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Old 06-01-2011, 09:57 PM   #141
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A physicist about the risks: We 'came close to losing northern Japan' - CNN.com

Now this man, while an important physicist, does seem to love sensational language.
What I would like to know: is what he says wrong?
Some of it does seem sensationalized, others oddly the opposite. I wish it was a transcript, I went to listen again, but you get the ads, and then it advanced to the next segment and I gave up (finally did the original link again), but...

When he was describing the state of the nuclear material, I think he said it was like a bowl of granola with cream on it? That doesn't convey anything dangerous (though it clearly is!), so it was an odd choice of words (reminds me of the Stay-Puft Marshmallow man!). Even the interviewer seemed to try to get him to change tact there.

I had trouble accepting that the reason the company was reluctant to throw sea water on it was that they hoped to salvage anything. I can't imaging there is anything to salvage there.

I think the bigger questions is - would it have ever got to the point of a meltdown, really, or would they have seen that and pumped the sea water in before then? I don't know enough to even guess, but the fact is that it didn't go that far. Maybe that is sensationalizing this, maybe not if it really was close to getting much worse and there was no solution.

I got lost when he said there was a 100% meltdown, but then also says that if they didn't put seawater in it would have been worse. I guess the seawater kept it cool enough to contain, but I thought 100% meltdown meant melting through (down to China!)

What may well be sensationalizing (again, I can't know for sure), is that 20x number for radiation for children versus workers. Without some more info, that might just be some clever statistics munching, but maybe not. I assume he meant a normal exposure for workers, not the elevated numbers they are using for rescue workers. Hard to say.


It is tough to know enough to talk about what could have been. But again, to put this in perspective, how many deaths are traceable to this being a nuclear plant, versus the total deaths from this tragedy? And how many deaths would there be if those were some other type of power source? There were injuries/deaths at some oil refineries in Japan. And those other power sources have deaths associated with them even when nothing extraordinary like an earthquake happens:

India child labor: In India coal towns, many miners are children - latimes.com

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"A big stone fell on a friend at a nearby mine last year, and he died," said Sharan Rai, 16, taking a break near the entrance with his friend Late Boro, 14. Both started mining when they were 12. "The owners didn't pay the family anything. I try and check if the walls look strong before I go in."

Thousands of children, some as young as 8, are believed to toil alongside adults in the northeast mines; their small bodies are well suited to the narrow coal seams.
I'm hoping that the next gen of nukes are smaller units, scattered across the grid. It's easier to make something small hold up to a disaster, and by spreading them around there is more redundancy and less loss to the grid.

-ERD50
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Old 06-02-2011, 12:02 AM   #142
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Originally Posted by Tigger View Post
A physicist about the risks: We 'came close to losing northern Japan' - CNN.com

Now this man, while an important physicist, does seem to love sensational language.
What I would like to know: is what he says wrong?
Yes.

(This should produce an entertaining response. String theory != nuclear engineering.)

Northern Japan is pretty big. Three reactors, the old GE BWR models there, with the cores completely melted down (the industry slang is corium), won't sustain a fission reaction. The careful geometry needed to sustain a reaction is lost. The water, which slows neutrons to the point where they can be captured and sustain a fission reaction was lost.

The reaction products still continue to release decay heat. Without coolant that produces a meltdown. They might melt 5-7 cm of the reactor vessel inner lining. It will not be a nuclear explosion. It will be one heck of a cleanup mess within the containment, and as we saw, with a huge earthquake and tsunami exceeding the historical record and the design limits of the facility, the containment was damaged and there was some contamination outside the facility.

Dumping in seawater in the presence of fuel element failures was risky, and ran a risk of making the contamination problem worse. They had no instrumentation that could inform them that the core might be completely dry and in meltdown, though. They could detect fission products in the gases vented from the primary plant, which told them that they did have a fuel element failure. How big was unknown. The safest action in the face of that unknown state was to provide some form of emergency cooling. (BTW, the professor was wrong about that, too. Reactor Plant Manuals do describe the use of seawater as a coolant of last resort. It's very corrosive compared to the normal ultra-pure water, and as seen here spreads contamination from corroding the fuel charge.)

So, no nuclear explosion possible, some local contamination problems, and one heck of a mess to clean up on site. Nothing as hideous as Chernobyl, where there was no containment, and the actual fuel matrix (graphite) was on fire!

Tell me how that would lose Northern Japan. We Americans dropped two freaking nuclear bombs on Southern Japan, with horrendous results, but that didn't 'lose Southern Japan.'

If I wanted to look at something that darn near lost Northern Japan, I'd look at that tsunami. That was horrific, the sort of thing that gives civil engineers and disaster contingency planners nightmares.
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Old 06-02-2011, 01:14 AM   #143
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I think the physicist was trying to say that the core melted down to rearrange itself into a critical mass of uranium that could have caused a nuclear explosion. That just doesn't happen, and thank goodness it doesn't or the terrorists would have made a mess of the world by now.

The reactor reality is that the uranium spews a lot of neutrons and heat while it's slagging down into a puddle at the bottom of the pressure vessel, and both are pretty much wasted by dissipating into the surrounding structure. The only way to put those neutrons to good fission use would be to somehow stop them, turn them around, and shoot them back into the uranium mass: like banking a raquetball off the side wall. Turns out that's accomplished with... water. So dumping a bunch of water onto a mass of uranium can still get pretty exciting if you do it wrong. But even if something critical (so to speak) happened, the whole mass would just heat up again and spread out some more to waste all those fission neutrons-- again.

Half the challenge of building a nuclear weapon is holding the critical mass together long enough to gain full effect from the fission neutrons. That only happens with a huge ball of pressure wrapped around the critical mass... like slamming one piece into another (the "gun" method) or compressing it with a dynamite explosion. In the case of the reactor, though, the best they could hope for would be a little more enthusiastic bubbling and spewing.

I'd sure love to hear his fellow physicists "reviewing" his hypothesis with him tomorrow in their coffee lounge... maybe this is why he's "popularizing" science instead of sticking to string theory.

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Yes.
I'm glad I bit my tongue and waited for you to offer your answer first. You were much more restrained than I could've been. But spouse still got my eyeball-glazing dinnertime rant on the sad state of teaching reactor core physics in American public schools.
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Old 06-02-2011, 05:49 AM   #144
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Well done to both of you. I have nothing to add.
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Old 06-02-2011, 07:35 AM   #145
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All this anti-nuclear fear-mongering seems ridiculous to me. What's the death toll from Fukishima so far? 3? Have we already forgotten the 11 souls who died on Deepwater Horizon just a year ago drilling for oil? Or the 29 miners who died in New Zealand just this past November, drilling for coal?

You simply cannot make a case against nuclear based on risk of death when you compare the death tolls for nuclear against oil or coal. Coal mining alone has literally killed thousands. It's pure emotional sensationalist fear-mongering.
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Old 06-02-2011, 07:59 AM   #146
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The careful geometry needed to sustain a reaction is lost.
Hmm. My (admittedly layman's) understanding of nuclear physics is that in the absence of a neutron "brake" (eg., the control rods), the reaction will occur uncontrollably. In fact, even with the "brake" fully applied (the control rods fully inserted, absorbing a large percentage of stray neutrons), the nuclear fuel still generates an enormous amount of decay heat (7-10% of full capacity). So much so that they still need to be cooled by being completely immersed in water. And that water gets so hot that if it were not in a sealed pressure vessel, it would be boiling. In fact, in cases where the water level dropped and the water did in fact boil, even the tiny "voids" in the water (the bubbles) were enough to compromise the cooling effect of the water, and overheating occurred.

My point is that even with the control rods fully inserted, the reaction is still occurring. If the cooling agent (water) drops below the tops of the fuel rods, they overheat and melt, even when only producing "decay" heat. Thus, the reaction is "sustained," even after the geometry of the fuel rods disintegrates into a puddle of molten uranium.

As I understand it (and again, I'm no physicist), the only requirements for a sustained fission reaction is proximity/density of fuel, and absence of a neutron absorber. In a molten state, at the bottom of the reactor chamber, I would expect that both of these conditions would be met, and the reaction would indeed be sustained indefinitely.

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The water, which slows neutrons to the point where they can be captured and sustain a fission reaction was lost.
I'm not sure what you mean here - you seem to be saying the water helped facilitate the fission reaction. My understanding is that the water actually discourages the nuclear reaction (by absorbing stray neutrons, preventing them from perpetuating more reactions), in addition to providing a cooling effect to avoid meltdown.

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The reaction products still continue to release decay heat.
Sure, but I believe that that "decay heat" is itself a fission reaction, and in the absence of a cooling agent, will generate enough heat to reduce the fuel to a molten state.

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Tell me how that would lose Northern Japan. We Americans dropped two freaking nuclear bombs on Southern Japan, with horrendous results, but that didn't 'lose Southern Japan.'
To be fair, we're talking about dramatically different amounts of nuclear fuel here. The amount of raw nuclear material in the Hiroshima and Nagasaki bombs were about the size of a softball. The nuclear reactors in question contain much, much more fuel than that. Also, the bombs were only able to fission about 10% of their material before being dissipated too broadly for the reaction to continue. Left undisturbed, the reactor core material will continue to fission to a much higher percentage, unless we can extract it and separate it from itself.

Again, I'm not a physicist, everything I just wrote could be completely wrong.
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Old 06-02-2011, 10:11 AM   #147
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It will not be a nuclear explosion.
Should I listen again? I didn't hear Kaku saying either directly or by implication that there would have been a nuclear explosion if seawater had not been used. He said that northern Japan might no longer have been habitable, and I took him to be referring to contamination.
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Old 06-02-2011, 10:35 AM   #148
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You simply cannot make a case against nuclear based on risk of death when you compare the death tolls for nuclear against oil or coal.
That's a non sequitur. There can be risk of future death even when there have been no past deaths at all. In the Kaku discussion, we are considering hypothetically what might have happened had seawater had not been used to cool the troublesome reactors, and then, how likely in the future a private company would be to make what, in this case, turns out to have been the right decision.
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Old 06-02-2011, 10:57 AM   #149
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Hmm. My (admittedly layman's) understanding of nuclear physics is that in the absence of a neutron "brake" (eg., the control rods), the reaction will occur uncontrollably. In fact, even with the "brake" fully applied (the control rods fully inserted, absorbing a large percentage of stray neutrons), the nuclear fuel still generates an enormous amount of decay heat (7-10% of full capacity). So much so that they still need to be cooled by being completely immersed in water. And that water gets so hot that if it were not in a sealed pressure vessel, it would be boiling. In fact, in cases where the water level dropped and the water did in fact boil, even the tiny "voids" in the water (the bubbles) were enough to compromise the cooling effect of the water, and overheating occurred.
Decay heat for a uranium or MOX reactor starts off at 6.6% of the previous sustained power level and drops off on an exponential function from there, to 1.5% after an hour, down to 0.2% after a week.

The decay curve function:




The fission chain reaction in these plants relies on thermal neutrons, neutrons produced by a fission of uranium or plutonium and slowed by a 'moderator' until they are moving so slowly that they can be caught and absorbed by another uranium or plutonium nucleus. There's a thing called the 'absorption cross-section' that indicates how readily the neutron is captured. Fast moving neutrons are not readily captured by the nuclear fuel in these reactors, and need to be slowed down.

In the Chernobyl plant, the moderator that slowed down neutrons was the graphite blocks the fuel was embedded in. That meant that unless the control rods could be fully inserted, the nuclear chain reaction would continue.

The water-moderated reactors, used for almost all nuclear power applications currently, rely on the water to slow the neutrons. With the water removed, the fast neutrons rapidly escape the reactor core and are absorbed in the surrounding neutron shield. (Typically a jacket including boron, which has a high absorption cross-section for neutrons but doesn't become radioactive.)

With the water removed, the nuclear chain reaction that supports continued fission stops.


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My point is that even with the control rods fully inserted, the reaction is still occurring. If the cooling agent (water) drops below the tops of the fuel rods, they overheat and melt, even when only producing "decay" heat. Thus, the reaction is "sustained," even after the geometry of the fuel rods disintegrates into a puddle of molten uranium.
The fission reaction is stopped at this point, as the water 'moderator' is removed. The control rods, metal alloys that absorb neutrons so as to damp the chain reaction, sill in the event of a core meltdown become part of that puddle in the bottom of the reactor vessal.

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As I understand it (and again, I'm no physicist), the only requirements for a sustained fission reaction is proximity/density of fuel, and absence of a neutron absorber. In a molten state, at the bottom of the reactor chamber, I would expect that both of these conditions would be met, and the reaction would indeed be sustained indefinitely.
Proximity of fuel, and presence of a neutron flux sufficient to be absorbed and maintain a chain reaction, to be specific. Removing the moderator results in a loss of the neutron flux. The remaining fast neutron flux is insufficient to maintain a chain reaction. The core being collapsed and spread out over a reactor vessel bottom doesn't provide the needed proximity. (The fuel region looks like a disk, rather than a concentrated sphere, and is laced with neutron absorbing material from the control rods.)

Note that when water was eventually injected for cooling, it included a charge of boric acid. The boron atoms are potent neutron absorbers, 'poisoning' any neutron chain reaction. This is done to prevent the coolant from acting as a moderator for a partial meltdown configuration that might otherwise sustain a local chain reaction in part of the core.

There are reactors, such as the fast breeder design, that can maintain a chain reaction using a fast neutron flux. These use a different fuel mix, and special materials to reflect and concentrate the neutron flux in the core. A precise core geometry is also needed to keep the reaction going. The conditions to sustain this reaction simply do not exist in a thermal neutron reactor such as the Fukushima plant uses.


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I'm not sure what you mean here - you seem to be saying the water helped facilitate the fission reaction. My understanding is that the water actually discourages the nuclear reaction (by absorbing stray neutrons, preventing them from perpetuating more reactions), in addition to providing a cooling effect to avoid meltdown.
Pure water does facilitate the reaction, by slowing neutrons so that that can be absorbed by the fuel. Borated water, and to some extent ordinary sea water will damp the reaction by absorbing neutrons.

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Sure, but I believe that that "decay heat" is itself a fission reaction, and in the absence of a cooling agent, will generate enough heat to reduce the fuel to a molten state.
Decay heat is not a fission reaction. Decay heat comes from radioactive decay processes. Often the products of a fission reaction are two atoms, with roughly similar masses about half that of the fissioned atom. (That is, fission typically breaks a big atom in half, so we get two lighter atoms, and maybe a couple of free neutrons.) These fission product atoms often have an extra neutron or two in their nucleus compared to stable atoms of the same element. These atoms will 'decay' by emitting an electron from their nucleus, effectively trading a surplus neutron in for a proton and becoming more stable.

This process results in the release of heat, but it is not a chain reaction. As shown above, the decay heat from these secondary nuclear reactions drops off pretty quickly.

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To be fair, we're talking about dramatically different amounts of nuclear fuel here. The amount of raw nuclear material in the Hiroshima and Nagasaki bombs were about the size of a softball. The nuclear reactors in question contain much, much more fuel than that. Also, the bombs were only able to fission about 10% of their material before being dissipated too broadly for the reaction to continue. Left undisturbed, the reactor core material will continue to fission to a much higher percentage, unless we can extract it and separate it from itself.

Again, I'm not a physicist, everything I just wrote could be completely wrong.
Actually, the reactor core contains a much more dilute form of the active nuclear material (Uranium-235) than a weapon, and as I have indicated above, the fission reaction stops in the absence of the moderator, or the insertion of control rods. (It also stops pretty quickly if one simply stops managing the reaction at all courtesy of a thing called the negative temperature coefficient of reactivity, but that is beyond the scope of the current topic.)

Oh, I do have a degree in physics, and taught reactor plant operation for two and a half years. I think I was fairly successful at it, as Nords and Gumby never did manage to vaporize central Idaho...

* Graphs, formulae, and links courtesy of Wikipedia. It's nice that someone else scanned in the texts and graphs.
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Old 06-02-2011, 10:58 AM   #150
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That's a non sequitur. There can be risk of future death even when there have been no past deaths at all. In the Kaku discussion, we are considering hypothetically what might have happened had seawater had not been used to cool the troublesome reactors, and then, how likely in the future a private company would be to make what, in this case, turns out to have been the right decision.
Great. So now second-guessers won't be satisfied until we also consider what would happen if operators repeatedly do dumb things? This should be fun. I'd suggest that hydoelectric power won't do well under this new paradigm (unless we are prepared to vacate all
structures downstream of them right now). And when this new "analysis" tool is used to examine the commercial air transport industry, I think we'll have some changes coming. But, we'll still be able to travel across the country. Wagons, Ho!!

It's useful to consider alternate scenarios in order to find ways to mitigate risk (e.g. the operators at Fukushima could have used more sensor redundancy as a means to make better decisions). But there's clearly a limit as to how far we need to go down the "and then they make 22 consecutive incorrect decisions, what about that?"
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Old 06-02-2011, 11:05 AM   #151
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Should I listen again? I didn't hear Kaku saying either directly or by implication that there would have been a nuclear explosion if seawater had not been used. He said that northern Japan might no longer have been habitable, and I took him to be referring to contamination.
That's the usual cognitive jump folks make when hearing about a reactor problem, and someplace becoming uninhabitable.

The contamination from the Fukushima plant is pretty much already a worst case for this type of reactor and containment. Northern Japan was not lost, or rendered uninhabitable.

There was this tsunami last March, though, which caused a tremendous amount of damage, and a horrifying number of deaths. Please, please folks, don't forget about that.

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Old 06-02-2011, 11:28 AM   #152
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Decay heat for a uranium or MOX reactor starts off at 6.6% of the previous sustained power level and drops off on an exponential function from there, to 1.5% after an hour, down to 0.2% after a week.

(...)

Oh, I do have a degree in physics, and taught reactor plant operation for two and a half years. I think I was fairly successful at it, as Nords and Gumby never did manage to vaporize central Idaho...
Aha. I was starting to wonder if you were running a clandestine reactor in your basement.
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Old 06-02-2011, 11:38 AM   #153
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Great. So now second-guessers won't be satisfied until we also consider what would happen if operators repeatedly do dumb things?

Well, people do tend to make a lot of mistakes. Human mistakes (dumb behaviour, if you will) are involved in most accidents, nuclear and other.


Three mile island:

"There wasn't enough coolant in the core, so the Emergency Core Cooling System automatically turned on. This should have provided enough extra coolant to make up for the stuck valve, except that the reactor operator, thinking that enough coolant was already in the core, shut it off too early."

"There still wasn't enough coolant, so the core's temperature kept increasing. A valve at the top of the core automatically opened to vent some of the steam in the core. This should have helped matters by removing the hot steam, but the valve didn't close properly. Because it didn't close, steam continued to vent from the reactor, further reducing the coolant level. The reactor operators should have known the valve didn't close, but the indicator in the control room was covered by a maintenance tag attached to a nearby switch. Because the operators didn't know that the valve had failed to close, they assumed that the situation was under control, as the core temperature had stopped rising with the first venting of steam from the core. They also thought that the coolant had been replaced in the core, because they didn't know that the pump outlets were closed"

"A few minutes later the core temperature began to rise again, and the Emergency Core Cooling System automatically switched on. Once again, an operator de-activated it, thinking the situation was under control. In reality, it was not."

"Soon, because of the coolant lost through the open valve at the top of the reactor, the core temperature began to rise again. At this point the fuel rods started to collapse from the intense heat inside the core. The operators knew something was wrong, but didn't understand what it was."



Chernobyl:

"What caused the accident? This is a very hard question to answer. The obvious one is operator error. The operator was not very familiar with the reactor and hadn't been trained enough. Additionally, when the accident occurred, normal safety rules were not being followed because they were running a test. For example, regulations required that at least 15 control rods always remain in the reactor. When the explosion occurred, less than 10 were present. This happened because many of the rods were removed to raise power output. This was one of the direct causes of the accident. Also, the reactor itself was not designed well and was prone to abrupt and massive power surges."

Source: http://library.thinkquest.org/17940/...disasters.html
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Old 06-02-2011, 12:21 PM   #154
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Hmm. My (admittedly layman's) understanding of nuclear physics is that...
Yes, you have a layman's understanding. But unless you're also a string-theory physicist who fools an adulterous ex-politician "anchor" that he knows what he's talking about, then you still have credibility.

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Originally Posted by kombat View Post
... in the absence of a neutron "brake" (eg., the control rods), the reaction will occur uncontrollably.
It can. What usually happens a few microseconds afterward, though, is that the resulting rise in power overheats the system until things boil & melt. The components (including the uranium fuel) tend to fly apart from this pressure/heat, which spreads out the fuel. The geometry becomes once again a sub-critical mass. There are still plenty of neutrons and fission products, but the nuclear reaction is no longer critical.

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Originally Posted by kombat View Post
In fact, even with the "brake" fully applied (the control rods fully inserted, absorbing a large percentage of stray neutrons), the nuclear fuel still generates an enormous amount of decay heat (7-10% of full capacity). So much so that they still need to be cooled by being completely immersed in water. And that water gets so hot that if it were not in a sealed pressure vessel, it would be boiling. In fact, in cases where the water level dropped and the water did in fact boil, even the tiny "voids" in the water (the bubbles) were enough to compromise the cooling effect of the water, and overheating occurred.
True.

"Nucleate" boiling (water phase-transforming into small steam bubbles forming on the metal surfaces and quickly detaching) is an extremely efficient way to transfer heat. It'd be great if we could control it. Unfortunately it usually mutates into "departure from nucleate boiling" where the water heats up and transforms into sheets of bubbles that grow into steam blankets around the metal surfaces, greatly reducing heat transfer and allowing the decay heat to melt the metals. We can't reliably keep NB from turning into DNB so we try to avoid both of them. Usually the only way to get things back under control is to raise the pressure to collapse the bubbles back into really hot (but not quite boiling) water. If the pressure vessel has holes (or if all the high-pressure pumps are out of service) then you can't keep pressure high enough to control/avoid boiling. Nukes get really upset about losing pressure control because the DNB means heat can no longer be effectively removed from the core.

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My point is that even with the control rods fully inserted, the reaction is still occurring. If the cooling agent (water) drops below the tops of the fuel rods, they overheat and melt, even when only producing "decay" heat. Thus, the reaction is "sustained," even after the geometry of the fuel rods disintegrates into a puddle of molten uranium.
Semantics issue. The fission chain reaction has stopped and quickly subsides. The decay of the fission fragments continues, which is still a heat-removal problem, but that's not fission. That's just decay.

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As I understand it (and again, I'm no physicist), the only requirements for a sustained fission reaction is proximity/density of fuel, and absence of a neutron absorber.
Incorrect. Proximity/density is one requirement, and absence of a neutron absorber is another. But the uranium's fission neutrons usually fly out of a nucleus with too much energy to be absorbed by nearby uranium nuclei. The neutrons zip out of the fuel mass and don't sustain the chain reaction. The trick is to slow the fission neutrons down enough to allow nearby uranium nuclei to absorb them. This is done by letting the neutrons ricochet around enough to lose some energy. The best ricochet material is hydrogen atoms, and there are two of them in every water molecule. So the fission neutrons zip out, bang around in the water molecules until they lose enough energy to slow down, and then smack into another uranium nucleus with just the perfect amount of energy to be absorbed to cause a new fission.

The proximity and number of hydrogen atoms is critical. Just messing with their density will disrupt their ability to slow down the fission neutrons. If the core heats up the surrounding water, the water molecules try to spread out and they let too many neutrons escape. So as a core's fission raises its power and heats the water, the water lets more neutrons escape and the fission reaction levels off. This negative-feedback loop in a pressurized-water reactor is what lets the nukes control the core's power.

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In a molten state, at the bottom of the reactor chamber, I would expect that both of these conditions would be met, and the reaction would indeed be sustained indefinitely.
Nope. Definitely not enough water around to slow down the fission neutrons. Fission stopped as soon as the water boiled, but decay heat generation (and a lack of heat transfer) started the melting. Of course the melting leads to boiling off the water and the loss of even more heat transfer, so the molten mess gets hotter. Lots of decay fission products are gases, and if the mess stays hot then it keeps offgassing radioactive fission products which eventually leak out of the containment vessel. So even though there's no more fission, and even though decay heat is dropping off (eventually), nukes still have to get excited about cooling the molten mass to stop the offgassing.

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I'm not sure what you mean here - you seem to be saying the water helped facilitate the fission reaction. My understanding is that the water actually discourages the nuclear reaction (by absorbing stray neutrons, preventing them from perpetuating more reactions), in addition to providing a cooling effect to avoid meltdown.
Water does both. In the right core geometry, pressure, and temperature, it will help slow down fission neutrons to create more fissions.

In a big cooling tank, without the right core geometry ("critical mass"), the water slows down neutrons as always. However those neutrons don't find a nearby uranium nucleus in time to keep them from experiencing more water collisions and slowing down to the point where they can no longer cause a fission. Or the neutrons slow down enough to get captured by other materials (like the hafnium in control rods, or the boron in core poisons) before they get to uranium atoms.

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Sure, but I believe that that "decay heat" is itself a fission reaction, and in the absence of a cooling agent, will generate enough heat to reduce the fuel to a molten state.
Another semantics issue. The fission products are still spewing off parts of themselves but it's usually in the form of neutrons, high-energy electrons, gamma rays, and alpha particles. Nothing like the big chunks that come from a fissioning uranium nucleus.

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To be fair, we're talking about dramatically different amounts of nuclear fuel here. The amount of raw nuclear material in the Hiroshima and Nagasaki bombs were about the size of a softball. The nuclear reactors in question contain much, much more fuel than that. Also, the bombs were only able to fission about 10% of their material before being dissipated too broadly for the reaction to continue. Left undisturbed, the reactor core material will continue to fission to a much higher percentage, unless we can extract it and separate it from itself.
It's not just the "amount", but also the density of the U-235 atoms and their geometry.

In the 1930s everybody knew how to fission U-235. It just wasn't sustainable, the same way today's laboratory fusion reactors aren't sustainable. The problem was purifying enough of U-235 into a big enough ball to cause the fission chain reaction to be sustainable instead of a short-lived "fizzle". The only effective way to purify the 0.7% of U-235 from the uranium ore (mostly U-238) was by the centrifuging banks developed at the Oak Ridge labs (and duplicated all over the world today).

Commercial nuclear reactors can be built out of natural uranium. Geologists even discovered a formation in Africa where a rich uranium deposit was soaked in water in just the right conditions to cause a "natural" chain reaction-- but one that quickly melted the formation and ended the fission. Navy nuclear reactors use an unbelievably highly purified U-235 fuel in a very small space which, ironically, is that much more easily handled by the control rods and the surrounding water because of its small size.

So "left undisturbed", the core material will fission, but it usually produces enough heat (and pressure) to mess up its geometry. In effect it separates itself and can no longer maintain criticality. The big challenge for weapons designers is using unbelievably high pressures to keep the critical uranium together long enough to cause all of it to fission-- even a couple of microseconds makes a difference.

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Again, I'm not a physicist, everything I just wrote could be completely wrong.
Like the physicist, it's enough correct information with enough comprehension errors to cause all sorts of confusion.

This stuff ain't easy. A fission chain reaction is a combination of physics and chemistry complicated by thermodynamic heat transfer. It happens so rapidly that even the math defies analysis and is dependent on statistics & probability.

The learning process is brutal and Darwinian. I graduated pretty high up in my USNA class and still got beat up by Admiral Rickover before he grudgingly admitted I might be worthy of the chance. My six months of nuke power school class lost 20% enroute graduation, and that was just classroom lectures & essay tests. By then I was middle of the pack. My six months of shore-based nuclear power plant training was in upstate New York (even back then we actively avoided M_Paquette's Idaho classroom) for more studying, essay exams, and watchstanding. By then I'd sunk down to the lower third of the pack and I actually failed a final exam. (Two of my classmates put themselves into psychiatric care during this period, and were no longer with us.) I managed to pull it all together by my final boards and graduate, but I was not among the sharper tools in the shed.

Confirmed masochists like Gumby tired of the Navy and entered the civilian nuclear power industry, where they were treated like nuclear newbies and made to suffer the training & qualification process all over again. Because those civilian designs have different physics, chemistry, and thermodynamics characteristics than the Navy plants-- let alone the different control systems and missions.
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Old 06-02-2011, 12:31 PM   #155
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Aha. I was starting to wonder if you were running a clandestine reactor in your basement.
The local HOA rules do not allow nuclear materials or weapons in the home. It's clearly written in the by-laws.
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Old 06-02-2011, 12:41 PM   #156
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So, no nuclear explosion possible, some local contamination problems, and one heck of a mess to clean up on site. Nothing as hideous as Chernobyl, where there was no containment, and the actual fuel matrix (graphite) was on fire!.
I've listed to the video again. Kaku says that if seawater hadn't been poured in, melted uranium would have plunged on the floor probably causing a steam explosion/a hydrogen gas explosion, blowing the lid of the whole thing and then you "would have had Chernobyl".

So, indeed no nuclear explosion.

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Three reactors, the old GE BWR models there, with the cores completely melted down (the industry slang is corium), won't sustain a fission reaction. The careful geometry needed to sustain a reaction is lost. The water, which slows neutrons to the point where they can be captured and sustain a fission reaction was lost.
I just read this elsewhere (possibly inaccurate):

"In a complete reactor meltdown, the extremely hot (about 2700� Celsius) molten uranium fuel rods would melt through the bottom of the reactor and actually sink about 50 feet into the earth beneath the power plant. The molten uranium would react with groundwater, producing large explosions of radioactive steam and debris that would affect nearby towns and population centers."

Some argue the molten core can't get in contact with ground water, because the reactors have a "core catcher" that prevents molten core from getting out.

Now I do hope that Fukushima I has a core catcher, but that doesn't seem to be certain. It's a BWR-3 and construction started in 1967.
Rumors that Fukushima doesn't have one have been circulating for months. John Beddington says the Fukushima reactors don't have a core catcher but says the effect of a meltdown could be an explosion that sends materials 500m high (in Chernobyl it was 10km), so it wouldn't fly farther than 20km (bad, but not as bad as Chernobyl).

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Northern Japan is pretty big.
Indeed.
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Old 06-02-2011, 01:35 PM   #157
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I've listed to the video again. Kaku says that if seawater hadn't been poured in, melted uranium would have plunged on the floor probably causing a steam explosion/a hydrogen gas explosion, blowing the lid of the whole thing and then you "would have had Chernobyl".

So, indeed no nuclear explosion.
No steam explosion. The hydrogen explosion actually happened. That's what blew the roof off over the top of the containment and the spent fuel tank. TEPCO failed to do the ventilation retrofit recommended for these plants when the risk of hydrogen accumulation was identified.

A molten core won't melt through the bottom of the reactor vessel. To have the case of "melted uranium would have plunged on the floor" would require a shear failure removing and displacing the bottom of the reactor vessel, along with the containment base and the suppression ring vanishing.



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I just read this elsewhere (possibly inaccurate):

"In a complete reactor meltdown, the extremely hot (about 2700� Celsius) molten uranium fuel rods would melt through the bottom of the reactor and actually sink about 50 feet into the earth beneath the power plant. The molten uranium would react with groundwater, producing large explosions of radioactive steam and debris that would affect nearby towns and population centers."
That won't happen. There's not enough stored heat and decay heat energy to do this. The old GE BWR design is flawed by current standards, but not THAT bad.

Note that I would prefer to see these old BWR designs replaced with a more modern PWR design, or any Generation III reactor, really. I do not favor extending the operating permits beyond the original timeframe to squeeze out additional profits, which TEPCO has repeatedly done.
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Old 06-06-2011, 11:35 AM   #158
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Thanks, Nords, I appreciate all the clarification. However, there is one point I was hoping you could expand on just a little.

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[T]he uranium's fission neutrons usually fly out of a nucleus with too much energy to be absorbed by nearby uranium nuclei. The neutrons zip out of the fuel mass and don't sustain the chain reaction. The trick is to slow the fission neutrons down enough to allow nearby uranium nuclei to absorb them. This is done by letting the neutrons ricochet around enough to lose some energy.
Again, you're the expert, but from my limited understanding of fission physics, the Uranium nucleii are not "absorbing" the stray neutrons, but rather are being "shattered" by them. That is, the stray neutrons smash into the Uranium nucleii, "fissioning" the uranium atom into two, lighter isotopes, and releasing a spray of still more loose neutrons (and, of course, a tremendous amount of energy). These newly-freed neutrons smash into still other uranium nucleii, continuing the reaction, until there are no more uranium atoms left to fission, or the density of stray neutrons drops to the point where they're "missing" the uranium nucleii, and instead are just flying off into space.

When a sufficiently high density of stray neutrons is achieved and maintained, this results in a fission chain reaction.

I wasn't aware any neutrons were being absorbed in the process, or that there was a speed limitation on the stray neutrons. Can you clarify this misunderstanding?
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Old 06-06-2011, 02:08 PM   #159
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...from my limited understanding of fission physics, the Uranium nucleii are not "absorbing" the stray neutrons, but rather are being "shattered" by them.
Nope, the neutrons are actually absorbed. The new heavier nucleus then often decays by fissioning into two roughly equal mass nuclei, along with some free neutrons and gamma rays.

Uranium 235, for example, absorbs a neutron to become Uranium 236 with the nucleus in an excited state. About 82% of the time it will decay by fission, and about 18% of the time it will just dump the excitation energy as a gamma ray and the nucleus will return to a ground state with a half-life of around 23 million years.
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Old 06-06-2011, 08:28 PM   #160
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Again, you're the expert, but from my limited understanding of fission physics, the Uranium nucleii are not "absorbing" the stray neutrons, but rather are being "shattered" by them. That is, the stray neutrons smash into the Uranium nucleii, "fissioning" the uranium atom into two, lighter isotopes, and releasing a spray of still more loose neutrons (and, of course, a tremendous amount of energy). These newly-freed neutrons smash into still other uranium nucleii, continuing the reaction, until there are no more uranium atoms left to fission, or the density of stray neutrons drops to the point where they're "missing" the uranium nucleii, and instead are just flying off into space.

When a sufficiently high density of stray neutrons is achieved and maintained, this results in a fission chain reaction.

I wasn't aware any neutrons were being absorbed in the process, or that there was a speed limitation on the stray neutrons. Can you clarify this misunderstanding?
What M_Paquette said!

Neutrons are heavy & dense, but against other nuclei they behave more like ping-pong balls than like bullets. The neutron is "captured" by the U-235 nucleus, so the neutron has to have a narrow range of speeds (energy) and collision angles in order to get absorbed. The subsequent fission happens (if it's going to happen) so quickly as to be nearly simultaneous.

Some of the fission neutrons have too much energy, hence the need for the water acting as "moderator" (as in moderating the neutron's speed) to slow it down enough for another U-235 nucleus to be able to absorb it... and to also redirect it to those U-235 nuclei.

The high-energy neutrons that escape the core still have enough mass/velocity to damage the crystal lattice of the metal structures around them. And a few of them even escape the reactor compartment's shielding to interact with the watchstanders or their dosimeters.

Nukes wear dosimeters which are sensitive to gamma radiation. IIRC though nobody gave a darn about documenting neutron exposure, either from the reactor or from the nuclear warheads in the ICBMs or TOMAHAWK cruise missiles. I think the logic was that it was too low to matter (as far as we know!), but M_Paquette might have a better response to that.

Nuclear power school was six months long. Subjects included core physics, overall reactor plant physics, thermodynamics, materials, chemistry, radiation, "aspects of reactor plant operations", and electrical engineering. Students who showed up before the official start date had to either burn leave (highly inadvisable) or study two weeks of calculus & physics. Classes ran daily for eight hours (with a 30-minute lunch). Mandatory study was four hours per weekday (highly inadvisable) although most students put in 40 hours per week (yes, five-six hours/day in addition to class time). Weekdays I was churning & burning in the classroom from 0600-2200, packing in two meals and a change of clothes. Weekends were for studying, catching up on chores, and running errands. (I have no idea how the married couples dealt with this workload.) Exams were several hours long and were both essays & problem solving. Never a short-answer or multiple-choice question... the idea was to extract every bit of information in your head (the hard way) so that they could decide whether to pass or fail you. Some of the essay questions took an hour to answer.

That was just the classroom portion of the training. The shore-based reactor-plant training was another six months of qualifying on plant-specific systems and how to operate them. Most of the qualification process here was by oral interviews, so it was difficult to bluff your way through any of it. Then you spent about six-eight months on your submarine repeating the qualification process (usually on different gear)... all for the privilege of standing watch, running drills, and being inspected. By the time you got to the submarine, everyone was reasonably confident that you knew how to study, pass exams, manage your time, complete a qual card, and stand a watch.

Then the real training started!
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