Japanese authorities launched an urgent attempt Thursday to avert a nuclear disaster by air, but initial attempts to drop tons of water at the quake-ravaged Fukushima Daiichi plant appeared to do little to lower potentially perilous radiation levels. Helicopters made four passes in about a 20-minute span Thursday morning, dropping 7.5 tons of seawater each time on the facility's No. 3 reactor in order to cool its overheated fuel pool. Experts believe that boiling steam rising from that pool, which contains at least partially exposed fuel rods, may be releasing radiation into the atmosphere. But hours later, the Tokyo Electric Power Company -- which runs the plant -- told Japan's Kyodo News that the operation didn't appear to lower radiation levels. The report suggested levels actually rose to about 3,000 microsievert per hour. It takes a year for a person to be naturally exposed to that level of radiation.