Authorities also signaled a new area of potential worry: temperature at the remaining two reactors, both not operating at the time of the earthquake and so far not the subject of concern, has risen slightly, prompting the need for close monitoring. Officials were also probing a report that boiling water had been spotted near where the fire broke out Tuesday morning, another potential worrying sign.

But the conditions at the plant appeared to have stabilized in some aspects Tuesday afternoon. While radiation levels at the plant remain elevated, they have dropped significantly from earlier in the day, ruling out the continuation of unstoppable large-scale leaks.

Radiation levels in downtown Tokyo—which had also risen earlier Tuesday, though they remained well below levels that could damage human health—also fell sharply later in the day…

Speaking at a press conference late in the afternoon, chief cabinet secretary Yukio Edano said officials now suspect that those extremely high readings may have come from the debris from Monday’s explosion coming to rest by the gauge.

Authorities also continued to pour water in the No. 2 reactor, where an explosion early Tuesday appeared to have damaged its all-important containment structure, which keeps radiation inside from leaking out. “I think we need to see how things will go at the No. 2 units for a little bit longer before we can call it stable,” Mr. Edano said.