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PostPosted: Oct 15, 2008 12:42 pm 
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Nathan2go wrote:
David, I noticed that in your recent presentations, you have snuck in a second potential solution to "plumbing problem". At least as I interpret the nature of the problem, the problem is not so much that the two-fluid reactor plumbing is un-buildable, but rather that it has poor service life (particularly designs which use graphite). It's basically a "first wall" problem of neutron damage (hence the clever single fluid design with no wall between core and blanket).

That suggests that graphite moderation (and/or complicated plumbing) is fine as long as we provide a solution that makes replacement fast and affordable. Your recent slides state that lowering the temperature (to 600C or so) would extend the life of graphite and allow use of stainless steel, which to me sounds a lot cheaper to replace than carbon-carbon (the most expensive part of the Space Shuttle's skin) or nickel-based hastelloy (asssuming the whole reactor is discarded with the graphite moderator). Sounds like a winner to me! I would think graphite blocks in a steel tube would last longer (because it could be allowed to degrade more) than a graphite tube that had a structural function as well as moderating.

Can you talk a little more about why you don't like graphite anymore? Isn't it the case that the graphite's lifetime is only somewhat shorter than other barrier materials (like hastelloy)? Isn't it more dangerous (for spills etc) to have the fissile load so high as to not require moderation, particularly in a long-skinny core?


Nathan

In term's of Oak Ridge's original "plumbing problem" of complex graphite tubes interlacing the two fissile and fertile salts, it is much more than the limited graphite lifetime that causes a problem. The graphite shrinks and then expands and the changing volume fractions of the two salts within the core was a nightmare to design for. So the term "un-buildable" you use is probably true for the traditional Oak Ridge graphite Two fluid design.

In general I have tended to drift away from favoring the use of graphite but I am certainly keeping the door open. I am also very interested in simple Single Fluid designs running as converter reactors (no fuel processing but don't quite break even), for those concepts, it is likely that graphite might be used.

Be careful in your assumptions, you are worried about the fissile concentration needing to be so high that we don't need moderation. It turns out that in a two fluid design that has a complete blanket (don't need to worry about leakage) that we can get the fissile concentration down in the fuel salt just about as low as with a graphite moderated design (i.e. about 0.2% U233F4, a bit higher if we have a metal barrier that steals some neutrons). The salt itself does a lot of moderating. Thus I don't see any real increase in danger of accidental criticality during spills. You'd need some way for a fairly large diameter mass to form (i.e. close to a meter in diameter or thickness). Since the salt is doing a lot of moderating already if the fissile concentration is low, there isn't that much of a concern about a spill finding moderation (like water).

It is true that a 2 Fluid design that tried to go to a fast spectrum by having a much higher fissile concentration (i.e. well above 1%) then you would really need to be more concerned about accidental criticality. Even with a single fluid design (thorium in the fuel salt) if you have a high concentration of U233F4 then you have to worry about spills finding moderator and hitting criticality. No one likes to even think of this but I come back to the point that even in a case like that, very little would probably happen beyond enough of the salt flashing to vapor until it was sub critical again.

Getting back to graphite in general. A few reasons I tend to want to avoid it include the fact that it makes your core much larger, it introduces a chemical potential (it can burn) and it gives a large volume of radioactive waste to worry about. If we want to use graphite, I'd have a higher preference towards graphite balls which we can put pyrolytic coatings on so we don't need to worry about the fire issue and we can also cycle them in and out.

David L.


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PostPosted: Oct 19, 2008 4:10 pm 
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Quote:
In term's of Oak Ridge's original "plumbing problem" of complex graphite tubes interlacing the two fissile and fertile salts, it is much more than the limited graphite lifetime that causes a problem. The graphite shrinks and then expands and the changing volume fractions of the two salts within the core was a nightmare to design for. So the term "un-buildable" you use is probably true for the traditional Oak Ridge graphite Two fluid design.



Since the Oak Ridge LFTR Days, it is been determined that not all reactor grade graphite is the same.

http://www.journalarchive.jst.go.jp/jnl ... nlabstract

Graphite volume changes vary based on orientation of crystallites and change in accommodation factor.

The nuclear grade graphites under consideration in the reference work were graphitized at almost the same temperature, and the variation of crystallite size among the samples arises from the difference of starting materials, and not from the variations of crystallite size due to the difference of heat-treatment temperatures like the case of carbon materials.

And acceptable reactor graphite might be found that can minimize the "plumbing problem”. IMHO, it is worth the R&D money and effort to find this graphite variant because graphite is imperious to high temperature fluoride and fluoride salt corrosion ( see figure "Corrsion behavior of ceramic materials"). The most challenging materials problem is that posed by the heat exchanger and finding a optimized carbon materials approach here is worth the effort.

One reason why I say this is that SiC is not good in a fluoride environment (see figure "Corrsion behavior of ceramic materials") whereas carbon is optimal. A SiC SiC heat exchanger is likely not to work. Running the LFTR very hot is enabled by the use of carbon in its construction. In addition, carbon can be used as a common construction material in both the U233 converter reactors and the thorium reactor. This simplifies design which is always a top priority goal.


Quote:
Getting back to graphite in general. A few reasons I tend to want to avoid it include the fact that it makes your core much larger, it introduces a chemical potential (it can burn) and it gives a large volume of radioactive waste to worry about. If we want to use graphite, I'd have a higher preference towards graphite balls which we can put pyrolytic coatings on so we don't need to worry about the fire issue and we can also cycle them in and out.



The “fire issue” may kill graphite; we want an absolutely safe design. SiC will not work because of fluoride corrosion and a simple design will not allow for a cycle of graphite balls in and out. Let’s look for a better way.

If Carbon doesn’t workout, then there is the pair of very high temperature metals: molybdenum and tungsten. Molybdenum has a low nuclear cross section in the thermal spectrum nuclear reactions and tungsten has a low nuclear cross section in the fast spectrum. In other words, molybdenum can be used in the thorium only LFTR and tungsten can be used in the U233 converter reactors.

Both resist corrosion in a fluorine environment; molybdenum more then tungsten at high temperatures that I prefer (see figure "Corrosion of Moly and tungston"); molybdenum is comparable to nickel (i.e. excellent) in terms of corrosion resistance in a fluoride salt environment (See figure “Corrosion of various metals”).

Both have a low coefficient of thermal expansion that is well matched to carbon.

Why is this important? I would like to see both metals coated by ether Nastelloy N via explosion wielding or vapor disposition. Or even better, coated with a layer of carbon via vapor disposition to form a protective diamond outer layer; or both coatings in a multilayered composite.

A molybdenum/tungsten sub-straight will give Nastelloy N the strength to withstand up to 1000C. But if that Nastelloy N coating does not workout, then a diamond coating alone will make the reactor/heat exchanger pair imperious to any corrosion over an indefinitely long time frame.

One last point; nuclear reaction will free fluoride to corrode the reactor material. To keep this free fluoride in check, a good supply of free Beryllium should be circulating in the molten salt to capture and neutralize this powerful corrosion menace.

In summation, R&D in LFTR materials is the first order of business in LFTR development. Perfect materials will make a perfect, elegant, and simple LFTR; a product that we will have no problem in championing and defending.


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PostPosted: May 18, 2016 2:42 pm 
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Molten salt reactors: A new beginning for an old idea

Nuclear Engineering and Design 240 (2010) 1644–1656

Article history:
Received 24 September 2009
Received in revised form 3 December 2009
Accepted 18 December 2009

Thanks, Dr. David. Your review of the reactor design helped me to understand a complex subject.

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