At the Fukushima Daiichi nuclear plant, nothing is more problematic right now than the contaminated water that covers the basement floors, leaks into the environment and endangers any worker who goes near it.

After dousing its reactors for 2¹ / ₂ months in jury-rigged cooling efforts following the March 11 earthquake and tsunami, the Tokyo Electric Power Co. must deal with the severe side-effects of that strategy by removing at least 15 million gallons of water — enough to fill the first five floors of the Empire State Building.

But engineers planning that unprecedented clean-up job face questions about where they’ll put the water and how effectively they can filter its radioactive particles.

Tepco’s problem “resembles a board game with 16 squares and one empty spot,” said David Lochbaum, a nuclear engineer who directs the Nuclear Safety Project of the Union of Concerned Scientists.

Workers must inject the reactor cores with water to keep them cool. But that step guarantees that water will leak through the quake-damaged plant and into the basement-level turbine rooms. And the resulting radioactive water makes repair work all the harder. Which means that workers, still struggling to fix the usual re-circulation system, must continue to “feed and bleed” the reactors from above.

Which means water levels continue to rise down below.

“They’re just perpetuating the problem and making a bigger and bigger mess,” said Lake Barrett, a nuclear engineer who directed the cleanup of of the hobbled Three Mile Island plant in Pennsylvania.

A potential turning point comes roughly two weeks from now, when Tepco plans to begin a treatment process in which water is sucked from the basement rooms and fed into a special tank, then treated with chemicals that eliminate its radioactivity. The process creates a byproduct of radioactive sludge, which is generally mixed with bitumen, poured into drums, then sealed and buried. The water itself can either be cycled back into reactors or discarded into the ocean.

The treatment system is being set up by Areva, a French company that uses the technology at its La Hague nuclear reprocessing plant, off the Normandy coast. Since 1997, Greenpeace — after taking water samples from La Hague’s discharge pipe — has made repeated claims that the supposedly decontaminated water in fact contains radioactivity levels above the regulatory limit.

The process “is not 100 percent, but it’s better than nothing,” Lochbaum said. “The alternative: you let the water simply evaporate and radioactivity carries to all parts far and wide.”

Japan already has experienced substantial environmental problems from the failure at Fukushima, with authorities at the plant discharging contaminated water into the Pacific on at least three occasions. During a visit to Japan last week, Greenpeace officials presented data showing higher-than-legal in seaweed and shellfish that were collected more than 12 miles from the plant. The samples’ high concentrations of iodine-131 — which has a half-life of eight days — indicated that leaks from Fukushima Daiichi were ongoing, and “much larger than has been declared by Tepco so far,” said Jan Vande Putte, a Greenpeace radiation expert.