A third explosion has rocked Japan's Fukushima Daiichi power plant, in what is being called the worst nuclear disaster since Chernobyl. Residents living nearby have been evacuated and emergency workers were removed from the plant. With fears of radiation exposure and a full meltdown, workers are continuing to pump seawater on the reactors in an effort to continue the cooling process.   Andrew Kadak, a former professor of nuclear science and engineering at MIT says that the power plant was constructed to withstand a tsunami, but not one as big as the one that struck. He helps explain what happened at the plant and the steps that are necessary to avoid a meltdown. Robert Hernan, author of "This Borrowed Earth: Lessons from the Fifteen Worst Environmental Disasters Around the World" weighs in on how this disaster compares to Three Mile Island and Chernobyl.