Yet those explosions did not puncture the steel containment vessel – shaped like an inverted light bulb – now keeping high amounts of radioactivity bottled up, the plant owner said. Even so, all three reactors’ cores have been exposed to air, and so are assumed to have melted or become damaged to some degree, Japanese and international authorities reported.
Detection of the highly radioactive cesium-137 and iodine-131 outside the Fukushima I plant suggests that fuel has melted and at least some cores have sustained damage. On Monday morning in Japan, the No. 2 reactor joined the other two in losing coolant for a period, thus exposing to the air fuel rods that must be kept underwater in order to avoid partial or complete melting.
To get water into the reactor to cool it, authorities are now forced to vent radioactive steam. The plant is now emitting as much radiation in an hour as it would normally release in six months.
“The possibility that a large amount of radiation has been released is low,” Yukio Edano, the chief government spokesman, said at a news conference.
Cooling the three reactors isn't the only problem. Cooling spent fuel stored in an adjacent spent-fuel pool at one troubled reactor could be an emerging issue, according to a TEPCO press release on March 13.
"We are currently coordinating with the relevant authorities and departments as to how to secure the cooling water to cool down the water in the spent nuclear fuel pool," the release said. Just how serious the cooling problem is remains publicly unknown, but raises concerns for some US nuclear experts.