"In some respects, it's not that big of an issue what they call it," explained David Lochbaum, a veteran nuclear engineer who works as a safety advocate for the Union of Concerned Scientists. "It's still the same mess." (See related: "Latest Radioactive Leak at Fukushima: How Is It Different?")

Workers for Tokyo Electric Power Co., which operates the plant, discovered on August 19 that 300 tons (nearly 72,000 gallons) of highly radioactive water apparently had escaped from a holding tank into the ground over the previous month. The water had been used to cool one of Fukushima's damaged reactors, in the wake of the 2011 earthquake and tsunami that caused one of the worst nuclear accidents in history. (See