TOKYO — A damaged reactor at the Fukushima Daiichi nuclear power plant suddenly began releasing steam again, but the operator of the plant said Thursday it did not appear to be a result of renewed nuclear reactions — a worst-case situation that could lead to a large new release of radioactive materials.

Still, the steam was another indicator of the fragile state of the plant more than two years after the worst nuclear disaster after the accident at Chernobyl. The latest news adds to revelations of a slew of other problems at the plant, including indications that radioactive water may have been leaking into the ocean since the disaster.

The issues have led critics to question the stewardship of Tokyo Electric Power Company, or Tepco, which operated Fukushima Daiichi before the triple meltdowns there and is now in charge of its cleanup. Distrust has also run high because of Tepco’s continued tendency to play down or hide problems at the plant.

Tepco said it based its conclusion that there was no new chain reaction at Reactor No. 3 on its failure to find xenon, a byproduct of fission that lingers for only a few hours and would be an indication of new nuclear activity. Tepco also said the temperature remained stable.