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OK, on a scale of Three Mile Island to Chernobyl, where are we now?

Officials in Japan say the situation at Fukushima Daiichi nuclear plant now depends on the condition of reactors two, three and four. We're getting mixed

Officials in Japan say the situation at Fukushima Daiichi nuclear plant now depends on the condition of reactors two, three and four. We're getting mixed reports on reactor No. 1, and cooling systems for Nos. 5 and 6 are being powered by diesel generators.

Of the other three reactors, Nos. 3 and 4 present the greatest worry. Rachel tweeted a link this morning to a World Nuclear News report that includes, among other things, the update that helicopter pilots say they've seen some water in the spent fuel pool at No. 4. Some water is better than none, because it's when the rods are uncovered that they quickly fall apart and release radiation. BBC reports that the courageous helicopter pilots are continuing to drop water, including now on reactor No. 2.

Last night on the show, Rachel looked at where we are now on a scale from Three Mile Island to Cherynobyl. The short answer, of course, is nowhere you want to be. Longer, more technical answer is after the jump.

Adapted from Rachel's opening block last night:

Here's what happened at Three Mile Island, in 1979: One of the reactors at Three Mile Island about half melted down. Because of human error and technical failures and some bad design, the cooling system at Three Mile Island failed and the water levels inside that reactor fell. That meant that the super hot and radioactive fuel rods inside the reactor were no longer being cooled by water covering them up. And so, they started to melt.

The fuel rods are long, skinny metal tubes with pellets of uranium inside them. And they are hot. They are thermally hot, like "touch the stove" hot. And they are also radioactively hot, too.

Once those things at Three Mile Island were not being cooled by water anymore, once they heated to a couple thousand degrees Fahrenheit, the metal tubes holding the uranium pellets started to breakdown. When the temperature got even hotter, the uranium fuel itself started to meltdown, too. And melting fuel rods like that can release a ton of radioactivity.

They are radioactive, on purpose. That's how they make nuclear power. But when they breakdown, they release that radioactivity into the atmosphere. They release radioactive stuff and they also hydrogen gas. Now, hydrogen is not radioactive, but it can be explosive.

So, that's your bad combination, right? You're releasing radioactive stuff and something that explodes. Those two things are being emitted at the same time together.

At Three Mile Island, faced with that problem, authorities did release into the air some of that radioactivity that was being emitted from those damaged fuel rods. They did that in order to relieve pressure so the reactor wouldn't explode. They also at one point had a huge hydrogen bubble they were worried could blow up. The risk of an explosion, of course, is that all the radioactivity being emitted by those messed up, hot, damaged fuel rods would just get emitted directly into those skies over Middleton, Pennsylvania.

They were worried that it was going to blow up, but the hydrogen bubble did not blow up. It did not cause an explosion. The containment vessel around the plant held, and the only big release of radiation in the Three Mile Island accident was what they released on purpose when they felt they had to do it to relieve pressure to prevent an explosion.

So, that's what happened at Three Mile Island. No deaths and no injuries to anyone who worked at the plant. There was some exposure to radioactivity by people who lived in the vicinity of the plant. Authorities generally have not attributed any major health consequences to that exposure. But for many years, many local residents have begged to differ.

Three Mile Island situation is the worst accident we've ever had at a commercial nuclear reactor in the United States.

It was not until 1990 that they finished getting all the radioactive fuel out of Three Mile Island. It took 11 years. The rubble from the Three Mile Island meltdown is sitting in radiation containment cask at Idaho National Laboratory in Idaho right now.

Now, Chernobyl, in 1986. At Chernobyl, there was no containment building at all around the reactor that blew up. What seems to have been a poorly designed experiment or test resulted in a big surge of power and the reactor exploded. It exploded and it burned.

Roughly 180 tons of radioactive fuel in that reactor at Chernobyl that blew up sent radioactive cesium, and iodine and lots else into the air. About 50 people on site were killed by radiation relatively quickly. Hundreds of thousands of people were evacuated. More than 200,000 people were permanently moved.

A 19-mile radius around the plant is still called the zone of alienation. It is not considered fit for human habitation. Hundreds of square miles of Eastern Europe were hit by radioactive fallout from Chernobyl. The health consequences were significant, particularly in terms of thyroid cancer. Rates of thyroid cancer are 50 to 100 times higher than they were before the disaster in Ukraine and in Belarus.

These things take time, but ultimately, thousands of cancer deaths that would not have otherwise taken place will be blamed on the radioactive fallout from the Chernobyl disaster. That's what we're talking about here. We tend to think of Three Mile Island and Chernobyl almost as metaphors, as big, bad ideas, but they are specific things.

When you've heard people saying that what's happening in Japan so far is worse than Three Mile Island, but not as bad as Chernobyl -- well, OK. It's good to understand that, but it's also good to understand that there's a lot of room between the consequences of Three Mile Island and Chernobyl -- not just the magnitude and the type of accidents themselves, but the consequences of those accidents, how much radioactivity was released, and what it did to people.

The ongoing crisis in Japan is about trying to minimize the amount of radiation that's going to be released from these reactors at Daiichi. The spent fuel pool in reactor No. 4 contains about 130 tons of material. For reference, that's about 28 percent less fuel than what blew up at Chernobyl.