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PostPosted: Dec 05, 2008 12:42 pm 
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Hello, all ! I'm new here!

From an other thread
viewtopic.php?f=5&t=308&start=0&st=0&sk=t&sd=a
I don' t really understand which could be the proliferation issues in molten salts (chloride) fast breeders; at last, correct me if I wrong, even in a uranium-plutonium cycle all the plutonium is produced and burned inside the reactor and never exit the reactor to be diverted for other uses, for example reprocessing. I'd like to know your own opinions


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PostPosted: Dec 05, 2008 12:51 pm 
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The short answer is that plutonium produced in a fast spectrum reactor has very high isotopic quality (nearly all Pu-239). The reason for this is that plutonium-239 will preferentially fission rather than absorb a neutron in the fast spectrum, so production of plutonium-240 (which degrades isotopic quality) is very limited.

That fast breeders tend to be very attractive to large nations like the US and Soviet Union who wanted to build reactors AND produce excess weapons-grade materials for a growing nuclear stockpile.


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PostPosted: Dec 05, 2008 12:59 pm 
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I believe any two fluid molten reactor could have U238 in the blanket rather than Th232. If such a reactor is a fast breader, then the 239Pu in the blanket would be realitively pure and be a good way to build weapons. I'm thinking all two fluid reactors should have Th232 in the blanket for this reason.


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PostPosted: Dec 05, 2008 1:03 pm 
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Lars wrote:
I believe any two fluid molten reactor could have U238 in the blanket rather than Th232. If such a reactor is a fast breader, then the 239Pu in the blanket would be realitively pure and be a good way to build weapons. I'm thinking all two fluid reactors should have Th232 in the blanket for this reason.

Yes, but at least in a fluoride reactor there's no simple way to remove bred plutonium from a uranium blanket like there is to remove bred uranium from a thorium blanket. Fluorination would remove the uranium blanket preferentially rather than the plutonium. As far as removal techniques in a chloride reactor blanket, I'm not sure.

My preferred embodiment for a liquid-chloride fast reactor would be to have a two-fluid reactor where the core fluid consisted of transuranic-chlorides to be burned, and the blanket consisted of thorium-chloride for U233 production. The U233 would be removed as it was produced and used to start LFTRs. The only purpose of the chloride reactor would be the destruction (through fission) of TRUs.


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PostPosted: Dec 05, 2008 1:08 pm 
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Kirk Sorensen wrote:
The short answer is that plutonium produced in a fast spectrum reactor has very high isotopic quality (nearly all Pu-239). The reason for this is that plutonium-239 will preferentially fission rather than absorb a neutron in the fast spectrum, so production of plutonium-240 (which degrades isotopic quality) is very limited.

That fast breeders tend to be very attractive to large nations like the US and Soviet Union who wanted to build reactors AND produce excess weapons-grade materials for a growing nuclear stockpile.


From Alexander deVolpi's Google Knol NUCLEAR WEAPONS PROLIFERATION: Controversy About Demilitarizing Plutonium,
Quote:
Fast-spectrum breeder reactors can generate byproduct plutonium that averages about 80% fissile, also well below the utility level for military weapons. Laser-isotope improvement of such plutonium might eventually be practical, but now it is still in the “iffy” stage.


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PostPosted: Dec 05, 2008 1:44 pm 
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One other thing with the blanket running 232Th versus 238U. It is easier to separate 233U from 232Th than 239Pu from 238U so even proliferation issues aside we should strongly prefer 232Th blanket for cost reasons.


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PostPosted: Dec 05, 2008 4:38 pm 
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Kirk Sorensen wrote:
The short answer is that plutonium produced in a fast spectrum reactor has very high isotopic quality (nearly all Pu-239). The reason for this is that plutonium-239 will preferentially fission rather than absorb a neutron in the fast spectrum, so production of plutonium-240 (which degrades isotopic quality) is very limited.



Yes, I know, but as far I understand there is no need in molten salt reactors to extract fuel in the blanket for future reprocessing, this is instead a problem with solid fuel liquid cooled fast breeders or am I wrong? In chloride fast breeders the fissile fuel is simply recirculated from the blanket inside the reactor or maybe there is something I don' t understand... It seems me as a not expert, maybe I wrong, that you consider a proliferation issue the simple fact there is an high quantity (and quality, of course) of plutonium in the blanket; I could agree with sodium breeders but NOT with molten salts breeders....

Kirk Sorensen wrote:
...
My preferred embodiment for a liquid-chloride fast reactor would be to have a two-fluid reactor where the core fluid consisted of transuranic-chlorides to be burned, and the blanket consisted of thorium-chloride for U233 production. The U233 would be removed as it was produced and used to start LFTRs. The only purpose of the chloride reactor would be the destruction (through fission) of TRUs.


Of course it is a very interesting and useful application, but I guess that one of the goal at least in the long term should be to harvest the enormous quantity of uranium 238 stored everywhere both as depleted uranium in the uranium enrichment tails (about 0,2%) and in low enriched uranium in LWR or CANDU spent fuel (about 1% or 0,2%, respectively). Only a fast breeder operating in the uranium-plutonium cycle can do that and as far as I understand the molten salts fast version is - at least - the most proliferative resistant


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PostPosted: Dec 05, 2008 5:28 pm 
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Alex,
A reactor with a blanket salt will have local processing to transfer the newly generated fissile material to the fuel salt.
The choices for the blanket fertile material are 232Th or 238U. Both are very available and we use them in minute quantities so there is no concern over supply (and by the way no particular advantage that Th is 3-4x as abundant as U there is sufficient for every concieveable deployment of either).

When transfering the fissile from the blanket salt we must first separate it from the fertile in the blanket salt.
It is much easier to separate 233U from 232Th and transfer it over than to separate 239Pu from 238U.

Proliferation concerns come up if someone can easily modify the reactor to produce weapons. For weapons grade 239Pu you want to keep it clean from other isotopes of Pu. If the 239Pu is left in the reactor some will fission and some will capture a neutron to become 240Pu (and capture more neutrons to become 241Pu, and 242Pu). In an LWR the fuel is left in the reactor for 2-3 years and the quality of Pu gets to be relatively poor for weapons. Not theoretically impossible but hard enough that one would choose another way to make a weapon. In an MSR it is much more feasible to rapidly extract the Pu from the blanket and have almost pure 239Pu. We really don't want to advance the knowledge of how to do such things.

There is some proliferation concern about using pure 233U coming from the blanket as well. Fortunately, it will naturally get contaimentated with 232U (and daughter products) that make it a painful material to build weapons with. Further, we can keep a bucket of 238U handy to dump into the blanket should someone try to take a reactor by force. This will make the reactors U similar to natural U (except for all the fission products making it hard to handle). So bottom line, while there is some proliferation concerns with 233U I think there are sufficient safeguards to address them.


The other thing to note is that normally (not always) there is fertile placed into the fuel salt as well. If using the existing uranium stockpiles is a goal you can use 238U as the fertile in the fuel. This has some advantages in the chemical processing (as it is fairly difficult to deal with using 232Th as the fertile in the fuel). The main concerns with this approach are:
1) long lived dangerous radioactive wastes - a reactor using 238U as the fertile will have 239Pu as the fissile and will be a fast spectrum. As such, the inventory of transuranics will be 20x higher than a reactor using 232Th as its fertile. The recycling process needs to be better understood to know what impact this higher TRU inventory has on the amount of TRU's that leak into the waste flow.
2) political. Plutonium has a bad public image so there is an uphill battle to fight in the public's mind that it is OK to have 7-8 tonnes of Pu inside a reactor. Makes for an easy soundbite ad against us.

One political advantage is that we could run on spent fuel uranium rather than natural or depleted uranium. It is a pretty attractive political message to be able to say that we can run for 150 years on the spent fuel already onsite at the reactors and consume virtually all of the uranium and plutonium there. I know as an engineer that the spent fuel uranium isn't nasty but I'm pretty confident that the opposition will try to scare the public with a message that the stuff is dangerously radioactive.

As an engineer I want to be sure we properly understand the implication on the waste flow of using 238U as the fertile. My guess is that it will add an interation (or multiple interations for liquid bismuth reduction) to the recycling process to get the TRU waste flow down to the same level as if we are on a 232Th cycle. If not, then we should use the 232Th cycle. If so, we need to look to the costs of recycling for the two options. We need to be much further along before we can properly find the costs and compare them.

Assuming that for the same waste flow the 238U and 232Th cycles are similar costs then we would need to decide which is the easier political sell.

By the way, this entire discussion applies equally to chloride or fluoride fast reactors. Personally, I lean to fluoride simply to keep the R&D costs down and get to the finish line faster. Kirk leans toward chloride because it gives a faster spectrum and hence will have a lower inventory of higher actinides.


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PostPosted: Dec 07, 2008 9:44 pm 
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Quote:
My preferred embodiment for a liquid-chloride fast reactor would be to have a two-fluid reactor where the core fluid consisted of transuranic-chlorides to be burned, and the blanket consisted of thorium-chloride for U233 production. The U233 would be removed as it was produced and used to start LFTRs. The only purpose of the chloride reactor would be the destruction (through fission) of TRUs.


I know that using chloride in the core fuel fluid makes for faster neutrons & better fission of the trans-uranics, but I don't see what advantage there would be to using chloride rather than fluoride in the thorium blanket.

BTW my mental picture of the chloride reactor is something like Davids double wall pipe, with the fissionable fuel in a cylinder ~1m diameter surrounded by a blanket of fertile material a meter or 2 thick.

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PostPosted: Dec 07, 2008 9:50 pm 
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Jim Baerg wrote:
BTW my mental picture of the chloride reactor is something like Davids double wall pipe, with the fissionable fuel in a cylinder ~1m diameter surrounded by a blanket of fertile material a meter or 2 thick.

A chloride reactor might need a fairly different core geometry strategy than a fluoride reactor, with the main issue being control of recriticality. Eric Ottewitte proposed a core geometry that looked something like this,

Image

immersed in a cylindrical tank of blanket fluid.


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PostPosted: Dec 07, 2008 10:43 pm 
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Given that core design I can see that chloride rather than fluoride in the blanket is needed.

Would the U233 be extracted from the blanket by fluorination or would something else have to be done with a ThCl mix?

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PostPosted: Dec 08, 2008 9:32 am 
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Lars wrote:
Alex,
A reactor with a blanket salt will have local processing to transfer the newly generated fissile material to the fuel salt.
The choices for the blanket fertile material are 232Th or 238U. Both are very available and we use them in minute quantities so there is no concern over supply (and by the way no particular advantage that Th is 3-4x as abundant as U there is sufficient for every concieveable deployment of either).When transfering the fissile from the blanket salt we must first separate it from the fertile in the blanket salt.
It is much easier to separate 233U from 232Th and transfer it over than to separate 239Pu from 238U.


My personal concern is NOT about supply; simply, we have an almost infinite reserve of low enriched o deplete uranium already mined

By the way, why is much easier to reprocess and separate uranium 233 from thorium, instead plutonium from uranium 238? If I remember correctly, al least for solid fuel, is quite the opposite....



Lars wrote:
One political advantage is that we could run on spent fuel uranium rather than natural or depleted uranium. It is a pretty attractive political message to be able to say that we can run for 150 years on the spent fuel already onsite at the reactors and consume virtually all of the uranium and plutonium there.


Indeed, spent fuel is mostly uranium deplete or low enriched, see above.
However, I understand that a thorium breeder is less complex and less problematic from the proliferation point of view than a fast plutonium cycle breeder


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PostPosted: Dec 08, 2008 10:06 am 
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Alex P wrote:
By the way, why is much easier to reprocess and separate uranium 233 from thorium, instead plutonium from uranium 238? If I remember correctly, al least for solid fuel, is quite the opposite....


You can just fluorinate bred U from the blanket salt by bubbling F though it. Trivial compared to solid fuel anything ...


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PostPosted: Dec 08, 2008 10:32 am 
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Kirk Sorensen wrote:
Lars wrote:
I believe any two fluid molten reactor could have U238 in the blanket rather than Th232. If such a reactor is a fast breader, then the 239Pu in the blanket would be realitively pure and be a good way to build weapons. I'm thinking all two fluid reactors should have Th232 in the blanket for this reason.

Yes, but at least in a fluoride reactor there's no simple way to remove bred plutonium from a uranium blanket like there is to remove bred uranium from a thorium blanket. Fluorination would remove the uranium blanket preferentially rather than the plutonium. As far as removal techniques in a chloride reactor blanket, I'm not sure.

My preferred embodiment for a liquid-chloride fast reactor would be to have a two-fluid reactor where the core fluid consisted of transuranic-chlorides to be burned, and the blanket consisted of thorium-chloride for U233 production. The U233 would be removed as it was produced and used to start LFTRs. The only purpose of the chloride reactor would be the destruction (through fission) of TRUs.


I think that fast spectrum 238U-239Pu liquid fuel reactors should be used to burn up not only Pu and TRUs but also vast quantities of 238U in spent fuel and depleted uranium. Let all such power reactors in the world be under IAEA safeguards to alleviate proliferation concerns. IAEA could also act as world's Nuclear Regulatory Commission to supervise safety and provide a uniform blend of control and expediency. There should be no restriction on any country on Nuclear power so long as they abide by IAEA rules. Non-government design proposals may also be examined and approved for benefit of all member countries. Only uranium-poor countries shall bother to develop thorium fuel which could later be used by anyone interested.


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PostPosted: Dec 08, 2008 11:55 am 
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Whether to use 232Th or 238U or no fertile at all in the fuel salt is a trade to be made.

232Th in the fuel makes the job of removing the fission products more difficult which will translate to higher costs.

238U in the fuel means 20x as much Pu present in the reactor. At a minimum this will mean more processing to be sure that we don't have 20x as much Pu/Am/Cm in the waste stream. This will add cost. Processing is an immature art at the moment, paper studies, computer simulati on, and some lab work using grams of stuff mixed to resemble spent fuel. I can find very few references to work done on an industrial scale. While it is politically attractive to consume the spent fuel uranium there really is no waste problem advantage with this unless we have trouble separating the TRU's from the U in the spent fuel. The other minor advantage would be that we would not have to have any mines. There is a sufficient supply of SNF + depleted uranium to never mine any more uranium or thorium - period. However, we already are reducing mining by more than 100x so the driver in this decision should be the cost and the waste.

No fertile in the fuel at all means the doppler term for thermal reactivity coefficient changes from negative to positive. We would need to ensure the total thermal reactivity coefficient remains negative for this configuration to even be considered.

In other words, it is premature technically to choose between these. Personally, I think Th in the fuel salt is probably going to be the winner based on better waste performance for similar cost but time will tell.


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