Tokyo Electric Power Company has found that the pressure inside one of the reactor containment vessels at the Fukushima Daiichi nuclear plant has dropped.
The utility said the earthquake that hit the area on February 13 may have been to blame. TEPCO said no radioactive material has leaked.
The nuclear plant in Fukushima Prefecture in northeastern Japan is now undergoing decommissioning.
TEPCO said it confirmed at around 6 p.m. on Sunday that pressure inside the vessel of the No.1 reactor dropped to 0.9 kilopascals from the usual 1.2 kilopascals. Nitrogen is being added to the vessel to raise the pressure and reduce the risk of a hydrogen explosion.
TEPCO found that after the quake the level of water in the containment vessel had dropped.
It said the part damaged in the 2011 nuclear accident may have been exposed after the water level dropped, possibly causing an air leakage and the drop in pressure.
TEPCO said it is continuing to pump water into the vessel and there should be no safety problem.
It said there have been no abnormality in the measurements at radioactivity monitoring posts in the compound.
The utility said it will keep watching the data at the vessel and other facilities.