Tokyo Electric Power Co (TEPCO), which operates the plant in eastern Japan, discharged 850 tons of formerly contaminated water it had extracted from the ground near the plant into the sea, saying a filtration process had now made it safe.
Today was the first time the plant, whose reactors went into meltdown after being hit by a huge tsunami in 2011, has released once radioactive water into nature after a years-long battle with fishermen, who feared it could destroy their livelihood.
"The risk that you run is that you have all these tanks full of water," Klein told AFP in an interview.
"The longer you store the water, the more likely you are going to have (an) uncontrolled release," he said, adding that he would like to see the supplies released from storage in the next three years.
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Four years later it is still extracting some 300 tons of contaminated water from the ground every day, which had been stored in tanks before TEPCO started releasing it into the sea after purification today.
The move is a milestone for the company, which said its Advanced Liquid Processing System, which removes highly radioactive substances like strontium and caesium, meant the ground water was now safe to release into the natural environment.
They had fought to stop the water being released into the sea, even after it is filtered, but eventually bowed to pressure from TEPCO, which is struggling to find space to store the tainted supplies.
But it has yet to find a solution to deal with another highly radioactive 680,000 tons of water that was used to cool the reactors during the meltdown, which is still stored on site.
Fishermen are opposed to the fluid being released into the sea, even after it is filtered.
Torrential flooding this month in an area not far south of the plant added to contamination concerns, flushing away at least 293 plastic bags of plants and soil that had been collected in the clean up.