Many people tonight appreciated Rachel's explanation of what is happening within the ailing nuclear reactors in Japan. I've taken what Rachel said tonight and adapted it a little for blog format for those of you who want to share it or read through it at your own pace.
The nuclear fuel rods in a reactor like the ones that are in trouble in Japan are about 12 feet long -- and they're skinny. Calling them rods isn't exactly right because they're not solid. They're straws. They're hollow.
The straws themselves are made of a metal called zirconium, and inside that metal straw is uranium.
When the reactor is working, the uranium pellets inside the fuel rods are creating fission. They're creating a nuclear reaction in order to generate heat.
The whole point of nuclear power is that you create an environment in which fission happens -- a nuclear chain reaction happens, but it's controlled so it doesn't produce an explosion. It generates heat in a controllable way instead. You use that heat to boil water. The boiling water makes steam, the steam spins a turbine, and that makes electricity. That's the basic idea behind these forty-year-old reactors in Japan that are in so much trouble now.
When a reactor is shut off, either in the normal course of events, or because of something like an earthquake, the nuclear reaction is stopped. To do that, a bunch of control rods are moved in among the fuel rods. The control rods stop the nuclear reaction from happening, in an orderly way.
Did you ever have the chance to develop a photo in a real darkroom? You put the photo in a developer solution until the image comes in the way you want it, and then once it's the way you want it, you put it in a different chemical bath to stop the developing. That's like the control rods. They stop the nuclear chain reaction.
But those fuel rods, those pellets of uranium inside the metal straws, even when the reactor is turned off, are still wicked hot. And so even though the reactor is off and it's not being used to boil water to make steam to spin turbines to make electricity anymore, the fuel rods still have to stay underwater. Because otherwise they're just too hot to keep it together -- to maintain their integrity. They'll melt.
The first thing that happens is the metal on the long metal tubes oxidizes -- it essentially just rusts really, really fast. And as you know from what happens in normal life when something rusts, it breaks down. It bubbles, cracks, splits. Damage to those fuel rods is a partial meltdown.
As the water level drops and the fuel rods are exposed to the air and the metal straws of the fuel rods oxidize, one of the gases produced is hydrogen, which can be explosive.
Authorities believe that a hydrogen explosion is what happened in two of the reactors at the Fukushima Daiichi plant. When we were told this weekend after the first explosion at the Daiichi nuclear plant, that authorities had detected radioactive iodine and cesium in the atmosphere, what did that mean? Why were they detecting those radioactive elements in the atmosphere? Those detections were an indication that the fuel rods had been damaged, which let some of the radioactive elements in that nuclear fuel, get released.
That is not good. That is a partial nuclear meltdown.
That is also not the same as a total nuclear meltdown, which they're still working to stave off.
The explosions that happened at two reactors at the Daiichi nuclear plant in Fukushima on Saturday and on Monday Japan time are not nuclear explosions. They're explosions caused by the inability to keep those fuel rods underwater and cool. The radioactivity released so far has been a sign that fuel rods were damaged. The steam that's being vented -- that's being let out into the atmosphere to avoid the pressure building up too high around the reactor -- that's mildly-radioactively-contaminated steam, contaminated by the fuel rods being at least partially compromised.
They have kept up efforts to submerge the fuel rods in seawater even after they knew they were damaged -- in order to prevent any further damage. As I said -- what they think they've got, is a partial meltdown. They're trying to avoid an uncontrolled total meltdown, in which the fuel rods break down totally -- and the uranium slumps down into the bottom of the reactor in what is essentially like radioactive lava -- burning (I think) at several thousand degrees Fahrenheit.
If that happens, that pile of hot radioactive goo will burn through most everything around it, and we'll be hoping that these last-line-of-defense containment structures are actually capable of containing that melted fuel and keeping it from the outside world and from the Earth. Because if they're not, that would entail a much larger release of radioactivity.
Tonight, just hours ago, we got word that a third explosion had been heard onsite at the Fukushima plant.
There are three reactors there they were worried about at Daiichi. Two of them had hydrogen explosions on Saturday and early today Japan time. Now it appears that the third reactor has suffered an explosion as well. The first two reactor explosions, authorities say, did not result in an unconstrained large radioactive release -- containment vessels had survived the blast.
Tonight, reports from Japan indicate that the third blast may be more serious -- that it may have resulted in a breach of the internal containment vessel. For the first time.
An explosion is not the same as a meltdown. An explosion at a nuclear reactor is not the same as a nuclear explosion. What's happened is that the inability to keep enough water circulating over those fuel rods to keep them cool has resulted in damaged nuclear fuel rods. And that's caused three explosions so far.
And that has provided us with the only conceivable thing that could make the whole world look at the horrible devastation in Japan and think we're as-yet worried about something else, about what might happen next.