TOKYO (AP) -- 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 a single, enormously distracting problem: coping with the vast amount of contaminated water _ a mixture of underground water running into recycled water that becomes contaminated and leaks after being pumped into the damaged reactors to keep their melted cores from overheating.

A number of buildings housing water treatment machines and hundreds of huge blue- and gray-colored industrial tanks to store the excess water are rapidly taking over the plant, which saw three of its six reactors melt down following the 2011 earthquake and tsunami. Workers were still building more tanks during a visit to the complex Wednesday by a group of 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, located 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, while about a dozen others were removing fuel rods from a cooling pool.

Most of the rest were dealing with contaminated water-related work, said Tatsuhiro Yamagishi, a spokesman for Tokyo Electric Power Company, or TEPCO, the utility that owns the Fukushima plant.

The work threatens to exhaust the supply of workers for other tasks, since they 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 environmental impact before the decommissioning work gets closer to the highly contaminated core area.

40 YEARS

The plant has six reactors, three of which were offline when disaster struck on March 11, 2011 _ a magnitude-9.0 earthquake that 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 of the 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 situated at the top of the reactor buildings. The entire job still requires finding out the exact conditions of the melted fuel debris and developing remote-controlled and radiation-resistant robotics to deal with them, and is expected to take at least 40 years.

500,000 TONS

The main problem is abundant inflow of underground water into the contaminated water that doubles the volume and spreads it to vast areas of the compound.

Workers have jury-rigged a pipe-and-hose system to continuously pump water into the reactors to cool the clumps of melted fuel inside.

The water becomes contaminated upon exposure to the radioactive fuel, and much of it pours into the reactor and turbine basements, and maintenance trenches that extend to the Pacific Ocean.

The plant recycles some of the contaminated water as cooling water after partially treating it, but groundwater is also flowing into the damaged reactor buildings and mixing with contaminated water, creating a huge excess that needs to be pumped out.

So far, more than 500,000 tons of radioactive water have been stored in nearly 1,000 large tanks that workers have built, which now cover most of the sprawling plant. After a series of leaks from the storage tanks last year, they are now being replaced with costlier welded tanks.

That dwarfs the 9,000 tons of contaminated water produced during the 1979 partial meltdown of Three Mile Island 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.

10 TRILLION YEN

An estimated 2 trillion yen ($18 billion) 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.

The work threatens to exhaust the supply of workers for other tasks, since they must stop working when they reach annual radiation exposure limits. 

About 500 workers are digging deep holes in preparation to build a taxpayer-funded 32 billion yen ($290 million) underground ``frozen wall'' around the 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 their daily capacity of 2,000 tons when they enter full operation next month.

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