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PostPosted: Sep 24, 2012 3:24 pm 
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Cthorm wrote:
Is there a particular reason why the secondary loop needs to be a salt? Why not a liquid metal like gallium (large liquid range, low vapor pressure, low thermal expansion, no worry of freezing)?


Gallium dissolves nickel (and most other metals). So that rules out Hastelloy N, Incoloys etc.

It's compatible with molybdenum though. Should be compatible with SiC and carbon-carbon composites as well.

One thing with gallium is that it's quite rare, whereas a second loop needs a lot of coolant (hundreds of tonnes/GWe). This may rule it out if we want to build terrawatts of LFTRs.


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PostPosted: Sep 25, 2012 1:54 pm 
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Cyril R wrote:
Cthorm wrote:
Is there a particular reason why the secondary loop needs to be a salt? Why not a liquid metal like gallium (large liquid range, low vapor pressure, low thermal expansion, no worry of freezing)?


Gallium dissolves nickel (and most other metals). So that rules out Hastelloy N, Incoloys etc.

It's compatible with molybdenum though. Should be compatible with SiC and carbon-carbon composites as well.

One thing with gallium is that it's quite rare, whereas a second loop needs a lot of coolant (hundreds of tonnes/GWe). This may rule it out if we want to build terrawatts of LFTRs.


Well those are pretty good reasons. Thanks. But if you use SiC or carbon-carbon composites, I imagine that greatly opens up the materials options...maybe even the liquid form of some really cheap metals like zinc, cadmium, lead, aluminum etc.


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PostPosted: Sep 25, 2012 3:32 pm 
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Cthorm wrote:
Cyril R wrote:
Cthorm wrote:
Is there a particular reason why the secondary loop needs to be a salt? Why not a liquid metal like gallium (large liquid range, low vapor pressure, low thermal expansion, no worry of freezing)?


Gallium dissolves nickel (and most other metals). So that rules out Hastelloy N, Incoloys etc.

It's compatible with molybdenum though. Should be compatible with SiC and carbon-carbon composites as well.

One thing with gallium is that it's quite rare, whereas a second loop needs a lot of coolant (hundreds of tonnes/GWe). This may rule it out if we want to build terrawatts of LFTRs.


Well those are pretty good reasons. Thanks. But if you use SiC or carbon-carbon composites, I imagine that greatly opens up the materials options...maybe even the liquid form of some really cheap metals like zinc, cadmium, lead, aluminum etc.


Not aluminium probably, but yes all the poor metals would be candidates. Lead is in my opinion the most realistic, cheap and good, though lead eutectics offer the advantage of melting point reduction. There are some low melting eutectics of lead, tin and cadmium, mp. slightly under 100 C. Lead-tin is also good, if cadmium's toxicity is a problem.

These coolants also have the advantage of low chemical reactivity with air and steam, and would be easily removed from the primary loop as well (in the event of a leak).


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PostPosted: Sep 25, 2012 5:57 pm 
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"These coolants also have the advantage of low chemical reactivity with air and steam, and would be easily removed from the primary loop as well (in the event of a leak)."

Those are the main reasons why I think a non-salt secondary loop would be useful. FLiBe isn't cheap, and any mixing of coolant with the fuel salt would effectively require replacement of the coolant salt. Ideally the secondary coolant avoids these issues and provides extra protection against things like coolant freezing & errant tritium.


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PostPosted: Sep 26, 2012 8:32 am 
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Another advantage of at least some liquid metals is they won't mix with the salts if there is a leak as they are immiscible and will separate. Lead does this and ORNL took a pretty hard look at using it, even as a direct contact method of heat transfer. In this mode the entrainment of salt into lead or visa versa can give big headaches. Having lead on the other side of a heat exchanger or as a buffer liquid is a different story (but still similar corrosion issues as many materials OK for the salt are not OK for lead).

David LeBlanc


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PostPosted: Sep 26, 2012 9:24 am 
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David wrote:
Another advantage of at least some liquid metals is they won't mix with the salts if there is a leak as they are immiscible and will separate. Lead does this and ORNL took a pretty hard look at using it, even as a direct contact method of heat transfer. In this mode the entrainment of salt into lead or visa versa can give big headaches. Having lead on the other side of a heat exchanger or as a buffer liquid is a different story (but still similar corrosion issues as many materials OK for the salt are not OK for lead).

David LeBlanc


ORNL's work was usually brilliant, but they must have been on serious drugs when considering the direct cooled steam/lead idea, maybe they had something to celebrate and had a wild party.

In any case, a lead coolant only works under the condition that carbon-carbon or SiC HXs are available. Lead does not react with carbon, neither do fluorides, which is nice. If such HXs are not available, then lead coolant doesn't fly.


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PostPosted: Sep 26, 2012 3:27 pm 
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Perhaps, but if you've got a national lab full of smart people don't you want them trying new promising things like MSR even if it seems a bit whacky.


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PostPosted: Sep 26, 2012 4:21 pm 
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Lindsay wrote:
Perhaps, but if you've got a national lab full of smart people don't you want them trying new promising things like MSR even if it seems a bit whacky.


Yep, we'd be better off if the national lab system still had that risk-taking attitude. Direct cooling could be a big advantage if it turned out to be workable: fewer complex heat exchangers, fewer places for systems to break down. Now we have labs working on ADSRs b/c they have public relations advantages, despite severe physics/mechanics/economic disadvantages.


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PostPosted: Sep 26, 2012 4:25 pm 
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Lindsay wrote:
Perhaps, but if you've got a national lab full of smart people don't you want them trying new promising things like MSR even if it seems a bit whacky.


Yes, and promising meaning having obvious potential to solve problems - not obviously creating many more problems to solve one problem that doesn't exist! Some things are just the worst options even before expensive R&D. Trying to cool a nuclear reactor with gasoline for example. Direct contact lead cooling is another of those ideas. Sure it might work, but you lose all the safety and economic advantages of the MSR: low pressure core, high temperature, etc.


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PostPosted: Sep 27, 2012 6:28 pm 
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Cyril R wrote:
Yes, and promising meaning having obvious potential to solve problems - not obviously creating many more problems to solve one problem that doesn't exist!

Direct contact lead cooling is another of those ideas. Sure it might work, but you lose all the safety and economic advantages of the MSR: low pressure core, high temperature, etc.

I have no opinion to offer on direct molten lead cooling, although it does sound interesting, my key point was that I am amazed at the things that the national labs were able to achieve in the 50's and 60's some of which were completely and absolutely bonkers, yet amazing; my all-time favourite being the Tory IIC 500 MWth nuclear ramjet http://en.wikipedia.org/wiki/Project_Pluto Completely bonkers, arguably quite insane, but wonderful engineering and perfect execution. In 2012 we talk about high temperature reactor design, but on 14 May 1961 the Tory IIC air-breathing nuclear ram jet ran 500 MWth at ~1350C (IIRC) for 5 minutes in the desert at a place called Jackass Flats (I sometimes wonder if there is more to that name than just geography). Staggering, amazing, wonderful, quite terrifying and definitely bonkers. More please.

I hope that you appreciate the irony of this comment "...promising meaning having obvious potential to solve problems - not obviously creating many more problems to solve one problem that doesn't exist!"

We have MSR tech as consequence of someone trying to build a nuclear powered strategic bomber, one could reasonably argue that was a problem that didn't need fixing, yet look at what it has given us.

That said, for anything MSR I generally agree with KISS and to that end my efforts are focussed on the engineering solutions that provide the simplest, cheapest and most robust solutions that do not require the development of new stuff, just applying what we have already learned. (Shame on me for being so boring, but then I don't have the resources of a national laboratory). The good news is that I seem to be making progress.


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PostPosted: Sep 28, 2012 12:30 pm 
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I don't see the irony. MSRs are useful because of their inherent advantages as reactors, whether or not a nuclear bomber is a bit crazy.

Direct lead cooling is not useful at all. It doesn't solve any major problem, it only produces new ones and enlarges existing ones. You need a high pressure vessel to contain both lead and steam. There's the lead carryover problem into the turbine. What is the advantage? No steam generator HX surface area. This is not much of an advantage at all, since you still need a high pressure vessel to contain the steam, which is actually a lot harder than lots of little thin steam tubes. Sometimes in trying to solve 1 problem, you end up with 20 new problems, while not really solving the 1 problem in the first place. It reminds me of the floating roof oil storage tanks in my business. They were designed to prevent explosive gas areas, but they produce so many safety and operational problems that it simply isn't worth it. If you can solve all the problems relating to floating roofs, then you can easily solve the primary problem (explosive gas areas) in the first place.

It's just silly. Running a good R&D program is not about trying everything. It's about making choices. When all other choices are better than the last, drop the last and go along with the other options. This is part of an efficient and effective R&D program.


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PostPosted: Sep 28, 2012 8:49 pm 
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Not to defend poor old lead but I was only mentioning the idea of direct contact between primary fuel salt and lead so you get rid of the primary heat exchanger that otherwise would become quite activated. Like ORNL I'd assume after that you are going through a traditional heat exchanger to go from lead to steam. Not that I'm saying even that is a good idea...

David L.


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PostPosted: Sep 29, 2012 2:40 am 
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David wrote:
Not to defend poor old lead but I was only mentioning the idea of direct contact between primary fuel salt and lead so you get rid of the primary heat exchanger that otherwise would become quite activated. Like ORNL I'd assume after that you are going through a traditional heat exchanger to go from lead to steam. Not that I'm saying even that is a good idea...

David L.


The issue with the PHX activation is more that the secondary coolant activates. The HX itself isn't a big deal. Put shielding around it. It needs shielding anyway in normal operation.

Lead would help a lot with activation. But please, put a HX barrier in between the first and second loops. Even that makes me uncomfortable. I'd like another HX after that, with a truely nonradioactive liquid coolant, especially for an open cycle turbine.


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PostPosted: Oct 11, 2012 11:56 am 
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Strange that I've also been mulling over the idea of heat exchange with an open gas turbine for a generator. In limited-size applications (probably <50MW); especially for mobile generators where water may be difficult to access for e.g. steam production and as a heat sink. They could replace the larger diesel generators and smaller gas-turbine plant in areas where fuel supply would be difficult for one reason or another.

While the Brayton cycle "likes" to be as hot as possible at the turbine inlet, it's always limited by the highest temperature that the turbine blade material will tolerate under high stress, for a reasonable time.

When jet engines first went to the skies en masse in the 1940's, their turbine inlet temperatures were around 700°C (e.g. Jumo 004A) to 775°C (Jumo 004B), where in the latter, the blade material's rated temperature was already exceeded. Thermal efficiency with that sort of temperature is "poor" compared to what can be achieved by other means.

If one can get the heat exchange from primary to secondary salt and then secondary salt to air good enough to achive that sort of temperature, it'll do for a start. As the simplest basis for providing generating plant in an area where providing access to a larger power station is unfeasible. Direct-contact heat exchange seems the most effective, researched heavily for solar-thermal generating plant. It would inherently pressurise the secondary salt to approximately the compressor outlet pressure. (That may not be a bad thing.)

Keep in mind that the exhaust stack temperature is well over 300°C so one can add a heat recovery system for e.g. process heat where required. Even a steam generator for a coupled steam turbine may be viable in some locations.

As materials develop and permit higher salt temperatures, cycle efficiency can improve.


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PostPosted: Jan 23, 2013 4:09 pm 
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I think we should either dream small, and use stainless steel or nickel running a comfortable Rankine cycle. Maybe even steam cylinders to keep the capital costs down.

Or, dream big: With TZM (molybdenum alloy) components, the upper limit is around 1350C. This seems like more than enough to drive combined-cycle air/steam turbines.


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