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PostPosted: Jun 20, 2016 11:08 am 
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Just ran across another possible way to deal with the Xenon off gas. I don't know what will happen if you cycle radioactive xenon through the MOF. I hope this is useful to those designing new reactors.

http://www.nature.com/ncomms/2016/160613/ncomms11831/full/ncomms11831.html

Mike


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PostPosted: Jun 20, 2016 11:18 am 
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Sorry for the duplicate reference. The paper was already mentioned in the Gaseous fission products discussion


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PostPosted: Jun 25, 2016 11:20 am 
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Michael, true. Maybe you meant this one:

Re: Gaseous fission product removal - Posted: Jun 13, 2016 by Jim L
Jim L. wrote:
This material looks promising for capturing the Xe and Kr in an off-gas processing system: http://phys.org/news/2016-06-material-potential-nuclear-fuel-recycling.html

The article just considers reprocessing applications using used LWR solid fuel rods, but of course it should be great for a MSR. I wonder if this MOF will work better than the charcoal beds that are normally considered for this application.
You included the paper itself that is downloadable. Jim's is an article on the paper. Sidebar: Jim, your posts are great! But to have the actual paper is very useful.

I wonder if Kirk's LFTR design will use MOFs.

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PostPosted: Jun 25, 2016 5:36 pm 
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Perhaps I did not read the study careful enough.

In a molten salt reactor it is necessary to clean the fuel from gaseous fission products quite often. The heat generation from the radioactive decay is one of the main issues for the separation of these gaseous fission products. According to the study it is necessary to keep the temperature < 100°C to work efficient. I did not see anything about the heat generation and the cooling of the material to absorb these noble gasses. I assume the study was performed with non radioactive substances.

But anyway a new method of separating noble gasses is a great opportunity for nuclear facilities including MSR.


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PostPosted: Jun 28, 2016 10:33 am 
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Holger, as far as I know at this moment, the nuclear engineers on this forum have not begun discussing MOFs for use in the MSR off-gas handler.
HolgerNarrog wrote:
The heat generation from the radioactive decay is one of the main issues for the separation of these gaseous fission products.
I believe your statement here is incorrect. The worst neutron offender is xenon that kills the reactor. Krypton forms along with xenon and both are gases that in a fluid design can be continuously removed by helium sparging.

MOF capture and separation and isolation of krypton-85 looks promising.

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PostPosted: Jun 28, 2016 2:56 pm 
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Hi Tim, if you remove the Xe and Kr perhaps some I and Br with it by helium sparging you have to seperate these gasses from the He before putting it back in the sparging. This is currently planned to be done by kyrogenic process.

My assumption is that the metal - organic frame might be an alternative to the kyrogenic method.


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PostPosted: Jun 28, 2016 3:19 pm 
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Hi Holger! I believe their goal is to use cheaper better MOFs expressly to avoid the expensive cryogenic processes, yes? I cannot comment on the I and Br.

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PostPosted: Jun 29, 2016 6:52 pm 
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Holger, I don't think you have to separate the gases for the purpose of sparging. The only one to worry about is 135-Xe. Just keep the gases in a separate tank for 90 hours and 99.9% of that rains out as 135-Cs. The rest can be pumped back into the salt. The bubbles increase the surface area of the salt/gas interface, so more gas can bubble out of the salt. Xe or Kr may be less efficient than He for that purpose, but I suspect it doesn't worth separating the gases to attain pure He.

However, some other processes may require pure He. In the Two fluid Breeder design doc they mention putting about 2 cu ft/min through an unspecified cleaning process, while 10 cu ft/min of mixed gas goes directly back to the sparging spritzer. I suspect this cleaning process is primarily designed to remove Tritium. They also hold up the gas for only 48 hours, and the second holding tank includes a charcoal bed. The first holding tank (one hour) is there primarily to disperse heat from short lived gas isotopes and noble metal "smoke". See ORNL 4528, p 48-49 for some heat dispersion values if you are interested in that, especially with respect to using metal-organics in place of the charcoal.


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PostPosted: Jul 04, 2016 5:15 pm 
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Dear Steve...the helium sparging is usually done to get the noble fission product gasses and perhaps as well the noble metal fission products that are acc. to several reports separated as well with the helium sparging gas out of the fuel. In serious reactor concepts it is planned to get the fuel thru the sparging every couple of minutes till each 1/2 hour.

These gasses have an enormous heat generation. It is as well strange if you plan to use a tank for 90h!!. It would be a very large tank that would require a strong cooling system.


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PostPosted: Jul 08, 2016 3:36 pm 
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Holger,
I only used the 90 hour figure because it's ten half lives. I can remember 99.9% that way. ORNL planned to hold up the gases for 48 hours, so "my" tank is only twice as big as that. (Note: The reactor I am building in my basement is only a simulation!)

The point I was making was that Xe or Kr should work just as well as He for making bubbles. You don't have to separate them for that reason. 83-Kr and 131-Xe have moderate cross-sections for capturing neutrons, so you might eventually (years?) need to discard old bubble gas. Even that becomes moot if the modest gas cleaner from ORNL 4528 is used. That doc mentions "other parts of the reactor" that need clean gas, but I don't know what those parts are.


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PostPosted: Jul 08, 2016 4:45 pm 
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Myself I like the idea of "vacuuming" the cover gas through a couple of one way gas locks and then sparging it through a water tank - you could arrange things so that the water is a solution designed to trap as many of the non noble products as possible, and some of the noble gasses would stay in solution.

It would much simpler to keep the material cool and contained - but it would be potentially troublesome to stop any moisture infiltrating the core through the gas locks.


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PostPosted: Aug 05, 2016 3:38 pm 
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Put a column filled with metal sponge above the reactor such that the top of the column is near vacuum. The metal sponge will tend to extract the noble metal FPs and the vacuum will extract the Kr and Xe. No parging needed. The cleaned fuel salt will then flow done a pipe where it can be pumped to a HEX and returned to the reactor. Someone has suggested a stir rod to agitate the fuel salt. Having one at the top of the column might be advantageous.

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