Entomb? Cement pumps flown in to nuke plant
Entomb? Cement pumps flown in to nuke plant
More here:
http://www.msnbc.msn.com/id/42359020/ns/world_news-asiapacific/
(Msnbc usually doesn't keep their links active for very long so after a week, I'm sure you won't find the rest of the article here.)
TOKYO Some of the world's largest cement pumps were en route to Japan's stricken nuclear plant on Thursday, initially to help douse areas with water but eventually for cement work including the possibility of entombing the site as was done in Chernobyl.
Operated via remote control, one of the truck-mounted pumps was already at the Fukushima Dai-ichi site and being used to spray water.
More here:
http://www.msnbc.msn.com/id/42359020/ns/world_news-asiapacific/
(Msnbc usually doesn't keep their links active for very long so after a week, I'm sure you won't find the rest of the article here.)
Comments
The expect it to be too hot to be returned.. wow..
OBC
The 50 workers at the plant are expected to die according to what the news has said. I think that losing them all is dangerous because they've been trained the longest and someone is going to have to get on the job training if they are going to stay and that would only be temporary under the current conditions.
If they're sleeping there, they aren't taking shifts.
http://www.msnbc.msn.com/id/42371032/ns/world_news-asiapacific/
"The so-called Fukushima 50, who actually are a group of about 300 people who have been working in shifts of 50, have become heroes in Japan and are known as atomic "samurai.""
I have been reading enough to be dangerous.
The idea of the puddled uranium fuel melting through the concrete does not appear to be a given unless we know a lot more about the concrete in question. Basically (from what I've read), the uranium will melt aound 2000*F. The concrete will lose all of its moisture and oxygen (not positive about the latter) somewhere around 1000*F.
However, sand and gravel will remain. The sand and gravel will not melt until about 2600*F . . . several hundred degrees above the melting point of the uranium. I wonder whether IF the original concrete--thus the resultant sand and gravel remnant--were thick enough THEN IF the uranium wouldn't just lie on top of this material? IF it tended to spread out, that would be even better.
IF this is accurate, then entombment might be a viable option. (Lots of IFs.)
There are probably supercomputers all over the world running models of this idea, if it has any merit at all.
--Bill
The uranium might melt at 2000 degrees F, but once it melts, I don't think it necessarily stops generating more heat, in which case it can continue getting hotter and hotter until it reaches its boiling point, which is about 7500 degrees F. (Just because water melts at 32 F, doesn't mean it can't get hotter than that.) But I'm sure you're right about needing supercomputers to even ballpark the actual mechanics behind all of this. It's a mindboggling tragedy happening in relatively slow motion.
--Bill
I guess a lot depends on the temperature and the extent of the melting. Considering that there are very few things more dense that uranium, I suppose it's possible for companion melted materials to simply float to the top of the molten mass and leave the almost pure uranium blob to burn straight down. As I understand it, the reason Chernobyl did not result in such a "China Syndrome" is because the sudden build-up of its energy in the reactor caused the reactor to explode, and this explosion actually dispersed parts of the core, scattering it enough that it actually helped slow the chain reaction. Other fires and explosions that followed then made closing off the fuming open wound of the reactor exceedingly difficult.
Consider the fact that they have radioactive water leaking into the ground and the ocean. One concrete pump may actually be deployed for pumping concrete for repairs in unrelated areas whilst the first one is continually pumping water to thermally stable-ize a problem reactor.
Of course I wish it was all as simple as that, but they are actually calling in concrete pumps from more than one source - an indication that they are very short on resources for cooling. If the reactors were stable, they wouldn't need to call in more equipment on an immediate basis.
Some of these concrete pumps may just start repairs and to create shielding in related work areas while the big ones remain tasked with cooling. I wish this were simple and under control, but the situation seems to still be evolving.
A melt down causes the fuel rods with their casings to melt. But from there,I suspect the results go pretty much into a thermodynamic chemical reaction and explode in a sub-nuclear type of explosion - dirty, but not so powerful as to melt concrete.
Nonetheless, we don't need a dirty explosion of any sort. I suspect they are trying to arrive at a thermodynamically stable situation by cooling and hoping that they won't have to immediately go in and disassemble the mess of melted fuel rods. Eventually, with some sort of robot, they may pull apart the mess to a less critical configuration. But since they likely have no where to move all this radioactive junk, it will most likely be entombed on the site and maybe in place.
As I said before, you can make concrete with steel shot rather than aggregate in order to gain greater radioactive shielding (It has been used on the Hanford Nuclear Area).
http://www.cavendishscience.org/bks/nuc/quests.htm ...yes.
http://www.oldenbourg-link.com/doi/abs/10.1524/ract.2000.88.9-11.657 ...Maybe.
this last one is not all about reactors, it just has a funny twist that I haven't heard before.
http://www.buffalobeast.com/?p=2493 ...this one uses the phrase,."Like a uranium dart thru concrete"
don't go to this one if you hate conspiracy theories..
That's a very good point. But it looks like uranium dioxide has a specific gravity of almost 11 (which means it's sbout 11 times denser than fresh water). Concrete I think is about 2.6 and most earth crust materials, such as bedrock, do not exceed about 3, I think.
In any case, I wonder if uranium oxide can break down into its elemental forms if it gets hot enough? Or does it just start boiling and turning to vapor before it chemically breaks down? Either way, it sounds like localroger is correct: the chances of a steam explosion sound greater than anything else, and then it's just a radioactive fumarole you end up dealing with, and whether it's above ground or below ground probably doesn't make much difference if you're one of those poor dudes who's volunteered to put it out.
I hope I'm wrong about all of this.
It's an interesting topic. You might want to have a look at this:
http://en.wikipedia.org/wiki/Geothermal_gradient
Potassium 40 is naturally so abundant that I've heard of people buying common rock salt to help calibrate their radiation instruments.
The key point is that it's not that there's so much radioactivity naturally occurring in the earth, but that the thermal insulation of so many miles of rock allows the radiogenic heat to build up over time, so the rock below the crust remains molten. That was my original point about trying to entomb the reactor cores before they are taken apart - I don't think it will do any good to bury the reactors unless the core materials are sufficiently spread apart so they can properly cool.
Part of the problem is what also makes nuclear waste so messy. Let's suppose you are a Uranium-235 nucleus, sitting around minding your own business, perfectly content to stare at your 92 electrons as they whiz around and, through their electric repulsion, keeping the other nuclei at a respectable distance. Then, along comes this neutron which just barges right through your electron shell and just WHACKS you. You roil for a bit and then (insert appropriate Simpsons sound effect) your left and right halves split up. You're no longer a Uranium nucleus. You're two new nuclei whose atomic numbers add up to 92, both of which are probably radioactive as hell because as a Uranium nucleus you had a lot more neutrons than would be needed to hold together the smaller nuclei you've now become.
Also, you're now two very positively charged particles very close to one another, and as Lise Meitner famously observed when she was figuring all this out, "if you did somehow make two particles like that, they would fly away from one another with considerable energy." A good fraction of the speed of light, in fact, it turns out. So your newly minted halves fly apart and go barreling through the solid material of whose crystalline matrix you used to be a part. Since your halves are electrically charged this plays havoc with the electrical bonds of all the other atoms you pass and they grab at you and slow you down, and this sets them in motion which is how your atomic death is transformed into simple heat energy; all the atoms you pass are set in violent motion by your passage and this motion is the very definition of heat.
But you left a hole in the solid material, and worse when your halves come to rest they will push other atoms aside. Nature abhors such a strong positive charge sitting around naked so the electron situation will sort itself out but the bottom line is that atoms tend to maintain a relatively constant distance from one another and where once there was one nucleus, there are now two plus a vacant hole. This isn't so bothersome if you live in a liquid but it really screws up a solid, which is why fuel rods get distorted.
Sintered oxides are preferred for nuclear fuel because they have room for this expansion and they've been studied to death so their behavior is understood.
But the halves aren't finished. Both of them have way more neutrons than is normal for such a much smaller nucleus and your splitting isn't even so there is a chaotic range of possibilities for the proton/neutron mix of the two new nuclei. Each of the dozens of possible splits has ramifications, some of which have truly horrific implications. The end result is truly an evil brew, physically distorted and chemically messed up and shot through with both chemical toxins and sizzling radioactivity. The reason zircalloy cladding is used is to hold this unpredictably toxic and physically unstable mess together as it's removed from the reactor and disposed of. Zircalloy is used because it has favorable properties for the reactor functioning, but as we've seen it also does not respond well to very high temperatures. The zircalloy cladding on Fukushima D1 units 1 thru 3 is bye-bye. This means we have these pellets that started out as sintered ceramic granules and look like Gaia knows what today sitting loose in the sometimes boiling cooling water.
Some of those half atoms are, unlike Uranium Oxide, very highly water soluble. That's another reason the cladding is used.
And the kicker is that not all of the resulting meltdown pile may be the same. You may have hot spots, there might even be accumulative physical processes we don't understand at work. (Or, as seems to luckly happened at Chernobyl, processes that spread the mass out along a wall.) This is what the nuclear wonks mean when they use the phrase "unkown configuration." That sounds innocent enough and it's meant to; after all, I don't even know the configuration of the Smile under the hood of my car all that well, ha-ha. But to someone versed in nuclear skills that is a truly terrifying phrase, because it means true unpredictability, in a mass of matter that is capable of releasing vast energy and vast toxicity.
Localroger, 'Doh!' Great illustration.
The nuclear inhibitors are already in place... the reason the reactors are still hot is due to residual heat, however no new heat is being produced. The control rods moved into place to stop the nuclear reaction as the quake was still shaking (in a process called "scramming",) but because the fuel rods were already hot, it can take several weeks/months for them to actually cool down. Until then, they must be bathed in water to keep that cooling process working. If they stop that cooling, or the water goes away, the rods can melt, but even if they do, they won't produce a nuclear reaction simply due to the design of the reactor structure itself. One other important reason for the water is to also help inhibit the nuclear reaction. BTW, the control rods will continue to prevent enough of the neutrons needed to sustain the reaction... even if they melt.
Entombing the reactor core in concrete is actually quite a nice short term solution... it's highly doubtful that the reactors cores would melt even through the current containment vessles... however, it does limit what you can do with the facility for several millinia afterwards. In the future, once the travelling wave reactors come online, we will be able to take those control rods (melted or not) and use them to help fuel that new generation of reactors. The current designs can only use about 5% of the fuel without the need to reprocess the fuel... the new Travelling Wave Reactors can use almost 95% of the fuel... so spent fuel rods are a great fuel for it.
Bill
Bill,
I wouldn't be so sure about that. First, you have to ask yourself why spent fuel rods need to be continuously cooled in pools. If those rods are "spent" and are separated enough to stop nuclear reactions, why do they still get hot enough to melt themselves years after being taken out of service just because their pools are allowed to boil away? Without a cooling system, even the relatively small amount of radiogenic heat continues to build up to dangerous levels. In fact, radiogenic heat is what powers many space-based thermoelectric generators, some of which have been working for decades.
http://en.wikipedia.org/wiki/Radioisotope_Thermoelectric_Generator
As a demonstration, take a 25 watt light bulb and wrap it in fiberglass insulation and turn it on for half an hour. Don't walk away, though, because it might just melt and set itself on fire. Though it's a tiny amount of heat being input, the insulation can allow it to build up to fairly high temperatures. It's the same reason electrically powered "heat tape" (like what you wrap around a frozen water pipe) should not be allowed to double up on itself - it's designed to work in a single layer and it can melt itself if it's folded up.
If the cores have been melting, as all evidence seems to indicate, then how can the control rods remain in place? How well do control rods work if they've been turned into mush or set on fire after coming into contact with the air? Cadmium metal, for example, is about 70% the density of uranium oxide - if melted along with the uranium oxide, would it not want to ooze to the top?
I'm not suggesting I know the answers to these questions, but I'm also not sure I believe some of the things some of the nuclear power representatives have been saying concerning "residual heat" and so forth.
http://en.wikipedia.org/wiki/Decay_heat
Spent rods only continue to produce residual decay heat for about 3 years that needs to be managed... after that, the decay heat is too low to worry about and they no longer need the cooling system. Again, the heat generated is not sufficient (even without cooling) to cause a nuclear chain reaction... it's only enough to melt the material. If it does melt, it still isn't enough to cause a chain reaction... it would just be a molten mass of hot uranium (which isn't pure, I might add.) Once sufficiently cooled, they can then safely be moved for reprocessing.
The control rods are at the lower portion of the unit. Even in a melt down, that material will impeed the nuclear reaction enough to cool it below the temperature requried to melt through the containment vessle. Without the water, which is a nuclear moderator, there isn't enough fuel to sustain the reaction by itself... the water actually controls the amount of high energy neutrons in the system, that are required for sustained fission. If the water heats up, it expands, which actually slows down the reaction. If the water vaporizes, the reaction will automatically stop because there is not enough uranium to sustain it by itself. The control rods act simply as a throttle in the normal case... but with them in place, there simply isn't enough energy to sustain it, regardless if they've melted down or not.
The residual heat is not enough to allow the core to melt through the containement vessel. (Or secondary vessel) It's only enough the melt the control rods. Three Mile Island is a perfect example of this... there was a melt down and the primary containment vessel was able contain everything. That's the way the system is designed.
Bill
This is from a few days ago, so I don't know if it remains relevant to Fukushima, but the comment below does suggest such a thing is possible:
"The potential for limited, uncontrolled chain reactions, voiced yesterday by the International Atomic Energy Agency, is among the phenomena that might occur, Chief Cabinet Secretary Yukio Edano told reporters in Tokyo today." http://www.bloomberg.com/news/2011-03-30/record-high-levels-of-radiation-found-in-sea-near-crippled-nuclear-reactor.html
Of course, this re-criticality would not mean that the reactors would explode like an atomic bomb, but it does suggest they can start heating up again during melting. One question I have: when uranium oxide melts into a glassy state, will it alloy with the control rod material? or would those materials remain separate, with control rod materials forming something like a dross?
On Wikipedia there's an interesting rundown on the events unfolding:
http://en.wikipedia.org/wiki/Fukushima_I_nuclear_accidents
It's been established that this is happening; very radioactive water is escaping through a crack in one of the basins. The nuclides in that water have short half-lives indicating that they were recently part of the core, which means that despite all the promises there is obviously a path for core water to leave containment. (Some of us were saying that what with the earthquake, tsunami, and multiple explosions the piping might not be in such good shape two weeks ago, but we're just anti-nuclear crackpots don'tcha know.) Last I read they attempted to plug this crack by pumping in concrete but that failed, presumably because a proper repair would require forms, reinforcement, and other things that are kind of hard to arrange when your human workers would get a lethal dose of radiation in an hour, and that assuming things don't continue to get worse.
This also explains some of the worries over the spent fuel ponds. If I remember right, there is normally about 14 feet of water that resides over the spent fuel. This isn't only there there to keep the rods cool as they break down, but it also keeps the radiation from them to a minimal. If the ponds get too low, they will start to emit gamma radiation which will make working near them too dangerous and refilling them quite difficult.