Nuclear dry cask have vents to keep the spent fuel cool. When the bottom vent is covered by water???

even if it isn't following the same route of circulation.

The fuel is still hot. It heats up the air. The hot air leave through the top vents and cooler air enter at the bottom.

When the bottom is coverd the air flow stops.

That's why they are working so hard to keep it flowing on the cores at Fukushima.

1% Carbon steel 43 watts per (metre x degrees Kelvin)

Stainless steel 16 W/(m K) at 25 degrees C

Medium Density concrete 0.4 - 0.7 W/(m K)

High density concrete 1.8 W/(m K) at best

Water 0.58 W/(m K)

Water has a high heat capacity whilst still being liquid. This makes it good for active cooling. It is very poor for passive cooling unless convection currents remove the heat rapidly. If those flasks are only semi-submerged there will be very little in the way of convection. Even if the flasks were fully submerged, because the system is designed for cooling using air, they will not set up efficient convection currents because water is far more dense than air.

Source: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

How is that substantially different than what I wrote in 10 words?

"Steel and concrete are good thermal conductors, so is flowing water."

The topic was carrying heat away from the fuel - not a ranking between steel, concrete and flowing water. As I understand it, the specific threat is a river with a 15mph current, therefore the inclusion of the word "flowing".

But I am grateful for the details; thank you. BTW, air is 0.024.

...but I'm with Kristopher on this one. Just look at the numbers:

Water 0.58 W/(m K)
Air 0.024 W/(m K)

Water conducts heat twenty times better than air. If the casks are designed to be sufficiently cooled by simple air convection, there is simply no way that they can overheat with two or three feet of water flowing around them. My bet is that the casks will actually be cooler than normal due to the flooding.

There is a huge difference between conductivity and heat capacity. Water has vast hear capacity so, if flowing, it can cool. Air has low heat capacity but can move rapidly transferring the heat elsewhere, it is also very easy for air to set up the convection currents that would carry heated air elsewhere.

If the flasks are semi-submerged convection would stop.

If the flasks are fully submerged convection of water in the very confined space between inner and outer flask would not be sufficient to transfer the heat from the flask to the environment.

Now add in the problems of corrosion ...

For some reason you are assuming that the water is not coming into contact with the inner flask. I think that is a poor assumption.

I have outlined the possibilities precisely and in the semi-submersed situation it does not matter whether water is in contact with the flask or not - convection, the primary mode of cooling - will shut down.

Conduction is not sufficient to cool these flask because water, air and concrete are abysmal conductors of heat.

Anytime there is a temperature differential in a fluid there will be convection. You simply cannot have a situation where water is in contact with the hot inner flask and not have convection currents created that will carry the heat away. That's just simple fluid dynamics.

Fluids near thermal equilibrium do not have strong convection currents because there is not sufficient energy differential driving that mechanism. The air in semi-submerged flasks approaches thermal equilibrium because no cooler air enters the bottom of the casing. There will be a very minor, turbulent, exchange of gases at the upper vents but that will be insufficient to provide cooling. The water at the bottom will not have sufficient area in contact with the air inside the flask to provide cooling. In addition there concrete casing will already near thermal equilibrium and so will not cool the air.

Simple experiment. Take a pipe, about 4' of 3/8 copper plumbing pipe should do, stand it vertically and place a water jacket round it. Insulate the outside if the water jacket and the pipe. Fill said jacket with boiling water and wait about 5 mins. Now shine a very strong light across the top of the pipe and project it onto a screen. The convection currents should be seen clearly in the projection of the shadow. If you have difficulty forget the projection just, gently, shake cornflour from a sifter neat the upper end of the chimney. Now put a bung in the bottom of the pipe - and see what happens to the strong flow of air through the pipe.

I understand your example, but it does not describe the situation the casks are in. The casks have several vent holes at the bottom, and those vent holes have been covered by flood waters. People here have assumed there is two or three feet of water, and although I haven't seen a source for that assumption let's just assume that is accurate. What will happen is this: water will flow in through the vent hole and come into contact with the warm inner flask, creating a temperature differential between the water in contact with the inner flask and all the water flooding the cask area. This temperature differential will create a convection current and cause water to flow in and out of the vent holes until all the water in the area reaches the same temperature of the inner flask--a near impossibility give the amount of water involved. The convection current will work best if water can flow in one vent hole and out another, but it is impossible to see from the pictures if the is possible. Even if it is not, convection currents will still be created in which water flows in the bottom of the vent hole and out the top. I suspect that even the simple movement of water in and out of the vent holes will be sufficient to keep the inner flask cool, but just for grins let's assume it's not. What will happen then is the water will heat up until it boils (100C), steam will rise up and exit the vent holes at the top of the casks, and new cold water will replace what has boiled off. At the very worst then, the temperature of inner flask cannot exceed 100C by very much. That temperature is well within the design parameters of the casks.

However, vent blockage has been identified as a probable cause of cask failure.

I can't imagine steam and steel at those temperatures as being good for corrosion. How long will the steel hold up in that type of environment? Who knows???? What if the water blocks the vents but does not reach the steel???

What if the water only reaches the bottom inches of the steel and it boils? Will is sound like a train steam whistle? or maybe a boat? Or will it go Pop Pop Pop as the steam forces the water back out the bottom. Maybe it will just simmer with steam coming out the top vents.

Any way you look at it, its not something I want to see on the evening news.
Double, double toil and trouble;
Fire burn, and caldron bubble

From the reports linked to elsewhere in this post I think the key is how long the casks have been there. Apparently due to the declining nature of radioactive decay, the casks put out 100 times as much heat in their first year as they do in subsequent years. As a result the question of heat being a problem is entirely dependent on how long ago the casks were place, and no one has posted anything about that.

I agree with your comments on corrosion and I certainly hope after the waters recede the casks are inspected for damage.

Convection currents flowing horizontally - are these flasks standing in a gravity anomaly so that the less dense water flows sideways?

The rest of your post is just as science free.

Maybe I'm wrong, but it looks pretty much like the situation we have with the casks. The heat source on the left is similar to the inner flask, and what they are calling a "sill" creates a geometry that matches up pretty well with the idea of the vent holes. It's titled "Horizontal Convection with a (small) Sill"

http://www.youtube.com/watch?v=SF53agDbvDo&NR=1

I wrote showing that steel and concrete are far from "good" thermal conductors. You want "good" see the equivalent numbers for copper and silver.

I also identified that water is a very poor thermal conductor but that because it has a high heat capacity, for a liquid, it can cool if flowing. In addition I then pointed out that in a semi-submersed situation neither water nor air would generate the convection currents needed to cool the flasks. It is also doubtful that the water would provide any cooling effect if the flasks were fully submerged because the cooling ducts in the flasks are designed to allow gaseous convection currents not liquid ones. At no point did I comment about the thermal conductivity of air, which is very poor, but if I had I would also have noted that the low density of air allows for rapid heat transfer by convection in appropriately designed channels.

Let me now give you the obvious conclusion, if these flasks are partially or fully submerged they will heat up. There will be insufficient convection to provide a cooling effect because either the channels are blocked to air flow or because they are full of water which will not be able to circulate properly to provide a cooling effect. In addition the concrete outer case will transfer very little heat to any water flowing outside the flasks because concrete is a poor thermal conductor.

Now stop trying to obfuscate by confusing conductivity with heat capacity.

As an afterthought it is likely that the flasks will be damaged to a greater or lesser extent after the floodwaters subside. Silt and detritus will block any vents at the bottom of the flask and possibly even the internal channels. In the event of water reaching the inner flask it may well be that corrosion will have occurred.

If the current material is "air" and that's replaced by something else... the comparative conductivity is what is relevant. Not how that material compares in conductivity to other materials that are not involved.

If you want to use the term "good" as a comparative feel free but it does not detract from the initial falsehoods and the shoddy and misleading use of perfectly sound physical principles and parameters.

1) Air, water and concrete are poor conductors in absolute terms (see the original link);
2) In absolute terms steel is a fair conductor but no more than that;
3) The important issue is NOT conductance but heat transfer.

For effective heat transfer from the steel inner flask to the outside world you need movement of the working fluid, whether that is air or water. These flasks have been designed to be cooled by air (the working fluid) that is convecting, they have not been designed with water as the working fluid.

A) If water or silt is blocking the lower air vents then there will be little of no convection to transfer the heat from the inner flask, i.e. heat will build up and the temperature of the inner flasks will rise.

B) If water replaces air as the working fluid there will be little or no convective heat transfer because the flasks are not designed for that method of cooling, i.e. heat will build up and the temperature of the inner flasks will rise.

D) If semi-submerged conduction will not keep the flasks cool (transfer heat) because air is a very poor conductor, i.e. heat will build up and the temperature of the inner flasks will rise.

E) If fully submerged the rate of heat build-up will be marginally slower because water is a marginally better conductor of heat than air, i.e. heat will build up and the temperature of the inner flasks will rise.

When the bottom holes are blocked by water no or very little air circulation is happening so the container inside and contents are heating up. That passive cooling is stopped.

There's the movement of air around the cask itself and the new (unintended) passive cooling of running water being in contact with the bottom few feet of the cask.

instead of air and last I remember water is a better median for transferring heat from one place to another than air is.
I take anything you're replying to me with a grain of salt mr FBaggins cause I find you to be so, well, I'll not say for fear of getting this removed. You seem to be only interested in lessening the fact that nuclear energy is not a good choice and you care less about the facts of the matter.

Have a good day anyway
I plan to

Ok... "full of water" meaning "entirely under water far enough to circulate".

But while water is a better conductor of heat than air is, only a portion of the cask is submerged. So the upper portion would only benefit from the water as that heat conducts through the fuel/steel/concrete.

One factor that's missed by many is that the heat that a given substance will conduct away isn't a constant, it's also a function of the difference in temperature. So the hotter the fuel gets, the greater the amount of heat that will be removed. But the amount of heat generated by nuclear decay in that spent fuel is (effectively) a constant. So you eventually reach a new equilibrium temperature at a higher point.

I take anything you're replying to me with a grain of salt mr FBaggins cause I find you to be so, well, I'll not say for fear of getting this removed. You seem to be only interested in lessening the fact that nuclear energy is not a good choice and you care less about the facts of the matter.

Sorry. You've constantly misread me to that end. I don't agree that "nuclear is not a good choice", but I don't debrudge others that opinion. My comments are usually focused on correcting innacurate spin when it gets too wild or when the science behind the post is just flat wrong. In this case what we've seen it the opposite effect and I'm sure that you can see it. Some people are so obsessed with their fear of nuclear power that anything nuclear is perpetually on the verge of killing hundreds of thousands of people. Flood waters rise to the level of some spent fuel casks and my goodness... the BEST case scenario is that they'll be shifted out of position and probably need to be hunted downstream. Did you see the site that said that Fort Calhoun was equal to twenty Fukushimas (which the author claimed was itself equal to twenty Chernobyls)?

The plant has been shut down for months and some people really think we're looking at 400 Chernobyls?

The fuel can go up to 450C...about 840F

There isn't a chance that they get anywhere close to that unless they're blocked for multiple weeks (their vent-blockage scenario assumes 20 years).

And even that temperature wouldn't do a thing to damage the fuel, the steel, or the concrete.

If you read the citation from your link, they make clear "None of the blocked vent or other thermal scenarios resulted in conditions that could fail the MPC."

FBaggins, you've been wrong throughout the Fukushima disaster. IIRC, It's INES 7, baby.
The NRC and the nuke industry as a whole has been wrong throughout the Fukushima crisis. It's never supposed to happen. It's not being mitigated.

So...

Do I trust your assessment of things? Nope.
Do I trust their assessment of things? Nope.

This is the problem with an industry the deceives the general public.
"We've got studies...", yeah, whatever.

:rofl:

The probabilistic risk assessment indeed looked at a 20 year period for the casks, but when they looked at blocked vents they determined the elevated steady-state temperatures would be reached in about 25 days or so:



This is from page 4-14 (p. 73 of the .pdf file) of http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1864/">NUREG-1864, "A Pilot Probabilistic Risk Assessment of a Dry Cask Storage System at a Nuclear Power Plant".

Nevertheless, my skim of this report suggests that the most likely accident scenerios they identified involved dropping casks. The simulations of blocked vent scenarios and 3-hour external fires did not predict loss of overall cask integrity.

The one from 2010 merely references the one from 2007. It doesn't add anything to it.

And they didn't say it would cause a failure, they said that event was one of the things they needed to look into... and they did.

is over 1600 degrees F, and it doesn't melt then.

Drying of the concrete is a concern, but that is a long term issue. Strangely, being partially submerged in water doesn't really help the long term drying out thing all that much, according to my civil engineering co-workers.

Not even after multiple decades.

Can't make it any simpler for you.

The independent spent fuel storage installation (ISFSI) facilities I am most familiar with are the horizontal cask systems by Transnuclear (NUHOMS-24P and
NUHOMS-32PT). Neither is designed as anything near a permanent solution, and have an estimated life of about 100 years. Additional temperature rise due to loss of convective cooling will reduce this working life somewhat, but it is an effect that will manifest itself in decades, not weeks or months.

During power operation, the fuel has a centerline temperature of between 1600 and 1800 degrees F, depending on the Fuel Element Assembly's physical location in the core. The fuel surface temperature is usually between 530 and 580 degrees F, with an average of around 550 degrees F.

Or at least it was for me.

Other issue is you will still have some air circulation due to thermal expansion. Hot air rises and displaces cold air, so some air will be drawn in from the top when the lower vents are covered. If the fuel gets hot enough to boil, the resulting steam generation will then leave from the upper vents, draw additional water in through the bottom, and maintain the fuel cell assemblies at less than ~240F due to conduction heat transfer. Need to hit around 2800F to start worrying about cladding failure, so we're highly unlikely to see that.

I'm much more concerned about corrosion, but they likely have sacrificial anodes attached to handle that issue.

I don't think it will melt, just damage the cask in some way. Particularly, if it was left that way for weeks.

It would probably be better if it was submerged completely.


This link is working. It worked when I previewed it.
http://spectrum.ieee.org/energywise/energy/nuclear/case-for-accelerating-dry-cask-storage-of-spent-nuclear-fuel-

Steel and concrete in suffiecient amounts (and of sufficient quality) to take a strike by a train...


...what temperature do you think it takes to damage same?

Point is, water at atmospheric pressure boils at 212F, which means that no component inside it will be capable of going above 212F plus whatever temperature gradient is established in order to transmit that heat to the film layer of the water. I estimated it at 240F, considering the dramatically higher melting point for steel, the fuel cell assemblies, and the other cask components, there is unlikely to be any deformation caused. This of course assumes that the cask is capable of reaching that temperature due to conduction heat transfer and decay heat generation balance. Heat will still pass through the cask materials into the surrounding materials without needing air convection heat transfer, and if the ambient is kept at 60-80F that is a permanent heat sink even without air convection.

TL;DR: Even if the interior which is cooled by air circulation does start to heat up, it won't be able to heat up enough to damage the cask.

Does 2 + 2 = 4 yet?

:banghead:

You live with it.

http://en.wikipedia.org/wiki/Palo_Verde_Nuclear_Generating_Station

Kind of hypocritical, isn't it? :shrug:

Maybe you sould take a cask. Just make sure no rats make a nest and block the air flow.

It must be burning its way through those casks right now!

What happens when it gets to the water table???

:rofl:

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for storing the spent fuel.

Kind of hypocritical, isn't it? :shrug:

We have a replica of the Eiffel Tower, slot machines, and washed-up entertainers.

Now give us the Colorado River and the right to whine about someone else's problem, which we helped create.

Boo Frigin Hoo! :D

and force Nevada to take it

and have taxpayers foot most of the bill

yup