More than three years into the massive cleanup of Japan's tsunami-damaged nuclear power plant, only a tiny fraction of the workers are focused on key tasks such as preparing for the dismantling of the broken reactors and removing radioactive fuel rods.

Instead, nearly all the workers at the Fukushima Dai-ichi plant are devoted to an enormously distracting problem: a still-growing amount of contaminated water used to keep the damaged reactors from overheating. The amount has been swelled further by groundwater entering the reactor buildings.

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Fukushima, where a nuclear power plant was damaged in a 2011 quake and tsunami, is in northeastern Japan. (Google)

Hundreds of huge blue and gray tanks to store the radioactive water, and buildings holding water treatment equipment are rapidly taking over the plant, where the cores of three reactors melted following a 2011 earthquake and tsunami. Workers were building more tanks during a visit to the complex Wednesday by foreign media, including The Associated Press.

"The contaminated water is a most pressing issue that we must tackle. There is no doubt about that," said Akira Ono, head of the plant. "Our effort to mitigate the problem is at its peak now. Though I cannot say exactly when, I hope things start getting better when the measures start taking effect."

The numbers tell the story:

• 6,000 workers

Every day, about 6,000 workers pass through the guarded gate of the Fukushima Dai-ichi plant on the Pacific coast — two to three times more than when it was actually producing electricity.

On a recent work day, about 100 workers were dismantling a makeshift roof over one of the reactor buildings, and about a dozen others were removing fuel rods from a cooling pool. Most of the rest were dealing with the contaminated water, said Tatsuhiro Yamagishi, a spokesman for Tokyo Electric Power Company, or TEPCO, the utility that owns the plant.

The work threatens to exhaust the supply of workers for other tasks, since employees must stop working when they reach annual radiation exposure limits. Experts say it is crucial to reduce the amount and radioactivity of the contaminated water to decrease the risk of exposure to workers and the environmental impact before the decommissioning work gets closer to the highly contaminated core areas.

• 40 years

The plant has six reactors, three of which were offline when disaster struck on March 11, 2011. A magnitude-9.0 earthquake triggered a huge tsunami which swept into the plant and knocked out its backup power and cooling systems, leading to meltdowns at the three active reactors.

Decommissioning and dismantling all six reactors is a delicate, time-consuming process that includes removing the melted fuel from a highly radioactive environment, as well as all the extra fuel rods, which sit in cooling pools at the top of the reactor buildings. Workers must determine the exact condition of the melted fuel debris and develop remote-controlled and radiation-resistant robotics to deal with it. The process is expected to take at least 40 years.

• 500,000 tons

The flow of underground water is doubling the amount of contaminated water and spreading it to vast areas of the compound.

Workers have jury-rigged a pipe-and-hose system to pump water into the reactors to cool the melted fuel inside. Exposure to the radioactive fuel contaminates the water, and much of it pours into the basements of the reactors and turbines, and into maintenance trenches that extend to the Pacific Ocean.

The plant reuses some of the contaminated water for cooling after partially treating it, but the additional groundwater creates a huge excess that must be pumped out.

Currently, more than 500,000 tons of radioactive water is being stored in nearly 1,000 large tanks which now cover most of the sprawling plant. After a series of leaks last year, the tanks are being replaced with costlier welded ones.

That amount dwarfs the 9,000 tons of contaminated water produced during the 1979 partial meltdown of the Three Mile Island nuclear plant in the United States. At Three Mile Island, it took 14 years for the water to evaporate, said Lake Barrett, a retired U.S. nuclear regulatory official who was part of the early mitigation team there and has visited the Fukushima plant.

"This is a much more complex, much more difficult water management problem," Barrett said.

• $18 billion

An estimated 2 trillion yen ($18 billion US) will be needed just for decontamination and other mitigation of the water problem. Altogether, the entire decommissioning process, including compensation for area residents, reportedly will cost about 10 trillion yen, or about $90 billion.

All this for a plant that will never produce a kilowatt of energy again.

About 500 workers are digging deep holes in preparation for a taxpayer-funded 32 billion yen ($290 million) underground "frozen wall" around four reactors and their turbine buildings to try to keep the contaminated water from seeping out.

TEPCO is developing systems to try to remove most radioactive elements from the water. One, known as ALPS, has been trouble-plagued, but utility officials hope to achieve its daily capacity of 2,000 tons when they enter full operation next month.

Officials hope to treat all contaminated water by the end of March, but that is far from certain.

With files from Reuters