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PostPosted: Oct 29, 2008 12:59 pm 
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jaro wrote:
You could actually just take the heavy metals (U + TRU) from LWR spent fuel, convert them to fluorides and stick them in an


I was thinking about fluorinating U out from SNF and using only the TRUs in a fast small compact high temperature unit.
Is that out right crazy or no? I know that we dont have precise x-sections for TRUs yet, but that is being measured as we speak :)


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PostPosted: Oct 29, 2008 1:26 pm 
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I'm sure that would be feasible, but you'd be throwing away a lot of good U, with fissile content higher than natural.
That is the remaining enrichment investment: Less than HEU from weapons disposition, but not all that much less, since the biggest effort is at the low end of enrichment.
Once you're up to a few percent, there is much less material to process....


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PostPosted: Oct 29, 2008 4:21 pm 
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What did you intend to do with the fission products - were you thinking of just putting them into the MSR as well?

In the chemical processing, generally we have been thinking about extracting the good stuff and sending the rest to waste. But what if we extract only the worst of the fission products and send everything else back to the reactor? At least for a faster spectrum there are only 5 of them worth extracting and removing 50-75% would be sufficient. Any chemists out there who could comment on removing Cs, Ru, Rh, Tc, and Pd (Ru, Rh and Pd being part of the platinum group)?


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PostPosted: Oct 29, 2008 4:48 pm 
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I think you can get Ru, Rh, Tc, and Pd all out by fluorination.

Which is pretty simple.


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PostPosted: Oct 29, 2008 5:24 pm 
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With a faster spectrum the fission products become transparent faster than the fissile or fertile fuels. There are ideas floating of very slow reprocessing (16-30 years) to reduce the cost of the chemical plant. If we can easily remove most of the really absorbing fission products perhaps we can extend the salt lifetime to match the reactor (60 years) and the local chemical processing plant is easier. The nice thing about removing the fission products this way is that removing 75% of them is almost as good as removing 99.9999%. Any idea how well we can separate any fluorinated U, Np, and Pu from the fluorinated fission products?


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PostPosted: Oct 30, 2008 12:45 pm 
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What is the minimum core salt volume that can support 500MWt in the LFTR and what does the associated reaction look like?

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PostPosted: Oct 30, 2008 2:00 pm 
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I would expect the core salt itself to scale pretty well. The French are using 20m^3 for a 2.5GWth plant. I think the core salt then would be something like 4m^3. I suspect that going more power dense is problematic for the heat exchanger. They budget 1/3 of the fuel salt out of the core at any moment. If you go below this controllability will be reduced since you will lose a larger portion of the delayed neutrons - still might be OK but needs to be checked.

The blanket still needs the same thickness in one dimension to usefully absorb the neutrons - the French use a 40cm thick blanket. I'm inclined to think it would be better to double this and skip separating the Pa. You'll notice that the minimum size is getting set by the blanket. An alternative might be to use a neutron reflector outside the blanket and use a thiner blanket. Warning I have not seen any information on the efficiency of neutron reflectors - just that they are used and they do reduce the critical mass required for bombs.


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PostPosted: Oct 30, 2008 4:34 pm 
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Lars wrote:
Warning I have not seen any information on the efficiency of neutron reflectors - just that they are used and they do reduce the critical mass required for bombs.


well you only need to know cross-sections~ f(E_neutron), in this case most importantly neutron scattering x-section. This is known quite well. Many reactor designs use reflector movement for reactivity control, IIRC Toshiba's S4 has a moving reflector to reach long life time between refueling; and EBR-1 only used its reflector for reactivity control, no rods, if I recall correctly.

This may be very useful for LFTR, as we can vary blanket salt concentration, changing efficiency of the reflector.


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PostPosted: Oct 30, 2008 6:19 pm 
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so can I estimate neutron losses in the reflector as (absorption cross section ) / (absorb + scatter) or is it more complicated than that?


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PostPosted: Nov 05, 2008 9:56 pm 
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I am confused by the thread. Is and E-M pump for Fl salts possible? Useful?


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PostPosted: Nov 06, 2008 10:28 am 
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Lars wrote:
so can I estimate neutron losses in the reflector as (absorption cross section ) / (absorb + scatter) or is it more complicated than that?


It's more complicated than that. Reflectors can do some non intuitive things. As most know, they can increase reactivity and lead to less fissile concentration needed in the core but at the same time they actually increase neutron leakage. That fact surprises many and it is often "rediscovered". Reflectors help reflect back neutrons hitting the outer edge but they also increase the power density near the edge which means a lot more neutrons trying to leave (a lower fraction, but a greater total number succeeding). In a core that is not reflected the outer edge power density can be quite low which also can sometimes help with the lifetime of the outer vessel.

In 2 Fluid designs with fully encompassing blankets we don't need to worry about reflectors. In the French TMSR work that only has a radial blanket they have to deal with lots of issues due to their axial reflectors.

David L.


Last edited by David on Nov 06, 2008 10:33 am, edited 1 time in total.

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PostPosted: Nov 06, 2008 10:32 am 
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robert.hargraves wrote:
I am confused by the thread. Is and E-M pump for Fl salts possible? Useful?


I don't think we actually know how possible or practical they'd be. I am not sure if the conductivity of the salts is known well enough to tell either way. It would be very nice if we could use them though as they should be pretty much maintenance free.

David L.


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PostPosted: Nov 06, 2008 1:26 pm 
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David wrote:
robert.hargraves wrote:
I am confused by the thread. Is and E-M pump for Fl salts possible? Useful?


I don't think we actually know how possible or practical they'd be. I am not sure if the conductivity of the salts is known well enough to tell either way. It would be very nice if we could use them though as they should be pretty much maintenance free.

David L.


Quote:
I am not sure if the conductivity of the salts is known well enough to tell either way.


There has been extensive research work done in the electrical conductivity of sodium, potassium, and lithium fluoride salts at high temperatures (1000C – 2000C) done for the aluminum smelting and refining industry. They use high electric currents to separate pure aluminum from aluminum ore at high temperatures. And many components of aluminum ore is close to LFTR salt is its composition.

At the beginning of this thread I referenced a research article on fluoride salt electric conductivity by the aluminum industry.

http://www.electrochem.org/meetings/sch ... 4/3003.pdf

IMHO, EM pumps should be a top priority in LFTR design. It enables the use of double wall pipe were the core salt flows down the center of the pipe and the blanket salt surrounds it on the outside layer, see

viewtopic.php?f=3&t=1014&p=10029#p10029

This double wall pipe configuration allows the following advantages:

The core salt heats the blanket salt to help achieve isothermal heat distribution throughout the total reactor salt volume. This makes for a very safe LFTR design.

Double walled pipe provides maximal thermal mass and inertia for the salt reaction. This is very important if we want to downsize the amount of salt to fill a very small LFTR (100mw).

The blanket salt protects the core salt from overheating or freezing, and in general minimizes rapid salt temperature fluctuations in the piping, i.e. heat flashing or cold plugs.

This double wall pipe captures all neutrons that are produce in the core salt when in circulation in the piping system effecting increased U233 production. Besides the capture of these delayed neutrons, it also reduces external reactor radiation including residual gamma radiation.


Since the pipe material: diamond, SiC, and molybdenum are paramagnetic, magnetic pumping can move both core and blanket salt in the double pipe at the same time to a common heat exchanger (see post viewtopic.php?f=3&t=1017) thereby reducing the number of pumps and pipes required by a factor of ½.

Double wall pipe protects the core salt from leaking due to primary salt pipe wall failure.

This new LFTR design paradigm will not be recognized and seriously explored by the conservative members of the LFTR design community. The priority is to get a working LFTR out in use as opposed to providing a dominant competitive through more risky technical solution.

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PostPosted: Nov 06, 2008 4:29 pm 
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And again: Low inertia! If you lose power to the pump the salts stop circulating more suddenly (so a Loss Of Flow accident becomes essentially alike to a blocked rotor accident). This results in a high and fast insertion of reactivity...which is an unwanted effect and you have to make sure that this won't result in any potential damage. Furthermore it's much easier to proof to and to convince a safety organization that the possibility for such a fast increase is very unlikely (for rotating pumps only in blocked rotor accident).

If you want to have a prototype MSR design, go conservative!

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PostPosted: Nov 06, 2008 4:33 pm 
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STG wrote:
And again: Low inertia! If you lose power to the pump the salts stop circulating more suddenly (so a Loss Of Flow accident becomes essentially alike to a blocked rotor accident). This results in a high and fast insertion of reactivity...which is an unwanted effect and you have to make sure that this won't result in any potential damage. Furthermore it's much easier to proof to and to convince a safety organization that the possibility for such a fast increase is very unlikely (for rotating pumps only in blocked rotor accident).

If you want to have a prototype MSR design, go conservative!


Depending on the strength of the temp coefficient of reactivity, the amount of positive reactivity inserted by a pump stopping may be rather untroubling, at least in terms of the mean temperature increase. I'm sure this situation was faced during the operation of the MSRE. What do the documents indicate?


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