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PostPosted: Aug 20, 2010 10:16 am 
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Reactors require very high purity fuels and materials to limit parasitic neutron capture and their many ill effects. We often hear the quote that while fluorides have well established purification methods, such as fluorination-hydrofluorination, chlorides do not have established methods to get to sufficiently high purity. Apparently chlorination-hydrochlorination does not achieve the required purity levels. However, chlorine cations readily displaces fluorine cations. So, one possible refining method would be to first do the fluorination/hydrofluorination trick, then chlorinate the purified fluorides. Trace fluorides shouldn't be a major problem for a fast chlorides reactor, as the materials of construction will be similar (online chlorination is unlikely since overlapping UClx volatilities and lower boiling points favour vacuum distillation).


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PostPosted: Aug 20, 2010 11:13 am 
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1) Sorry, chlorine does not displace fluorine - other way round, F>Cl>Br>I for stability of salts / reactivity of elements

2) Fission produces almost every element up to gold in some trace quantity. However clean you get the salt to start with, it won't stay that way for long. Since all the Cl will have to be subjected to isotope separation anyway, chemical contaminants will be a relatively minor problem / cost compared to >99.99% 37Cl, or whatever is determined to be acceptable for avoiding 36Cl production

3) If for some reason you have to purify further, borrow from the semiconductor industry and zone refine the salts. Not cheap, but it's only once in the reactor lifetime - and I'd be surprised if it turned out to be necessary.


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PostPosted: Aug 20, 2010 12:41 pm 
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Luke wrote:
1) Sorry, chlorine does not displace fluorine - other way round, F>Cl>Br>I for stability of salts / reactivity of elements

2) Fission produces almost every element up to gold in some trace quantity. However clean you get the salt to start with, it won't stay that way for long. Since all the Cl will have to be subjected to isotope separation anyway, chemical contaminants will be a relatively minor problem / cost compared to >99.99% 37Cl, or whatever is determined to be acceptable for avoiding 36Cl production

3) If for some reason you have to purify further, borrow from the semiconductor industry and zone refine the salts. Not cheap, but it's only once in the reactor lifetime - and I'd be surprised if it turned out to be necessary.


Thanks for your response Luke.

1) Who said the reaction would be exothermic? Gibbs figures of the salts are close enough for the chlorides/fluorides for practical production I'm guessing. Wrong?

2) Fission doesn't produce every element stable enough to matter much. There are also some impurities (various oxides if memory serves) that would be small concern in fission product yield, but are present in the chlorides. The noble stuff is probably the easiest to capture. The offgas stream gets out the xenon and krypton, and probably a sizeable amount of the volatile salt seekers - which tend to be more volatile than their fluoride cousins (eg zirconium chlorides). Noble metals can be made to plate out outside the core where we want them to. Perhaps more important is we don't worry too much about neutron burden of fission products in a very fast chloride reactor. But we do care about oxygen and sulphur impurities.

3) Very interesting. Is this done at all for halides? It thought it was just for (semi) metals. But maybe there is no reason why it couldn't be used for the halides?


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PostPosted: Aug 20, 2010 2:05 pm 
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Hmm, perhaps Luke is right. If we're bothering with high purity chlorine-37, then its not a big bother to buy high purity sodium and make ourselves some NaCl rather than digging or dissolving the impure stuff from salt layers. This should not be an issue for the fuel salt in terms of economics. The secondary coolant would be an issue, though fluorides would be fine there, so we can save costs. It would still be too bad if we can't make very pure chlorides cheaply though, because I want to use them for low cost high temperature thermal energy storage. But maybe they can be pure enough with protective coatings on the structural materials.


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