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PostPosted: Jul 23, 2011 1:16 am 
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https://docs.google.com/viewer?a=v&pid= ... m&hl=en_US
A Swiss paper study.
https://docs.google.com/viewer?a=v&pid= ... m&hl=en_US
Russian prospective design.


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PostPosted: Jul 23, 2011 6:33 am 
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jagdish wrote:
https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B6OcA2av_W7hODkzM2JiMTctYWExYy00OWMxLTgzYjQtMWRkYmI3NGM3MzJm&hl=en_US
A Swiss paper study.
https://docs.google.com/viewer?a=v&pid= ... m&hl=en_US
Russian prospective design.


These are the same link


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PostPosted: Jul 23, 2011 6:44 am 
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Lindsay wrote:
Thanks Alex I do have that one, but I will go back and read it again. If you come across any other MCFR papers, please let me know as I am 'really interested in fast chlorides'.

Cheers


You're welcome.

By the way, as far you know, which are the actual limits of fast chlorides, besides obvious things like proliferation and/or lower tech maturity and experience vs fluorides ? I don't know, I mean chemical separation and reprocessing, material need and/or corrosion and so on ?

At least on paper, the fast chloride seems a very interesting option in terms of waste incineration (including some fission products, not only transuranics) and breeding capability (a breeding gain even of 0,6-0,8 is achievable with the plutonium-uranium cycle, even if a thorium blanket and a Th/U-233 cycle is more preferable) or both at the same time


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PostPosted: Jul 23, 2011 9:52 am 
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Chemical seperation is easier on the one hand because of the lower boiling point of chlorides; you can even distill thorium chloride quite easily. It does mean that more fission products are going to come along as well, but that's not such a big deal in a very fast spectrum.

Materials needed are very similar to fluorides. Corrosion mechanism is the same, fuel salt chlorides are slightly less stable than their fluoride counterparts - but so are the structural material chlorides so its not much of an effect really. The big exception on corrosion with chlorides is that some sulphur is made via neutron-proton reactions. It's not so bad with enriched chloride though, and sulphur is easily removed via online hydrochlorination (H2+HCl sparging).

Its possible to go for lower melting salts with chlorides so you might end up easier going with chlorides (lower temp operation means stronger metals).

The fast spectrum is demanding though so going for two fluid seems risky in that respect (blanket-fuel barrier deteriorating). On the other hand with a single fluid you will end up losing lots of neutrons to leakage. Can't have an undermoderated outer region as there is no moderator.


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PostPosted: Jul 23, 2011 1:35 pm 
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Alex P wrote:
which are the actual limits of fast chlorides, besides obvious things like proliferation and/or lower tech maturity and experience vs fluorides ?

My concern is that there may be some reactor control issues that tend to get swept under the carpet, so to speak.

We all know that solid-fuel fast reactors tend to have very small cores, compared to thermal spectrum reactors.
That's despite the fact that liquid sodium has a density similar to water and graphite (roughly speaking!), and quite low compared to liquid fuel salts like chlorides (and even fluorides) -- tending to increase neutron mean free path, and the minimum critical size.
But the fuel rod lattice in SFRs is very tight, which works in the opposite direction, and which tends to dominate (on top of that, some SFRs use metal or carbide fuels, to really boost density).

Water and graphite moderated thermal reactors typically have much larger minimum critical size than SFRs (at least with NU/SEU/LEU oxide fuel).
Even so, making them very big eventually runs up against operating stability problems, when the overall volume is large enough to include many critical lattice volumes (zones).

An MCFR would tend to have a small minimum critical size, due to high material density and short neutron mean free path. A graphite-free ("fast") LFTR would have a similar tendency.
Making either of these reactors "big" may be asking for trouble: Whereas "zone control" in large thermal reactors is readily feasible, due to the very long delayed neutron lifetimes (especially in HWRs), I bet that this is NOT feasible with fast reactors, due to short delayed neutron lifetimes. Hence the preference for several small "modular" cores, rather than one big one.

An interesting counter-example may be that of the lead-cooled fast reactor -- where the coolant density is, obviously, very high, leading to decreased neutron mean free path and minimum critical size.
Yet there are some Gen-IV proposals for large LFRs.
One will note however, that these large LFRs typically have a very different fuel rod lattice: very loose and open. This has the effect of increasing neutron mean free path and minimum critical size, thus allowing design of a (relatively) large but stable reactor core.

Another example relevant to this discussion is that of the bi-modal reactor -- which has fast spectrum inside fuel channels, because they contain a high-density material (such as molten fluoride salts - ideally free of moderating carrier salts like FLiBe) and because the channel diameter is made to be a significant fraction of the minimum critical size of a salt-only reactor (but actually far from critical on its own, due to low unit fissile loading, as compared to fast reactors).
At the same time, the thermal spectrum in this bi-modal reactor dominates the overall dynamics, making for a large minimum critical size, due to the sparse fuel channel lattice, in a low-density moderating medium, combined with the ease of operability that comes with long delayed neutron lifetimes.


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PostPosted: Jul 24, 2011 4:58 am 
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Jaro, why would you care about zone control in a homogeneous liquid fuel reactor? Zone control is important if you want to get the peak power density lower with the purpose of protecting the solid fuel+cladding (and/or graphite if that is your moderator). It seems unimportant for a molten salt only core. Getting a high power density in the center of the core with molten salt fuel looks attractive since it reduces leakage neutrons. Flux flattening is ineffcient.

Its true that control is always an important feature, but with dilatation coefficients as negative as with molten chlorides, what seems to be the problem?


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PostPosted: Jul 24, 2011 6:17 am 
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Cyril R wrote:
Its true that control is always an important feature, but with dilatation coefficients as negative as with molten chlorides, what seems to be the problem?

Instability.... flux/power oscillations....

....a brief power burst in a zone may extinguish itself by thermal dilatation, but the ensuing pressure wave to the surrounding fuel might cause a power burst there: in a fast spectrum things can happen quickly -- I'm thinking there might be a possibility of some nasty vibration.


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PostPosted: Jul 24, 2011 9:49 am 
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jaro wrote:
Alex P wrote:
which are the actual limits of fast chlorides, besides obvious things like proliferation and/or lower tech maturity and experience vs fluorides ?

My concern is that there may be some reactor control issues that tend to get swept under the carpet, so to speak.

We all know that solid-fuel fast reactors tend to have very small cores, compared to thermal spectrum reactors.
That's despite the fact that liquid sodium has a density similar to water and graphite (roughly speaking!), and quite low compared to liquid fuel salts like chlorides (and even fluorides) -- tending to increase neutron mean free path, and the minimum critical size.
But the fuel rod lattice in SFRs is very tight, which works in the opposite direction, and which tends to dominate (on top of that, some SFRs use metal or carbide fuels, to really boost density).

Water and graphite moderated thermal reactors typically have much larger minimum critical size than SFRs (at least with NU/SEU/LEU oxide fuel).
Even so, making them very big eventually runs up against operating stability problems, when the overall volume is large enough to include many critical lattice volumes (zones).

An MCFR would tend to have a small minimum critical size, due to high material density and short neutron mean free path. A graphite-free ("fast") LFTR would have a similar tendency.
Making either of these reactors "big" may be asking for trouble: Whereas "zone control" in large thermal reactors is readily feasible, due to the very long delayed neutron lifetimes (especially in HWRs), I bet that this is NOT feasible with fast reactors, due to short delayed neutron lifetimes. Hence the preference for several small "modular" cores, rather than one big one.

An interesting counter-example may be that of the lead-cooled fast reactor -- where the coolant density is, obviously, very high, leading to decreased neutron mean free path and minimum critical size.
Yet there are some Gen-IV proposals for large LFRs.
One will note however, that these large LFRs typically have a very different fuel rod lattice: very loose and open. This has the effect of increasing neutron mean free path and minimum critical size, thus allowing design of a (relatively) large but stable reactor core.

Another example relevant to this discussion is that of the bi-modal reactor -- which has fast spectrum inside fuel channels, because they contain a high-density material (such as molten fluoride salts - ideally free of moderating carrier salts like FLiBe) and because the channel diameter is made to be a significant fraction of the minimum critical size of a salt-only reactor (but actually far from critical on its own, due to low unit fissile loading, as compared to fast reactors).
At the same time, the thermal spectrum in this bi-modal reactor dominates the overall dynamics, making for a large minimum critical size, due to the sparse fuel channel lattice, in a low-density moderating medium, combined with the ease of operability that comes with long delayed neutron lifetimes.

Chlorides of thorium and uranium, besides lower melting points, have lower densities, less than half of oxides and more than a third of it chlorine.
If you have 7 or 19 fuel channels in a triangular pattern, it will ensure a significant but lower than critical size. You could provide them with expansion bulbs at end. They could be immersed in a coolant tank of clean salt or dense Pb-Bi or light Al-Mg eutectic. Size of the core can be adjusted with spacing of these channels.
The moderator, if you insist, can be Beryllium oxide or carbide balls immersed in the coolant.


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PostPosted: Jul 24, 2011 10:55 am 
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Thank you all.
Correct if I'm wrong, but it seems me that almost all the limits of a fast chloride is mainly in the fast/faster spectrum rather that in the nature of chloride salts vs fluoride itself, isn' it ? If it's so, why don't try to soften energy spectrum or for example to lower power density in some way ?


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PostPosted: Jul 24, 2011 12:14 pm 
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Much of the work on chlorides is theoretical and that which isn't is grossly optimistic about the materials challenge. So you see very fast spectrum (physicists like high breeding with U/Pu) and high power density (cool and looks economical).

With chlorides the spectrum can't be softened too much since you lose more neutrons to the salt. But with Th-U a less crazily fast spectrum can be used that isobreeds if you don't leak too many neutrons. U234 likes to fast fission so there is still an advantage to a faster spectrum.

One development pathway is to make the reactor single fluid and very big, both physically and in thermal output, and let the leakage neutrons be absorbed in B4C blocks surrounding the vessel, then hope that a non-flux flattened tank of salt core can break even on breeding.


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PostPosted: Jul 25, 2011 1:00 am 
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Alex P wrote:
Thank you all.
Correct if I'm wrong, but it seems me that almost all the limits of a fast chloride is mainly in the fast/faster spectrum rather that in the nature of chloride salts vs fluoride itself, isn' it ? If it's so, why don't try to soften energy spectrum or for example to lower power density in some way ?

There are some bona fide materials issues and we have yet to see a combination of materials demonstrated that will provide the low corrossion rates and longevity desired. That said I beleive that there are a number of promising lines of enquiry, but for the MCFR fans out there it is important to acknowledge that the materials for use with fluorides are far more mature with demonstrated materials options that haven't yet been demonstrated for chloride systems yet.

Jaro, thank you for your post on the potential for power fluctuations, that's an angle that I had not considered and we know that can happen with fast spectrum reactors (apparently Superphenix suffered from power fluctuations).

If we look at the core described by Taube in EIR 332, it has an outrageous power density of 750 kW/L, if you do the math you find the core volume is very modest 3.65 m3 for 2.8 GWt. That means for a temperature range through the core of 650/750C the circulation rate is 8.5m3/s on a 3.65m3 core, so every 0.43s the core volume is changed. Reading Jaro's post alongside those numbers it seems that power fluctuations could be an issue.

However if we look at the REBUS-3700 the power density is much lower at 100kW/L while still providing options for reasonable BR, by my estimate REBUS with a blanket could have a BR of up to 1.39. I would expect but cannot prove that with a lower power density core should have more inherant stability than a small 'hot rod' core producing a lot of power out of a very small volume. The other area for hope is that one should be able to model the stability of the reactivity response to temperature and determine if power fluctuations are likely. I beleive that that they did this for MSRE and got very good correlation between the modelled and experimental results (Edit: See Figure 4.4 of ORNL-4812 for details). So it would seems that the tools are available to examine that issue, and no need to build one first and then hope.


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