Japans nuclear crisis intensified on Wednesday after the authorities announced that a second reactor unit at the stricken Fukushima Daiichi plant in northeastern Japan may have ruptured and appeared to be releasing radioactive steam. The break, at the No. 3 reactor unit, worsened the already perilous conditions at the plant, a day after officials said the containment vessel in the No. 2 reactor had also cracked. Such were the radiation levels above the plant, moreover, that the Japanese military put off a highly unusual plan to dump water from helicopters a tactic normally used to combat forest fires to lower temperatures in a pool containing spent fuel rods that was overheating dangerously. The operation would have meant flying a helicopter into steam rising from the plant with potentially high radiation levels. However, in one of a series of rapid and at times confusing pronouncements on the crisis, the authorities insisted that damage to the containment vessel at the No. 3 reactor the main focus of concern earlier on Wednesday was unlikely to be severe.