Japan's top government spokesman has warned of the risk of a second explosion at the quake-hit Fukushima nuclear plant, but said that reactor No. 3 could withstand it as reactor No. 1 did a day earlier.
An 9-magnitude earthquake struck off Japan's northeastern coast at 2.46pm on Friday, sending a 10-metre tsunami through towns and cities.
About 200,000 people have been evacuated from the area around the plant.
Meanwhile, police have warned that the death toll in tsunami-hit Miyagi prefecture alone could exceed 10,000.
"There is the possibility of an explosion in the third reactor, as in the case of the first reactor," Chief Cabinet Secretary Yukio Edano told a televised press conference.
He added that the reactor would survive and that there would be no effect on the health of nearby residents.
The No. 3 reactor is the only one at the plant that uses the plutonium fuel mix, which makes the problem potentially more serious.
Radioactive steam to be released
Technicians have opened the valves at reactor 3, allowing small amounts of radioactive vapour to escape in a bid to reduce the pressure in the unit reports the BBC.
They performed a similar operation at the Fukushima No. 1 reactor, hours before it exploded.
The earthquake damaged the cooling system at the Fukushima No. 1 reactor and an explosion on Saturday blew away the concrete walls and roof of a building at the plant, though did not damage the container housing the reactor.
The Tokyo Electric Power Company spokesman earlier said the amount of radiation to be released would be small and not of a level that would affect human health.
The power company is using sea water as an emergency coolant, public broadcaster NHK says.
Japan's nuclear safety agency said on Sunday that as many as 160 people may have been exposed to radiation. The World Health Organisation has said public health risk from the radiation leak appeared to be "quite low".
Thousands moved away from power stations
An estimated 200,000 people have been evacuated from the areas around Fukushima No. 1 plant and Fukushima No. 2 plant.
The International Atomic Energy Agency (IAEA) said in a statement an estimated 170,000 people have been evacuated in the 20km radius around Fukushima No. 1 plant.
It said 30,000 people have been evacuated in the 10km radius around Fukushima No. 2 plant and that full evacuation measures have not been completed.
Reuters reports that workers at evacuation centres wearing white masks and protective clothing used handheld scanners to check everyone arriving for radiation exposure.
The authorities prepared to distribute iodine to people in the vicinity to protect them from exposure.
Iodine can be used to help protect against thyroid cancer in the case of radioactive exposure in a nuclear accident.
The safety agency has rated the incident at the plant as a 4 on the International Nuclear and Radiological Event Scale of 1-7. Three Mile Island was rated 5 while Chernobyl was rated 7.
On Saturday night, a 6.4 aftershock occurred near the nuclear plant, although there were no reports of further damage to the facility.