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PostPosted: Sep 09, 2010 10:24 am 
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I'm going to guess that we can directly use SNF which has been declad and fluorinated as fissile feed. That should be a lot cheaper than separated Pu.


Iain, my guess would be the other way that SNF wouldn't give a net reactivity gain if we tried to simply fluorinate it and add it to a molten salt reactor. It just gets burned down too low in U235 content (used to be about 1%, now higher burnups has that much lower). The Pu has nice fissile but the absorption cross section of the fertile Pu isotopes is so high that it doesn't have as high a net reactivity as you might expect.

However, what is likely is that if one fluorinates SNF and simply gets rid of the uranium that comes out (very low radiotoxicity) then the remaining fission product and transuranic "ash" would indeed give a nice net reactivity gain. This has been proposed by several people. One study I saw assumed that 5% of the uranium would stay behind with the ash but would still have a good long burnup ability if used again as solid fuels (which of course are hard to fabricate but trivial to use for molten salt reactors). This idea is a bit of a pain for a DMSR design since it would bring with it a fair amount of fission products but interesting angle nonetheless since it might be a quite inexpensive feed of fissile material.

David L.


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PostPosted: Sep 09, 2010 10:30 am 
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Luke wrote:
The U is too low in fissile to be used, unless you are running U/Pu cycle and it is just a source of fertile. It might make reasonable CANDU fuel.
Absolutely !
....a cheap way of making use of LWR SNF in HW-MSRs !
Who doesn't want cheap here ? ("cheaper than coal")


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PostPosted: Sep 09, 2010 10:38 am 
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Cyril R wrote:
Re Nickel: Ni58 neutron, alpha barely shows on the nndc databases. Less than 0.001 barn @ 2 MeV and downhill from there.

Yes the Cx is 0.001 barns but the boron concentration is only 0.01% so there are more than 10^4 more atoms of nickel than boron in the wall. At 2MeV we will 95% of the generated helium is from 58Ni(n,alpha). The French had problems with wall lifetime due to 58Ni(n,alpha) and their spectrum is considerably slower than a chloride salt reactor. They plan on a mixed approach where the top and bottom are reflectors (they use thick Hastalloy as the reflector) and they plan on periodically replacing the reflector. Theirs is a 1.5 fluid design and is 2.5meters in diameter.

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Re the wall. It is big because of single fluid design being a big reactor (roughly similar total core power density as PWR).

What is the diameter you are thinking of?
What is the fuel concentration? How much fissile are you planning on?

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Re neutron leakage effect. My thinking is that leakage affects burnup in this reactor because more leakage means the once through cycle has to stop earlier as the conversion ratio will drop sooner and we don’t want to add fissile. So indirectly it affects fuel evolution with less pu being burned out. That makes sense to you?

Yes that makes some sense but it is more complex to think about if the reactor fuel is never steady state. Allow me to think out loud.

The idea is to start with clean salt and an initial fissile load of lots of SNF/plutonium and LEU20 with enough thorium added to make the reactor just critical. As the initial fissile burns you generate both u233 and pu239 and consume th232 and u238. You keep adding thorium to keep the machine just critical. Initially, I think this means you add more thorium than is consumed due to the high eta of plutonium and the lack of fission product absorptions. The hope is that fuel evolves to consume almost all of the plutonium and u238 so that the machine is running on 232Th and 233U. All this while fission products are building up and reducing the reactivity of the machine so that later in life we have to add less thorium than what is required for replacement to keep it critical. Eventually the machine stalls out under the fission product absorption load.

The trick is going to be the balance of plutonium and LEU20. As I understand it, you don't want to add fissile once the reactor is started and you don't want to have fancy chemical processing (specifically the ability to remove thorium from the reactor w/o removing plutonium). As the fertile (232Th and 238U) transmutes your reactivity will go up, as the fission products build up it will go down.

Seems like the higher the initial concentration of fissile the longer your reactor will run. Also higher plutonium content relative to LEU20 in your startup fissile will result in higher eta and thus more u233 produced initially allowing more thorium to be added and again resulting in a longer life reactor.

It sounds possible for a single fluid reactor. I suspect that the wall and economics remain a challenge but if we can put absorbers in front of the wall then it may well work. Maybe I'm too conservative but I'm still worried about getting sufficient R&D funds over enough time to make this happen. It seems hard enough to get there starting from MSRE and a chloride reactor would start considerably further behind.


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PostPosted: Sep 09, 2010 12:18 pm 
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Lars, how does the helium production rate compare to that of an epithermal machine? I'm currently thinking the best material is high iron alloy running at a bit lower temp. There will be a coating for added corrosion protection. A thick tungsten or pyrolitic carbon/diamond coating would be ideal. Nickel might be a more conservative option if we're starving for R&D money (but why go for rapid online processing if you're on a tight R&D budget?).

For the size I'm thinking something like the REBUS reactor. Its leaky with IIRC almost 0.4 n/f lost. But with Pu startup and spectrum above 100 keV averaged it seems like we have the neutrons to spare. The REBUS assumed natural chlorine but without online fission product lanthanide removal we'll need the enriched chlorine for sure.

For easier replacement one would like to have a tube reactor. Something like Jaro's but imagine no heavy water and the tubes getting very fat in the middle where it will be critical. This also seperates the reflector mechanically. It would still be single fluid because we want to avoid the blanket processing step in this configuration.

For the denaturant U238 the LWR SNF Jaro mentioned will be fine. No enrichment required. In fact more enrichment means more U238 has to be added intitially to keep things LEU with all that U235. Besides we get more waste eating sales pitch.

David mentions something very interesting about FP coming with the initial TRU loading. Big benefit in my mind is that the fast chloride reactor can be started up with greater tolerance to fission products, so the TRU initial loading is easier (cheaper) to get. That might offset some of the expense for higher fissile loading. Direct chlorination & chloride vacuum distillation is probably adequate purity, and the lower temperatures may make it cheaper.

David does win all prizes for lowest R&D money, almost certainly. But if some group were to develop a once through chlorides reactor with the purpose of TRU conversion to U233, that would be a very promising development, IMHO.


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PostPosted: Sep 09, 2010 4:08 pm 
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Cyril R wrote:
Lars, how does the helium production rate compare to that of an epithermal machine?

I haven't run the numbers but I'm thinking there is a minimum rate between thermal where the production is dominated by the two step 58Ni->59Ni->He and the fastish spectrum of the French where it is dominated by 58Ni->He. The optimal point will varying depending on how long you want the wall to last since the thermal end is quadratic in time and the fast end is linear.
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I'm currently thinking the best material is high iron alloy running at a bit lower temp. There will be a coating for added corrosion protection. A thick tungsten or pyrolitic carbon/diamond coating would be ideal. Nickel might be a more conservative option if we're starving for R&D money (but why go for rapid online processing if you're on a tight R&D budget?).

This comes to which you think takes more R&D getting a new vessel material qualified or online processing. Personally I'm thinking getting new materials qualified will take 10x the R&D budget that a vacuum distiller will take. But in honest my opinion on this isn't worth anything.
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For the size I'm thinking something like the REBUS reactor. Its leaky with IIRC almost 0.4 n/f lost. But with Pu startup and spectrum above 100 keV averaged it seems like we have the neutrons to spare. The REBUS assumed natural chlorine but without online fission product lanthanide removal we'll need the enriched chlorine for sure.

Assuming you do chlorine enrichment you gain 0.17 neutrons/fission. But fissioning u233 rather than Pu239 loses you 0.4 neutrons/fission at 1MeV. There isn't much breeding gain so it seems like you will have to cut the neutron losses to leakage in half. You could do that by doubling the salt volume but they are already at 5 tonnes fissile / GWth (contrast this with 1-2 tonnes fissile / GWe of LFTR). Seems like a cost challenge.

Quote:
For easier replacement one would like to have a tube reactor. Something like Jaro's but imagine no heavy water and the tubes getting very fat in the middle where it will be critical. This also seperates the reflector mechanically. It would still be single fluid because we want to avoid the blanket processing step in this configuration.

I'm not getting it. I thought you were doing a single fat cylinder like REBUS (3.8m diam/3.5m high).
Quote:
For the denaturant U238 the LWR SNF Jaro mentioned will be fine. No enrichment required. In fact more enrichment means more U238 has to be added intitially to keep things LEU with all that U235. Besides we get more waste eating sales pitch.

I'm confused again. If you aren't adding u235 for startup and using only Pu then what is the denaturant that you are adding.
Quote:
David mentions something very interesting about FP coming with the initial TRU loading. Big benefit in my mind is that the fast chloride reactor can be started up with greater tolerance to fission products, so the TRU initial loading is easier (cheaper) to get. That might offset some of the expense for higher fissile loading. Direct chlorination & chloride vacuum distillation is probably adequate purity, and the lower temperatures may make it cheaper.

Certainly making fuel from SNF for an MSR should be much cheaper than making MOX (and more productive too).


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PostPosted: Sep 10, 2010 7:10 am 
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David wrote:
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As for #4. What is the cost/kWh levelised for the 10x larger fissile startup? From what I've read its very tiny and 10x tiny is still not much. We need to get rid of the TRUs one way or the other. If we put in more its good public relations. Important to market this aspect well.


As an example, the REBUS 3700 chloride fast reactor design (3700 MWth) needed 11 tonnes of fissile Pu per GWe to start up. That is 18 to 20 tonnes of LWR Pu which typically quoted as costing about 100$ a gram to process out of spent fuel. Perhaps cheaper processing methods could be used for molten salt reactors (especially fluoride based) but a high end of 2 billion$ per GWe is certainly not a tiny expense in my view! The IFR typically assumed startup on 20% LEU because of the high cost of Pu (something only found in the fine print!).

David LeBlanc


Thanks David. Cetainly no tiny expense. About 1.5 cents/kWh with 20 year operation @ 10% interest rate. With 5% interest its 1 cent/kWh. A significant cost but not a show stopper in my mind. We save a bit on the cost of online processing. (especially development money & time seems important to me). No fuel fabrication either, like any fluid fuel – this cost saving is of similar magnitude.

Odds are cheaper TRU can be had, since with the fast reactor we’re less discriminating on fission product contamination (and not discriminating at all on U contamination). In fact some fission products would help with proliferation concerns early on. We don’t care much about the level of transplutonium either. That is a pain for solid fuel fabrication, we don’t care that much with fluid fuel.

Not using PUREX makes things cheaper and safer too. Since we need fluid fuel, fluorination or chlorination would be fine. Keep in mind also that if we’re going to take the majority of the SNF TRU worldwide, there will be economies of scale. It’s hard to see how the cost will be so high even then.


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PostPosted: Sep 10, 2010 9:38 am 
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Cyril R wrote:
.....Not using PUREX makes things cheaper and safer too. Since we need fluid fuel, fluorination or chlorination would be fine. Keep in mind also that if we’re going to take the majority of the SNF TRU worldwide, there will be economies of scale......
This presentation of some Japanese work outlines a molten salt route for processing spent LWR fuel (dirty UO2) into IFR fuel. We can miss out the salt removal and fuel fabrication steps.


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PostPosted: Sep 10, 2010 1:14 pm 
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The electroprocessing chlorides papers usually mention a cadmium anode bath. How much cadmium ends up in the fuel?


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PostPosted: Sep 11, 2010 4:15 am 
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If they know the residual Cd level, they aren't saying, at least in what I have read. For very fast spectrum reactors (IFR or chloride) traces of Cd aren't that bad - they steal a few neutrons, but it would be tolerable. In a thermal spectrum, where the X-sec goes up to 10^5 - 10^6 barns, it would be a real problem.

We may not even need it. The Cd electrode is used to give some selectivity between U and the TRUs. It lets you deposit nearly pure U on the main electrode (for IFR blanket) while pulling out U+TRU that is rich enough for IFR core into the Cd. We may not need this. We can pull out U (happens naturally), leaving the TRUs in the salt - where we want them anyway.


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PostPosted: Sep 11, 2010 4:27 am 
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So then you just electrolyse at low voltage to get the noble metal fission products reduced to metallic state, scoop off the chloride mixture, and vacuum distill that to get most of the lanthanide chlorides out?

What is the cost target for the chlorination/distillation route to TRU extraction for the MSR? This has got to be MUCH cheaper than PUREX, and much less messy too (cleanup costs are a big cost component for PUREX).

(Oops, I wrote 1-1.5 cents/kWh but its 2-3 cents/kWh actually, for David's 2 Gdollar worst case)


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