Tokyo Electric Power Co has said it has found substances in a Fukushima reactor which could be due to nuclear fission.
This could mean a setback in efforts to safely shut down the nuclear plant by the end of the year after it was hit by the Japan earthquake and tsunami in March.
Plant operator Tokyo Electric Power (Tepco) said that it discovered xenon, a substance produced as a byproduct of fission from the No 2 reactor, and had poured in a mixture of water and boric acid, an agent that helps prevent nuclear reactions, as a precaution.
"It can be assumed that isolated criticality took place for a short period of time judging from the presence of xenon," Tepco spokesman Junichi Matsumoto said.
Criticality is a state when controlled nuclear reactions take place and nuclear power plants harness the resulting heat to produce electricity.
The amount of detected xenon was small and the nuclear fuel in the No 2 reactor is unlikely to have melted down again, Tepco said.
The fuel in the No 2 reactor, along with two other reactors, had melted down early in the crisis after the tsunami knocked out the plant's cooling system triggering the world's worst nuclear crisis since Chernobyl 25 years ago.
The Fukushima plant has released radiation into the atmosphere since the earthquake, but analysts said there was minimal risk of further radiation.
Kazuhiko Kudo, a professor of nuclear engineering at Kyushu University, suggested two possibilities.
"Some of the lumps of fuel that melted off early in the accident may have caused the nuclear fission. I would not rule it out completely but this possibility is highly unlikely as many elements, like temperature and the amount water, have to be at a right balance for fission to occur."
The other possibility, Kudo said, was tiny radioactive elements produced by the nuclear reaction early in the crisis colliding and moving neutrons inside the reactor, in turn causing the neutrons to collide and split uranium, causing tiny nuclear fissions.
"The initiating amount in this case is so small that any nuclear fissions would not leave a significant impact," Kudo said.
Tepco has used various cooling efforts to bring down the temperatures at the three damaged reactors from levels considered dangerous and said temperature and pressure at the No 2 reactor remained stable.
It hopes to declare a cold shutdown - when temperatures are stable below boiling point - this year.
Tepco said last month that the amount of radiation being emitted from the complex had halved from a month earlier, in the latest sign that efforts to bring the facility under control are progressing.