Japan Does Not Face Another Chernobyl

Ok, I misremembered and my comment was wrong. For about 40 minutes all the cooling pumps were turned off, and during the following 7 hours cooling was ineffective due to hydrogen bubbles. (http://www.threemileisland.org/downloads//195.pdf). The core partially melted. Sorry.

This doesn't make the original article any more correct though. During most of the TMI accident the core was being actively cooled, and in fact it never got uncovered enough to even completely melt. So this still does not tell us anything about how containment would work in the absence of cooling.

> Repeatedly we've seen commentators and outside observers saying "this can be no Chernobyl because...".

Yes, because this can't be a Chernobyl. I do understand that you don't exactly have time to train yourself on the ins and outs of nuclear energy. But if you're really interested in the topic, Wikipedia actually has good articles on Chernobyl and the Chernobyl disaster, including a lot of information on the design flaws that were present in the Chernobyl design, flaws which are not inherent to either nuclear plant design in general, or the Fukushima boiling water reactors in particular.

> But the situation at Fukushima-Daiichi is still very dynamic and many questions remain.

I agree. But that does not mean that any disaster conceivable is in the set of possible outcomes of the casualty. Although I know it may not seem like it, especially with all the confusing and contradictory information being put out, but what has occurred up to this point has occurred almost like clockwork from the initial conditions leading up to the casualty and the actions taken (and unable to be taken) thereafter.

A full nuclear fuel meltdown has been the final disaster since the start. The bad things that have happened since then have taken TEPCO progressively closer to that stage but as I said before, it's not like there's a multitude of different paths for the casualty to go down.

> These cores still have a lot of energy left to dissipate and thus each still seems to have a life of its own.

Yes, but the response is completely predictable. Everything that has gone and is currently going on (as far as the nuclear core response goes) is due just to the laws of nature. Decay heat is a well-understood concept (it is, after all, why so many redundant systems were part of the design in the first place), and the decay heat itself depends fully on things which can be figured out before shutdown, such as average power level, operating time, etc. It's not like some nuclear fuels have randomly different heat generation profiles and the Fukushima plant happened to pick up the super-reactive uranium.

The only variable so far has been how much cooling could they provide? So far it hasn't been enough, but that hasn't changed the underlying phenomena, and the worst case is completely predictable (i.e. no cooling could be provided whatsoever under any circumstances).

This is not like the weather where the phenomenon interact in such a chaotic way that it's effectively unpredictable: Decay heat production is easy to predict (to at least a general approximation) and even the decay of that residual decay heat over time is predictable. Even the hydrogen production from steam-zirconium reaction is a well-known and predictable effect, even during the time of Three Mile Island. The hydrogen explosions themselves were unpredictable, but even they were unsurprising.

If this is all surprising to you because it's the first you've been exposed to nuclear theory then don't feel bad, but that doesn't make what's happening at the Fukushima-Daiichi plants at all unpredictable or chaotic, or for that matter, philosophical.

I've actually read a good bit about it. E.g.: http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf

I know it's not voodoo and obviously the laws of physics are well understood, but it also seems like there hasn't been enough experience with actual meltdowns that these things are entirely predictable.

I understand that the Chernobyl design contained a large quantity of graphite and little containment beyond the vessel itself.

But I still have questions:

* If the engineering is well-understood, then why did the buildings blow up due to accumulated hydrogen?

* Do these reactor designs (GE Mk I and II) include a "core catcher"? Some analysts act like it's obvious that they do, but I find no mention of it in the NRC docs. Mainly it's counted as a feature of more advanced designs.

* What does it mean for the suppression pool to be damaged? Was that optional equipment? It's being described as the "last line of defense" by a smart-looking fellow from Tokyo U.

* Can the control rods fall out? They don't drop in from above, they're pushed up from the bottom hydraulically.

* Do the materials of the fuel and the control rods melt at different rates? It seems like this could result in the materials separating as they melt and a partial core re-igniting as a lump at the bottom of the vessel. Or was this possibility considered and designed against in this '60s-era reactor?

* What is the real implication of the plutonium in the MOX fuel in Fukushima Daiichi reactor No.2? They were mostly exposed for 2+ hours.

And so on...press conference on NHK world right now (available on ustream.tv nhk-world-tv).

Now these are better questions. :)

As far as hydrogen goes, the building itself was not designed to be the containment (instead there's a hefty steel containment around the reactor and primary piping from what I understand), so I don't think they bothered trying to make the building resistant to explosion from the inside (since they didn't have to). It would still be better to have the building in place, but I think the answer is as simple as it wasn't necessary for the meltdown casualty analysis.

I'm not sure if the Mk I and II containments contain a core catcher or not. Modern designs do from what I understand, but I would imagine the Mk I does not. Perhaps the Mk II does.

The Three Mile Island core suffered much greater fuel melting and still didn't make it past the reactor vessel, but a lot of that depends on core geometry as melting progresses, and the TMI reactor vessel was much stronger than the Fukushima plants since TMI was a pressurized-water reactor. On the other hand, the Fukushima designs have the steel containment that would catch any core leakage out of the primary initially and as long as they have seawater available they can keep the steel containment intact, and without having to send that seawater into a high-pressure system.

I'm not sure what they mean by damage to the suppression pool. The suppression pool is a torus of water that steam from the primary system is ducted to when pressure relief valves lift, which acts to condense the steam and help with filtering some of the radioactive contamination to keep it from going airborne. Doing this heats up the suppression pool, which will itself boil eventually.

I assume this is what they meant when they talked about venting the containment (as opposed to the reactor or primary). Anyways, if the suppression pool is damaged I assume that means there is a leak of the water inside of it, which would leak to the bottom of the steel containment. Without water in the suppression pool any venting of the reactor would not be immediately condensed, and would cause higher levels of airborne radioactivity inside the containment building for Unit 2 (intact AFAIK). This would not affect the ability of venting the reactor to lower pressure though, which is important for allowing water to be forced into the coolant system.

Anyways, some of my IRC buddies have clued me more into a blast at Unit 2's suppression pool? I'll try to read up on that. Until then:

According to http://rushi-langaliya.blogspot.com/2011/01/boiling-water-re... control rods are inserted into the core from the bottom. I'm not certain of BWR design but obviously if all the metal is melted then the control rod could fall out (this was actually a concern with Chernobyl which used metals with very low melting points to hold up control rods). I will say the plant I was on had about 5 different ways to keep control rods in position whether there was power or not so I'd be very surprised if the control rods weren't latched in place now at the very least. But I don't know for sure, don't want to tell you otherwise.

* I don't know what their control rods are made out of, but the good assuming is that control rods melt after fuel does. That actually means the fuel would melt down away from the control rods before lumping up. If there is coolant there, the fuel shouldn't melt further. If there is no coolant, the nuclear chain reaction can't go critical again since the fuel is not enriched enough (the coolant actually also helps aid the neutron chain reaction). I can't speak to reactor protection analysis for BWR to that level of detail, but again I'd be very surprised if that's something that wasn't considered at some point between the 1960's and the last couple of years.

If they had considered it and concluded there was a risk of re-criticality they would have taken measures to preclude it. That might be why they're already using boric acid for instance.

As far as plutonium, it has a fairly long half-life, which means it's not very radioactive. With that said it is an alpha-emitter so it is very important that it is not inhaled. As long as it's not inhaled it's not a large concern from there radiologically. As with any other heavy metal though, it is highly dangerous chemically.

Hopefully better information gets put out at the press conference.

Man, wtf happened at Unit 2? I keep repeating the NHK broadcast and I'm not sure whether the suppression pool itself suffered damage and not the reactor vessel, or both, or what.

They mentioned radiation going up right after the explosion, which could indicate a reactor vessel or other primary piping rupture, but could also just be from trapped radioactive contamination in the suppression pool being thrown out by the explosion.

Either way it's important to make sure they don't allow the concrete building shell to get damaged at Unit 2 if they want to avoid further airborne release.