Japanese nuclear safety officials say they suspect the reactor core at one unit of the troubled Fukushima Dai-ichi nuclear power plant may have breached.
That raises the possibility of more severe radioactive contamination to the environment.
Hidehiko Nishiyama, a spokesman for the nuclear safety agency, said "something at the reactor may have been damaged" in Unit 3 of the six-unit plant.
He said "our data suggests the reactor retains certain containment functions", an implication that the damage may have occurred in the reactor's core, but that it was limited.
Officials said the damage could instead have happened in other equipment, including piping or the spent fuel pool.
Operators have been struggling to keep cool water around radioactive fuel rods in the reactor's core since the massive earthquake and tsunami cut off power supply to the plant and its cooling system.
Damage could have been done to the core when a hydrogen explosion blew apart Unit 3's outer containment building a few days after the disasters.
This reactor, perhaps the most troubled at the six-unit site, holds 170 tons of radioactive fuel in its core.
Previous radioactive emissions have come from intentional efforts to vent small amounts of steam through valves to prevent the core from bursting. However, releases from a breach could allow uncontrolled quantities of radioactive contaminants to escape into the surrounding ground or air.
Operators stopped work today at units 1 to 3 to check on radiation levels.