I'm really kind of sick of listening to people talk about how everything is hunky-dory. I heard one supposed expert on CNN basically state that a nuclear meltdown is no big deal. The lesson to learn here is when a government tells you everything is ok regarding an event with potential regional impact, get the hell out of town.
I'm assuming the reactors are melting down and are out of control. Pumping seawater in may be keeping the thing from blowing up, but obviously isn't cooling the reactors down. So what happens now? Do they keep melting down indefinitely, or do the reactions eventually fizzle out?
I heard one supposed expert on CNN basically state that a nuclear meltdown is no big deal.
He's right, in a sense. Modern nuclear reactors are designed so that if the fuel melts it will end up in a wide concrete tray which sits under the reactor, where -- thanks to the fact that the tray is very wide and not at all deep -- it will cool down to a safe temperature.
All good engineering works on the principle of defence in depth. This is the last step to safely contain the hot fuel, after the many redundant cooling systems fail, and it's certainly not ideal -- but it's nothing like the mythical "China syndrome". (Or like Chernobyl, which involved a graphite fire causing fuel to go up rather than down.)
"All good engineering works on the principle of defence in depth."
That line is as much a sound-bite at this point as anything you'll hear on cable television. Yes, good engineering has many levels of fall-backs and redundancies. But when you've reached the point that you've evacuated the last fifty people from your site because it's no longer safe for them to be there, then you've exhausted your defenses. There's no more depth.
Let's stop whistling past the graveyard: a meltdown is, in fact, a really big deal. Maybe the "wide concrete tray" will capture the waste. Maybe it won't. But in the meantime, you're hoping that there's not a secondary fire from the heat, or a steam explosion, or some other kind of explosion that flings radioactive particulate for miles around. You're hoping that the fuel won't melt, form a critical mass in the bottom of the reactor, and re-initiate a reaction that's hard to stop. You're hoping that the containment doesn't breach, and that vast quantities of radioactive waste aren't exposed to the elements before the whole system calms down again. You're hoping that the whole system calms down again.
The point is: they've lost control. The 'engineering' that they're doing right now is desperate and hacky, and they're very nearly out of options. It might be comforting to pretend that this whole thing is scripted out on some intricate Japanese checklist somewhere, but that's really nothing more than a fantasy. I certainly hope that things aren't as bad as they sound, but this isn't just a matter of bad PR by some pessimistic, nuclear-energy skeptics. These guys are actually in trouble.
Maybe the "wide concrete tray" will capture the waste. Maybe it won't.
Maybe the laws of physics will change, but I'd be willing to bet that they won't.
But in the meantime, you're hoping that there's not a secondary fire from the heat
Concrete doesn't burn.
or a steam explosion
At the point when the nuclear fuel melts, all the water has boiled off 1000 degrees ago.
or some other kind of explosion that flings radioactive particulate for miles around.
Explosions don't just happen for no reason.
You're hoping that the fuel won't melt, form a critical mass in the bottom of the reactor, and re-initiate a reaction that's hard to stop.
Nuclear reactors don't hold enough fuel to form a critical mass. In order to become critical, they need a moderator (usually water) which thermalizes neutrons.
"Maybe the laws of physics will change, but I'd be willing to bet that they won't."
There's absolutely nothing about the "laws of physics" that guarantees that this particular reactor design is going to be able to contain a full meltdown, because it's never happened before. Nobody knows.
"Concrete doesn't burn."
No one said it did. There's plenty of other stuff around that does burn readily, which is why the plant is currently on fire. A lot of that stuff is radioactive.
"Explosions don't just happen for no reason."
Indeed. But red-hot piles of radioactive waste are a good way of making explosions happen, particularly when there's lots of hydrogen gas floating around from the breakdown of the cooling water and the fuel. That's why there have been several explosions at the plant.
"Nuclear reactors don't hold enough fuel to form a critical mass. In order to become critical, they need a moderator (usually water) which thermalizes neutrons."
You're assuming an intact core. Criticality is a function of density, shape and temperature, in addition to mass. Melt the fuel rods, and the guarantees of that nice, well-moderated behavior are off.
In general, you're making lots of simplistic assumptions about a nicely behaved, engineered, controlled system. What they've got now is far messier. Moreover, a lot of the stuff that you're saying can't happen, is actually happening right now. The reality of the situation trumps your theories of the situation, however confident.
There's absolutely nothing about the "laws of physics" that guarantees that this particular reactor design is going to be able to contain a full meltdown, because it's never happened before. Nobody knows.
Physics isn't biology/medicine. The laws of physics are not discovered by running experiments to enumerate every possible combination or permutation of configurations.
You're assuming an intact core. Criticality is a function of density, shape and temperature, in addition to mass. Melt the fuel rods, and the guarantees of that nice, well-moderated behavior are off.
The optimal shape for criticality is a sphere - surface/volume is the key factor here. A wide, shallow puddle at the bottom of the containment chamber is the least dangerous shape.
Temperature affects things because higher density makes achieving criticality easier. I.e., the colder things get, the more likely criticality is to be achieved.
"Physics isn't biology/medicine. The laws of physics are not discovered by running experiments to enumerate every possible combination or permutation of configurations."
Wanna bet? Guess how we know most of what we know about criticality and neutron cross-sections? People like Louis Slotkin, who spent hundreds of hours poking at piles of radioactive material in the lab, to derive those mathematical models that you're leaning upon. Critical mass calculations, in particular, are so fiendishly complicated that the entire field of stochastic simulation (i.e. monte carlo methods) were invented to address them. So tell me again about the "laws of physics", and how they're not tested through pemutation.
"The optimal shape for criticality is a sphere....a wide, shallow puddle at the bottom of the containment chamber is the least dangerous shape."
Prove it. It's pretty amazing how everyone wants to cite "physics" to prove that there's no problem with a meltdown (in the face of overwhelming empirical evidence to the contrary), but nobody is doing much more than hand-waving allusions toward their undergrad physics textbook in defense of their assertions.
A sphere is definitely a shape where we have good calculations to model critical mass. Otherwise, we don't really know much that wasn't determined empirically. We know that criticality depends strongly on density. We've assumed that the structure of this reactor will prevent that density change from occurring. We don't actually know what will happen.
I can almost understand why a community of nerds is so strongly interested in maintaining the self-delusion that the world is a fully knowable, controllable place, but I don't understand how so many people can ignore so much real-world evidence for so long. If you're seriously telling yourself that a meltdown isn't a big deal, you need to go back and re-examine what you know about the situation, and why you think you know it.
So tell me again about the "laws of physics", and how they're not tested through pemutation.
Ok. You generally perform a sequence of experiments, construct a low entropy theory, and then apply that theory in the future. Kind of like what Louis Slotkin did.
He doesn't need to redo them on a train, a plane, in a car, at the bar. The fundamental principles discovered tend to be pretty solid.
Prove it.
Not that hard. Take a fixed volume, convolve it with the 1/r kernel of the neutron diffusion equation. If the volume of uranium is a sphere, you get the spot neutron density at the center is [(3V)^{2/3}]/2. If the volume is a disk of height dz, radius R, you find the the local density is 2(pi V dz)^{1/2}. The smaller dz gets, the smaller the local density of neutrons is, and the further from criticality you are.
(Computing the volume at someplace other than the center is left as an exercise for the reader. However, the maximum principle shows that it always goes down.)
Now plug this into the standard soliton machinery (i.e., use Duhamel's principle, L^p-L^q estimates, etc) and you'll always need a bigger source for a flat soliton than a spherical one.
Yes, I'm skipping a few steps. You can find them in Cazenave's book on solitons (that's where I learned it) and most likely any book on nuclear engineering (but with much less of a mathematical bent). No, it's not the "undergrad physics textbook" you seem to think I'm referring to.
It's pretty amazing how everyone wants to cite "physics" to prove that there's no problem with a meltdown (in the face of overwhelming empirical evidence to the contrary), but nobody is doing much more than hand-waving allusions toward their undergrad physics textbook in defense of their assertions.
What is the "overwhelming empirical evidence" that criticality will be achieved?
The physical principles behind criticality calculations are not fiendishly complicated. The computations are computationally intensive, yes (Slotin was around in a time where experiments were cheaper and easier than simulations), and (maybe -- I don't know) the exact nature of physical materials involved was not well known, and needed to be measured.
Prove it.
Why don't you prove it? It's not other people's job to do all the work for you. It is provable that a sphere is the optimal shape. If somebody on the internet suggests that you're wrong, you don't win the argument by saying it's their responsibility to do all the hard work of convincing you you're right. You're still the one who is wrong.
It's a shame that cperciva's nonsense is being modded up.
> "Nuclear reactors don't hold enough fuel to form a critical mass."
Come on. Each reactor core holds over 100 tons of uranium, of which about 3-5 tons is U-235. Critical mass under perfect conditions is 50 kilograms. Conditions are not perfect for forming a critical mass, but you've got 100 times as much U-235 as is strictly necessary, and there is no fucking way of knowing what will happen when an entire reactor core melts and flows together, because no one has ever been stupid enough to try it. Chernobyl experienced a criticality event, and there is absolutely no guarantee that Fukushima will not.
Moreover, each reactor has a spent fuel pool with five times as much fuel in it as the reactor itself has. And no containment vessel. And no water being supplied. And when the water boils away from those pools, and that fuel melts and flows together...
Each reactor core holds over 100 tons of uranium, of which about 3-5 tons is U-235. Critical mass under perfect conditions is 50 kilograms.
Critical masses don't work that way: the additional U-238 absorbs neutrons making more difficult to achieve criticality. In fact, even an infinite amount of unmoderated natural uranium cannot sustain a chain reaction. See the figure 3.1 in [1] for more information about unmoderated critical masses for enriched uranium.
Chernobyl used UNENRICHED uranium, containing only .7% of U235, and went kablooey. Which you just said can't happen, so I suppose Chernobyl didn't actually happen. /thread
This thread has brought out the worst aspects of Hacker News - reasonably bright people who feel compelled to opine, in their usual arrogant "I am always right" manner, about subjects where they have no clue.
Please read carefully before making absurd accusations. I said: "even an infinite amount of unmoderated natural uranium cannot sustain a chain reaction". The nuclear excursion at Chernobyl occurred in a graphite-moderated reactor core.
I don't claim any special expertise in nuclear power but, as you don't seem to know the importance of moderation in criticality, I think you should refrain from accusing others of cluelessness in this topic.
Criticality is a function of density, shape and temperature, in addition to mass. Melt the fuel rods, and the guarantees of that nice, well-moderated behavior are off.
It is impossible for the uranium in a nuclear power plant to become critical in the absence of a moderator. Even if it's shaped into a sphere and supercooled. Add heat, and it gets further away from criticality (mostly due to doppler broadening; partly due to thermal expansion). Change the shape, and it gets further away from criticality (because there's more surface area to lose neutrons).
And if they are not within the containment, aren't we f..ked?
Isn't the greatest risk of nuclear plants not the reactor but the proper waste disposal? Storing it near a nuclear plant seems to be idiotic.
EDIT: Most news articles read like there was a waste disposal next to the plants and outside of the containment
As is every technological and scientific achievement humankind has ever reached and is relying upon. Replace nuclear reactions with, say, human flight and your statement remains just as valid. I don't see many people crying out for abolishment of planes, though.
I don't see many people crying out for abolishment of planes, though.
I think there's a tiny difference between a plane falling out of the sky and a nuclear reactor melting down. I'm sure you can spot it, too, if you think long and hard.
According to [1] roughly 15k people died in plane accidents over the past decade.
It doesn't seem unreasonable to assume that Chernobyl killed many more than that.
Furthermore a nuke meltdown renders a large area uninhabitable and leaves an economic footprint "slightly" bigger than a few planes coming down every year.
Quote from [2] (page 33):
Coping with the impact of the disaster has placed a huge burden on national
budgets. In Ukraine, 5–7 percent of government spending each year is still
devoted to Chernobyl-related benefits and programmes. In Belarus,
government spending on Chernobyl amounted to 22.3 percent of the
national budget in 1991, declining gradually to 6.1 percent in 2002.
Total spending by Belarus on Chernobyl between 1991 and 2003 is estimated
at more than US $13 billion.
Other Quote (same page):
Belarus, for instance, has estimated the losses over 30 years at US $235
billion.
I looked at your first source, and immediately noticed that the 2001 death toll does not count those who died in the WTC collapse (article states for 2001, 200 accidents, 1534 dead, so it doesn't include ground zero casualties). In order to make an apples-to-apples comparison, you really need to include the deaths of those in the buildings, those in the planes, the rescue workers with respiratory problems, and anyone else exposed to pollutants at ground zero.
After all, we're counting the fallout from Chernobyl, so we have to count the fallout from 9/11. Both are huge, (hopefully) once-in-a-lifetime occurrences with massive secondary casualty counts.
And as far as economic costs, I believe you have to address the 9/11 costs.
Yes, I already regret jumping in on the math-game, when the real point I wanted to make was that this is an invalid comparison (apples/oranges) - sorry for that.
Plane crashes are created relatively equal in rate and magnitude.
Nuke crashes are nothing like that. We have only a single datapoint to draw from, and one that in hindsight almost seems like a relatively lucky one.
9/11 was a plane crash that wasn't like any other before it.
Yes, but think of the scales.
How many 9/11's does it take to match a worst-case scenario involving Tokyo (~35 million people)?
When I say "relatively equal" then I mean somewhere between 500 and perhaps a few thousand deaths from a plane crash. Whereas a nuke accident may range from 600k exposed to.. well, let's hope Fukushima gets its act together.
Yes, I am serious. I said the plane falling happens more often, not that it is worse as a singular event.
Unfortunately it is a problem with human psychology that we respond more strongly to a singular large event than to consistent low-level events.
Some of the discussion on this page has suggested Chernobyl killed around 60k, directly and indirectly. Certainly a tragedy, and a huge one at that, but it is also the only major nuclear disaster. The second worst (until we fully understand the current Japanese incident at least) remains Three-mile island, where there are no confirmed deaths I am aware of.
So, we're comparing approx 60k deaths, over the course of approximately 60 years, which works out as about 10k deaths a decade, against aviation accidents of, as you said yourself, approximately 15k a decade.
By those numbers, approximately 50% more people die each year from aviation accidents than from nuclear-power-related injuries. It's just that Chernobyl is a big story, but we hear about plane crashes all the time.
Unfortunately it is a problem with human psychology that we respond more strongly to a singular large event than to consistent low-level events.
I agree with that in general, although my opinion on this particular comparison differs.
we're comparing approx 60k deaths, over the course of approximately 60 years
Sorry to be nitpicking, but I'm not sure where the 60 years are coming from. However, since there's no hard data to rely on I'll even concede that plane accidents may have accounted for the same or slightly more "directly related" deaths in the same timeframe.
But: This is only a single datapoint. And a relatively "lucky" one.
The surrounding area around Chernobyl was sparsely populated and quickly evacuated. This is not representative for the locations of the majority of nuke plants, and certainly not for the Fukushima area.
If we imagine a worst-case scenario in Japan, with Tokyo right around the corner, then the second data-point could already change the equation in a drastic way.
This is why I think analogies to plane-crashes or traffic-accidents are invalid.
I don't see many people crying out for abolishment of planes, though.
But a lot of people, especially those who know anything about planes are always crying out for more safety. Both in mechanical terms, more plane inspections, and in human terms, more rest for pilots.
And I don't think many people here are arguing for the abolishment of nuclear power. That is a straw man.
I think what we all want is the safest possible nuclear power.
And this argument is between the people who willing admit knowing little to nothing and think things can always get worse, versus the people who keep arguing things are peachy and there's no way they can get worse, even as the situation has continued to deteriorate.
I think this boils down to an argument of "Shit happens!" vs. "Nah uh, engineering is magic!"
I suspect your parent did not realize a nuclear meltdown simply means the melting of the fuel, and instead (like most people) simply directly associates the words with "huge disaster".
(From that frame of reference of course- "a huge disaster would not be a big deal!"- it DOES sound ridiculous)
A few short days ago, we were told that everything was a-ok, no worries. Then the buildings started exploding -- still, everything is under control. On Monday we hear that fuel rods are fully exposed, but that is ok too.
Now we're talking about the staff evacuating the site to avoid acute radiation poisoning and dropping seawater from helicopters. At what point do you stop accepting the best-case scenario as the most likely one?
Well, I imagine when people are dying. The quake and resulting waves killed a lot of people. This hasn't, yet. Excuse me for thinking the most about those.
I respect your faith in good engineering, but I haven't seen alot of evidence of good engineering here.
There is no evidence to support your assertion that this is nothing like a "China syndrome" type issue. This facility is located a few miles from the eqicenter of the 4th largest earthquake in recorded history. None of the safety systems worked, and the power company and Japanese government has no clue about what to do.
So the future of (at a minimum) an entire region of Japan is dependent on a slab of concrete after a massive earthquake. It is a facility built in a known earthquake and flood hazard zone that requires continuous access to the utility grid.
Doesn't sound like good engineering by any standard to me.
EDIT:
nvm, found the answer by re-reading the article.
In short, even without chain reaction, substantial (7% of normal output early after shutdown) heat is produced, so some cooling is needed.
--
The original post:
Why are (or were) they pumping water in anyway? Wasn't the whole point of reactors moderated by water that once the moderating water escapes (evaporates or flows out), the unmoderated neutrons don't sustain chain reaction anymore and it basically ceases to produce heat?
There's already no chain reaction - the control rods, plus the boron in the water they're pumping in absorb too many neutrons to sustain a reaction. The heat is not from uranium fission, but rather from the fission products made while the reactor was running undergoing spontaneous decay. This process cannot be stopped; you can only wait for it to slow down and cool to a reasonable level.
The reason they want to keep it cool is to prevent the fuel rods from melting. While melting won't immediately mean a release, it would mean the loss of two of the five barriers to a radioactive release, as well as quite a lot of heat for the remaining barriers. It would also significantly complicate later cleanup. As such, it is preferable that the fuel rods remain intact if at all possible, and this means attempting to cool them.
CNN seems to have been doing everything in their power to make things more confusing and worse than it actually has been. To make matters worse, right in the middle of them stating that the reactors were experiencing a meltdown (this is over a day ago), they cut back and forth between videos of the explosion and videos of a natural gas refinery on fire. Classy act, that; clearly they're interested in journalistic integrity and accuracy. The expert they had on was attempting to do away with some of the insane sensationalism they were trying to lead him to admit had minor accuracies in them.
According to the IAEA, Reactors 1-3 have been in cold shutdown since yesterday, which means they finally are where they were supposed to end up, in a stable situation where no water is boiling off anymore, so all the risk is gone.
It seems that the IAEA is not a useful source of up-to-date information here. Apparently they're bound by agreements not to say anything without TEPCO's approval. The little they've said has been hours or days late.
Unit 2 and 3 are believed to have lost containment integrity. Unit 3 is currently billowing smoke/steam.
> The Agency shall seek to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world.
Now why would an Agency which is interested in "accelerating" the use of atomic energy for "prosperity" around the world say anything too negative about all this mess?
If one reactor gets so bad (meltdown) forcing permanent evacuation - all the remaining reactors won't be maintainable/saveable and thus in a domino effect - if one goes, all go (meltdown).
It is unlikely that a meltdown through the core will occur but if that happens - the scary term China Syndrome - the melted radioactive fuel will melt through steel, concrete, rock, maybe even bedrock.
For the USA and rest of the world: Irradiated food supplies will be a real concern if hundreds of tons of radioactive melted fuel merge with the outside environment
To put it in perspective: If you were in Japan near Tokyo or closer to the affected nuclear plant, would you be more skeptical of the seemingly contradictory news coming out.
Because of fire, explosions, and excessive radiation exposure to workers.
The meltdown would still be contained
Containment is believed to have failed in reactor 2 and likely 3 at this point. Smoke or steam has been billowing from number 3 for hours now and its fuel is in the reactor, not the spent fuel pool.
I'm assuming the reactors are melting down and are out of control. Pumping seawater in may be keeping the thing from blowing up, but obviously isn't cooling the reactors down. So what happens now? Do they keep melting down indefinitely, or do the reactions eventually fizzle out?