Energy From Thorium Discussion Forum

It is currently Aug 15, 2018 4:24 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 53 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
PostPosted: Sep 27, 2011 2:45 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
I don't know the relative difficulty of the two - I think they are the same functionally.
Personally, I'd guess a two tank system with simple valves would be preferable. We have the decay products and they build up to a significant volume over time so it seems like they might clog a delay bed (as well as present a cooling challenge). I had to rethink the off-gas system when I realized just how intense the heat is in the xenon.

It is clear that a holding tank with free mixing is not sufficient - I ran that one. As best I understand this was ORNL's baseline design.
Even splitting the one hour tank into a 30 minute fill, and a 30 minute hold (presuming the emptying time is almost zero) makes a bit difference in the offgas heat load.

You have expressed a thought of not sparging with He. My impression is that ORNL wanted the extra surface area of the bubbles to compete with the heat exchanger surface area for collection of the tritium. If we do not sparge with He will we end up with more tritium passing into the HX?

I wonder what Dr. Peterson is planning for tritium? Are they planning a He sparge?


Top
 Profile  
 
PostPosted: Sep 27, 2011 3:39 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
I think the holding tank can be a calandria. With vertical tubes welded in it to allow buffer salt to flow through, cooling the tank with natural circulation. You need a similar surface area to the PHX which can be done with several hundred tubes. It would look a lot like a CANDU calandria.

I have no idea on the effect of no helium to outcompete for diffusion into the HX metal. Pyrolitic carbon or SiC coating on the HX surface will probably be good enough to limit tritium diffusion (and eliminate corrosion as well). I'm not too worried about it because of the binary nitrate third loop that would reduce tritium diffusion by a factor 10000 and the possibility of using titanium in the steam cycle to hydridize the tritium (even modern condenser tubes are made of titanium which will grab tritium as well).

ORNL mentioned that bubbles of helium could cause a reactivity problem. In an extremely unplausible scenario, but still better to avoid multiple phases in the salt as much as we can. Its a major advantage to just have one phase (liquid) at all times. I say let's remove the Xe and Kr quickly and not use helium to keep it single phase.

But Per Peterson's test reactor setup does not use a nitrate loop, and it looks like it will possibly be an open cycle gas turbine or at least air cooled. Looks like he won't be using the fluoroborate either (FLiNaK secondary loop). That's bad news for tritium management.

Dr. Peterson is a clever guy. Surely he's thought of something clever.


Top
 Profile  
 
PostPosted: Sep 28, 2011 12:04 am 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 823
Location: Calgary, Alberta
I was wondering if one could use the dump tank and add semi-permeable bulkheads to slow the passage of the off gas from one side to the other to allow the 137Xe time to decay, then compress the remaining off gas into formed tubes submerged in salt or some other heat transfer media. One very long pipe from which that gases emerge some time later having had time to go through a number of decays.

I haven't looked at the idea in any detail, but it seems that we need compression at some point to reduce the volume as well as a very competent heat removal system that is preferably self powered (natural convection or boiling liquid). However it's done it would be nice to harvest the significant decay heat (~1%) and add it to the main power cycle if possible.


Last edited by Lindsay on Sep 28, 2011 3:02 pm, edited 1 time in total.

Top
 Profile  
 
PostPosted: Sep 28, 2011 4:32 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
It will be difficult to prove the safety case if you use the decay heat actively. Perhaps it can be done for Xe-137, if you have a system that has inherent thermal capacity to soak up all the decay heat (about half an hour, starting at 20 MW). Perhaps if you have enough secondary coolant to soak up all the heat in the event of a station blackout the regulators will be sufficiently pleased.

I prefer to always lose a small percentage, maybe 0.6-0.7 percent of the decay heat, through the hot cell walls (like the AP1000 passive containment heat rejection). And then we can use the offgas heating to keep the buffer salt from freezing during normal operation. The Xe-137 will decay quickly so your total decay heat will replace it with very similar thermal balance over the entire system (maybe close to isothermal in fact). Very gentle on the components I think. And very safe. Walk away safe.


Top
 Profile  
 
PostPosted: Sep 28, 2011 1:45 pm 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 911
Location: Ottawa
I've been doing a lot of thinking about the off gas system and possible changes as well. Keep up the discussion as it is a very important area and likely the challenge is under appreciated in general.

I'd remind something I don't think has come up but even if you filter out noble metals you also get a large mass of solid fission products in the off gas system that at some point came out of the salt as either Xe or Kr. By far the biggest concern is that Cs 137 (30 year) and Cs 135 (3 million) will be born from Xe precursors. If Cs is in contact with the fuel salt it will pick up a fluorine and become stable. If the gas just goes to bottles though it will stay in its elemental form and be volatile (I'm sure there are solutions). For the MSBR the first stage was the decay tank so presumably if periodically held the fuel salt it could clean out any Cesium. As well there are a whole boatload of other fission products coming from Xe and Kr precursors but all are stable after awhile.

If you want to learn more, ORNL 4541 has one of the best sections I've found on all this.

David LeBlanc


Top
 Profile  
 
PostPosted: Sep 28, 2011 2:02 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Hi David, I've proposed to use a bed of silver or copper fluoride to bind the cesium in the offgas to a fluoride form. AgF + Cs = CsF + Ag. If we're lucky we can also use this to grab iodine. 2Ag + I2 = 2AgI. Silver iodine is not soluble in water so can be buried. Luke pointed out that this does not work at high temp as silver iodine dissociates at 550 Celcius. But we can also use high temp carbon filters to grab the iodine and later silver-bind any I-129 that is left in the carbon filter after the other iodines have decayed.


Top
 Profile  
 
PostPosted: Sep 28, 2011 3:53 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
David wrote:
I've been doing a lot of thinking about the off gas system and possible changes as well. Keep up the discussion as it is a very important area and likely the challenge is under appreciated in general.

I'd remind something I don't think has come up but even if you filter out noble metals you also get a large mass of solid fission products in the off gas system that at some point came out of the salt as either Xe or Kr. By far the biggest concern is that Cs 137 (30 year) and Cs 135 (3 million) will be born from Xe precursors. If Cs is in contact with the fuel salt it will pick up a fluorine and become stable. If the gas just goes to bottles though it will stay in its elemental form and be volatile (I'm sure there are solutions). For the MSBR the first stage was the decay tank so presumably if periodically held the fuel salt it could clean out any Cesium. As well there are a whole boatload of other fission products coming from Xe and Kr precursors but all are stable after awhile.

If you want to learn more, ORNL 4541 has one of the best sections I've found on all this.

David LeBlanc

The ORNL design had salt overflow also going through the dump tank so the dump tank was continually being washed with fuel salt that was continually being pumped back into the reactor. Unfortunately, this also means that we are taking lots of fission products that were separated from the fuel salt and reintroducing it into the reactor (about 200kg/GWe-yr). So it would be nice to have a fluorine donor in the offgas tank to stablize the Cs but to keep the contents out of the fuel salt since it has such a concentration of fission products.


Top
 Profile  
 
PostPosted: Sep 28, 2011 5:28 pm 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 823
Location: Calgary, Alberta
Cyril R wrote:
It will be difficult to prove the safety case if you use the decay heat actively.
I think that it achievable, you just have to passively switch the heat sink on failure of the normal heat sink. I'm not proposing active heat removal only ever passive heat removal, but a choice of heat sink within a passive self powered system. I'd agree that an active heat extraction system is not the way to go, just not reliable enough. Gravity, I like gravity, I've found it very reliable in the past.


Top
 Profile  
 
PostPosted: Sep 28, 2011 10:34 pm 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 911
Location: Ottawa
Lars wrote:
David wrote:
I've been doing a lot of thinking about the off gas system and possible changes as well. Keep up the discussion as it is a very important area and likely the challenge is under appreciated in general.

I'd remind something I don't think has come up but even if you filter out noble metals you also get a large mass of solid fission products in the off gas system that at some point came out of the salt as either Xe or Kr. By far the biggest concern is that Cs 137 (30 year) and Cs 135 (3 million) will be born from Xe precursors. If Cs is in contact with the fuel salt it will pick up a fluorine and become stable. If the gas just goes to bottles though it will stay in its elemental form and be volatile (I'm sure there are solutions). For the MSBR the first stage was the decay tank so presumably if periodically held the fuel salt it could clean out any Cesium. As well there are a whole boatload of other fission products coming from Xe and Kr precursors but all are stable after awhile.

If you want to learn more, ORNL 4541 has one of the best sections I've found on all this.

David LeBlanc

The ORNL design had salt overflow also going through the dump tank so the dump tank was continually being washed with fuel salt that was continually being pumped back into the reactor. Unfortunately, this also means that we are taking lots of fission products that were separated from the fuel salt and reintroducing it into the reactor (about 200kg/GWe-yr). So it would be nice to have a fluorine donor in the offgas tank to stablize the Cs but to keep the contents out of the fuel salt since it has such a concentration of fission products.



Yes there is a lot of fission products passing through Xe or Kr. However, except for a couple Cs isotopes they almost have quite low neutron absorption cross sections so they actually don't make too big a change in the overall picture (but certainly would be nice to see things in detail). For MSBR studies things ended up all over though, Cs 137 in the decay heat tank along with Sr89 (50 day). In the 47 hr delay bed, most of the Cs 135 would be born and trapped. In the end there is a whole mix of Cs,, Y, Ba and La isotopes from Xe and Rb and Sr isotopes from kr in various places, luckily almost all the rest have pretty short half lives (well not good for decay heat but good for long term disposal).

David L.


Top
 Profile  
 
PostPosted: Sep 29, 2011 3:29 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
In the French modelling on TMSR, they found that the effect of neutronics between a 1 minute sparge and a 1 day sparge is very low. Thermal spectrum reactors are worse of course but even with 1 hour sparge you'll let almost all Xe137 decay in the fuel salt.

So you have another option: sparge slower to let almost all of the Xe137 decay to Cs137 before removing the gasses.

Pro: easier sparging, use Xe137 and Cs137 in the fuel salt to use the heat productively rather than being a problem. You get 20 MWth extra in the fuel salt.
Con: Cs137 not seperated. Will accumulate to steal a minor amount of neutrons and it will be more difficult to use for other applications (land and ocean based RTGs).

I've never felt that a 1 minute sparge is possibly anyways. This is cubic meters per second processing. With 60 cubic meters fuel salt it is sparging 1 cubic meter per second. Way optimistic. The rapid sparging is considered 'easy' but looking at the details it does not appear to be so. The solubility of the noble gasses is very low, but not zero.


Top
 Profile  
 
PostPosted: Sep 29, 2011 7:43 am 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 911
Location: Ottawa
Cyril,

Was the 20 MWth you mention just an estimate (I"m guessing based on what the MSBR off gas had to deal with)? It would be good to know the effects of slow purge more precisely. A one day sparge would leave almost all Xe + Kr decay products behind (except a lot of Cs135(3 million year) since its precursor Xe135 (6%yield) has a 9.14 hour half life. Of course it is Xe135 that they really wanted to get out in the first place so certainly not an insignificant effect on the neutron budget (i.e. likely unthinkable for a breeder but OK for converter). If this started to mean we'd have Xenon poisoning effecting shutdown's like LWRs this could be a problem though.

David L.


Top
 Profile  
 
PostPosted: Sep 29, 2011 8:07 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
You're right David lets find some more accurate numbers.

From the ORNL-4396. ORNL-DWG-69-6006, curve D:

http://www.energyfromthorium.com/ORNL4396_sec05.html

Curve starts at 20 MWth. Between 10e1 and 10e2 seconds is the normal sparging time suggestion. This would lead to almost the full 20 MWth in the offgas (18-19MWth).

If in stead we remove the gas at 10e4 seconds cycle, which is 2.78 hours, we have dropped the heat load to 4-5 MWth.

So we get an equilibrium heat load that is 4x lower by sparging at a 10000 second cycle. If Xe135 and daughter Cs135 is what you want to pull out then this system does almost as well since its half life is 3x longer than the processing cycle.

And the system is realistic unlike the crazy 30 second reprocessing assumption. An important fact.


Top
 Profile  
 
PostPosted: Sep 29, 2011 8:32 am 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 911
Location: Ottawa
That is probably a good compromise for sparge time. That estimate should be pretty close, of course since it is a continuous system the real numbers will be a little different since a fraction of very young Xe and Kr will go out with the sparge even with a long cycle time. Would also drive up the net efficiency from 44.4% to 44.7%. A nice couple million dollar bonus per year per GWe plant too (at 4cents/kwh).

David L.


Top
 Profile  
 
PostPosted: Sep 29, 2011 9:19 am 
Offline

Joined: Jul 28, 2008 5:01 am
Posts: 461
Location: Teesside, UK
If you're not sparging fast enough to get the Xe-135 out before it steals a neutron, is there any point having a sparging system at all? Just let the Kr and Xe diffuse out of the salt, and pipe it away.

The French TMSR design is 20 m^3, not 60, but 1 min for sparging is still unreasonable for the metals. I might just about believe it for the xenon.

The Xe decay 1/2 life isn't the relevant comparison. We don't mind if it decays. It's the 1/2-life for capture, given the neutron spectrum and flux, that has to be >3X the sparging extraction time to avoid most of the losses.


Top
 Profile  
 
PostPosted: Sep 29, 2011 10:00 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Good points David and Luke.

Where do I find capture half lives for a given power density?

More specifically, does anyone know what it is for Xe-135 at DMSR and MSBR power densities?

It occurs to me that a DMSR would have less losses to Xe and any fission product for that matter because of the lower power density (similar to lower losses to Pa). As such it might do with a slower sparging system. That's good since DMSR is likely to be a first MSR development.

The French group has good modelling (raytrace) for captures. If the French TMSR has little effect on captures with a few days sparging time, then that is good news for a fastish MSR, and a thermal MSR with a few hours should do about as good.

I'd also like to know, what the rate of 'self-sparging' would be - how fast does the Xe and Kr remove itself if we don't sparge?


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 53 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group