Energy From Thorium Discussion Forum

It is currently Dec 16, 2017 12:29 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 34 posts ]  Go to page Previous  1, 2, 3  Next
Author Message
PostPosted: Nov 05, 2013 4:48 pm 
Offline

Joined: Sep 22, 2013 2:27 pm
Posts: 262
Dear Cyrill,

it is correct...

If the Te is available as metal it will be extracted after 1h in average (new concept version not yet in the summary) by helium bubbling and the share in the reactor will not reach 1.2ppm.

If it is available as TeCl4 it will not cause grain boundary corrosion.

Most probably a share of the Te will be available as TeCl4 and another as Te metal. I will have to think how to put it on paper.

Holger


Top
 Profile  
 
PostPosted: Nov 10, 2013 11:40 am 
Offline

Joined: Sep 22, 2013 2:27 pm
Posts: 262
Dear Cyrill,

another point that is not in favor of using UCl4 in a chloride reactor is the boiling point of UCl4. It is 791°C at 1 bar.
The power density in such a reactor is not equal but in the center x-fold higher than at the edges. The temperature in the upper center of the reactor will most probably let the UCl4 boil.

Best regards

Holger


Top
 Profile  
 
PostPosted: Nov 11, 2013 10:41 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
HolgerNarrog wrote:
Dear Cyrill,

another point that is not in favor of using UCl4 in a chloride reactor is the boiling point of UCl4. It is 791°C at 1 bar.
The power density in such a reactor is not equal but in the center x-fold higher than at the edges. The temperature in the upper center of the reactor will most probably let the UCl4 boil.

Best regards

Holger


As long as the UCl4 is a minor salt constituent it will be relatively non-volatile.

I actually consider UCl4's higher volatility to be a major advantage in reprocessing. ThCl4 also is reasonably low boiling. So you could recover these by simple distillation at much more reasonable temperatures than the fluoride counterparts...


Top
 Profile  
 
PostPosted: Nov 11, 2013 12:16 pm 
Offline

Joined: Sep 22, 2013 2:27 pm
Posts: 262
Hi Cyrill,

the chlorination of UCl3 to UCl4 is indeed a strong option to separate uraniumchloride from plutoniumchloride in a reprocessing unit some years after taken from the reactor.

In the same way as it evaporate in a distillation equipment it will gas out in the reactor. Gases might concentrate in dead edges or form bubbles and could create oszillations to the nuclear reaction. This is something no one would like in a reactor.

Holger


Top
 Profile  
 
PostPosted: Nov 11, 2013 12:40 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
HolgerNarrog wrote:
Hi Cyrill,

the chlorination of UCl3 to UCl4 is indeed a strong option to separate uraniumchloride from plutoniumchloride in a reprocessing unit some years after taken from the reactor.

In the same way as it evaporate in a distillation equipment it will gas out in the reactor. Gases might concentrate in dead edges or form bubbles and could create oszillations to the nuclear reaction. This is something no one would like in a reactor.

Holger


Like I told you, if you have a salt with only a minor fraction UCl4, then the total salt volatility is very low. Most of the salt would be NaCl or other carriers.

This is particularly so for thorium chloride cycles; I really wonder why these are not considered more often. Thorium chloride is very easy to recover, unlike thorium fluoride in fluoride reactors. ThCl4 also produces many more attractive low melting eutectics with NaCl and other carriers than U/Pu produces. In addition, the thorium cycle has very little uranium in it, since it is high quality fissile, which means that even if it is say 90% UCl4, it will only be at most a few percent of the total salt charge. Hence a very low volatility is created. For small fractions, you can assume the volatility increase is equivalent to the volatile species relative to its fraction, added to the volatility of the rest of the salt. So adding 1% of a salt that has 10000% the volatility of the rest of the salt will only double the volatility (from a very low number).

Distillation will be done at subatmospheric pressure, not necessarily strong vacuum but definately below 1 atm.


Top
 Profile  
 
PostPosted: Nov 11, 2013 3:42 pm 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 826
Location: Calgary, Alberta
Sorry I can't quote any references, but I expect quite low proportions of UCl4. If you have a high proportion of UCl4 that indicates that the redox potential is oxidising and not reducing which is likely to promote corrosion. I think that Eric Ottewitte's MCFR thesis expressed concerns regarding the corrosion chemistry if too much UCl4 was present with metal reactor components.


Top
 Profile  
 
PostPosted: Nov 11, 2013 4:31 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
Lindsay wrote:
Sorry I can't quote any references, but I expect quite low proportions of UCl4. If you have a high proportion of UCl4 that indicates that the redox potential is oxidising and not reducing which is likely to promote corrosion. I think that Eric Ottewitte's MCFR thesis expressed concerns regarding the corrosion chemistry if too much UCl4 was present with metal reactor components.


Yes, but ORNL's work on fluorides showed that corrosion dropped off rapidly at UF3/UF4 ratios of under 70. So a quite low corrosion rate is achieved with just 2-3% UF3, and it settles out at negligible rates at 10% UF3. ORNL actually found issues with too much UF3. Uranium was overreduced, producing a meangingful amount of uranium metal ions in the melt. These then reacted with pretty much anything, including graphite and (likely) carbon in Hastelloy.

Is there a reason to expect fundamentally different behaviour with chlorides? I think not, so a 90% UCl3, 10% UCl3 mix sounds pretty good for starting experimental investigations on corrosion.


Top
 Profile  
 
PostPosted: Nov 12, 2013 3:55 am 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 826
Location: Calgary, Alberta
In his UCLA Thesis Eric Ottewitte wrote:
3.3.3.3 Chemical behavior. UCl4 and UCl3 can coexist in equilibrium. In the core some UCl3 will pick up free chlorine to form UCl4. By itself UCl4 corrodes, but not when UCl3 predominates (i .e. when the stoichiometry is near 3 Cl atoms to 1 U atom).
Assuming that Eric is somewhere close to correct; 90% UCl3/ 10% UCl4 is probably a good place to start and move out from that point. Here there does seem to be a difference between a F and Cl based salts, UCl3 seems to be the almost neutral chloride and UF4, seems to the the almost neutral fluoride. I don't know why that is. For thorium its happy place is as ThCl4 or ThF4 so U behaves differently in a chloride system for reasons I cannot explain. Where's Nnadir when you need him.

Edit: added this quote also from Eric's thesis
Quote:
3.4.2.2 The effect of UCl4, presence in the core salt [31]. Pure UCl4 highly corrodes container metals, but both theory (stability of corrosion product chlorides) and laboratory experience show that dilute-UCl4 fuel salt attacks little: alloys of iron, nickel, or refractory metals such as molybdenum resist chloride fuel and blanket salts, providing the UCl3/UCl4 mix contains no more than a few percent UCl4.


Top
 Profile  
 
PostPosted: Nov 12, 2013 4:29 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
I am no chemist at all but I think there's a difference between whether +3 or +4 is more stable and whether a reducing environment is present.

Reducing environment is produced by for example adding some of the thorium as thorium metal. This will then mop up free fluorine.

It seems that Ottewitte never made that distinction. Certainly a salt that is not actively kept reducing by addition of metal or otherwise, would need nearly pure UCl3 since the fission chlorine demand is only about 3. So even with a tiny amount of UCl4 you then build up excess chlorine in the salt which will corrode. Not so with active redox control.

UCl4 is much less stable than NiCl2, so as long as there's no excess chlorine about (ie quickly reacted with thorium metal) there should be very low corrosion rates.

One thing is clear, the oxidation states of chlorides are often more complex than the oxidation states of the fluorides. But it seems to me that redox control would avoid this problem and may even make it more attractive as a corrosion control than with fluorides (more oxidation pairs?)


Top
 Profile  
 
PostPosted: Nov 12, 2013 4:50 am 
Offline
User avatar

Joined: May 24, 2009 4:42 am
Posts: 826
Location: Calgary, Alberta
One of the lessons that I take out chloride chemistry, having an 'always on' passive method of soaking up excess chlorine such as metallic thorium or uranium is highly desirable, assuming that is sufficient to keep the redox potential in the right place. If it is sufficient it sounds preferable to adding hydrogen of sodium metal as more aggressive reducing agents.

I think that the ratio of UCl3 to UCl4 is a symptom of redox potential in the mix, not a cause; excess Cl will promote more UCl4. Similarly if you actively reduce the mixture to get the ideal UCl3 to UCl4 ratio, the reduction process will bind Cl in the system and eventually change the UCl3/UCl4 equilibrium in favour of more UCl3.

But I think that I get your point, fissioning UCl4 liberates more Cl per fission than UCl3 does.


Top
 Profile  
 
PostPosted: Nov 12, 2013 5:24 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
Lindsay wrote:
One of the lessons that I take out chloride chemistry, having an 'always on' passive method of soaking up excess chlorine such as metallic thorium or uranium is highly desirable, assuming that is sufficient to keep the redox potential in the right place. If it is sufficient it sounds preferable to adding hydrogen of sodium metal as more aggressive reducing agents.


Yes. To be safe, the redox should be performed by the metal of the least stable major constituent in the mix. Sodium is clearly an effective reductant, but a little too effective. It may overreduce UCl3 to U metal. If thorium and/or uranium metal are used this is much easier to avoid. Sodium is also a bit tricky because it's a liquid at all reactor operating temperatures, so carryover of sodium into the reactor would be a design issue.

Quote:
I think that the ratio of UCl3 to UCl4 is a symptom of redox potential in the mix, not a cause; excess Cl will promote more UCl4. Similarly if you actively reduce the mixture to get the ideal UCl3 to UCl4 ratio, the reduction process will bind Cl in the system and eventually change the UCl3/UCl4 equilibrium in favour of more UCl3.


Yes, but it is more complicated than that. The excess chlorine from fission is initially produced as free chlorine, not as UCl3 or UCl4. So even with pure UCl3 there is no guarantee that chlorine will go to this UCl3 to make UCl4. UCl3 is quite stable actually. The free chlorine may react with other stuff as well such as whatever metal the reactor is made of. With thorium metal redox control this will not be an issue. Active redox control is simply mandatory for any molten salt reactor.

We really do need a nuclear chemist to chime in here.


Top
 Profile  
 
PostPosted: Nov 12, 2013 5:26 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
By the way. According to this document,

http://www-pub.iaea.org/MTCD/publicatio ... 85_prn.pdf

NaCl-ThCl4 has a eutectic at just 360 degrees Celsius. VERY attractive compared to NaCl-UCl3. And with the NaCl-ThCl4 system you can have all uranium as UCl3 if necessary (but this may actually be a bad idea as I speculated on previously since you have no margin to overreduction to uranium metal in the melt).


Top
 Profile  
 
PostPosted: Nov 12, 2013 5:40 pm 
Offline

Joined: Sep 22, 2013 2:27 pm
Posts: 262
Hi Cyril,

the paper has more than 400 pages and I do not find the data for the ThCl4 - NaCl phase diagram.

A melting temperature of 400°C would help a lot. It would reduce the corrosion issue by dimensions. It would make a lot of things easier.

It would be very easy to control the hm content. The ThCl4 becomes PaCl4 by neutron capture. The PaCl4 is gassed out and bred in a cooled tank to UCL4. UCl3 or UCl4 could be given in a controlled way to the reactor.

Best regards

Holger


Top
 Profile  
 
PostPosted: Nov 12, 2013 5:57 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
Do a ctrl+f on thcl4, you'll find figure 2 providing the ternary on ThCl4-NaCl-PuCl3. ThCl4-NaCl providing a 360C binary, with the plutonium ternary even lower than that, but it has too much plutonium too be of practical use. Takes about 5 seconds to find that diagram. Thanks to hotkeys.


Top
 Profile  
 
PostPosted: Nov 12, 2013 6:01 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5056
By the way, I highly doubt that PaCl4 will be gassed out to any noticeable extent. Neither will UCl4, for the simple reason there isn't much around and you never get to the boiling point @ 1 atm.

m.p. of 360C means the reactor likely operates at a low temp of 460C or lower, and high temp of under 600C. Quite a low temperature that is easy on the engineering. No need for that 1000C salt temp that theoretical impractical people all talk about (don't listen to these people, they are the fusion reactor people that are just not practical guys and gals, keep dreaming).


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 34 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