Comment by timr | Hacker News Reader

Comment by timr | Hacker News Reader

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"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.

Sure the laws of physics don't change. But the state of this complex system is way way out of understood and explored space at this point.

http://www.digitalglobe.com/downloads/featured_images/japan_...

This game was out of the playbook from the beginning, as soon as all the batteries and backup generators failed.

"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.

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.

[1] http://www.ornl.gov/sci/radiation_transport_criticality/Hopp...

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.

What the FUCKKKKKKKK? You DO know Chernobyl wasn't a nuclear explosion right? It was a STEAM explosion.

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.

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.

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.

"Why don't you prove it?"

Because I'm not the one making extraordinary claims. I'm also arguing that you can't 'prove' anything in this situation; there are too many unknowns.

> > "Why don't you prove it?"

> Because I'm not the one making extraordinary claims.

You are, though. You're the one suggesting a nuclear catastrophe, contrary to apparent scientific evidence.

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?

You're assuming an intact core.

No, I'm not.

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).

I may ask you about the spent fuel disposals that seem to be on fire.

Are they within the containment? This picture doesn't clear things up for me (http://en.wikipedia.org/wiki/File:BWR_Mark_I_Containment_ske...)? And if so and if they are on fire, does the containment still work?

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

EDIT 2: http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1300...

Your argument would hold a lot more water if all the other things that 'couldn't possibly' go wrong hadn't already gone wrong.

Concrete doesn't burn.

No. But concrete can crack during, say, an earthquake.

And I assume concrete can also lose integrity (melt? vaporize?) under very high temperatures.

Wikipedia has some details on the typical interactions here: http://en.wikipedia.org/wiki/Corium_(nuclear_reactor)#Corium...

The summary is that yes, typically concrete would melt at temperatures anywhere in the range that you'd expected a molten core to be at.

My understanding is that newer designs have a "core catcher" under the reactor made of something more resistant than concrete, but this unit doesn't.

Totally agree with this, it reminds me of impenetrable software and unsinkable ships.

Exactly, the bottom line is were playing with nuclear reactions and hoping everything goes as planned. It's arrogance.

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.

The plane falling happens a lot more often?

Are you serious?

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.

[1] http://en.wikipedia.org/wiki/Aviation_accidents_and_incident...

[2] http://www.iaea.org/Publications/Booklets/Chernobyl/chernoby...

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.

60 years is approximately how long we've had nuclear power for, I believe.

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.

No worries. I still think you are incorrect, based on this:

>Plane crashes are created relatively equal in rate and magnitude.

Not all of them are... 9/11 was a plane crash that wasn't like any other before it.

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.

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!"

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