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PostPosted: Dec 01, 2012 1:37 pm 
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Cyril R wrote:
Lars wrote:
Cyril R wrote:
I think you'd want boron carbide balls or blocks in the still bottoms in that case, to make sure no criticality is every remotely possible.

I wonder if the still we need is even large enough to contain a critical mass?


It might not be a problem. The ORNL still was about 150 liters if memory serves. Not much at all, and in a pure U233 cycle there would be many kg of neutron poison for every kg of plutonium. This will not be the case if the reactor is started on Pu or mixed TRUs, something to consider in the still design I think for a TRU startup. A 1.5 or 1 fluid design would have lots of thorium which helps here.

If we run the still hot enough to cause the thorium to become a gas then I don't think it matters whether we have a 1/1.5/ or 2 fluid design.

Seems like we won't need to run the still until we've run the reactor for a while. I'm assuming a thermal reactor. Suppose we want to clean the salt once per year and we have a means post distillation to recover the Pu. Assume a 1GWe reactor started with 2 tonnes pure 239Pu. At the end of the first year we have burned 1 tonne of 239Pu, converted another 500 kg to 240Pu, created around 1 tonne 233U, and created around 360kg of salt-seeker fission products. About 25kg of this is Zr which will leave in the distillation gases and another 7.5kg as 137Cs. Net we get 330kg fission products/year.

Via the distillation the Li, Be, Zr, Cs, U, Th all leave. Initially in a 1 fluid we may also have 80kg of Pa (unsure of the volatility of Pa). If we process one days worth per batch that would mean we have around 2 kg Pu239, 2 kg Pu240, 1kg various fission products, and 1/3 kg Pa that will decay to U233. Just doesn't seem like enough stuff to be worried about criticality over.

Perhaps if you pile up 30 days of this stuff you might get to criticality - but then the simple rule is you don't store more than X days together.

If there is a subsequent process (fluorination?) to remove the Pu then the you remove the fission products that absorb neutrons but the key feature I think is to limit the batch size so that criticality isn't possible.


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PostPosted: Dec 01, 2012 3:16 pm 
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ORNL operated the still in batch mode of 67 days. They wanted to run as long as possible, 67 days was the max their NaK heat removal system could move out (this seems like another processing safety issue that we need to discuss).

Certainly looks like there won't be a criticality problem at all, thanks for the figures. Considering all the cooling tubes that ORNL planned on, and possibly a nickel packed bed filler to improve the still efficiency, criticality seems impossible. At 67 days the amount of poison fission products in the still was 14%!! This will never go critical.

According to the DMSR work, you can actually operate the reactor for 3 to 4 years before having to add fissile makeup. So the same would count for a distillation unit in a reprocessing design.

All this does support my suggestion of a DMSR type reactor started with TRUs running on Th fertile. You could easily burn out 90% of the TRUs in 30 years with a modest TRU feed. Then bring in a vacuum still and recover Li, Be, U and hopefully Th if we can get a bit higher temp. CsF could be removed with the same still operating in reverse (keep the bottoms this time) at more mild vacuum at the modest cost of losing some BeF2. Then the entire batch is ready to start a LFTR! And we only needed a still.


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PostPosted: Dec 01, 2012 4:36 pm 
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Sounds good - though we should check out the behavior if we run the still more frequently (say every 7-15 years) as the neutron losses to salt seeker fission products go up quadratically with time. One argument we will run into is whether it is kosher to have HEU in the core. It is certain the argument will be made. Starting with TRUs and thorium will result in HEU in the core. We could add some LEU to start with so that we don't get HEU until the fission products have built up. If we have to run LEU in the core it is still viable but then we do want to stretch out the salt cleaning to the longer side as we lose a lot of fissile each time we clean the salt. Also, such a DMSR will not be an isobreeder.

It does look like we want two stages of still. One way as you suggest is to run hot keeping the gas, then run cool keeping the liquid. Another way would be to run cool to remove the Cs first, then run hot to get the stuff you want to keep.

I suspect we want to keep the still bottoms for a year then run the still again to remove the Pa that converts to U233.

Is there an advantage to bringing in the still rather than including it with the initial reactor? The still needs full containment and once used I doubt we would want to move it. I almost wonder if it wants to be submerged in the buffer salt?


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PostPosted: Dec 01, 2012 5:32 pm 
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It's true there will be HEU in the core, but like you said before, how is this different from a solid fuel reactor making weapons grade Pu239 in the first months of operation? The problem with solid fuelled reactors is not fixable because plutonium can't be diluted with nonfissile plutonium, so during the first weeks especially there's weapons grade plutonium and relatively little fission products to safeguard it. I guess a TRU started DMSR could start with some U238 (natural or depleted uranium) which dilutes the U233 in the first months. Later there is sufficient fission product and U232 safeguarding. One thing with a DMSR is it has a lot of U232 after 30 years of operation, when it will be reprocessed. I think the key will be to not have any way the U233 could be cleanly removed - another reason to like stills over fluorinators. Jaro thinks this makes any 1.5 or 2 fluid a nonstarter. But I'm not well versed in the actual specifics of proliferation regulations. I know HEU is often downgraded in enrichment or mixing facilities, so these clearly have a license to handle this (with a high oversight and safeguard level of course). One could consider a DMSR to be something similar in regulatory terms. There's no way to move out fissile, so it's essentially a downgrading operation that improves the proliferation situation, just like a mixing facility that mixes HEU with U238. Certainly much better than having to guard today's HEU and WGPu forever. Another selling point here for thorium is the Pu238 which essentially denatures the plutonium. So it is both a uranium and plutonium downgrader.

Obviously in terms of development, starting out with a DMSR gives you 30 years to develop a high temperature still, and if that doesn't work then the standard still can be used (temporarily forfeit recovering thorium). I've always assumed that focusing on the reactor (DMSR) is the quickest to an operating reactor, without any need to do detailed engineering on fuel processing. But recently, looking at the offgas system details, there's a need for considerable engineering there. The vacuum still looks to be about the same level of complexity as the offgas system. But even if the still is included in the design, a TRU started reactor would benefit by not operating it for as long as possible, to keep the plutonium. Developing a plutonium recovery pyroprocess seems harder and would likely take more time.


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PostPosted: Dec 05, 2012 11:25 pm 
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U232 is helpful in any amount from a non-proliferation perspective, but the IAEA requires 1% or more for U233 to be considered denatured by U232.

M Ragheb, p 21 wrote:
The IAEA criterion for fuel self protection is a lower dose equivalent of 100 cSv(rem)/hour at a 1 m distance. Its denaturing requirement for U235 is 20%, for U233 with U238 it is 12%, and for denaturing with U232 it is 1%
https://netfiles.uiuc.edu/mragheb/www/Global%20and%20USA%20Thorium%20and%20Rare%20Earth%20Elements%20Resources.pdf
It is my understanding that thermal MSR's struggle to make 1% U232 without the addition of Th230. I believe that denaturing is important, we can argue about how important it is, but I would recommend going for the IAEA 'Denatured' tick every time unless there are some extremely important reasons not to.

Edit: Kang says "However, it would require a level of 2.4 percent U232 before the U-233 would satisfy the IAEA's standard for reduced physical protection requirements (>100 rem/hr at 1 meter)"

Ralph Moir also refers to 2.4% U232 in connection to 100 rem/hour gamma dose, so I don't know why M Ragheb refers to 1% as being the IAEA requirement. Assuming the threshold is 2.4%, U232 that's an even harder target to get to than 1%.


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PostPosted: Dec 06, 2012 6:00 am 
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Lindsay wrote:
U232 is helpful in any amount from a non-proliferation perspective, but the IAEA requires 1% or more for U233 to be considered denatured by U232.

M Ragheb, p 21 wrote:
The IAEA criterion for fuel self protection is a lower dose equivalent of 100 cSv(rem)/hour at a 1 m distance. Its denaturing requirement for U235 is 20%, for U233 with U238 it is 12%, and for denaturing with U232 it is 1%
https://netfiles.uiuc.edu/mragheb/www/Global%20and%20USA%20Thorium%20and%20Rare%20Earth%20Elements%20Resources.pdf
It is my understanding that thermal MSR's struggle to make 1% U232 without the addition of Th230. I believe that denaturing is important, we can argue about how important it is, but I would recommend going for the IAEA 'Denatured' tick every time unless there are some extremely important reasons not to.


In that case, U232 will never be enough. It takes many many decades to get anywhere near 1% U232 even with a fast spectrum.

If you believe in all this nonsense, then you can't solve the TRU waste problem. You need to add loads and loads of U238. It's one of the many downsides in suffocating on proliferation. Of course, waste is another non-problem, so you'd effectively be mitigating one non-problem in exchange of enlarging another non-problem.

I have no problems with that sort of window dressing, though. Whatever gets a LFTR or DMSR developed.


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PostPosted: Dec 07, 2012 2:57 am 
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If you have a fast spectrum liquid chloride reactor, you can boil off protactinium at 400C followed by uranium at 791C and thorium at a higher 921C in fractional distillation. You can reject all the more or less volatile matter as waste. Any plutonium put in as initial fissile feed is better burnt off or extracted by electro-winning, if required.


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PostPosted: Dec 07, 2012 12:14 pm 
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DMSR is a non-solution.

If it's implemented in a weapons-state (US, Russia, UK, France, China) then it's pointless because they would never be running MSRs to make weapons-grade material anyway. You might as well run an efficient LFTR. Pa-sep or no Pa-sep, they'd never be using such a machine to make stuff for bombs.

If it's "given" to some state with nefarious, weapons-lusting motives (and I wouldn't recommend that) then they can stop denaturing it if they wanted to do something illicit.

It solves no problems at all. It was conceived to appeal to Jimmy Carter who was trying to look super-righteous about proliferation. Except Jimmy didn't even mess around with nuclear, just wanted to burn coal instead.


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PostPosted: Dec 07, 2012 12:31 pm 
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Beg to differ.
In the ORNL-7207 design, only 279 kg of pure U-238 was added (years 2 and 3).
Page 31, Table 17.
All the rest was 20% LEU.
So in this design there was only a very brief period in which
the fuel went slightly over the denatured limit.
The rest of the time our rogue nation has to add the fuel
we give him or the reactor shuts down.


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PostPosted: Dec 07, 2012 1:06 pm 
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DMSR can also operate on mined uranium only. Needs a bit more fuel but this is offset by the much lower enrichment needed (also makes fuel appropriation easier). FLiU needs only ~2% enrichment. Enrichment is actually quite expensive, much more expensive than fuel fabrication, surprisingly.


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PostPosted: Dec 07, 2012 1:41 pm 
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Cyril R wrote:
DMSR can also operate on mined uranium only. Needs a bit more fuel but this is offset by the much lower enrichment needed (also makes fuel appropriation easier). FLiU needs only ~2% enrichment. Enrichment is actually quite expensive, much more expensive than fuel fabrication, surprisingly.

I don't understand this statement. Are you saying that we could run a DMSR with no thorium but use only enriched uranium? If so, I agree one could do that.
OR
Are you making the claim that we can feed a DMSR with only natural (0.7% 235U) uranium? If so, could you show the neutronics please? What is required for startup, how long it lasts, and what is left over at the end, and what is passed onto the next batch?


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PostPosted: Dec 07, 2012 3:24 pm 
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Natural uranium probably won't go critical in fluoride salt with graphite moderator. But it will be close, especially with a good 7Li enrichment. Probably just under 2% enrichment will do if there's no thorium.

Such a fuel cycle would guzzle quite a bit of 7Li and slightly enriched uranium, though. It would be worthwhile to recover this, via vacuum distillation, perhaps in a central facility that serves a large number of such reactors. I think we can recover >99% of the 7Li and >98% of the uranium this way (vacuum, ~1100 degree C).


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PostPosted: Dec 07, 2012 4:17 pm 
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Over time you will build up 236U - are you re-enriching the uranium?
Or is this running in batch mode like an LWR where periodically throw away everything and start over?
Have you looked at the fuel evolution to see what it does?


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PostPosted: Dec 07, 2012 4:45 pm 
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Kirk Sorensen wrote:
DMSR is a non-solution.

If it's implemented in a weapons-state (US, Russia, UK, France, China) then it's pointless because they would never be running MSRs to make weapons-grade material anyway. You might as well run an efficient LFTR. Pa-sep or no Pa-sep, they'd never be using such a machine to make stuff for bombs.

If it's "given" to some state with nefarious, weapons-lusting motives (and I wouldn't recommend that) then they can stop denaturing it if they wanted to do something illicit.

It solves no problems at all. It was conceived to appeal to Jimmy Carter who was trying to look super-righteous about proliferation. Except Jimmy didn't even mess around with nuclear, just wanted to burn coal instead.



Sorry but I really have to challenge that. How would a DMSR owner "stop" denaturing? The only way to do so would be to find another fissile source to makeup the shortfall as it is a simple converter. If that owner could get their hands on another source like Pu or highly enriched uranium as top up then they'd just use that for illicit purposes. Besides that, even if an owner did stop adding Low Enriched Uranium it would takes years before the fissile content in the Uranium would be anything much above the vague denatured limit.

I agree of course, anything can be twisted to illicit purposes if a nation/owner tries hard enough. Same is true of every pharmaceutical factory in the world that could be converted to produce biological weapons. So I agree that in general we are probably over speculating on things.

David LeBlanc


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PostPosted: Dec 07, 2012 5:11 pm 
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Lars wrote:
Over time you will build up 236U - are you re-enriching the uranium?
Or is this running in batch mode like an LWR where periodically throw away everything and start over?
Have you looked at the fuel evolution to see what it does?


It will probably be re-enriched a number of times, until the U236 becomes too much, at that point treat it as depleted uranium. Some of the U236 would end up in the enrichment tails, so it should be possible to recycle a reasonable number of times.

Probably the way to run this kind of reactor is to first run a couple years on the first load, then add makeup fissile, but first remove as much fuel salt to make room for the fresh uranium. It could be a continuous process from then on, constantly adding new makeup fuel and constantly relegating some of the fuel salt to a decay tank. It would be best to use a bit higher enriched makeup fuel, maybe 5%, to limit the volumes, and compensate for FP buildup over the fresh core (which might have only 2% LEU).

Haven't looked at fuel evolution, no can do. Need more training in nuclear engineering for that.


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