For those folks who're completely lost in the current situation with Fukushima power stations Daiichi and Daini, here's something reat! Fellow Redditor "bpg131313" has provided a number of excellent posts explaining what's up in layman's terms. As other people in the thread have mentioned: how the hell is it possible that a few paragraphs online effectively explain what all those experts on TV have failed to make clear? Permalink 1 and permalink 2.
I'm reproducing his (her?) explanation here in full:
4 is in danger because the fuel rods were still being stored in the buildings, near the top of the facility in the holding tanks. Those tanks are essentially swimming pools that circulate cool water to keep the rods cool. Even though the rods are out of the reactor (and are sometimes referred to as "spent" or "depleted", those are really misnomers. These rods are still highly radioactive and still throw off a substantial amount of heat. A single rod (they are about 12 feet long, and about an inch or so thick - internal diameter) is made of a zirconium alloy that you'd think wouldn't have a problem with air, but it does (after all, people wear fake diamonds all the time). This stuff is different though. These rods are hollow. They are sort of like pipes. Inside each rod are these little pellets of highly enriched uranium. When these rods are separate, they are hot to the touch. It's when you bundle them together that they really get hot. Hot enough to turn liquid water into steam really quickly. That's how they produce power.
Well in #4, this pool where they kept all the rods near the top of the facility doesn't have a containment like the reactor itself does. It's a LOT less protected than the core. When the tsunami hit and wiped out all of the power to all of the facilities, the pumps that kept that swimming pool full of rods cool shut down and stopped working. The rods kept doing what they do best, they started making the water hot. It doesn't take all that long (a day or two) to literally boil off all of the water in that pool. That's not a good thing.
The zirconium on the outside of the rods gets insanely hot, and starts oxidizing when it hits air (which is why it has to stay under water). The oxidation process is like iron rusting, but really fast. This oxidation rips the oxygen out of H2O, and suddenly you're left with hydrogen. At that point all it takes is a spark.
4 has lost all of its water in the pool and those rods are fully exposed, generating enough heat to start melting down the outsides of the rods. The heat feeds itself and it keeps getting hotter and hotter. Eventually, the uranium will get hot enough to melt as well (if they don't get those things cooled off soon) and you can imagine what molten metal does. It will pool at the bottom just like any other liquid would. This stuff is highly radioactive, and unlike a reactor core, this stuff has relatively easy access to the open air. That's why #4 is such a bad thing. Once uranium gets hot enough to melt, it's obviously already burned off the zirconium sheath and, they call this radioactive lava at the bottom, "Corium". It can eat through concrete and cause all sorts of problems.
A level 7 event is as far as you can go in a nuclear incident when there is no detonation involved (like the atomic bomb). Most people are familiar with the reactor in Chernobyl that melted down and released radiation into the atmosphere. Most of that area is still evacuated today, though surprisingly enough they do offer “tours” that get close to the sarcophagus.
Chernobyl was a Level 7 event because it was actively releasing radiation into the atmosphere. These reactors are not the same design, and a different set of circumstances was involved in the Chernobyl event, but the most basic truth to a Level 7 event is active and sustained release of high levels of radiation into the atmosphere. Three Mile Island was a 5, by comparison.
Fundamental differences here are being faced by society for the first time. Multiple cores melting down simultaneously right next to one another. This has no precedence in history. There are concerns that the containment vessels have been compromised (or will be) starting with Reactor 3. I’m not able to confirm this, but from what I know of these systems, there’s a degree of likelihood that the containment vessel in reactor 3 is cracked or was somehow breached either by the explosion or through extreme heat generated by the uncooled core. If that’s the case, there’s not going to be a successful operation in cooling that core. It will melt down, it will burn through, and the Corium will find its way out.
Being downwind from a melted down core is not conducive to long life. Time, Distance, Shielding are essential.
Reactor 4 is the worst because the rods are so close to the open air (reactor 3 also has rods in storage in the upper pool, but I don’t know it’s water level -though I know that no water is being circulated, and that white smoke is coming from it, so it’s likely not doing well)
TL;DR - A Level 7 event is the worst you can have, and has immediate consequences for those exposed to radiation escaping into the atmosphere. There’s a very good reason why the US told it’s forces to withdraw 80 km. The helicopter water drops have been a failure. If the containments are cracked and the coolant isn’t effective, we’re going to witness multiple cores failing all at the same time. It’s never happened before.
This is all speculation at this point. There's literally no precedent for it. This has never happened before. Do I see it as being as bad as Chernobyl? Yes. There are population issues to deal with here. We're not talking about being out in the middle of nowhere with Pripyat being the closest town and not much else. We're talking about one of the most population-dense nations on the planet.
There's also the dirty little secret (that isn't really a secret) that the media hasn't been splashing on the headlines, and that is that Reactor 3 had both uranium, and plutonium in the core. Plutonium is just nastily toxic to living things, and being combusted and airborne is a nightmare most aren't willing to really contemplate. If the winds shift to the south, so that they are blowing toward Tokyo, there's a catastrophe in the making.
The reports indicate that 70% of the fuel in reactor 1 is melted (likely all of it by now, or close to it). 30% of the fuel in reactor 2 is melted (likely over 50% now) and people are being exceedingly quiet about reactor 3. US inspectors indicated that it's pressure went to 1 ATM, so that means to me that it's been breached. If that's the case, it's already beyond saving. Reactor 4, I discussed above. There are two other reactors on site, 5 and 6. These were offline at the time and we're only having to deal with the fuel rods in the storage tanks, though there aren't as many fuel rods, and the tanks are bigger so there's less of an immediate danger, though the temps are rising.
Do I think it'll get bad? Yes. Do I think Japan will have to build a sarcophagus? Yes. If the winds blow to the south, things will be far worse. Even though the reactors are different, there are four reactors in various stages of meltdown, versus one. Sadly, we're all going to find out if it's worse or not.
> Question: So, could we just shield the radioactivity by dumping lead pellets into the core?
It would sound like a good idea because people have been told forever that a lead shield will protect you from radiation. That’s true if you’re not talking about molten uranium rods. We’ve all been to the dentist, we know how it works.
The problem is that when uranium melts, it’s really hot. We’re talking in the ballpark of 2000 degrees F. It’s toasty. The problem with flooding it with water is that water, as we all know, boils at 212 degrees F, and if you were to try spraying down a pool of melted uranium with water, the water would evaporate before it ever actually touched the surface. There’s the deluge method, where you literally flood the stuff and eventually it’ll cool the surface enough to actually touch it, but we’re far from that point in this case.
They’ve also tried using Boric Acid to try and stop the chain reaction. Uranium has tons of neutrons, and boric acid gets in there and tries to slow them down. It works a lot better when it’s not really hot. Boric acid is actually a white powdery substance and isn’t all that acidic. Problem with it, is the same problem with the water. It boils off before it ever touches the melted uranium. It boils off at somewhere around 600 degrees F, which is just a bit shy of the 2000 degrees of the uranium (it actually does get hotter than that, but that’s the start of the melting process).
The short answer is that lead wouldn’t work in this case. There’s a degree of likelihood that steps will be taken that are similar to those taken in Chernobyl. They’ll have to encase the site permanently, and abandon the location. They might get lucky and get everything cooled down. I’m honestly hoping that’ll happen. I just think too much has gone wrong for this to be stepped-down.
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