Water from units 1-4 has contaminated adjacent seawater with iodine-131 and cesium-137. Engineers are processing about 60,000 tons of contaminated water found in the turbine buildings of units 1, 2 and 3, diverting it to reactor condensors and temporary storage tanks, with plans to send it to a radiation waste treatment facility before discharging the water into the sea.

A look at where things stand at the Fukushima Daiichi plant as of 2:30 p.m. Saturday, local time.

How dangerous is the radiation?

The highest reported level of radiation released from the Daiichi plant was 1,000 millisieverts per hour on March 27. The spreading contamination represents a critical safety concern for workers at the plant. On March 24, three workers were hospitalized for radiation burns to their legs and feet.

Effects of radiation

A blast of radiation often causes immediate, obvious symptoms, but damage from low levels of exposure -- generally 100 mSv or less -- may not appear for decades, if ever. Japanese officials have set a dose limit of 250 mSv for nuclear workers during emergencies.