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PostPosted: Jun 19, 2016 1:32 pm 
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If I understand correctly, in a two fluid design, that the protactinium generated in blanket salt is extracted using a reductive extraction column with metallic bismuth, and then can be separated out via a series of electrolytic cells, where it then be held in a decay tank to become U233, and then the U233 can be removed via fluoridation to be eventually reintroduced into the fuel salt.

In a single fluid design, the protactinium can't be easily removed can it, and leaving it in the reactor would lead to unwanted fission products?

I wanted to make sure my understanding was correct.

Also what other disadvantages does a single fluid design have over a two fluid?

Thanks very much in advance.


Last edited by matthewwight on Jun 21, 2016 12:21 am, edited 1 time in total.

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PostPosted: Jun 19, 2016 6:33 pm 
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I also have questions and thought I'd describe my understanding of the operation of LFTR while also seeking correction.
matthewwight wrote:
In a single fluid design, the protactinium can't be easily removed can it, and leaving it in the reactor would lead to unwanted fission products?

I believe the problem with leaving the protactinium in the core is that it can take valuable neutrons from the core. If the neutron is captured by protactinium then that neutron is lost due to not being used to convert thorium to protactinium. That neutron capture will result in a non-fissile element. It will then take another neutron to convert that atom to something fissile, and then another neutron to fission that atom. A process that would normally take two neutrons now takes four.

I recall two ways to deal with this problem. One way is to separate out the protactinium just as with a two fluid design, place it in a storage tank until it decays into uranium, and then reintroduce it to the fuel salt. The other method I recall being proposed is to have enough fuel salt for the reactor that the protactinium has an opportunity to decay outside the core before it is pumped back in.

Separating out the protactinium is problematic in that it requires greater complexity to the reactor design. By increasing the size of the "reserve" salt to the point that it allows much of the protactinium to decay outside of the core means taking on the expense of much more expensive lithium salt, and much more enriched fuel to start the reactor.

Another way to deal with this is to not use LFTR but a related design that requires enriched fuel. By using enriched fuel the loss of a few neutrons in reactions like protactinium soaking up a few extra neutrons is made up for by the abundance of fissile material like U-235 or Pu-239. There will be some breeding of thorium to U-233, which is expected and required for the reactor to operate. There will be some U-238 in the fuel because the enriched uranium will be a mix of fissile U-235 and fertile U-238, the U-238 will breed to fissile Pu-239. Some designs do without the thorium and breed U-238 only but the concept is the same. Flibe Energy advocates for the breeder LFTR designs, Terrestrial Energy advocates for the DMSR (denatured molten salt reactor) or IMSR (integrated MSR) burner designs.

matthewwight wrote:
I wanted to make sure my understanding was correct.

Likewise.

matthewwight wrote:
Also what other disadvantageous does a single fluid design have over a two fluid?

There is a trade off in both designs, which is why this is being discussed vigorously in the thorium for energy community. I also welcome others more knowledgeable than I to comment. One design variation I've seen, and which I believe has most chance for success, takes elements of both the single and dual fluid designs. These design variants are called the "1.5 fluid" reactor. This concept has a number of variants but what they share is that breeding takes place in both the blanket and the core salt, fission occurs only in the core though. The single fluid designs have breeding and burning in a single salt. The two salt reactor designs have burning in the center core salt, breeding in the blanket salt. The 1.5 fluid design has breeding in the blanket and core salt, and burning only in the core.

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PostPosted: Jun 19, 2016 8:29 pm 
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Double fluid designs have traditionally been hamstrung by the necessity of maintaining a seal between the two fluids in the horrendous thermal and radiation environment of an active reactor core.

Single fluid designs avoid that entire problem, as the walls of the reactor can be simple shapes and are also partially protected by self shielding by the fuel salt.
I believe this reason is the reason that most of the original MSR team's later work was focussed on the so-called DMSR.
Which still has surprisingly good neutron economy compared to existing reactors.


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PostPosted: Jun 19, 2016 9:44 pm 
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Reductive extraction using metallic thorium dissolved in bismuth will preferentially remove uranium and protactinium from a salt mixture, in that preference.

The blanket salt of a two-fluid design will have very little uranium and a small amount of protactinium.

The single salt of a one-fluid breeder design will have a quite a bit of uranium and a small amount of protactinium. Therefore they planned to fluorinate out the uranium first and then use reductive extraction to pull out residual uranium and any protactinium.

Protactinium can therefore be removed from either design, it's just that in the single-fluid breeder there's a lot of uranium that will come out first, or must first be removed by fluorination.


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PostPosted: Jun 19, 2016 9:51 pm 
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E Ireland wrote:
Double fluid designs have traditionally been hamstrung by the necessity of maintaining a seal between the two fluids in the horrendous thermal and radiation environment of an active reactor core.


You seem to know a lot about this. Have you been working this issue?


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PostPosted: Jun 19, 2016 11:21 pm 
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Thank you, Mr. Sorensen, for your reply. A question though about the extracted protactinium and uranium mix.

Kirk Sorensen wrote:
The single salt of a one-fluid breeder design will have a quite a bit of uranium and a small amount of protactinium. Therefore they planned to fluorinate out the uranium first and then use reductive extraction to pull out residual uranium and any protactinium.


At this point would there be another step to remove the uranium from the protactinium before being placed in a decay tank? I assume not. I also assume the uranium would be extracted from the decay tank by fluorination as the protactinium decays, which would then be reintroduced to the fuel salt.

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PostPosted: Jun 20, 2016 8:43 am 
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Kirk Sorensen wrote:
E Ireland wrote:
Double fluid designs have traditionally been hamstrung by the necessity of maintaining a seal between the two fluids in the horrendous thermal and radiation environment of an active reactor core.


You seem to know a lot about this. Have you been working this issue?

I had a go, problem is, as I am sure you know, the neutron flux experienced by the seperator is not actually constant across the reactor, which causes all sorts of differential expansion/contraction effects if you use graphite. Which causes lots and lots of problems unless you go for some type of core shape like a CANDU Calandria [simple independent tubes that can change shape relative to each other], but then you end up with thousands of end terminations where the tubes meet the face assemblies (although they are in a far lower radiation position).

In the end I kind of concluded that a single fluid design is probably too much easier to operate to bother.


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PostPosted: Jun 21, 2016 10:35 am 
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Is one advantage to minimize the accumulation of TRUs in the waste stream?

Is another fine control over neutron economy? What are the unbiased papers on this question?

Once a two-fluid system is selected, we find the ways to make it work; do the materials work and designs, correct? Did Dr. Weinberg prefer the two-fluid design?

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PostPosted: Jun 26, 2016 1:32 pm 
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Tim Meyer wrote:
Is one advantage to minimize the accumulation of TRUs in the waste stream?

Is another fine control over neutron economy? What are the unbiased papers on this question?

Once a two-fluid system is selected, we find the ways to make it work; do the materials work and designs, correct? Did Dr. Weinberg prefer the two-fluid design?


So if the protactinium absorbs a neutron before it finishes decaying to U233 wouldn't that lead to undesirable fission products?


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PostPosted: Jun 27, 2016 1:02 am 
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Main advantage of liquid fuel is escape/removal of gaseous fission product Xe.
Main advantage of thorium is higher production of superior fissile U-233.
Main function of second fluid is as a coolant. It is best combined with moderator and not blanket. It is best kept clean of nuclear fuel.
Blanket is best as metallic thorium which after irradiation can be electro refined to thorium and uranium.
Use of high boiling organic compounds as moderator cum coolant will keep down corrosion like an engine oil. All these together will keep down costs.


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PostPosted: Jun 28, 2016 10:07 am 
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matthewwight wrote:
So if the protactinium absorbs a neutron before it finishes decaying to U233 wouldn't that lead to undesirable fission products?
Pa-233 absorption forms Pa-234 that rapidly beta decays to U-234. Neutron absorption is undesirable.

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PostPosted: Jul 03, 2016 12:11 am 
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Pa-234 is an actinoid like the others, the transuranics. They are inevitable and the design should have a balance between their reduction and other benefits. It may be best to keep them in fuel' Thorium as fertile fuel is the best that could be done.


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PostPosted: Jul 03, 2016 7:55 am 
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U-234 has a neutron-capture cross-section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral. In a nuclear reactor, non-fissile isotopes capture a neutron breeding fissile isotopes. U-234 captures a neutron and transmutes to U-235 more easily and therefore at a greater rate than U-238 to Pu-239 (via neptunium-239) because U-238 has a much smaller neutron-capture cross-section of just 2.7 barns.
From Re: NNL Reports on Advanced Reactors and Thorium Fuel Cycle, Kirk Sorensen on September 17, 2012 wrote:
The blanket salt of the reactor (LiF-BeF2-ThF4) should have a low fission product inventory, particularly if protactinium isolation has been employed in reactor operation. If the fission product inventory is sufficiently low, then a simple fluorination step to remove any residual UF4 should be all that is necessary to prepare the blanket salt for recycling to the next LFTR reactor. If there is a more significant fission product inventory then additional purification steps will be necessary. Thorium is not extracted through fluorination, nor is it removed by distillation, so there is a strong incentive to keep fission product inventory at a minimum rather than to have to deal with the challenge of trying to separate lanthanide trifluoride fission products from thorium tetrafluoride salts. This challenge was considered by the Molten-Salt Reactor Program during their investigation of the "one-fluid" reactor concept and was found to be quite challenging; for this reason we have been strongly incentivized to investigate the "two-fluid" reactor concept that is the basis for LFTR.

Does the "two-fluid" design still hold today? Many feel the DMSR (one fluid) is the "best" design and TE's IMSR is moving forward for licensing in Canada.

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