Eight years after a tsunami knocked out power and caused a meltdown at three of its four reactors, Japan's Fukushima nuclear plant is still in crisis. The biggest challenge facing TEPCO is the buildup of radioactive cooling water. More than 1.2 million tons have already been stored in tanks built on the site, and space is running out. Meanwhile, the cleanup costs are set to be much higher than initially estimated, according to a private think tank.

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