Japan's nuclear safety agency is also set to order a plant operator of another plant to release slightly radioactive vapor to protect the reactor from damage.
Altogether, five reactor units — two at the Fukushima No. 1 plant and three at nearby Fukushima No. 2 plant — are in a state of emergency. All five plants have shut down after the massive quake Friday.
Officials said earlier that only one of the two Fukushima No. 1 plant's units had cooling problems resulting from power outages. They now say both units are troubled.