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> Conflicting information keeps coming out about possible worst case scenarios (and their relative likelihoods) with the nuclear reactors. There is only one wo

by mpyne 16y ago

> Conflicting information keeps coming out about possible worst case scenarios (and their relative likelihoods) with the nuclear reactors. There is only one worst-case scenario, so although I very much agree that the information coming out of Tokyo Electric Power Co. and the Japanese national government is inconsistent, that doesn't mean new "worst-case" scenarios become possible. IMHO the worst-case scenario is fuel melting through the reactor pressure vessel, through the thick steel containment which was not damaged in the hydrogen explosion, and through the bottom of the concrete containment (the top of which was damaged), and then from there into the environment. The worst-case plausible scenario from what I can tell (which again, has been true from the moment they lost cooling systems for decay heat removal) is fuel melting to the bottom of the reactor vessel, possibly breaching the primary boundary but getting trapped by the steel containment, with possible further release of gaseous radioactivity. I say this because cooling the fuel if it is outside the primary but inside the steel containment is much easier than cooling the fuel inside the primary: you must merely submerge the steel containment in seawater (which would boil away as it heats up), but this time you don't have to worry about pumping to high pressures. The fuel was transfer its heat across the steel containment to the seawater. As long as the steel containment is intact (as was true last I heard, but refer to my earlier point about TEPCO) then there would be no additional release of radioactive contamination from this. If there are air leaks in the containment that would be worse, but a portable radiation monitoring device would quickly identify such areas so they can be quickly patched or covered to at least keep contamination from spreading. So, I also disagree with "unimaginably bad". Unimaginably bad might be something like coal slurry breaking through its retention walls and killing all aquatic life in a nearby river for hundreds of miles (as actually happened in Kentucky in 2000 IIRC). At my last check all three reactors that had been operating at Fukushima-Daiichi have probably had fuel damage, which means that any steam they vent from the plant will contain radioactive contamination. So, definitely bad. Unfortunately I can't seem to find to many numbers for activity concentration (how frequently the radionuclides are actually decaying) or for dose rate (a measure of biological damage per unit time). A Guardian article mentions 680 microsievert per hour to the northwest of the site, which would be 68 millirem/hour, which is a value that is actually fairly high, but far below levels that would lead to immediate health risk. For a point of comparison, there are (inhabited ;) spots in the world (Ramsar, Iran) where you would receive 70 millirem/day, every day, which is far above what a trained radiation worker would be permitted to receive and yet still appears to have little (if any) effect on the local population. Of course any unnecessary exposure is unacceptable! But I also wouldn't come close to calling it "unimaginably bad", except in regards to the economic cost to TEPCO and the substantial loss of generating capacity (which as you mention, can have a substantial economic impact).

marshray 16y ago
By "unimaginably bad" I meant the earthquake+tsunami disaster scenario. This is now the unfortunate baseline against which the nuclear issues are, perhaps, the most significant remaining unknowns. Reading again what I wrote I see it was ambiguous. There is only one worst-case scenario, so although I very much agree that the information coming out of Tokyo Electric Power Co. and the Japanese national government is inconsistent, that doesn't mean new "worst-case" scenarios become possible. This is an interesting question. Repeatedly we've seen commentators and outside observers saying "this can be no Chernobyl because...". But the situation at Fukushima-Daiichi is still very dynamic and many questions remain. These cores still have a lot of energy left to dissipate and thus each still seems to have a life of its own. As much as we would like to know the current state of the reactor cores and all the possible paths they could take, the picture emerges that those officials in charge of the mitigation efforts don't have a perfect picture either. Whether there are multiple worst case scenarios, or there is only one that we can't see clearly yet seems to be a philosophical question about a highly physical phenomenon.
- mpyne 16y ago
  > 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.

by mpyne 16y ago

marshray 16y ago

By "unimaginably bad" I meant the earthquake+tsunami disaster scenario. This is now the unfortunate baseline against which the nuclear issues are, perhaps, the most significant remaining unknowns. Reading again what I wrote I see it was ambiguous. There is only one worst-case scenario, so although I very much agree that the information coming out of Tokyo Electric Power Co. and the Japanese national government is inconsistent, that doesn't mean new "worst-case" scenarios become possible. This is an interesting question. Repeatedly we've seen commentators and outside observers saying "this can be no Chernobyl because...". But the situation at Fukushima-Daiichi is still very dynamic and many questions remain. These cores still have a lot of energy left to dissipate and thus each still seems to have a life of its own. As much as we would like to know the current state of the reactor cores and all the possible paths they could take, the picture emerges that those officials in charge of the mitigation efforts don't have a perfect picture either. Whether there are multiple worst case scenarios, or there is only one that we can't see clearly yet seems to be a philosophical question about a highly physical phenomenon.

mpyne 16y ago
> 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.