According to TEPCO, the remaining contaminated water remains at the bottom of the storage tanks. It cannot be vacuumed by the original pumps but must be carefully removed for treatment as the tanks are disassembled.

Tokyo Electric Power Company (TEPCO) announced it had completed on Wednesday processing almost all the highly radioactive water stored in tanks at Japan's damaged Fukushima-1 nuclear power plant, the company said in a statement.

"This is a significant milestone for improving the environment for our surrounding communities and for our workers," TEPCO Chief Decommissioning Officer Naohiro Masuda said in a statement published on the company website.

According to TEPCO, the remaining contaminated water remains at the bottom of the storage tanks. It cannot be vacuumed by the original pumps but must be carefully removed for treatment as the tanks are disassembled.

In March 2011, the Fukushima Daiichi nuclear plant suffered a meltdown of three of its six reactors due to flooding caused by a tsunami that followed in the wake of a 9.0-magnitude earthquake. Radioactive material leaked into the sea, soil and atmosphere, and hundreds of thousands of people were evacuated from the region.

After three months elapsed, new automated reactor water-cooling systems were installed. At least 250,000 tons of water contaminated by radiation were then stored in tanks to await processing.

The radioactive water was to be filtered to remove 62 radioactive elements using an Advanced Liquid Processing System (ALPS).

TEPCO had originally hoped to have all stored radioactive water treated by the end of March 2015, but problems made the company announce a delay until May 31.