TOKYO — The stricken Fukushima Daiichi nuclear power plant came a lot closer to a full “China Syndrome” meltdown than previous company analyses had indicated, though there is no danger of further damage now, plant operator Tokyo Electric Power Co. (TEPCO) said Wednesday.

The nuclear fuel rods in reactor No. 1 likely melted completely, TEPCO and the Japanese government said for the first time — burning a hole through one surrounding vessel and eating through up to three-quarters of the concrete base at the bottom of a second containment vessel meant as a last barrier between the radioactive core and the outside world.

That brought the fuel closer than previously believed to breaching the containment vessel and continuing to burn through the ground below — a catastrophic scenario sometimes described as the “China Syndrome,” from the fanciful notion, popularized in a US film by the same name, that in a catastrophic meltdown, molten reactor fuel could sink through the earth until it reached China.

Earlier, TEPCO had said only that it thought unit No. 1’s fuel was more than half melted, and that some had fallen into the containment vessel.

The findings are the latest reminder of how dangerous the mid-March accident at Fukushima Daiichi was and how much remains unknown. They result from separate TEPCO and government analyses of the latest data on radiation and temperature readings around the reactors, which has helped TEPCO get a better idea of where the fuel is at the three units whose cores partially melted down.

TEPCO said the damage in units No. 2 and No. 3 was less severe than in No. 1, although some fuel did burn through their surrounding vessels to the concrete base of their containment vessels. In all three units, the fuel has now cooled to below the critical temperature of 100 degrees Celsius (212 degrees Fahrenheit), and thus poses no further threat, officials said.

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