White smoke rose from the No. 3 reactor at the Fukushima No. 1 nuclear plant and radiation levels rose at one point Wednesday, but the government later played down the possibility of grave damage to the containment vessel.

Correcting an earlier remark, Chief Cabinet Secretary Yukio Edano told reporters in the afternoon that the government now believes the water pool for spent nuclear fuel at the No. 3 reactor probably heated up, causing steam to rise.

The containment vessel is the last line of defense for containing lethal radioactive materials, and significant damage would pose grave safety concerns.