GWEN IFILL: Japan suffered two major blows today, as a desperate effort continued to head off a nuclear crisis on the heels of a catastrophic earthquake and tsunami.
A new explosion rocked a shut-down reactor at a plant that lost its cooling system in Friday's disaster. And the U.N. nuclear agency warned a second reactor was failing as well. Thousands of people had already been ordered out of a 12-mile exclusion zone around the plant. Today, another 140,000 people living 12 to 20 miles away were told to stay indoors, and officials imposed a no-fly zone around the site.
We have a series of reports from Independent Television News, beginning with Tom Clarke on the growing nuclear scare.
TOM CLARKE: Seen from space, this is the blighted Fukushima nuclear plant. In this photo, two square reactor buildings remain intact, but explosions since it was taken have left both resembling the smoldering carcasses of their neighbors.
Confirmation today that the containment around this reactor had been breached -- this is a look inside reactor number two. Engineers are struggling to keep the nuclear fuel rods covered with seawater. That exposes the fuel inside. And as it heats up, water turns to steam and pressure builds up.
Reports suggest that due to that pressure, this structure, called the suppression chamber, cracked. It contains much of the harmful radioactive material given off by the fuel rods. Steam mixed with hydrogen gas escaped and then exploded.
YUKIYA AMANO, International Atomic Energy Agency: There's a possibility that the suppression chamber that I explained may have been damaged. And there is also the possibility of core damage.
TOM CLARKE: And there wasn't just the explosion at reactor No. 2. Normally submerged spent fuel rods like these filmed at Fukushima before the disaster caught fire at the number four reactor site. The fire is now out, but it added a major spike in radioactivity around the plant.
Because it damages cells as they grow, radiation is most harmful to children and pregnant women -- thyroid cancer in children a particular worry, as radioactive iodine from nuclear reactions can concentrate there.
Has last night's blast of radiation increased the risk to health? Immediately after the explosion and fire, the level reported outside the reactors was 400 millisieverts per hour. Compare that to the exposure limit for a nuclear worker of 20 millisieverts a year.
According to the World Health Organization, exposure to 100 millisieverts in a year is the minimum level that can lead to cancer. Within the exclusion zone, a level of five millisieverts per hour has been recorded. At that point, 20 hours of continuous exposure would raise the danger of illness. All these levels have now been falling, so continued exposure probably won't happen.
The mandatory evacuation around the plant will reduce the risk of harm to the general public but not to the 50 people now fated as heroes in Japan who remain at the plant. Radiation releases have, at regular intervals, forced them to abandon efforts to avert meltdown, but in their struggle, they will inevitably risk fatal radiation exposure.