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PostPosted: Aug 13, 2010 1:03 pm 
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We often read in documents about how slow fuel reprocessing can be for a very fast spectrum chlorides reactor. This begs the question whether very fast chloride reactors can be built without fuel reprocessing at all (single fluid of course) while still breeding at least breakeven. Certainly for the denatured U/Pu cycle, but, given that Pa-233 and U-234 have significant fast fission cross sections, a combined U/Pu Th/U cycle might also be attractive. David LeBlanc's calculations on breeding ratio for thermal/epithermal DMSR were quite promising.

I think this setup has some advantages in not having to develop chloride fuel reprocessing. The electrochemical processes we have today isn't very impressive, its messy and very lossy on plutonium. Chlorination also has some issues due to UClx volatility overlap. One might still want noble stuff extraction, notably offgas removal and noble metal plateout out of core, but even they are not as bad compared to a thermal reactor (especially xenon). And likely one would use very pure enriched Cl-37 for neutron budget, longterm radwaste and sulphur corrosion reasons (I think the chloride seperation problem is manageable if you look at state of the art isotopic seperation technology).

So, is it possible to get to CR=1 at equilibrium in this setup, burning NU or DU as fertile?


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PostPosted: Aug 13, 2010 1:29 pm 
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Interesting thought. I would imagine you still want to remove the Kr, and Xe - though this might be simply to use some jets in the pump space and collect the gas above it. I also think providing a deliberate place for noble metal plate out would be better than letting it plate out all over the pipes and HX.

The salt-seekers will build up. True that they don't have the same impact and we could process more slowly. It may be possible (actually I'd guess likely) that you could get iso-breeding over a 60 year life but the fuel salt at the end would need to be cleaned before using in the next reactor. Is this a substantial benefit?

A single fluid fast reactor means we have higher leakage. MSBR's trick of having a lesser moderated outer area won't work if the inner section isn't moderated. Most reflectors are moderating and would tend to cause a spike of fissions with any reflected neutrons causing more fissions at the edge and thus more neutrons lost. No blanket to soak up those neutrons means we lose them. In a fast spectrum we've got some extra neutrons to lose so it could be OK. The wall will be stressed though and you can reduce that stress only gradually by making the reactor core larger. This exacerbates the problem of such a reactor requiring a large initial fissile inventory - likely around 1/2 of how much an IFR needs (and 4-8x what a LFTR needs). No idea of the material that handles lots of fast neutrons, hot chlorine with a thermal gradient, and fission products.

Seems like significant technical risk compared to LFTR.

One benefit is that it can consume TRUs.


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PostPosted: Aug 13, 2010 2:35 pm 
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Quote:
The salt-seekers will build up. True that they don't have the same impact and we could process more slowly. It may be possible (actually I'd guess likely) that you could get iso-breeding over a 60 year life but the fuel salt at the end would need to be cleaned before using in the next reactor. Is this a substantial benefit?


I think it is substantial. The conservative thing to do is to assume the fuel salt to be fully depreciated with zero salvage at the end of its useful life. It can then be stored. If good techniques have been developed by then to reprocess, it gets better, if not its ok and you base the business model on that.

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A single fluid fast reactor means we have higher leakage. MSBR's trick of having a lesser moderated outer area won't work if the inner section isn't moderated. Most reflectors are moderating and would tend to cause a spike of fissions with any reflected neutrons causing more fissions at the edge and thus more neutrons lost. No blanket to soak up those neutrons means we lose them. In a fast spectrum we've got some extra neutrons to lose so it could be OK.


True, though there are non-moderating reflectors compatible with the fuel salt such as steel, tungsten, etc. Materials durability for any fast reactor are a big issue though I also think that there are sufficient extra neutrons as long as the spectrum is kept very fast (based on Taube and others). I'm not too worried about the high fissile load for this reactor, the cost @ 10 dollars per gram fissile is like 0.1 cent/kWh levelised (10% discounting).


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PostPosted: Aug 13, 2010 3:55 pm 
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I don't like the idea of no recovery at the end of the reactor life. The inventory would be a HUGE waste source if we don't recycle it. However, my impression is that chloride processing would be similar to fluoride processing (perhaps easier).

One concern is that the melting point of UCl4 is 590 and boiling at 790C. It probably changes once its dissolved but that is a narrow working range and I think creates some safety concerns that would have to be worked.


The biggest issue is that a chloride reactor is a few decades behind the fluoride one in terms of actual experience so you'll need a strong incentive push ahead with chloride over fluoride.


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PostPosted: Aug 13, 2010 5:31 pm 
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I don't know if chloride reprocessing is easier than fluoride, but it is in some ways better known. The IFR program used NaCl/KCl eutectic for electrochemical recovery of TRUs from the spent fuel. The process probably works just as well with fluorides, but for a mix of U + TRU chlorides, there is no doubt.

One of the advantages of chloride systems is that the trichlorides of both the lanthanide fission products and the actinides are lower melting and much more soluble in chloride carrier salts than the equivalent trifluorides. If you don't like UCl4 for being to volatile, use UCl3, mp 837C, Bp 1657C

There is also the issue of 37CL(n,2n) 36Cl. Small X-section, but there's a lot of Cl, and 36Cl is nasty - long-lived and very soluble.


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PostPosted: Aug 13, 2010 7:20 pm 
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Luke wrote:
I don't know if chloride reprocessing is easier than fluoride, but it is in some ways better known. The IFR program used NaCl/KCl eutectic for electrochemical recovery of TRUs from the spent fuel. The process probably works just as well with fluorides, but for a mix of U + TRU chlorides, there is no doubt.

One of the advantages of chloride systems is that the trichlorides of both the lanthanide fission products and the actinides are lower melting and much more soluble in chloride carrier salts than the equivalent trifluorides. If you don't like UCl4 for being to volatile, use UCl3, mp 837C, Bp 1657C

There is also the issue of 37CL(n,2n) 36Cl. Small X-section, but there's a lot of Cl, and 36Cl is nasty - long-lived and very soluble.


In many ways, besides the Cl-36 problem, which in my mind is a show stopper, chloride is more problematic for reprocessing than fluoride.

People tend to look at reprocessing as a "one size fits all" case, all electrochemical, all solvent extraction, all this, all that. This is not particularly useful in my view.

The wonderful thing about fluoride salts as opposed to chloride salts, besides the obvious advantage of there being lots of volatile metal fluorides is that there are so many water insoluble fluorides.

You can't beat that.

Although many people like to talk in exotic terms, there is still no solvent quite like water in the universe.

It happens that I'm an atheist, but if there were anything in the universe that would make me want to believe in God, the chemistry of water would be it.

I still insist that a chloride reactor is a very bad idea, and wholly unnecessary.

The big advantage of fluid phase reactors is in my view, that one can avoid the terrible waste of neutrons that is involved in leaving fission products in a mixture, a feature that would be wholly negated by any scheme to leave them in for any reason.


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PostPosted: Aug 13, 2010 7:33 pm 
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NNadir wrote:
The big advantage of fluid phase reactors is in my view, that one can avoid the terrible waste of neutrons that is involved in leaving fission products in a mixture, a feature that would be wholly negated by any scheme to leave them in for any reason.

You must be reading my mind 8)

....plus, its not just the loss of neutrons, but also discarding the advantages of separating the issues of reactor operation from radionuclide hazard management -- the tried & true strategy of "divide and conquer".

PS. the chloride fast reactor protagonists can point to the undeniable fact that fission products have a far smaller impact in a fast spectrum than in a thermal one -- this has been proven with solid-fuel SFRs....


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PostPosted: Aug 13, 2010 9:13 pm 
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jaro wrote:
You must be reading my mind 8)

....plus, its not just the loss of neutrons, but also discarding the advantages of separating the issues of reactor operation from radionuclide hazard management -- the tried & true strategy of "divide and conquer".

PS. the chloride fast reactor protagonists can point to the undeniable fact that fission products have a far smaller impact in a fast spectrum than in a thermal one -- this has been proven with solid-fuel SFRs....


Well, people are always acting as if fluorine is a great moderator. It really isn't.

Its mass number is 19.

Its nuclear properties aren't dramatically different than sodium, with a similar scattering cross section and a mass number of 23.

In historical fluid phase reactors, to thermalize the system, they sank graphite in the salt.

I like the fast spectrum, I really do. I regard it as essential to the complete utilization of our uranium resources. I just don't like the chloride ion for this purpose.

(I know that this is the thorium website, and I like thorium, but I'm not prepared to throw the baby out with the bathwater. Uranium is truly a renewable fuel, and I'm not sure that thorium is quite as renewable as uranium is over many millenia.)

Some fission products have quite strong absoption resonance absorptions in the fast and epithermal regions and none of them are risk free in from a neutron economy standpoint in any spectrum.


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PostPosted: Aug 14, 2010 1:31 am 
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NNadir wrote:

People tend to look at reprocessing as a "one size fits all" case, all electrochemical, all solvent extraction, all this, all that. This is not particularly useful in my view.

The wonderful thing about fluoride salts as opposed to chloride salts, besides the obvious advantage of there being lots of volatile metal fluorides is that there are so many water insoluble fluorides.

You can't beat that.

Although many people like to talk in exotic terms, there is still no solvent quite like water in the universe.

It happens that I'm an atheist, but if there were anything in the universe that would make me want to believe in God, the chemistry of water would be it.

I still insist that a chloride reactor is a very bad idea, and wholly unnecessary.

The big advantage of fluid phase reactors is in my view, that one can avoid the terrible waste of neutrons that is involved in leaving fission products in a mixture, a feature that would be wholly negated by any scheme to leave them in for any reason.

Talking of solubilities or lack of it, how to separate the actinides from fission products? The best bet I have come about is use of 3.5N HCl to dissolve lanthanide s and leave thorium and uranium undissolved in rare earth extraction. I am of the opinion that it can be done in wet ground SNF with reverse concentrations of the two. Chlorine and hydrochloric acid are also less corrosive to metals and ceramics. So the chlorides have a lot going for them in reprocessing chemistry.


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PostPosted: Aug 14, 2010 2:37 am 
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NNadir wrote:
jaro wrote:
You must be reading my mind 8)

....plus, its not just the loss of neutrons, but also discarding the advantages of separating the issues of reactor operation from radionuclide hazard management -- the tried & true strategy of "divide and conquer".

PS. the chloride fast reactor protagonists can point to the undeniable fact that fission products have a far smaller impact in a fast spectrum than in a thermal one -- this has been proven with solid-fuel SFRs....


Well, people are always acting as if fluorine is a great moderator. It really isn't.

Its mass number is 19.

Its nuclear properties aren't dramatically different than sodium, with a similar scattering cross section and a mass number of 23.


Elastic scattering isn't the issue, its inelastic. Chlorine is very low, while fluorine, iodine and bromine are all very significant. This means your neutrons will quickly lose their fast spectrum and accumulate likely somewhere in bad resonance regions. To get the fullest from a fast spectrum you need a homogeneous very fast spectrum, because you want to get to the eta and fast fission ‘thresholds’. There are actually medical treatment moderators employing fluorine (IIRC Teflon) to quickly get out of the MeV region.
I agree that fluoride reactors are a far more attractive preposition, but if you want to make an 800 kEv average neutron energy reactor, I’m not convinced it can be done with fluorides. I just wanted to discuss details for a theoretical very fast reactor with no fuel reprocessing.

Quote:
Some fission products have quite strong absorption resonance absorptions in the fast and epithermal regions and none of them are risk free in from a neutron economy standpoint in any spectrum.

Yes but the total effect will be much less worse for a VERY fast spectrum. You still get to remove most of the noble stuff, a large portion of fission product absorptions, and you get the usual fluid fuel advances of dilatation coefficients and high power density.


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PostPosted: Aug 14, 2010 2:57 am 
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Luke wrote:
I don't know if chloride reprocessing is easier than fluoride, but it is in some ways better known. The IFR program used NaCl/KCl eutectic for electrochemical recovery of TRUs from the spent fuel. The process probably works just as well with fluorides, but for a mix of U + TRU chlorides, there is no doubt.

One of the advantages of chloride systems is that the trichlorides of both the lanthanide fission products and the actinides are lower melting and much more soluble in chloride carrier salts than the equivalent trifluorides. If you don't like UCl4 for being to volatile, use UCl3, mp 837C, Bp 1657C

There is also the issue of 37CL(n,2n) 36Cl. Small X-section, but there's a lot of Cl, and 36Cl is nasty - long-lived and very soluble.


Don't really like the IFR, its messy and not as efficient as the advocates claimed it would be. But maybe its better than nothing, and for sure getting 60 year life out of your fuel inventory helps a LOT with wast volume compared to LWR. And yeah, NaCl/MgCl2 and UCl4 have no solubility limits, and IIRC neither does PuCl3. There is a NaCl (MgCl2) UCl4 eutectic which melts at 368 degrees C. This means you can operate at lower temperature as well, say 500-650 C min-max. The thing to look out for is what the fission product 'salt seekers' are going to do to melting temperature.

I checked the NNDC sigma plots, ROSFOND database, it has Cl-36 in its library. n,2n for Cl-37 doesn't show up at all. The neutron,gamma cross section for Cl-36 is bigger than Cl-37, though not as big as Cl-35. The question is what is the equilibrium concentration of Cl-36 if you leave the salt in the reactor for a very long time. May be that it is very tiny if you start up with high purity Cl-37. You need to do that anyway, to avoid major sulphur corrosion problems and improve the neutron economy. And I don't yet buy the isotope enrichment is expensive argument, with all respect to NNadir, if you look at cost/kWe or kWh, most enrichment processes are very affordable. If not we'd only be seeing graphite moderated natural uranium reactors and the opposite is the case. Axil had some cool stuff about laser enrichment too.


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PostPosted: Aug 14, 2010 3:05 am 
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Also, what happens to the fission product 'salt seekers' if you leave them in the reactor for such a long time? They will tend to transmute to heavier (unstable) stuff, and then what? Will we end up with much different elements once they are stable (or stable-ish)?


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PostPosted: Aug 14, 2010 3:30 am 
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Hmm, you know this thread brings up more questions than answers :lol:

Re the vapor pressure of UCl4. Thinking about the very high temperature still idea that Luke had for fluorides, could this work for chlorides? The higher vapor pressure could mean lower still temperatures, especially for a U/Pu chlorides cycle (no thorium chloride). PuCl3 is also lower boiling point than PuF3. If we use Jaro's vacuum siphon idea, the vapor pressure can also be used as a safety mechanism, ie boiling unpriming the siphon. There wouldn't be any heavy water, obviously, and a very tight lattice, with the tubes perhaps just being steel.


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PostPosted: Aug 14, 2010 3:38 am 
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Interesting! Thorium chloride is quite low melting and a lot more volatile than I would have expected:

http://en.wikipedia.org/wiki/Thorium%28IV%29_chloride


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PostPosted: Aug 14, 2010 9:28 am 
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Since 37Cl(n,2n) reaction isn't in any of the NNDC databases I have to presume that the reaction is extremely rare so I don't think I'd shy away from the chloride based reactor for that reason.

I don't think the fission products will alter the melting/boiling point of your liquid significantly since even after 60 years they will still be a minor portion of the salt (roughly 300 kg/yr * 60 yrs or 1.8 tonnes while the fissile portion will need to be on the order of 10 tonnes and the fertile around 80 tonnes and then I presume you have some dilutant beyond this).

I would still be concerned that UCl4 wants to evaporate on you. This seems to be troublesome to me.

The other area to watch for is controlability. Since your spectrum is fast your prompt neutron lifetime is very short. Since the spectrum is very fast the proportion of fissions that happen with slow neutrons presumably is very small so likely you have almost no negative temperature coefficient contribution from resonance. I'm guessing any mechanical control system will be too slow for this style of reactor. If the generational lifetime is shorter (or even about the same) as the time for the salt to expand out of the core region then where does the control for this reactor come from? In particular, in the safety studies for LFTR the extreme (on paper only) test is to suddenly stop all the fuel pumps. Did Taube do safety studies of the chloride reactor? What is the control mechanism?

Actually I can't think of a way to suddenly (1mS) stop the flow - in fact I would guess cutting all power to all the pumps simultaneously would still take several seconds due to shear inertia of the salt (your moving something like 10 tonnes of material per second). So maybe the test case is bogus.


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