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 Post subject: Gilding the LFTR?
PostPosted: Oct 28, 2015 11:42 am 
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Ok, maybe a dumb question but here goes:

I keep hearing about a "corrosion problem" with materials used for MSRs. Has anybody ever studied the use of gold as an inerting layer atop the vessel and piping material? I mean, the value of the output would certainly pay for the small amount of gold needed.

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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 28, 2015 1:51 pm 
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Coatings work pretty well in theory, but in practice end up being scratch prone, that then provides a corrosion interface. The real question then is does the resulting local corrosion de-laminate or otherwise degrade the coating. Actual cladding, typically weld-overlaid sheet at least 3 mm thick, is a safer bet. Gold is too expensive for such thickness, of course, so you typically use nickel or some high nickel alloy as a realistic compromise. From the chemistry it looks like copper is even better as a cladding, but it may have other issues for high temperature salts in a high flux environment.

ORNL used gold plated apparatus for all kinds of aggressive salt-chemistry stuff. Gold is much nobler than even pure nickel, so it can take the nasty stuff like fluorinations, much better.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 28, 2015 1:57 pm 
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I should probably point out, likely repeating myself, that I'm not convinced of any 'corrosion problems' for MSRs. The Hastelloy N from the MSRE showed very shallow surface cracking from fission product tellurium, and even then only when badly strained. And when ORNL operated a bit more reducing (more UF3) the cracking was >99% reduced. In fact when such chemistry control is employed even kitchen-sink grade stainless steel show nil corrosion. The whole corrosion argument appears to be a prejudice. After all, its salt, and as we all know salt water is corrosive. People forget that there is no water in the molten salt reactor and that close chemistry control (that you could never do when dealing with an open ocean or sea system), combined with suitable stainless or higher alloy construction, makes a world of difference.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 28, 2015 6:12 pm 
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MSR are run at temperatures between 550 and 850°C. In case of a failure of the pumps the temperature in reactor might increase due to fission product heat by 150°C above the operational temperature quickly.

Gold is a very soft material. In the temperature range mentioned its mechanical properties are insufficient.

An interesting alternative for plating are platinum or palladium. According to my knowledge palladium has the better mechanical properties within this temperature range.

Are there any reports about using palladium or platinum for platings on nuclear rector components?


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 29, 2015 12:30 am 
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Let us think in terms of an intermediate, an enamel. I wonder which would be better, nickel fluoride or silicon carbide?


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 29, 2015 2:14 am 
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How about ceramics?

I've been doing a bit of work with ceramic seals and these are: Very hard, chemically inert (maybe not in fluorides), and operate at very high temperature.

Silica is another option. That would also allow radiative heat transfer. Or an enamel as pointed out above.

If the temperature of the lining and heat-exchanger can be increased, can the molten salt temperature also be increased? I'm trying to see if the secondary loop can reach 850C to allow thermochemical production of hydrogen. The primary salt in the moltex reactor is over 1000C, but I believe the secondary salt is quite a bit cooler, and that keeps the steel tubes within operating range.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 29, 2015 6:06 pm 
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Dear Alex,

in general ceramics are very brittle compared to metal. Temperature changes might cause cracks and the cracks does not stop in ceramics as in ductile metals. That is the main reason that ceramics are not more popular.

The most popular ceramic for construction is SiC. It has a good heat conductivity and is not that brittle. But it is according to some tests not resistant to chloride and fluoride salt corrosion.

Plansee in Austria developed an extremly corrosion resistant ceramic coating for molybdenum alloys...SIBOR. It contains boron. Boron will generate under neutron radiation alpha/helium particles.

The most corrosion resistant ceramic is alumina (AlO3). It is extremly brittle and has it has a different coefficient of thermal expansion as the base material it will easily crack.

That is the main reason to look for noble metal coatings.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 30, 2015 5:39 am 
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Holger - they are brittle compared to metals, but maybe good enough.

SiC is used for hardness - the second hardest material after C. But we're looking for corrosion resistance.

Our capabilities in working with glass have improved significantly over the last decade or so. Could silica glass work?

Enamel coatings are an option. Normally this is done with low temperature glass, but how about vapour deposition of pure silica onto a steel vessel? That way the steel can be chosen for temperature and neutron performance (Hastalloy?) with the glass handling the corrosion.

Or how about electro plating with titanium?


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 30, 2015 5:44 am 
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KitemanSA wrote:
Ok, maybe a dumb question but here goes:

I keep hearing about a "corrosion problem" with materials used for MSRs. Has anybody ever studied the use of gold as an inerting layer atop the vessel and piping material? I mean, the value of the output would certainly pay for the small amount of gold needed.


Coming back to the original question, the University of Manchester has looked at this. The result of their research is that corrosion (at current MSR temperatures) appears not to be a problem as long as the salt is kept slightly electronegative. This can be done by adding Zirconium (as planned by Moltex).

It may be an issue for high temperature MSRs for fuel production - but that isn't being looked at yet.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 30, 2015 8:33 am 
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Regarding silica glass - that will not work in a fluoride environment. I've worked in a fused quartz facility, and HF for example eats quartz - and there will be some amount of HF and fluorine in the salt(s). Also, the oxygen will be freed and we do not wish to have that in the system either.

I believe that titanium is not an option either, it forms titanium fluoride very easily, and in fact I think I've read in the ORNL reports about Ti alloying elements in Hastelloy being leached out.

All that said, I believe that controlling the redox environment of the salts will eliminate/greatly reduce what little corrosion was found by the MSRE. I'm not a fan of introducing Zr into a thermal breeder since it absorbs too many neutrons, but I do like using Be for redox control. I think ORNL used Be turnings sitting in a basket in the salt, and I can picture perhaps a bundle of thin-walled Be tubes inside a section of pipe to perform the same task.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 30, 2015 11:37 am 
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As a Chemist, I find HF absolutely terrifying.
It eats through almost anything - including silica and borosilicate glasses (ie. Pyrex).

We don't have 'fluorine martyrs' for nothing.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 30, 2015 12:44 pm 
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Agree glass is no good with FLiBe. HF is used to etch glass. But maybe with NaBe.

The Zirconium addition is planned by Moltex who are looking at Fast spectrum. The amount was very low.


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 Post subject: Re: Gilding the LFTR?
PostPosted: Oct 30, 2015 12:55 pm 
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Hi Alex,

the electrodeposition of palladium and platinum on stainless steel, molybdenum.. is done in the manufacturing of catalysts.

Holger


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 Post subject: Re: Gilding the LFTR?
PostPosted: Nov 01, 2015 2:01 pm 
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Location: Iowa, USA
KitemanSA wrote:
Ok, maybe a dumb question but here goes:

I keep hearing about a "corrosion problem" with materials used for MSRs. Has anybody ever studied the use of gold as an inerting layer atop the vessel and piping material? I mean, the value of the output would certainly pay for the small amount of gold needed.


Another dumb question:
I seem to recall that noble metals produced in fission were a problem in early MSR testing. The metals would tend to collect in low spots in the piping as well as plate the cooling pipes, causing restricted flow. Do I recall correctly?

I was under the impression that the corrosion of containment in MSRs was largely solved. I seem to recall talks on MSRs where they've done testing and were able to show a number of materials with acceptable corrosion, the problem was more of finding a supplier willing to make enough product at a low enough price to produce a full scale reactor.

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