The assessment, provided to The Times on background, suggests that the plant is unlikely to suffer a complete meltdown, in which uranium fuel gets so hot that it melts through the bottom of the reactor and containment vessels, spewing high-level radiation into the plant’s underlying foundation.

“We are a long way from a point where anybody would say this is stable,” a senior administration official said. “But it is not a runaway. For a long time, we will be at a declining level of risk.”…

The most telling evidence about the condition of the reactors is the absence of heavy radionuclide contamination around the plant, which would indicate that uranium fuel became so overheated that it vaporized heavy fission products such as strontium and technetium, experts said.