Steam was seen around the fifth floor of the building housing Reactor No. 3 shortly after 9:00 am local time, Tokyo Electric Power Co (TEPCO) said, adding workers were continuing with the ongoing operation to inject cooling water into the reactor and a pool storing nuclear fuel.
TEPCO said monitoring equipment showed no significant changes, including in the levels of potentially cancer-causing radioactivity the broken reactor is releasing.
It said it was looking at the possibility that accumulated rainwater had been the source.
TEPCO said the steam had disappeared by early afternoon.
"We judged that the steam is not present anymore, after not having observed it from 1:30 pm to 2:30 pm," the utility said in a statement, adding they would continue to examine possible causes.
The reactor, devastated by a massive tsunami in March 2011, is too dangerous to approach, and workers had seen the steam on a camera feed, the utility said.
