Workers of Tokyo Electric Power Company (TEPCO), the plant's operator, struggled to remove some 25,000 tonnes of highly radioactive water in and around the No.2 turbine building, which has an extremely high level of radiation exceeding 1,000 millisieverts per hour, Kyodo news agency reported.
The toxic water, which has accumulated in the basement of the turbine building and a tunnel connected to the No.2 reactor, needs to be moved quickly, as it could leak into the nearby sea.
The total amount of contaminated water accumulating in the plant's premises is estimated to be a little less than 70,000 tonnes.
Ahead of the operation, TEPCO sealed cracks in the walls of the facility and ensured that other measures were in place to prevent contaminated water from leaking.
After the government's nuclear safety agency checked procedures and safety measures, TEPCO began the critical mission this morning of moving the highly toxic water to a waste processing facility at the site, according to national broadcaster NHK.
TEPCO said it plans to move about 480 tonnes of the water a day and it will take about 26 days to move about 10,000 tonnes to the waste facility near the No.4 reactor.
The pools of toxic water at the site are believed to be a side-effect of a stopgap measure of injecting water into many of the plant's reactors and spent nuclear fuel pools, which have lost cooling functions since the March 11 magnitude 9 quake and tsunami, to prevent them from overheating.
TEPCO said it would also examine the seabed off the facility to ensure that no plutonium has leaked into the ocean, more than a month after the twin disaster left nearly 30,000 people dead or unaccounted for in Japan's northeast.
The power company said it would conduct the inspection as plutonium is heavier than other radioactive materials and could have accumulated on the floor.
TEPCO earlier detected small quantities of plutonium, a radioactive substance which could cause lung cancer if inhaled, in the soil around the plant. But it said the amount was too small to harm human health.
So far, no plutonium has been found in the air and sea water samples from around the plant.
Senior TEPCO official Junichi Matsumoto was quoted as saying by NHK that there is little doubt that plutonium has leaked from the plant.