaps the worst involving a nuclear plant since the Chernobyl disaster 25 years ago — as the nation was reeling from the aftermath of the largest recorded earthquake in its history.
The cooling systems at three other reactors at a second nuclear plant had also failed, officials said. While backup systems might still be revived, if they could not, these reactors too could require emergency cooling, they said.
For now, the most pressing concern stemmed from a radiation leak and explosion at the Fukushima Daiichi Nuclear Power Station on Saturday, which prompted the government to expand an evacuation order to affect 170,000 people in the plant’s vicinity.
By early Sunday, the plant’s operator issued an emergency notice that the second reactor at the aging plant had also experienced a critical failure of its cooling system. A makeshift way to inject water into the reactor to cool it was urgently being sought. But electricity outages from the 8.9-magnitude quake have hampered the efforts.
The plant was preparing to release vapor to ease pressure from the reactor so the water could be injected, said Naoki Tsunoda, a spokesman for the operator, Tokyo Electric Power. The vapor could contain trace amounts of radiation, the Japanese Nuclear and Industrial Safety Agency has warned.
Operators were urgently working to cool off three reactors at the nearby Fukushima Daini power plant, Mr. Tsunoda said.
At a press conference, Yukio Edano, the chief cabinet secretary, called for calm. “If these measures can be taken, we will be able to ensure the safety of the reactor,