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-km (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.

Repeated water leaks, as well as preventive measures, monitoring and water have caused higher levels of exposures among workers.

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.