Trying to cool down three molten reactor cores and stop radiation leaks to end a crisis set off when the 3-11-11 earthquake and tsunami crippled the plant.
The job is expected to take several more months, and is complicated by massive amounts of radioactive water that could soon leak into the sea.
About 110,000 tons of tainted water have accumulated, and it could start overflowing in early July 2011 unless workers get a trouble-plagued water treatment system working properly.
The system became fully operational a week ago but shut down after a few hours when one of the radiation absorbing cartridges reached its limit much more quickly than expected.
News from The Associated Press