"There is nothing like this, on this scale, that we have ever attempted to do before," says Robert Alvarez, a former assistant secretary of the U.S. Energy Department.

Japanese officials estimate that they already have accumulated about 15 million gallons of highly radioactive water. Hundreds of thousands of gallons are being added every day as the plant's operator, the Tokyo Electric Power Co., continues to feed coolant into the leaky structures.

Ultimately, the high-level radioactive substances in the water will have to be safely stored, processed and solidified, a job that experts say will almost certainly have to be handled on a specially designed industrial complex. The process of cleaning up the water could take many years, even decades, to complete. The cost could run into the tens of billions of dollars.