Japanese officials say the reactor cores at the Fukushima nuclear plant are still intact, as they try to allay fears of a radioactive leak after a second explosion in a reactor.
Six people have been injured in the explosion, the plant's operator says.
Technicians have been battling to cool the three reactors with seawater at the plant, 240km north of Tokyo, since the quake and tsunami knocked out its cooling system on Friday.
A large explosion on Saturday blew apart a building housing one reactor.
It was followed by another blast, involving a second reactor, on Monday.
Television footage showed smoke rising from the plant after that explosion, which Japan's nuclear safety agency says is believed to have been caused by the build-up of hydrogen.
The International Atomic Energy Agency says it has been told the second blast did not damage the plant's primary containment vessel.
A third reactor has now lost its cooling capacity at the plant, raising fears of another explosion.
Radiation levels around the plant rose to 751.2 microsievert per hour on Monday, a Tokyo Electric Power Co spokesperson said, slightly more than the level a person would be exposed to for a stomach X-ray.
Chief Cabinet Secretary Yukio Edano earlier said there might have been a partial meltdown of fuel rods at that reactor and engineers were pumping in seawater to try to prevent the same happening at a second reactor.
An exclusion zone around the entire complex was extended to a 20km radius on Sunday. Some 200,000 people have been moved from the area.
At Tokai No. 2 nuclear power station in Ibaraki prefecture, the cooling system is working though some generators used for cooling are out of order, the Japan Atomic Power company says.
At Onagawa nuclear plant, where high radioactivity levels had been reported, Japanese authorities said levels at the site boundary had returned to normal and that there had been no emission of radiation from the plant itself.
If reactors overheat and fail, the containers that house the core could melt or explode, releasing radioactive material into the atmosphere.
A Japanese official says 22 people have been confirmed to have suffered radiation contamination and up to 190 may have been exposed. Workers in protective clothing used handheld scanners to check people arriving at evacuation centres.
A UN official says health risks remain low and winds are likely to carry any contamination out to the Pacific without threatening other nations.
About a third of Japan's energy is from nuclear power, the BBC reports. In total, 11 of the approximately 50 nuclear reactors in Japan were shut down following the earthquake.