OKUMA, Japan (AP) -- The radioactive water that has accumulated at Japan's crippled nuclear power plant remains the biggest problem hampering the cleanup process three years after the disaster.
The Fukushima Dai-ichi plant has stabilized substantially since the March 11, 2011, earthquake and tsunami destroyed its power and cooling system, triggering meltdowns. Massive amounts of water are being used to cool the melted cores at three reactors, but some of the contaminated water has seeped through the ground into the Pacific and leaked repeatedly from storage tanks.
Plant chief Akira Ono said Monday that improving water management is crucial not only to the plant cleanup but also decontamination of the area so evacuees can return to their homes.
"The most pressing issue for us is the contaminated water, rather than decommissioning," Ono said during a plant tour for foreign media, including The Associated Press. "Unless we resolve the problem, fear of the society continues and the evacuees cannot return home."
Experts say the water leaks are spreading radiation across the plant and into the sea, hampering the cleanup process.
In order to mitigate the problem, TEPCO will build an underground ice wall around the four damaged reactor units to block contaminated water from leaking out while keeping underground water from flowing in -- a multibillion-dollar government-funded project.
On Monday, workers were making final preparations to activate an experimental ice wall at a test site at the plant. The test is set to start within days, then the nearly 2 kilometer (1.2 mile) wall around the four units would be built for use sometime next year. A similar method has been used at a U.S. nuclear plant, but one with this magnitude is untested, and some experts say backup measures should be installed.
The plant has accumulated 436,000 tons of contaminated water stored in 1,200 industrial tanks that have taken over large parts of the plant. TEPCO has developed a set of water treatment units that can remove all radioactive elements but tritium for safer storage of the water, and is currently working to build another set, while developing its upgraded version as part of a government-funded project. The plant is feared to run out of storage capacity sometime next year.
Dale Klein, former U.S. Nuclear Regulatory Commission chairman who heads a group of outside advisers for TEPCO's nuclear reform, said the space is limited and the water should be released into the sea eventually. He said that scientists agree that tritium is not harmful like bone-settling strontium and cesium, and can be safe in a controlled release.
"Storing the massive amounts of water in tanks is not sustainable," said Klein, who was also at the plant Monday. "My assessment is it's not science that needs to be developed, but it's public policy."
Repeated water leaks, as well as preventive measures, monitoring and water have caused higher levels of exposures among workers.
The number of workers who were exposed to more than 5 millisievert -- the benchmark annual exposure level for seeking labor compensation for developing leukemia -- had fallen to 98 in June 2013 but surged back as high as 398 in October before slightly leveling off to about 250 in January. More than 2,900 workers were exposed to 5 millisievert or above in March 2011.
Many workers who were at the plant early in the crisis had been removed from plant jobs or retired after reaching their exposure limits. TEPCO has acknowledged that securing a steady stock of skilled workers for the plant's decades-long decommissioning is a challenge.
In a landmark step toward decommissioning, the plant started removing fuel rods from Unit 4 storage pool in November. More than a quarter of its 1,533 fuel rods have been moved to a safer storage.
Japan still needs to develop robotics technology to assess the damaged reactors to start removing melted fuel from them by 2020, a target some experts say too optimistic.
The plant's operator, Tokyo Electric Power Co., acknowledged in July that contaminated underground water has been flowing into the ocean for some time, soon after the crisis began.
The disaster is the world's worst atomic accident since Chernobyl in 1986. More than 100,000 people have not returned home due to fear of radiation from the plant.