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PostPosted: Sep 25, 2011 11:28 am 
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To remove the most important fission product neutron poison, xenon, sparging the fuel salt with helium is typically assumed. This seems to be similar to blowing bubbles through a straw in a soda bottle: the CO2 from the drink is removed much more rapidly when you blow bubbles through it.

Of course, if helium is used in the sparging system, there will be a lot of helium that has to be stored along the xenon. That means more bulk.

There are lots of different ways to remove the CO2 from the soda bottle. Shaking it is very effective, but is not practical for a red hot highly radioactive fuel salt.

Stirring rapidly works great as well. Perhaps this is the best way to remove the xenon without using lots of helium to dilute the flow. Stirrers can be magnetic, so we can make a canned stirrer without seals. This seems like a much more elegant solution to the problem.


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PostPosted: Sep 25, 2011 12:33 pm 
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There is a second goal of helium bubbling: removing non-gaseous insoluble fission products like noble metals. How can this be accomplished by stirring?


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PostPosted: Sep 25, 2011 1:11 pm 
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Burghard wrote:
There is a second goal of helium bubbling: removing non-gaseous insoluble fission products like noble metals. How can this be accomplished by stirring?


Solids in the offgas system would give trouble over time. Given the quantities involved they could easily clog pipes and valves.

Better to remove the solid noble metal 'smoke' through a nickel wool filter system where it is trapped. Just pass a small sidestream of cold salt (from the HX outlet) through a big filter bed at low flow speed, say 1 cm/sec. A 1 or 2 cubic meter filter should be plenty for a big reactor.

So I think this would be an advantage rather than a disadvantage.


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PostPosted: Sep 25, 2011 1:33 pm 
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I while ago I have proposed a vacuum sparging system.

The high density of the salt (2 gram/ml even before adding the heavy metal) makes it possible to pump a side stream of salt a couple of meters up an insulated pipe into a vaccum chamber. In this chamber, the salt can be passed through a shallow tray before going down another pipe back to the core. The surface of the liquid in the tray can be agitated with high amplitude ultrasonic vibrations. This will result in a very fine mist rising above the liquid surface. Like this: http://www.chameleonpages.com/MistMaker.htm. In this case, though, the mist droplets will not form an aerosol because it's a vacuum chamber so the droplets will make short ballistic hops and fall right back into the liquid. The population of droplets in flight will still have a very high total surface area for the vapor pressure difference to do its work.

The vacuum chamber can be scrubbed by various pumps, adsorption beds or cold traps as suitable for the mix of gasses and vapors. Iodine can be immobilized almost instantly by silver coated porous blocks. Which components having a relatively low vapor pressure are present in the salt other than noble gasses and iodine? This type of setup can potentially blur the barrier between sparging and vacuum distillation.


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PostPosted: Sep 25, 2011 1:35 pm 
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I while ago I have proposed a vacuum sparging system:

The high density of the salt (2 gram/ml even before adding the heavy metal) makes it possible to pump a side stream of salt a couple of meters up an insulated pipe into a vaccum chamber. In this chamber, the salt can be passed through a shallow tray before going down another pipe back to the core. The surface of the liquid in the tray can be agitated with high amplitude ultrasonic vibrations. THe result is a very fine mist rising above the liquid surface. Like this: http://www.chameleonpages.com/MistMaker.htm. In this case, though, the mist droplets will not form an aerosol because it's under a vacuum so the droplets will make short ballistic hops and fall right back into the liquid. The population of droplets in flight will still have a very high total surface area for the vapor pressure difference to do its work.

The vacuum chamber can be scrubbed by various pumps, adsorption beds or cold traps as suitable for the mix of gasses and vapors. Iodine vapor can be immobilized almost instantly by silver coated nickel sponges.

Which components having a relatively low vapor pressure are present in the salt other than noble gasses and iodine? This type of setup can potentially blur the barrier between sparging and vacuum distillation.


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PostPosted: Sep 26, 2011 1:57 am 
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How about CO2 as the sparging gas? It could be condensed and recycled. Alternately, the Xe and Kr could be condensed and He recycled.


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PostPosted: Sep 26, 2011 3:50 am 
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jagdish wrote:
How about CO2 as the sparging gas? It could be condensed and recycled. Alternately, the Xe and Kr could be condensed and He recycled.


Adding oxide ions is probably a really bad idea.

The problem with the Xe is that it makes a lot of heat, which makes condensing it difficult and expensive.

Better is to immediately store the offgas in gas cylinders that are filled with silver fluoride particles. The cesium-137 that forms from decay of some of the Xe will become cesium fluoride and any iodine will tend to bind to the resulting silver.

The offgas cylinders can be at the bottom of the buffer pool salt in a pool type reactor. For cooling and shielding. Plus you can store under pressure equal to the hydrostatic pressure of the buffer salt.


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PostPosted: Sep 26, 2011 5:57 pm 
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Cyril R wrote:
....... and any iodine will tend to bind to the resulting silver....
Not very well, AgI decomposition temp is given as 550 to 555 C


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PostPosted: Sep 26, 2011 6:03 pm 
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Within a few months the gas is not giving off any significant heat. So, likely we don't try to store it hot.
You are right though that the He is a large volume to deal with for storage and the Xe decay makes cryogenic distillation tough - the heat itself is probably OK but it is all concentrated in the Xe so as soon as the Xe is liquid you get a fantastic heat density and I can't imagine how to keep it liquid long enough to separate it from the He.


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PostPosted: Sep 27, 2011 3:11 am 
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Luke wrote:
Cyril R wrote:
....... and any iodine will tend to bind to the resulting silver....
Not very well, AgI decomposition temp is given as 550 to 555 C



OK. Then we put in high temp active carbon to sorb the iodine (it is really good at this). Later on we pull the carbon and silver out and remove the cesium fluoride and iodine (it will be only I129 at this point) and bind the iodine to the silver for permanent burial.


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PostPosted: Sep 27, 2011 3:21 am 
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Lars wrote:
Within a few months the gas is not giving off any significant heat. So, likely we don't try to store it hot.
You are right though that the He is a large volume to deal with for storage and the Xe decay makes cryogenic distillation tough - the heat itself is probably OK but it is all concentrated in the Xe so as soon as the Xe is liquid you get a fantastic heat density and I can't imagine how to keep it liquid long enough to separate it from the He.


I was thinking to put the gas canisters on the bottom of the buffer salt pool. This way you get excellent passive cooling and gamma shielding. You can also store under pressure equal to the hydrostatic salt pressure and still be isobaric. 10 meters of buffer salt is +3 atmospheres. So we can put in more gas.

If the decay heat cooling system is always on for safety and simplicity, we’ll appreciate the heatup from the offgas in the buffer salt. It will keep the buffer salt at temperature without parasitic electric heating. If it is 20 MW of heat then we can design the hot cell to passively dump that amount at modest temperature and then rise gently and slightly (using the large thermal buffer) during a power blackout/other loss of forced cooling scenario to remove the decay heat.


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PostPosted: Sep 27, 2011 9:56 am 
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The canisters will take 2000 psi. Adding 3 atmospheres to that is an extra 45 psi.
I have been using 1000psi just to be arbitrary and put in extra margin.

Increasing the temperature by 36C will result in an extra 3 atmospheres pressure.

So putting them in the buffer salt makes some sense but it isn't a big delta.


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PostPosted: Sep 27, 2011 12:36 pm 
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Are you going to compress highly radioactive gasses to 2000 psi? Has this ever been done at all? Wouldn't you want a more gentle pressure, something like a propane canister?

These canisters would become RTGs as they accumulate Cs-137. Red hot if you put them in air. So we submerge them in the buffer salt (550 C) and reduce the pressure to reduce the Cs-137 loading per canister.

These things would be like spent fuel rods in rough decay heat generation. Those have to be stored for years under a cool pool of water before they can be stored passively.


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PostPosted: Sep 27, 2011 1:03 pm 
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The 137Cs comes from 137Xe which has a 3.8 minute half-life. So keep the gas at atmospheric pressure or slightly above for an hour or two and all the 137Xe will have decayed to 137Cs so the gas cylinders shouldn't have any 137Cs.


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PostPosted: Sep 27, 2011 2:21 pm 
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Lars wrote:
The 137Cs comes from 137Xe which has a 3.8 minute half-life. So keep the gas at atmospheric pressure or slightly above for an hour or two and all the 137Xe will have decayed to 137Cs so the gas cylinders shouldn't have any 137Cs.


Ok, thanks for pointing that out. So we first have a low pressure holding tank with silver fluoride particles to immobilize the cesium. Then after an hour the gas goes into the cylinders (but maybe first through a carbon filter). The holding tank could then be at the bottom of the buffer salt pool as well.

What are the specifics of this system? Will we use a swinging tank system with one tank in use and the other decaying, then move the gas to the cylinders after decay and swap tanks? Or will it be a continuous process with a delayed bed of some sort?


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