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PostPosted: Sep 17, 2016 5:57 pm 
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I'm hoping somebody can help me understand something. Assuming I have a vat of hot molten salt and that in this salt there are compounds of Pu239, U238, U235, U233, Thorium and various fission products. My simplistic thinking is that the compounds with the heavier atoms should tend to be near the bottom of the salt. And the scum (fission products) should tend to float towards the top. Even if the tendancy is only slight it still seems to be a somewhat reasonable assumption. And perhaps if we artificially increased gravity by applying some centrifugal force to the vat then the effect could be magnified.

Is this effect real? Is there any literature on this? Is the effect significant enough to influence the design of a MSR?


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PostPosted: Sep 17, 2016 6:53 pm 
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Even without forced circulation, convective circulation will dominate.
And while that is gravity driven, it overwhelms differences in molecular weight.
In other words, everything gets entrained in that flow.
Only gas bubbles may be light enough to stay at the top.


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PostPosted: Sep 17, 2016 6:55 pm 
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What if the heat source is at the top so convection is minimal?


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PostPosted: Sep 18, 2016 8:32 pm 
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TerjeP wrote:
What if the heat source is at the top so convection is minimal?


Let's think this through. How would the heat source end up at the top? The concern is that the heavier elements would collect toward the bottom, which may be a good thing because then you might get a greater chance of fission from the neutrons flying about. If the fission is at a greater concentration at the bottom, which may or may not be a safe assumption, then what is producing the heat at the top?

Can we assume the fission products will "float" to the top? You seem to agree we can. Would the decay heat be such that it exceeds the heat produced from the fission below? I recall that in a LWR it is something like 5% of the heat from the reactor comes from the decay of fission products. If we assume that this is similar for LFTR, and the heat and lower density of the material causes it to rise to the top, then I ask, how thick would this "skin" of fission products be? Is having these products floating to the top be bad in any way? Would it not in fact be beneficial? Many of these fission products would be fission poisons so having them move to the periphery should benefit neutron economy. If a neutron reaches the top of the salt, and is taken by a fission product, then there is no effective loss, right? If that neutron had not been taken by the fission product then it would have continued on and hit the containment structure or something.

I'd expect this "skin" to be quite thin since the quantity of the fission products should make up a small fraction of the total volume. The rest of the reactor salt below this "skin" is going to have convective forces acting on it, right? This convection is going to carry away some of this heat by conduction, right? If all of this is true then just how much hotter can the top "skin" be than the rest, and would this be of a concern? In thinking about this I have my doubts that this would be a concern.

I'm assuming that FLiBe salt is used to carry the fuel partly because of it's ability to conduct heat. It has to conduct something or it would be useless as a coolant. Just the conduction of the heat in the salt will not allow the top of the salt to be too much hotter than the rest. There will be some convection in some part of the salt, since it is a liquid. Further mixing will be done by the fission products floating to the top and the salt above being displaced down.

In order to allow for proper heat removal, since we are using this as a heat source, I see in many designs that the fuel salt is pumped through a heat exchanger. This will mix the fuel and spread the heat, likely with a much greater effect than any convection or conduction. In designs where the coolant fluid merely surrounds the reactor vessel this coolant movement will spread the heat as well.

You asked in your first post if this effect is significant enough to influence the design. Given what little I know of heat engine design I suspect the only effect it would have, if any, is in deciding which direction the coolant would flow. That decision depends on if the other aspects of the design call for keeping the salt temperature consistent throughout or if it is beneficial to allow the salt to develop a temperature gradient from top to bottom. That decision gets complicated real quick and I've pretty much reached the limits of my knowledge as it applies here.

I hope this helps and I welcome any corrections or counterarguments.

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PostPosted: Sep 19, 2016 9:42 pm 
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You seem to be leaping ahead to some assumed reactor design. I just said it was a vat. And I'm starting with the assumption that there is not any notable fission happening. I was interested in understanding if stratification occurs before contemplating how it might help or hinder a particular design. But for now it sounds like we are both guessing.


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PostPosted: Sep 20, 2016 12:20 am 
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TerjeP wrote:
You seem to be leaping ahead to some assumed reactor design.

Well, yes and no. I stated my assumptions, one where we assume a simple container where there is no mixing from pumping and another where there is.

TerjeP wrote:
I just said it was a vat.

You also asked if this would affect MSR design, so I assumed fission was occurring and heat being removed.

TerjeP wrote:
And I'm starting with the assumption that there is not any notable fission happening.

Well, that's not a reactor then, is it? :)
There would be heat generated still from radioactive decay, which I believe I stated in my assumptions. The heat from the top, like you suggested, plays well with the idea that the heating is from the lighter fission products. Conduction will still limit the heat differential. I'd think that convection would still happen if there is any heat removal, which would be difficult to prevent with a salt at such a high temperature. Heat will leak into the air, structure, or earth.
Wait a minute, is it safe to assume the vat is on Earth? :)

TerjeP wrote:
I was interested in understanding if stratification occurs before contemplating how it might help or hinder a particular design. But for now it sounds like we are both guessing.

Yes, I am guessing. I'd like to think that I'm making an educated guess. If the relative difference in mass was going to cause a significant stratification then I'd think this would be a show stopper on any MSR design. If my math is correct the mass difference between LiF and UF4 is 12x, depending on your perspective that is a lot or a little. Now that I think about it the stuff that floats to the top is not going to be the fission products but the lithium, assuming anything just floats to the top. The fission products would then end up in the middle somewhere.

Then you ask, what if the heat source is at the top so convection is minimal? A heat source at the top would likely induce more convection than prevent it. If we assume a vat of molten salt, where no pumping occurs, and no heat leaves that might cause convection, then stratification would quite likely occur. Even then the stratification is limited by mixing from Brownian motion. Heat generated from decay will mix things up too, and convection will happen as the heat from the decay moves things around.

The point is that the convection, pumping, cooling, heating, etc. that happens in a nuclear reactor is going to overwhelm any stratification tendencies. I recall Mr. Sorensen making the observation that the reactor core in a MSR will always be homogeneous. If there is fission then the core is going to be well mixed. If we assume that there is no fission then we can assume any stratification effect will not influence the design of a MSR. Is that a fair assessment?

I like your question as it is an interesting thought experiment. I'm not quite sure where this might occur in real life though. I suppose if someone were to dump some of the core salt into small (as in sub-critical mass) drums and let it sit in an extremely warm room then we might see this stratification effect as it slowly cools. Your question on how much stratification happens depends on a lot of factors. If we assume that there is any heat source at all, like fission product decay, and any cooling at all, such as to the air around the drum, then convective mixing will happen.

Again, as you point out, I'm guessing. If I've made a mistake somewhere then I'd appreciate a correction. I think this is a fun little experiment to discuss and don't take my lighthearted comments as anything other than trying to keep it fun for everyone.

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PostPosted: Sep 20, 2016 12:59 am 
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I'm thinking about isotopic separation. And yes it's a thought experiment.


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PostPosted: Sep 20, 2016 9:39 pm 
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TerjeP wrote:
I'm thinking about isotopic separation. And yes it's a thought experiment.


I recall reading about alloys separating out and how people deal with it. Depending on whether you want to maximize or minimize the effect one could take lessons from how it's dealt with there.

In casting a machine part there is a desire to keep it balanced, and separation of the different metals that make up the alloy is a problem. In casting a spinning part one way to keep it balanced is to have the axis of rotation vertical, the mass difference would be front to back rather than from one side to the other. Some more esoteric solutions I've seen is to do the casting in orbit or on the moon since gravity is lower. If you'd rather maximize the effect then go to Neptune or something, or use a centrifuge.

When it comes to controlling convective mixing I see that convection is usually controlled with barriers, but a total barrier in this situation would also prevent the isotopic separation. I can imagine a set of baffles in the tank that could keep convection to a minimum while still allowing heavier isotopes to sink.

When it comes to separating heavy elements like uranium the differences in mass are very very small. We're talking about ratios like 232/233 and 235/238. In that case I'd think that separation by using the half life differences would be quicker than waiting for the heavier isotopes to settle to the bottom. For example, if you want to separate U-232 from U-233 then put the salt in your theoretical vat and wait about 700 years and the U-233 will have settled to the bottom, because by then the U-232 remaining will be 1/1000th of what you started with.

There's an idea I'd like to play with, would it be faster to wait for the isotopes to separate out by their mass differences alone while molten and under 1G or to wait out the decay of the unwanted isotopes? This assumes the unwanted isotopes have a shorter half life than the wanted isotopes.

Just so I'm clear, I expect the rate of separation to be very very slow. So slow that the rate of radioactive decay becomes a factor. We are talking about mass differences that are very small, which means the difference in gravitational force acting upon them will be very small. The masses of the molecules are also quite large, as far as molecules go. With force equal to mass times acceleration the speed that these molecules will move in an otherwise undisturbed tank will be very very small.

Add into that equation that the molecules are at a high temperature, which means they are vibrating like crazy, then things get complicated. I don't know how to compute this exactly but I'd expect the rate of movement to be something like glacial, tectonic, or congressional.

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PostPosted: Sep 20, 2016 10:39 pm 
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Just for the record the surface gravity on Neptune is only a little higher than on earth at 11.15 m/s^2. This is in spite of the fact that the mass is 17 times the earths. People often think that large planets are going to have a proportionally larger surface level gravity but it is not the case. Even on the surface of Jupiter the gravity is only 2.5 times that on the surface of the earth.

I'm not sure the process would be as slow as you suggest. If you pour water into oil it will sink immediately based on density. But density is not the only consideration with various ionic bonds between the respective molecules.


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PostPosted: Sep 20, 2016 11:00 pm 
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Perhaps relevant is a question that was asked on Quora:

Is there more deuterated water at the bottom of an ocean or deep lake?
https://www.quora.com/Is-there-more-deu ... -deep-lake

The question is similar to the one proposed here, for the purpose of isotopic separation.
If we look at it from an entropy standpoint, the entropy of mixing is greater than the change in potential energy from gravity.

Edit: Forgot to add link!


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PostPosted: Sep 20, 2016 11:35 pm 
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TerjeP wrote:
Just for the record the surface gravity on Neptune is only a little higher than on earth at 11.15 m/s^2.

I did not expect my comment on going to Neptune to be taken seriously, merely as an example of the absurd means one might propose to reach the desired ends. Much like I would not expect my comment on Congress moving at the rate of glaciers to be taken seriously, as that would imply anything moves in Congress.

TerjeP wrote:
I'm not sure the process would be as slow as you suggest. If you pour water into oil it will sink immediately based on density. But density is not the only consideration with various ionic bonds between the respective molecules.


The difference is that oil and water do not mix while the different isotopes of uranium would. Comparing this to heavy water vs. light water, as Mr. Hughes suggests, would be more appropriate. If one is able to control the heat in and out of this vat very very carefully it may be possible to separate the isotopes by controlled freezing. The heavy isotopes would freeze before the light isotopes and fall to the bottom but this effect is also very small.

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