Comparing Fukushima, Chernobyl, Three Mile Accidents: Q&A

Radiation leaks from Tokyo Electric Power Co.’s earthquake-stricken reactors in northeastern Japan represent the worst nuclear power accident since the meltdown at Chernobyl, Ukraine, almost 25 years ago, scientists say.

Helicopters poured 30 metric tons of water today on pools used to cool spent fuel rods. No change in radiation was reported after four runs by the aircraft, Kyodo News said, citing plant operator Tokyo Electric Power Co. Once exposed, the spent fuel rods may catch fire and melt, spewing radiation into the atmosphere.