It means the same thing now that it meant on Friday morning.
A "meltdown" means that the rods inside of the reactor core overheat to the point of melting. This is bad because molten hot radioactive rod-goo can burn a hole through the reactor's containment vessel, which would allow radiation to escape the core.
A "meltdown" may have already occurred inside one of the reactors, but the containment vessel is still intact.
The cores of two reactors are still overheating, but they are intact.
In order for something Very Bad to happen to the people immediately surrounding the plant, the core's containment vessel will have to be breached. After that, there are a million factors that could affect how much radiation is leaked and how large of a geographical area it covers. But it is extremely unlikely that any other country will be affected by a radiation leak from Japan. Fukushima is on the Pacific coast of Japan so there is a gigantic mountain range and the entire Sea of Japan separating it from every other Asian country. It is also unlikely that anybody except the people actually working at the nuclear plant are in danger anymore, since such a large area around the plant has been evacuated. As of two hours ago NHK was reporting that everybody living near the evacuation zone is being tested for radiation poisoning but so far nobody has shown any signs of radiation exposure.
I think people are panicking because they see the word "meltdown" and they immediately think of a Chernobyl-like scenario, although that's not actually what a meltdown means.
no subject
A "meltdown" means that the rods inside of the reactor core overheat to the point of melting. This is bad because molten hot radioactive rod-goo can burn a hole through the reactor's containment vessel, which would allow radiation to escape the core.
A "meltdown" may have already occurred inside one of the reactors, but the containment vessel is still intact.
The cores of two reactors are still overheating, but they are intact.
In order for something Very Bad to happen to the people immediately surrounding the plant, the core's containment vessel will have to be breached. After that, there are a million factors that could affect how much radiation is leaked and how large of a geographical area it covers. But it is extremely unlikely that any other country will be affected by a radiation leak from Japan. Fukushima is on the Pacific coast of Japan so there is a gigantic mountain range and the entire Sea of Japan separating it from every other Asian country. It is also unlikely that anybody except the people actually working at the nuclear plant are in danger anymore, since such a large area around the plant has been evacuated. As of two hours ago NHK was reporting that everybody living near the evacuation zone is being tested for radiation poisoning but so far nobody has shown any signs of radiation exposure.
I think people are panicking because they see the word "meltdown" and they immediately think of a Chernobyl-like scenario, although that's not actually what a meltdown means.