A government-commissioned group of experts has concluded that a costly underground ice wall is only partially effective in reducing the ever-growing amount of contaminated water at Japan's destroyed Fukushima nuclear plant, and other measures are needed as well.

The plant's operator says the ice wall has helped reduce the radioactive water by half. The plant also pumps out several times as much groundwater before it reaches the tsunami-damaged reactors.

The panel agreed Wednesday that the 35 billion yen ($320 million) ice wall helps, but doesn't completely solve the problem.

The 1.5-kilometer coolant-filled underground structure was installed around the wrecked reactor buildings to create a frozen soil barrier to keep groundwater from flowing into the heavily radioactive area.

The groundwater mixes with radioactive water leaking from the damaged reactors.