Japan works to stop radioactive water leaking into sea
Workers at Japan's quake-hit nuclear plant are trying to prevent radioactive water from seeping into the sea.
Highly radioactive liquid has been found inside and outside several reactor buildings.
Small amounts of plutonium have also been detected in soil at the plant - the latest indication that one of the reactors suffered a partial meltdown.
Prime Minister Naoto Kan said his government was on maximum alert, and the situation remained "unpredictable".
Japan's Nuclear Safety Agency said there was still no confirmation that radioactive water has seeped into the sea from flooded tunnels within the Fukushima Daiichi plant.
Water levels in underground tunnels adjoining reactors 1, 2 and 3 had been stable, the agency said.
Workers from plant operator Tepco have been piling sandbags and concrete blocks around the shafts, which lie between 55m and 70m from the shore, the agency said.
Water in the tunnel linked to the No 2 reactor was measured at more than 1,000 millisieverts an hour, a dose which could cause radiation sickness. Radiation levels in water in tunnels adjoining reactors 1 and 3 were much lower.
Work to safely remove the contaminated water is a priority, government officials said, but stressed more water would need to be used to continue to cool fuel rods.
"We need to avoid the fuel rods from heating up and drying up. Continuing the cooling is unavoidable... We need to prioritise injecting water," Chief Cabinet Secretary Yukio Edano told a news conference.
Tepco and the safety agency say the exact source of the radioactive leak is unknown.
But, like the discovery of plutonium, the high levels of radiation found inside and outside reactor buildings are likely to have come from melted fuel rods.
Theories for the leak centre on two possibilities: steam is flowing from the core into the reactor housing and escaping through cracks, or the contaminated material is leaking from the damaged walls of the water-filled pressure control pool beneath the No 2 reactor.
The plutonium - used in the fuel mix in the No 3 reactor - is not at levels that threaten human health, officials said.
Engineers are battling to restore power and restart the cooling systems at the stricken nuclear plant, which was hit by a powerful quake and subsequent tsunami over two weeks ago.
Operator Tepco has been accused of a lack of transparency and failing to provide information more promptly. It was also heavily criticised for issuing erroneous radiation readings at the weekend.
On Tuesday, National Strategy Minister Koichiro Gemba said the government could consider temporarily nationalising the energy giant.
His comments came a day after shares in the company dropped to their lowest level in three decades.
The massive 9.0-magnitude quake and the tsunami on 11 March are now known to have killed more than 11,000 people, with at least 16,700 people still missing across north-eastern Japan.
The authorities are struggling to identify about 4,000 bodies in Iwate, Miyagi and Fukushima prefectures.
''They were collected at places far from their residential areas (due to being swept away by the tsunami), or their families as a whole must have been washed away by the tsunami,'' a senior official at the National Police Agency was quoted by Kyodo news agency as saying.
Police are posting information about clothes and physical appearance online, the report said.
Some 190,000 people are continuing to live in temporary shelters, many having to cope with food, water and fuel shortages.
The breakdown of local administration has also left municipal offices struggling to assess the damage and casualties in some coastal areas devastated by the tsunami, national broadcaster NHK reports.
Prime Minister Kan is expected to visit the city of Rikuzentakata, in Iwate prefecture, on Saturday.