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PostPosted: Nov 10, 2012 7:28 am 
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You seem to have done a mental optimization routine. Care to put some numbers on it?

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PostPosted: Nov 10, 2012 8:19 am 
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KitemanSA wrote:
You seem to have done a mental optimization routine. Care to put some numbers on it?


If we look at a David Leblanc tube in shell design, with a cylindrical core, 1 meter diameter. Then a cylindrical blanket around that of say 2 meters thick, that´s 5 meters total. The volume of blanket would be 24 times the volume of the core. For David´s 6.6 meters core there would be around 5 cubic meters fuel salt in core. With that much in the tapered plenum, HX, etc outside the core it is 10 cubic meters fuel salt. For around 1 GWth reactor. The blanket salt would be around 160 cubic meters, adding the end plenum volume gets to around 200 cubic meters. So total we have maybe 20x the blanket salt to add, over 400 cubic meters per GWe. That is a lot of salt to absorb heat from the core. And the vessel around that has large surface area to passive shed heat, too. You could do without buffer salt, but you do need a blanket internal PRACS as the surface area of the core cylinder is not sufficient to passively transfer heat into the blanket salt.


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PostPosted: Nov 10, 2012 10:02 am 
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I was thinking more like, twice the volume of salt at $X/in3 vs. one unit plus cost of extract holdout system (both of which have been shown to result in the same breeding ratio with the optimal %age of U232....what, you mean they haven't... then modify the designs...). You know, THAT kind of design optimization routine, with numbers. :shock:

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PostPosted: Nov 10, 2012 10:55 am 
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KitemanSA wrote:
I was thinking more like, twice the volume of salt at $X/in3 vs. one unit plus cost of extract holdout system (both of which have been shown to result in the same breeding ratio with the optimal %age of U232....what, you mean they haven't... then modify the designs...). You know, THAT kind of design optimization routine, with numbers. :shock:


The cost of the NaF/ThF4 eutectic is low enough that it doesn´t matter. I didn´t do the sort of optimization that you had in mind, but was simply pointing out the other advantages of going for a bigger blanket.

But, doing a quick check, 400 cubic meters blanket salt/GWe, 22.5% ThF4, this is 90 cubic meters ThF4. At a density of ThF4 of around 5 or 6, let´s take 6, this is 540 tonnes ThF4 containing 407 tonnes Th. At $100-kg high purity Th it is $41 million. Chump change for a multi billion dollar powerplant. Prices will likely be much lower once a big market is established. Cost of NaF is at least one order of magnitude lower, feel free to ignore this.

Now, you might be able to build a still for less than $41 million. And you could save a bit off the vessel cost as well. But you won´t get the thermal storage benefit, and have to bother with large ThF4 additions with an operating reactor...


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PostPosted: Nov 10, 2012 11:17 am 
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I don't have numbers for the processing end so it is hard to compare there. But for increasing the blanket here are some numbers. The fuel salt cost is dominated by the fissile content. For a 1GWe reactor you need somewhere between 500 and 5000 kg of fissile. I know this is a huge range. On the very low end you have Dr. LeBlanc's tube in tube reactor claims. On the high side you have the fast, graphite less, beryllium less reactor of the French group. Personally I think we should expect to have 2 tonnes of 235U if we start with 20%LEU. The fissile costs around $131 Million (higher in the early days when ordering 20%LEU without fuel fabrication is an unusual order). The old ORNL costs estimates for FLiBe was about 10% of the fissile cost for the MSBR (IIRC) which had a lot of fuel salt per unit of fissile. Note that the cost estimates for 99.999% 7Li were quite uncertain then and still are quite uncertain as the costs of depleting natural lithium of 6Li is unclear. But let's assume a cost of $13M for the 30 cubic meters of fuel salt. [Repeat this cost could be substantially off but if so we can switch to a different salt]

To stay conservative let's assume FLiBe for the blanket salt. We can save money in the later stages of planning by switching to a lower cost salt. The French design uses a 0.4meter thick blanket which leaks 20% of the neutrons that enter it. If we assume a 1.5m radius, 3m high core (20 m^3 for the core fuel salt plus another 10 m^3 external for the pumps/pipes/HX) then a 0.4m blanket is around 20 m^3 of salt ($9M). Increasing the blanket to 0.8m would reduce the leakage to 4% and increase the volume to 55m^3 and cost to $24M and cuts the Pa capture losses to 20/55. Increasing the blanket to 2m thick results in a leakage of 0.03%, volume of 248 m^3, cuts Pa capture losses to 20/248, and cost of $108M.

To properly complete this analysis one should dig up the Pa capture losses in the blanket and the neutron flux entering the blanket so you get a total for neutron losses.

Compared to Pa isolation the thicker blanket approach will save neutrons both from dilution of the Pa and from reduced leakage.

For a true two fluid approach, with no fertile in the fuel salt the blanket sees about half of all the neutrons (in a simplistic sense half the neutrons go to fission and the other half go to convert thorium to 233U and since there is no thorium in the fuel salt in this design then half the neutrons must go to the blanket). In such a design one requires a thicker blanket both to reduce the leakage and to reduce losses to Pa capture.

In a 1.5 fluid design about 60-80% of the thorium neutron capture happens in the fuel salt so the blanket only sees around 10-20% of the neutrons. Hence, the blanket contains a much lower inventory of Pa. Compared to a 2 fluid design both blanket Pa capture and neutron leakage are reduced 5-10x versus a 1.5 fluid design with the same thickness blanket. So in a 1.5 fluid design you would expect a thinner blanket to be the optimal point. Note that this is a minor difference in the 2 versus 1.5 fluid designs - the design drivers for making this choice aren't centered around the blanket thickness.

Incorporating Pa isolation in the design will also strengthen the arguments of the anti-proliferation crowd. Adding to that political battle represents an additional hurdle and risk to a design that incorporates Pa isolation.


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PostPosted: Dec 25, 2013 8:04 am 
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Hi, bumping an old thread here. I've been hearing arguments in favour and against separately storing protactinium. The main solution for not separating is decreasing the flux by increasing the salt (i.e. lowering the concentration of fuel). I've plotted a graph comparing the cross-sections of the actinides in the fuel (see attachment). There's clearly a large bias for Pa-233 capturing a neutron over Th-232. Wouldn't lowering the concentration have a bad effect on the breeding with Th-232 (thus the general U-233 production as a function of time) or am I missing something here?


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PostPosted: Dec 25, 2013 9:49 am 
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Gilliam wrote:
Hi, bumping an old thread here. I've been hearing arguments in favour and against separately storing protactinium. The main solution for not separating is decreasing the flux by increasing the salt (i.e. lowering the concentration of fuel). I've plotted a graph comparing the cross-sections of the actinides in the fuel (see attachment). There's clearly a large bias for Pa-233 capturing a neutron over Th-232. Wouldn't lowering the concentration have a bad effect on the breeding with Th-232 (thus the general U-233 production as a function of time) or am I missing something here?


Hi Gilliam - when you increase the salt volume you would increase the thorium inventory in the reactor and primary system as well to keep the same overall Th concentration in the salt. This will naturally decrease the relative concentration of the Pa-233 produced in the reactor, since essentially the same amount of Pa-233 will be produced, but with a much larger salt (and thorium) inventory in the system. Running the larger inventory through the reactor would then decrease the overall specific power and decrease Pa-233 capture. You can also increase the core size and operate the reactor at lower power density, which is also done to increase graphite lifetime. Of course this comes at a cost of lower power production, so it is a tradeoff that you need to make in your design process.


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PostPosted: Dec 25, 2013 2:03 pm 
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I see. I was under the impression that they meant adding a salt without fuel (pure FLiBe for example) to the core in turn decreasing fuel concentration, but this isn't the case.


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PostPosted: Dec 25, 2013 3:46 pm 
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Most of the previous discussion of increasing the salt really means increasing the blanket salt. Most specifically, increasing the thorium in the blanket salt (but generally also increasing the volume of the salt itself) so that the chances of thorium absorbing a neutron rather than protactinium go up proportionately. Increasing the thorium in the blanket is an easy tradeoff as the downsides are pretty minimal.

If you have a 1.5 fluid system then you also have protactinium losses in the fuel salt. However, increasing thorium in the fuel salt is more painful as this also implies increasing the fissile content proportionately and that is a much more expensive proposition.


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