The video images showed a small amount of vapor or steam, but the origin wasn't clear. It was detected in the morning and was continuing in the afternoon.
The reactor's spent fuel pool was stable and measurements of the temperatures and pressure have not changed significantly, Tokyo Electric Power Co. said.
Workers were continuing to inject water into the No. 3 reactor to cool it, the utility said.
The No. 3 reactor was one of three at the Fukushima Dai-ichi plant where the nuclear cores overheated and melted after the 2011 earthquake and tsunami. Radiation spewed from the plant and still contaminates the air, water and soil nearby.
TEPCO is relying on a makeshift system of hoses and pumps to keep the reactors from overheating, and the site still is littered with damaged vehicles, twisted metal and other debris left by the waves that swept through the plant and knocked out its power. The utility has estimated that shutting down the plant will take 40 years.
The Fukushima accident was the world's worst nuclear disaster since Chernobyl in 1986. Thousands of people have been unable to return to their homes near the plant because radiation levels are still high.
Most of Japan's nuclear reactors remain shut down for safety checks following the disaster.
Copyright 2013 The Associated Press.