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Units 4, 5 and 6 were shut at the time of the quake, but even offline reactors have nuclear fuel — either inside the reactors or in storage ponds — that need to be kept cool.

Meanwhile, the Nuclear and Industrial Safety Agency estimated that 70 percent of the rods have been damaged at the No. 1 reactor.

Japan’s national news agency, Kyodo, said that 33 percent of the fuel rods at the No. 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.

Yuasa reported from Tokyo. Associated Press writers Elaine Kurtenbach in Tokyo, David Stringer in Ofunato and Jocelyn Gecker in Bangkok contributed to this report.