it’s pretty clear that in TEPCO-speak, “possible” means “happened.”
I wouldn’t go that far, but TEPCO does seem to have had a M.O. so far of sticking with the best-case scenario until proven otherwise, and generally not being very forthcoming in their communication about Fukushima. This is e.g. headlining Asahi Shimbun English at the moment: TEPCO concealed radiation data before explosion at No. 3 reactor.
So I don’t think we have to assume that TEPCO is necessarily being forthcoming with their internal assessments of the likelyhood of fuel leakage until they know for sure.
Ooops, wrong link. Here’s the right one: Accurate data destroys optimistic TEPCO assessment, hampers cooling plan
Until further notice the “China syndrome” will now be known as the “Manhattan syndrome”
OR
the “GOD DAMN we are arrogant syndrome”
Not just TEPCO speak. It is BP oil speak. nuclear industry speak, oil energy speak and government who does not want to admit the damage speak.
Not quite! The whole reactor under normal operation is in a state of criticality, maintained by careful geometric arangement and spacing of the fuel rods, immersed in water that slows (“moderates”) the fission neutrons, which drastically increases their chances of causing secondary fissions in neighbouring fuel rods.
Any nuclear accident that disrupts the arrangement of the core will automatically end the state of criticality. Lose the cooling water and the fission neutrons are too fast to induce secondary fissions and the chain reaction stops. Move the rods too close to each other and there isn’t enough water between neighbouring rods to moderate properly and the reaction stops. Move them too far apart and too many neutrons escape without hitting any fuel… you get the picture. It is highly unlikely after the initial insertion of the control rods and reactor shutdown that a criticality spontaneously reoccurred.
The big issue is decay heat - all those fission fragment atoms in the fuel rods that are unstable and decay by alpha and beta emission over time, releasing energy in the process. At the moment of shutdown, decay heat is about 7% of total reactor output. This drops to 1% within an hour and continues to drop with time, but consider a 700 MW reactor with 33% generating efficiency. Full operating thermal load is 2100 MW of HEAT, and 1% of that is 21 MW of heat that has to go somewhere. Decay heat is indeed a problem and can severely damage a reactor if it isn’t continuously cooled, even after shutdown.
Now, what damage has actually occurred to the pressure vessel of reactor 1 remains unclear. All that is known is that water injection is failing to raise the water level in the pressure vessel so it isn’t holding water. It should also be kept in mind that in this pressure vessel the control rods are hydraulically inserted through holes in the bottom with wiping seals. Hot fuel at the bottom of the pressure vessel could damage the seals or enlarge/distort the insertion holes, causing water to leak. Whether any fuel has actually escaped is a tough question - if it actually became molten, then probably yes, but if it remained as solid pellets, probably no. If it did escape then it would be at the bottom of the drywell and the effects of that depend on how hot it became there, which depends on how much escaped water was sitting at the bottom of the drywell, etc. Until they get a robot into the drywell to inspect, we’re just not going to know.
Good question with a range of answers, none of which include a big hole through the Earth!
The reactor fuel is contained by three structures - the zirconium cladding of the fuel rods, which are in the big thick-walled steel reactor pressure vessel, which itself is within the containment building - a thick reinforced concrete shell.
A meltdown means that the zirconium cladding has failed for sure and the fuel will end up at the bottom of the reactor pressure vessel. This happened at Three Mile Island, probably heating the pressure vessel up to a good red heat but it remained intact. However, Three Mile Island was a presurised water reactor, where the control rods insert into the reactor pressure vessel through holes in its top. The boiling water reactors of Fukushima have control rods passing through the vessel wall through holes in its bottom and this may make them more vulnerable to failure during meltdown. In that case the fuel may have escaped the bottom of the reactor pressure vessel and be sitting on the concrete floor of the containment building, known as the drywell in this particular design. More modern reactors have a “core catcher” (aka “corium” catcher) layer beneath the pressure vessel that is supposed to just let the hot fuel sit there without going any further, but the Fukushima reactors do not. This means that the hot fuel can cook the concrete below it, releasing various gases and causing damage to the concrete. In the worst case, it passes through the concrete and contacts the ground below, but this doesn’t result in Japan becoming a ghost country or all the fish dying in the Pacific. Big chunks of red-hot Chernobyl core were sitting on the ground exposed to the world for days and while the resulting contamination was serious and widespread, it did not devestate the continent. In Fukushima, core material on the floor of the drywell still has a big concrete dome over it and is still subject to water cooling from various sources so I would say containment is still in place for the moment, just about.
It’s a land grab. Since this isn’t a real problem, then this is prime Japan real estate. So how do the rich remove the poor, they have a fake nuclear meltdown and get the poor to move without much fuss, “compensate” them and then seize the land.
Thanks for the crash course, very interesting! And my proposed scenario was quite off the mark, in no small part because I am - as xtisme appealed to - only an entusiastic amateur on the subject. My appologies.
So, if I’ve understood this correctly: decay heat may over time ultimately cause the fuel pellets to escape their casing, possibly melt, and end up at the bottom of the vessel in together with other molten materials from the inside of the vessel (e.g. control rod material, etc) in a lava-like form called “corium”. Here’s the question though: It is my understanding that critical mass could be achieved locally in the corium?
Thanks for the very interesting and illuminating info on pressure vessel layout at issue!
A nitpick: we’re assuming here that the drywell containment is intact and wasn’t damaged by the earthquake. But if I remember correctly Fukushima 1 is designed to withstand an earthquake of the magnitude that occured and more - most of the problems were caused by the tsunami - so maybe such an assumption is warranted.
In the reporting we’ve discussed here (Reuters, ENS, NHK, Asahi) there’s pretty consistent talk of the fuel as molten, which should indicate that that is TEPCOs assessment, but of course it may also be a case of sloppy journalism. But Matsumoto did in fact also say that the fuel pellets “melted” in the Reuters piece. But if they are sure of that, and how they came to the conclusion we don’t know.
What is your opinion? Assuming decay heat post-earthquake left without cooling - how likely is it that they melted into corium proper?
Here is an update from Russia Today. It is an interview with a nuclear expert in Hiroshima. This mess is no where near resolution.
It’s very much a question of timing. When the reactor is running (fissioning) the fission process is continuously creating smaller atoms that are themselves unstable (too many protons in their nuclei to hold themselves together.) These are then constantly decaying, contributing their decay heat to the energy also generated by the fission itself.
The moment the fission stops, the only source of energy is the decay heat and the various decay elements all have different lifetimes. The very short-lived ones disappear in seconds, the longer-lived ones in minutes, still longer-lived ones in hours etc. As I mentioned, in the first hour you get a reduction in heat load from 7% to 1% of the reactor capacity. If you have a complete loss of cooling right at the start then that 7% heat load is going to melt right through the pressure vessel. If OTOH you have several hours of successful water cooling before loss of cooling capacity (and remember at Fukushima they had diesel emergency power until the tsunami hit, and then battery power and various steam-powered emergency cooling for several hours after that) then the heat load is less and the uranium oxide pellets might not melt. They would be glowing like hot coals from their decay heat, but that heat could be conducting into the walls of the pressure vessel and radiating away. In Three Mile Island the fuel was indeed molten at the bottom of the pressure vessel but it stayed there without melting through.
Not a chance, especially if you’ve added control rod material to the melt. Once you’ve got to corium, you cannot achieve criticality because the neutrons are too fast for easy stimulated fission. The neutrons from any spontaneous fission are far more likely to zip right out of the corium rather than hit another uranium 235 atom and cause it to split. To get a criticality, you have to either:
-
devide the corium up into lots of little pieces and fill up the gaps with water so neutrons are slowed down zipping through the water from one piece to another. This makes them much more likely to split uranium 235 atoms in their neighbours (this is what a reactor core arrangement does) or
-
pile really large amounts of fuel together to catch enough of the fast neutrons (this is what happens in a “fast neutron” reactor). There isn’t enough fuel in a normal reactor to achieve fast-neutron criticality, and a big blob of molten corium isn’t going to have water all through it allowing slow-neutron criticality.
A pile of uranium pellets in water at the bottom of a pressure vessel might achieve momentary criticality, until the water between the pellets turned to steam, after which the moderating effect would vanish and the criticality would stop. When it cooled down and water filled the gaps between the pellets then you could get another momentary criticality so you could end up with “pulses” of criticality a few hours apart. However, for this to happen the neutron-absorbing control rods running up through the vessel would have to be withdrawn. If the control rods melted and became incorporated into corium then you definitely cannot get even an intermittent criticality, even with perfect geometry and water placement - the boron control rods will have “poisoned” the reaction.
The degree of damage to the drywell is indeed a good question. TEPCO have been planning to flood the drywell itself to raise the water level over the fuel whatever the state of the pressure vessel, but I don’t know how far they’ve got with that. What is known is that highly radioactive water has been ending up in the turbine halls next to the reactor buildings and this is almost certainly the “missing” water that has been injected into the pressure vessel. It is not clear to me if this is deliberate (the primary lpower-generating oop runs from the pressure vessel to the turbines and back again) to create a cooling flow-through of water or if it the result of leaking from the drywell into the trenches and conduits. If the latter, then they will have an ongoing radioactive water treatment/disposal problem until they can seal that drywell and set up recirculating cooling for it.
As to whether they actually have or have had molten corium in any of the reactors, really there’s insufficient data but it’s quite possible.
This blog is one of the best for the most up-to-date info from TEPCO press releases, JAIF, NISA etc. http://atomicpowerreview.blogspot.com/ The May 15th update has a timeline for events in Reactor Building No. 1.
OK, so did my question about where all that water is going get answered, and I’m just too dense to gather? Does it simply not matter? I suppose that there is little to fear from water that is simply exposed to radiation, or that nothing truly nasty is water-soluble. They say the temp is 100 degrees F so it isn’t boiling away. Could call it leakage, I guess, but I’m hard pressed to find that reassuring. 190 tons of leakage?
If I’m missing something, there is a slot available for the info.
TEMPCO have pumped thousands of tonnes of water from the trenches and turbine hall basements into storage, and yet the basements are still filling up. That must be where the water is going. See http://www.businessweek.com/news/2011-04-28/tepco-to-get-storage-float-in-may-decontaminate-water-from-june.html For reference, a million litres is a thousand tonnes.
Since the water has been in contact with unclad fuel it very likely contains water soluble iodine and caesium, and possibly strontium, all of which are nasty.
Japanese Evacuation Zone widened
But no worries. It’s a scientific fact that nuclear power is safer than kittens.
[QUOTE=tagos]
But no worries. It’s a scientific fact that nuclear power is safer than kittens.
[/QUOTE]
Could you quantify how safe kittens are? Because I’m unsure if this is a scientific fact. Seems an apples to oranges comparison to me. One would compare puppies to kittens, or energy sources to energy sources…unless one was trying to use ridiculous hyperbole and strawmen. Not something YOU’D ever do, I’m sure…
[QUOTE=matt]
This blog is one of the best for the most up-to-date info from TEPCO press releases, JAIF, NISA etc. http://atomicpowerreview.blogspot.com/ The May 15th update has a timeline for events in Reactor Building No. 1.
[/QUOTE]
How accurate have you found this source to be?
-XT
I thought the hystericists, as you call them, were merely saying that the core would melt right through the center of the Earth and come out the other side (see The China Syndrome), which in this case would be in the South Atlantic, a couple hundred miles from Rio de Janeiro.
There are no hysterics. That is well poisoning. We have discussed this without hysterics, just erroneous accusations of hysterics.
Now we have to debate the size of the openings? Good grief.
Thanks for your contributions in this thread. The last couple of threads were just wailing and gnashing of the teeth.
This isn’t true, iodine is a dietary requirement that is added to table salt.
The most common radioactive isotope formed in water under bombardment is N-16, which decays back into O-16 with a half life of just over 7 seconds.
Don’t you think he means it’s radioactive? I read thats why they stopped making iodized salt as the threat of nuclear war isn’t as big of a threat anymore.