Energy From Thorium Discussion Forum

It is currently Aug 21, 2018 8:40 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 35 posts ]  Go to page Previous  1, 2, 3  Next
Author Message
PostPosted: Sep 14, 2013 7:06 am 
Offline

Joined: Mar 16, 2010 1:48 am
Posts: 68
Location: Guangdong, China
Cyril R wrote:

Why do you think iodine will cause pitting? Materials of construction are designed for fluoride environment and the chemistry is reducing at all times. Any alloy that can take that environment should not be bothered by iodine.


Here pitting is not material corrosion, bu the iodine-135 decay to xe-135 cause reactivity decrease. If reactor is shut-down with control inserted, the I-135 decay and xe-135 concentration increase during several hours, so if we want restart reactor, it may not get to critical after control rod withdrawal.
There is iodine pit in PWR, so I thought msr also exist "iodine pit" if no iodine online removal, isn't it?
here is iodine pit explain :http://en.wikipedia.org/wiki/Iodine_pit


Top
 Profile  
 
PostPosted: Sep 14, 2013 12:01 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
longwei1221 wrote:
Cyril R wrote:

Why do you think iodine will cause pitting? Materials of construction are designed for fluoride environment and the chemistry is reducing at all times. Any alloy that can take that environment should not be bothered by iodine.


Here pitting is not material corrosion, bu the iodine-135 decay to xe-135 cause reactivity decrease. If reactor is shut-down with control inserted, the I-135 decay and xe-135 concentration increase during several hours, so if we want restart reactor, it may not get to critical after control rod withdrawal.
There is iodine pit in PWR, so I thought msr also exist "iodine pit" if no iodine online removal, isn't it?
here is iodine pit explain :http://en.wikipedia.org/wiki/Iodine_pit

In an MSR that has been off for a few hours indeed there will be more Xe in the fuel salt than normal and this will inhibit criticality. However, after a few minutes the He sparge will remove the Xe so that we get back to criticality.

Another point with iodine though. We should have something in the offgas system to bond with the iodine to reduce its volatility in the event of an accident. One of the big safety features of LFTR is that the fission products are generally not volatile and the reactor has no motive force to push them out into the environment. Iodine is one of the few dangerous fission products that is volatile in a LFTR. I'm thinking a silver sponge in the offgas system will lock up the iodine and remove its volatility.


Top
 Profile  
 
PostPosted: Sep 14, 2013 1:00 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Silver should work at low temperatures, but is dubious at high temperatures (ie, during an accident of sorts). I'd suggest an alkali or alkali earth to lock it up. Calcium or lithium should be the most stable compounds. We don't have any fluorine or fluoride in the offgas, so iodine won't have compete with its bigger younger brother that is bullying it around in the fuel salt.


Top
 Profile  
 
PostPosted: Sep 14, 2013 2:32 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
Cyril R wrote:
Silver should work at low temperatures, but is dubious at high temperatures (ie, during an accident of sorts). I'd suggest an alkali or alkali earth to lock it up. Calcium or lithium should be the most stable compounds. We don't have any fluorine or fluoride in the offgas, so iodine won't have compete with its bigger younger brother that is bullying it around in the fuel salt.


But don't we?! Part of the decay process in the offgas is Xe -> Cs and we want to lock up the Cs so I was thinking we want some fluorine donor in the offgas region to lock up the Cs.

Silver iodine seems to be good as far as volatility is concerned. But we need a better chemist than me to consider the proper donors so that we lock up both the Cs and I. Luke where are you?


Top
 Profile  
 
PostPosted: Sep 14, 2013 4:07 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Don't worry. Iodine can't compete with fluorine in terms of getting a donor, so first remove the Cs as CsF with a molecular F donor (NiF2 in a nickel vessel), when the Cs is bound up, another step for binding up iodine with an elemental donor (Ca or Li in stainless steel vessel).

Iodine won't react with NiF2, and residual Cs won't react with CaI or LiI. If residual Cs reacts with residual I-129 at the end of the process, that seems like a good thing, at least for temporary storage.


Top
 Profile  
 
PostPosted: Sep 14, 2013 5:15 pm 
Offline

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
But the Iodine is there first, no? Wouldn't we need to take the iodine out first in a method that won't create problems when part transmutes to xenon, then take the cesium out after the xenon transmutes some more?

_________________
DRJ : Engineer - NAVSEA : (Retired)


Top
 Profile  
 
PostPosted: Sep 14, 2013 6:15 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
KitemanSA wrote:
But the Iodine is there first, no? Wouldn't we need to take the iodine out first in a method that won't create problems when part transmutes to xenon, then take the cesium out after the xenon transmutes some more?


Good point. I suppose then the stages must be switched, first the iodine binding, then the Cs binding. Cs that has decayed in the iodine binding vessel has no fluorine donor around, but this could be solved by draining by gravity to the Cs binding vessel.

Offgas handling is a lot more complicated than advocates often tout, if you want to bind things up to non-volatile stable forms whilst retaining full passive cooling.

What complicates things firstly is the Xe-137 that is a real hot head. It comes from I-137 that is even worse. Trying to bind up the I-137, not a good idea. A lot of passive cooling will be needed to let these decay. Xe-137 decays to the infamous Cs-137, and it seems prudent to lock this up as soon as possible. Cs will be a liquid so it can drain by gravity to a vessel with fluorine donor in it.

So first there is an hour or so holdup period, where I-137 and Xe-137 decay in some vessel. Product Cs-137 is gravity drained from this and locked up in another vessel. Some of the Cs-137 will likely bind with iodine isotopes, and this might clog up the drain line, further complicating things (m.p. 621C). The noble metals that have been carried over will complicate this even more. Product gasses I, Br, Xe, and Kr move to the halide binding stage (like react with silver pellets), still a lot of heat and need for passive cooling. Do we still need another Cs binding stage after this? Maybe for Cs-134 (even with fast offgas removal a small amount will be made)?


Top
 Profile  
 
PostPosted: Sep 14, 2013 7:37 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
KitemanSA wrote:
But the Iodine is there first, no? Wouldn't we need to take the iodine out first in a method that won't create problems when part transmutes to xenon, then take the cesium out after the xenon transmutes some more?

I guess one could try to pass the offgas through the iodine collector first, then onto the holding tanks to collect Cs as it is created but I'm thinking it may be tough to collect all the iodine in one pass - especially as the gas is fast flowing and 20 MWatts worth of heat. If we collect the iodine first then you will have to deal with decay chains like:
137I (25 Sec) -> 137Xe (4 min) -> 137Cs (30 years). I don't think it would be possible to ensure that as the 137I decays in to iodine collector and converts to Xe that you can get all the Xe out before any of it decays. If you don't then you end up with elemental Cs in your iodine collector. I'd prefer to ensure all the Cs gets chemically bound up.

Rather, I'm thinking we end up with a continuous flow and various isotopes decaying all the time and we keep the chemicals present to tie up the Cs and I. This way if we don't get all the iodine immediately that is ok as we will collect it over time.

I don't see the need for passive cooling of the offgas. This stuff decays very rapidly so the total energy is likely pretty modest. I suspect we can use active cooling for normal operations and heat capacity for emergencies. Need to check the calculations.

Also, I don't suspect we get 134Cs in the offgas. Both 134Xe and 134Ba are stable so I think the only way we get 134Cs is directly from fission. If so, then it will be in the fuel salt but only trace amounts (from fuel salt mist) will be in the offgas decay products.


Top
 Profile  
 
PostPosted: Sep 14, 2013 9:01 pm 
Offline

Joined: Jun 05, 2011 6:59 pm
Posts: 1335
Location: NoOPWA
Lars wrote:
Also, I don't suspect we get 134Cs in the offgas. Both 134Xe and 134Ba are stable so I think the only way we get 134Cs is directly from fission. If so, then it will be in the fuel salt but only trace amounts (from fuel salt mist) will be in the offgas decay products.
We will also get some from neutron activation of 133Cs but since that typically comes from 133Xe, I don't suppose much will be in the core to be activted.

_________________
DRJ : Engineer - NAVSEA : (Retired)


Top
 Profile  
 
PostPosted: Sep 15, 2013 9:27 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
KitemanSA wrote:
Lars wrote:
Also, I don't suspect we get 134Cs in the offgas. Both 134Xe and 134Ba are stable so I think the only way we get 134Cs is directly from fission. If so, then it will be in the fuel salt but only trace amounts (from fuel salt mist) will be in the offgas decay products.
We will also get some from neutron activation of 133Cs but since that typically comes from 133Xe, I don't suppose much will be in the core to be activted.

Again though the 134Cs will form in the core and hence will rapidly become CsF and lose its volatility and this route also does not lead to 134Cs in the offgas system.


Top
 Profile  
 
PostPosted: Sep 15, 2013 11:18 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Lars, elemental Cs in the I trap can likely be accomodated. Cs doesn't react with silver. It will likely react with AgI, then the iodine will be absorbed onto some other silver and CsI takes its place. Doesn't appear to be a problem to me. Any Cs that doesn't react can be made to drain (it is liquid at all trap temperature ranges) by gravity to a NiF2 trap vessel connected and attached below the Ag trap vessel. This could even be in the same vessel, with the top being an Ag iodine trap, the bottom part having a NiF2 partition to immobilize drained elemental Cs.

The biggest problem is probably the decay heat as you mentioned. It could well cause heat spikes somewhere, and AgI has a relatively low melting point so the iodine trap could be damaged by local overheating in normal operation, or global overheating in transients.


Top
 Profile  
 
PostPosted: Sep 15, 2013 8:04 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
Cyril R wrote:
Lars, elemental Cs in the I trap can likely be accomodated. Cs doesn't react with silver. It will likely react with AgI, then the iodine will be absorbed onto some other silver and CsI takes its place. Doesn't appear to be a problem to me. Any Cs that doesn't react can be made to drain (it is liquid at all trap temperature ranges) by gravity to a NiF2 trap vessel connected and attached below the Ag trap vessel. This could even be in the same vessel, with the top being an Ag iodine trap, the bottom part having a NiF2 partition to immobilize drained elemental Cs.

The biggest problem is probably the decay heat as you mentioned. It could well cause heat spikes somewhere, and AgI has a relatively low melting point so the iodine trap could be damaged by local overheating in normal operation, or global overheating in transients.


Ag has a melting point of 962C, AgI of 1036C so it would seem that the silver would melt before the silver-iodide does and silver has a higher density. So I could imagine a couple of problems. If nothing melts then the silver-iodide would form on the surface of the silver - ? does it form a protective coat like aluminum oxide or a porous one like rust? If it does melt then does the silver-iodide float and form a surface seal? Probably we can make the silver be a replaceable item - a sponge that gets replaced every major service interval. With a sampling system of the offgas after going through the sponge to see if any iodine got through. I would picture a couple of stages - the first to capture the majority of iodine but exposed to the hot gas so it stands a plausible chance of developing a hole through it due to local heating. Then a second one after the gas has cooled for several hours that could mop up anything that gets by.


Top
 Profile  
 
PostPosted: Sep 16, 2013 2:42 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Wikipedia, Webelements, and Google all agree that that AgI has a melting point of 558 degrees Celsius. (perhaps your source said 1036 F). It will melt off before silver will melt.

One advantage at least is that silver conducts heat extremeley well, so local overheating may be very limited if we make it so that the silver bed touches a cooled vessel wall on all sides. But it's still a petty that we can't operate the bed above 558 C at all times. Or were you trying to say, perhaps, that it might be possible with large surface area, where the molten AgI doesn't drip off but wets the silver and sticks to it? But if that is the case, then Cs may not drain off either, which is bad. Looks like a good area for a non-radioactive experiment, using nonradioactive iodine and silver packed beds.

I'm not sure, but expect the AgI to be somewhat protective at lower temperatures, much less so closer to the melting point. But we can make the silver slivers very thin, like a sponge or a packed bed, so it isn't a big problem.

What about bromine? How are we going to trap that, or is there no need?


Top
 Profile  
 
PostPosted: Sep 16, 2013 4:57 am 
Offline

Joined: Jun 19, 2013 11:49 am
Posts: 1547
Bromine will be trapped by the same mechanism that traps iodine, since it will try to displace iodine in various salts.

I would have thought anyway, although I must admit high temperature solid state chemistry is not my field of expertise.


Top
 Profile  
 
PostPosted: Sep 16, 2013 9:55 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
E Ireland wrote:
Bromine will be trapped by the same mechanism that traps iodine, since it will try to displace iodine in various salts.

I would have thought anyway, although I must admit high temperature solid state chemistry is not my field of expertise.


Yes, you'd think that bromine has a slight advantage over iodine in its competition. But the silver iodine reaction appears a bit strange. It could favor iodine over bromine. Hence my question, is it trapped effectively on silver or will much of it stay volatile?

Based on the Wiki entry,

http://en.wikipedia.org/wiki/Silver_bromide

It looks like silver bromide is a stable compound up to around 1500 Celsius. So it should be trapped well by silver at high temperature. It does decompose at 1502 C according to the article, whereas silver iodide has a 1506 C boiling point with no mention of decomposition, suggesting silver iodide to be the more stable compound.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 35 posts ]  Go to page Previous  1, 2, 3  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