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What's the deal with spent nuclear fuel?

"The first segment of #maddow last night & tonight should be required viewing if want to know anything about nuclear reactors," @jkars tweeted last night. Will

"The first segment of #maddow last night & tonight should be required viewing if want to know anything about nuclear reactors," @jkars tweeted last night. Will adapted Monday's opening segment for the blog already. This post adapts the opening block from last night.


Nuclear fuel rods contain little pellets of uranium, somewhere between the size of Tootsie Roll and Cadbury eggs. Those uranium pellets are stacked inside thin, 12-foot-long metal tubes, which we call fuel rods.

That's essentially the business end of a nuclear reactor. Big, long fuel rods like that mounted vertically, are inserted into a steel containment vessel. What's happening inside when the reactor is going is a nuclear reaction, nuclear fission, atoms splitting. But instead of creating a nuclear explosion, it is a controlled reaction. Instead of blowing up, the nuclear reaction creates a lot of heat, and the heat is what we are after, because the heat is what these reactors use to make a whole lot of electricity.

To make that nuclear reaction that makes that heat, those uranium pellets are the fuel. And just like any fuel, it gets used up eventually. Your 12-foot-long fuel rod full of those uranium pellet, lasts about six years in a reactor, until the fission process uses that uranium fuel up. It becomes something they call "spent fuel." What they mean is that it is degraded enough that even though it's still wicked radioactive, it is no longer efficient for doing what nuclear power plants are supposed to do, which is generating a lot of heat, boiling a bunch of water, making a bunch of steam that spins a bunch of turbines that make electricity.

So, here's the problem -- after you've gotten your good six years out of your uranium pellet-filled fuel rods, what do you do with them?

What do you do with your expired fuel? What do you do with that spent fuel rod?

Even after it's been taken out of service, the spent fuel is still incredibly hot, thermally hot, like touching the stove hot. And it's also very, very radioactive.

What you do is put these hot, really radioactive fuel rods underwater. You put them in pools that, in the case of the Japanese reactors we're focusing on, are about 40 feet by 45 feet.

First of all, water just physically cools down the fuel rods. But the water also provides some shielding for their radioactivity. They're so hot that they need to be kept underwater. And the water can't just sit there either, it needs to be circulating so it is cooling these rods off. If the cooling system stops and the rods are hot enough, if that water stops circulating, the fuel rods are so hot, they will boil off the still water that is covering them.

If the rods boil off the water that is covering them, the water level drops and the fuel rods get exposed to the air, what happens? The same thing as in an active reactor that's been shut down. It's not good.

The uranium, remember, is in little pellets that are inside these long metal tubes. If those tubes are exposed to the air, the metal oxidizes and starts to breakdown. It's sort of like the same idea as rusting, but it;s not rusting. It's oxidization. But you can understand it because we're all familiar with what happens when something gets rusty, right? The metal breaks down, starts to degrade.

The combination of the heat and the oxidization works sort of like super-fast rusting. Exposing those zirconium fuel rods to the air causes oxidization to the metal holding on the uranium. It's like super-fast rusting on steroids, and that's trouble.

Between that oxidization and the heat, the metal starts to breakdown, and that allows the uranium to get exposed. The uranium so hot that it, too, begins to melt. The same thing that's true for fuel rods and active reactors that have to get shut down.

It's also true for spent fuel rods sitting in the pool. They're all hot and radioactive, right? These fuel rods have to be cooled for anywhere between five to 10 years before they're safe enough to be taken out of these pools and put into dry cast storage. Until they are safe enough for that, they need constant attention. They need a constantly operating cooling system to keep them covered up with that water, or we are talking about the same kind of meltdown that you see in an active reactor that has been shut down for some reason.

The difference is that with the spent fuel rods, it's probably worse. I realize this is a tough time to say worse. I'm not saying it to be upsetting. I'm saying it because I think it is frankly less upsetting to actually understand what's going on than it is not to understand.

This is understandable. The reason spent fuel rods could be even more dangerous than a shutdown active nuclear reactor is because of two things. First: a spent fuel pool that loses its cooling system and has all of its water evaporate is a potentially greater source of a radiation leak than a reactor is, simply because there are often more fuel rods in a spent fuel pool than there are in an active reactor.

The stuff has just to sit there for eight to 10 years, right? So, sometimes, they make it a lot of stuff just sitting there in the same pool, which means that if there is a loss of cooling system to that pool, there is more uranium to form a bigger radioactive mass that everybody hopes we don't have to contemplate.

The other reason, though, that spent fuel rods are potentially more dangerous even than a shutdown reactor is because of where the pools are. When a reactor shuts down, you have to worry about the cooling system over the fuel rods there. That's taking place inside an incredibly strong internal containment vessel. And that incredibly strong internal containment vessels is housed inside an incredibly strong external containment vessel.

One of our guests the other night described this as sort of a Russian doll type system. It's a containment vessel inside a containment vessel. The spent fuel poles that we've been talking about can be just as radioactive, can often have more fuel rods in them, but they're not necessarily kept in that Russian doll-style multiple containment system.

They don't want to move these fuel rods far from the reactors that they come out of. They are, of course, super hot and super radioactive. You don't really want to be trucking them across the country.

But in these reactors that are in trouble in Japan, where do they move those fuel rods to? They just move them to the top of building. They're essentially just protected by the one external containment wall. And that external containment wall is something that we're all very familiar with looking at pictures of right now -- external containment walls that have been blown off from various explosions over the past few days.

At the Daiichi nuclear plant in Fukushima, reactors one, two and three were on and working when the earthquake hit on Friday. Because of the quake, they've shut down. The difficulty in keeping those still very hot reactors cool is what we have been focusing on for the past few days. But there are three other reactors at Daiichi. We have not been talking about them as much because when the quake hit, they were not producing power. They were turned off already for maintenance.

The reason that we are talking about those reactors now is because they have spent fuel pools inside them. As far as we can tell, those spent fuel pools are just protected by those outer containment walls, which had two fires and an explosion at one of these reactors with spent fuel pools continue, with a spent fuel pool in it.

We've had reports of the water level dropping at two other spent fuel pools. At reactor four, there has been an explosion and there have now been two fires. We know that an explosion there cracked the roof. It appears that damage to the spent fuel rods is what is allowing for a release of radioactivity there.

Japanese authorities did report a large spike in radioactivity after the fire at number four. What that means is that either the force of that explosion or the smoke from the fire or both carried aloft radioactive particles being released by those damaged spent fuel rods, by those uranium pellets inside those big, now probably damaged metal tubes.

This is not a nuclear explosion. There are not nuclear chain reactions going on here. But this is a means by which radioactivity is being released into the atmosphere. And the question is whether or not those fuel rods, even if they're already damaged, can be resubmerged.