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PostPosted: Jul 17, 2011 11:14 pm 
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This is a cut and paste from somewhere else under a new topic, but it's good stuff.

jaro wrote:
That's interesting Lindsay,
Lindsay wrote:
two designs that I am familiar with only run 10 - 11% effective enrichment, i.e. the fissile loading is only 10 -11% of the total heavy metal in the reactor.
....maybe I missed it when reading various reports, but I don't recall ever seeing that 10 - 11% effective enrichment figure for MCFRs -- do you mind please citing the source ?
....the lowest I've seen - for LMFBRs - is 13%, with more typical range beeing 15 - 18% enrichment. Is it possible that MCFRs are so much more efficient with neutrons ? (10 - 11% is close to the absolute minimum for a large experimental pile of pure U metal).


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PostPosted: Jul 17, 2011 11:15 pm 
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Well those number are really interesting and in stark contrast to the numbers I was quoting, I think that you can see why I'm interested in the possibilities.

The two references are:
EIR 332 Table 2.18, this is in PDF archive, and
REBUS-3700, by Mourogov and Bokov, there's a copy posted in this thread http://www.energyfromthorium.com/forum/viewtopic.php?f=5&t=1333&st=0&sk=t&sd=a&start=15

The equivalent enrichments are 10.4 - 11.0% and 11.0% respectively :P

jaro wrote:
Is it possible that MCFRs are so much more efficient with neutrons ?
I don't know, I was thinking that power density may be part of it, but then the REBUS runs 100 kW/L while Taube is running 750 kW/L (potentially problematic IMHO), so it doen't seem to be a power density thing AND REBUS is running natural Cl, so there's some serious losses to Cl with that approach and yet it still runs and can breed.

The only other thought is that if you look at the mass of materials in the core, there's a powerfully large % of heavy metal by mass, perhaps that is helping the cause.


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PostPosted: Jul 17, 2011 11:16 pm 
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jaro wrote:
Lindsay wrote:
The two references are:
EIR 332 Table 2.18, this is in PDF archive, and
REBUS-3700, by Mourogov and Bokov, there's a copy posted in this thread http://www.energyfromthorium.com/forum/viewtopic.php?f=5&t=1333&st=0&sk=t&sd=a&start=15

The equivalent enrichments are 10.4 - 11.0% and 11.0% respectively :P

Thanks !

.....I think I can see where the difference comes from: Footnote 5 in the Rebus paper explains that "It means the enrichment normalized to reactivity worth of 239Pu" -- whereas normally I'm used to seeing enrichment normalized to reactivity worth of U235, which yields the bigger enrichment number....


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PostPosted: Jul 17, 2011 11:28 pm 
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jaro wrote:
....the lowest I've seen - for LMFBRs - is 13%, with more typical range beeing 15 - 18% enrichment. Is it possible that MCFRs are so much more efficient with neutrons ? (10 - 11% is close to the absolute minimum for a large experimental pile of pure U metal).

So due to reduced reactivity of U235 as compared to Pu239, the fissile requirements for a U235 started fast reactor could be about 50% more than for a Pu239/241 started core?

That's good to know and it's another pointer to using Pu to start fast spectrum reactors. Looking at the fast spectrum (n,f) cross sections U233 and Pu241 definitely sit at the top of the heap, another good reason to accumulate U233 and not destroy it.

However all this talk about TRU extraction for startup charges is nice, but unless than fissile is cost competitive with U, then I would say let's startup on uranium.
Attachment:
(n,f) for Pu239, Pu241, U233, U233, U238.png
(n,f) for Pu239, Pu241, U233, U233, U238.png [ 16.45 KiB | Viewed 2914 times ]


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PostPosted: Jul 18, 2011 1:05 am 
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I agree we should start on 20% LEU for thermal reactors UNLESS
a) there is strong political reward for starting with SNF - for example, there is considerable concern about spent nuclear fuel now and MOX really doesn't do much so it could be that in the US and England that the pathway to build the first LFTR is as a TRU burner.
b) if the LFTR is graphite-less then you need around 8:1 thorium to u233. So if you start with 20% LEU then there isn't much room for thorium (since most of the neutron budget for fertile is used by the 80% 238U). I suspect one would need to augment the fissile in 20% LEU with a similar amount of fissile in SNF/Pu.

I really am attracted to the graphite free design but for the initial units we likely will need to graphite to give us more latitude in the start-up fissile and to completely remove concerns about recriticality under accident conditions.


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PostPosted: Jul 18, 2011 6:49 am 
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There are other considerations than just cost.

An important one is that plutonium is different from uranium so you get high quality U233 rather than a bunch of U238 contamination. It’s not clear at all that you can isobreed by starting on LEU with that pesky 80% U238. Plus you get U236 poisoning as U235 captures more neutrons than U233 and U236 doesn’t fast fission (U234 does). It may be cheaper to use LEU20% for startup but if you can't go into a path to isobreeding this way then that is a heavy price to pay for this initial cheapness.

Plutonium is more expensive for startup but it is manageable for thermal or epithermal designs (remember the startup is slightly lower than LWRs). The real problem with plutonium startup, I think, is that we don’t know how efficient the processing will be or even what processing tech we need to use. If you use a no-processing MSR such as DMSR then this problem is solved.

Also remember that eating waste is an important source of funding. If you can manage completely private funding, which I highly doubt, then going for LEU and no thorium in a converter reactor seems the best path. With public funding I think that waste-eating by starting up on waste and then going to pure thorium makes a lot of sense. There's lots of ways to build a reactor, why build an MSR and where do you get the justification for public funds? Since MSRs are good at waste-eating, due to homogeneous liquid fuel and simple processing, that would be an argument to chose MSRs for waste-eating.


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PostPosted: Jul 18, 2011 9:44 am 
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Cyril R wrote:
There are other considerations than just cost.

An important one is that plutonium is different from uranium so you get high quality U233 rather than a bunch of U238 contamination. It’s not clear at all that you can isobreed by starting on LEU with that pesky 80% U238. Plus you get U236 poisoning as U235 captures more neutrons than U233 and U236 doesn’t fast fission (U234 does). It may be cheaper to use LEU20% for startup but if you can't go into a path to isobreeding this way then that is a heavy price to pay for this initial cheapness.

For a graphite less design it is doubtful that one can evolve the fuel enough to get to isobreeding starting with LEU20. If you have a blanket system you could feed the core until you have saved up enough 233U in the blanket, then remove the uranium from the core and insert the 233U as David LeBlanc has suggested. But I suspect this will fall into the same troublesome arguments as starting the core on HEU. However, for more thermal designs with a graphite core I'm sure we can start on LEU20 and evolve the fuel to be a isobreeder.
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Plutonium is more expensive for startup but it is manageable for thermal or epithermal designs (remember the startup is slightly lower than LWRs). The real problem with plutonium startup, I think, is that we don’t know how efficient the processing will be or even what processing tech we need to use. If you use a no-processing MSR such as DMSR then this problem is solved.

So, are you thinking of starting up on plutonium and then feeding LEU to keep the reactor going? Are you going to deliberately add 238U in the beginning to keep the content of the core LEU? How long would you plan to keep running with the same fuel salt and what would you plan to do with the fuel salt once that run is done?
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Also remember that eating waste is an important source of funding. If you can manage completely private funding, which I highly doubt, then going for LEU and no thorium in a converter reactor seems the best path. With public funding I think that waste-eating by starting up on waste and then going to pure thorium makes a lot of sense. There's lots of ways to build a reactor, why build an MSR and where do you get the justification for public funds? Since MSRs are good at waste-eating, due to homogeneous liquid fuel and simple processing, that would be an argument to chose MSRs for waste-eating.

Precisely my point a) and this is important if we are talking about developing LFTR in the US/Europe/Japan and possibly Russia. If we are developing LFTR elsewhere then burning plutonium isn't likely to be a political win and we should look to the lower R&D cost road for the first units. At this point I'm assuming we don't know where the money comes from and hence I'd like to keep both roads open.


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PostPosted: Jul 18, 2011 9:55 am 
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We had some discussions allong similar lines in the early days of the Aim High thread. Even the fast LFTRs (which aren't as fast as LMFBRs) will start on LEU, but they won't get CR>1 because the 238U steals too many neutrons and the spectrum isn't fast enough to breed on the U/Pu cycle. You need a mix of LEU, spent fuel transuranics and thorium to get a system that is self-sustaining from the start. This doesn't rule out LEU-only startup, though....

If you start with no thorium at all, you get a converter reactor that runs on ~13% U-235, and makes some plutonium. You feed it 20% LEU, and remove some ~13% LEU, which gets sent back for re-enrichment, a process which has the net effect of adding pure 235-U to the reactor, without ever handling anything above the LEU threshold. With no Th in the core, 232-U levels are low enough for the enrichment plant to cope, as with recovered uranium from reprocessing LWR fuel. Over time, plutonium and higher elements build up in the reactor. Once there are enough, you shut down, remove all the uranium, and replace with a mix of 20%-LEU and thorium, such that the system is now an iso-breeder. Over the next ~decade, most of the 238-U burns off and you're back to a pure Th-233-U cycle.

{Lars posted while I was typing}

As Lars said, a graphite-free design needs too high a fissile/fertile ratio to evolve to an isobreeder if fed only 20%-LEU, unless you have some way to remove the excess 238-U.


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PostPosted: Jul 18, 2011 11:03 am 
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Luke wrote:
Even the fast LFTRs (which aren't as fast as LMFBRs) will start on LEU, but they won't get CR>1 because the 238U steals too many neutrons and the spectrum isn't fast enough to breed on the U/Pu cycle.

If we're talking about MCFRs, as mentioned in the Rebus posts above, then the spectrum should actually be faster than LMFBRs, since chlorine is heavier than both oxygen (in UO2 LMFBR fuel) and sodium (coolant).
Consequently, it should also be a better breeder in the U/Pu cycle.


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PostPosted: Jul 18, 2011 2:31 pm 
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Quote:
So, are you thinking of starting up on plutonium and then feeding LEU to keep the reactor going?


Start on Pu, feed on Pu. The feeding requirements are modest and gradual compared to the startup so if we're going to use Pu for startup we might as well use it for top-up. Can also feed Am and Cm if required, along with the Pu.

This could be a graphite moderated single fluid reactor with no online fuel processing. Without all that U238 you actually do very well on trifluorides percent. Once you choke on plutonium or damaged the graphite too much you stop the reactor. At this point you've burned down 90+ percent of the Pu you've fed in, and get lots of goody U233 for isobreeder startup (assumed available at this later point). Not bad.


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PostPosted: Jul 18, 2011 2:35 pm 
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Quote:
Are you going to deliberately add 238U in the beginning to keep the content of the core LEU?


There's no need, the fissile is safeguarded in the irradiated fuel and there's no means of removing fissile present in the reactor system. Very similar to bomb grade plutonium in LWR cores after a fresh core loading, as you've mentioned before, this is allowed and considered perfectly proliferation proof.


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PostPosted: Jul 18, 2011 3:10 pm 
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Good. We are thinking along the similar lines.

I wonder if we have a fluorinator inside the reactor that handles small volumes if that would be acceptable. If so, then we could clean up the salt on a 10 year cycle. Of coarse, when we do so we will end up with the plutonium in with fission products. These may need to be sent to a central site for separation.


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PostPosted: Jul 18, 2011 9:59 pm 
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jaro wrote:
If we're talking about MCFRs, as mentioned in the Rebus posts above, then the spectrum should actually be faster than LMFBRs, since chlorine is heavier than both oxygen (in UO2 LMFBR fuel) and sodium (coolant).

Consequently, it should also be a better breeder in the U/Pu cycle.

Definitely, that breeding capability has been driving my interest, specifically the opportunity to breed U233 to support thermal reactors and reduce the total U238 in the thermal cores.

Specificially, EIR 332 shows a BR of 1.34 as a U233 machine staring up with 10.4% enrichment (showing what a great fuel U233 is even in the fast spectrum) and 1.58 as a steady state U/Pu burner breeding U233, so the U/Pu cycle is definitely more effective for this design concept.

And yes these cores are very fast, for REBUS there are 0.39 neutrons leak out of the core per fission event (could be used for breeding U233) and it still manages a BR of 1.03 without any blanket

The EIR 332 core has a steady state BR of 1.58 and U238 absorbs 30.6 n/100 n produced and then produces 25.45/100 n produced so the spectrum is fast enough that U238 is doing a lot of the heavy lifting. Pu240 normally considered fertile absorbs 2.79 n/100 and produces 3.37 n/100, so even Pu240 is working pretty hard for the cause. Heavy actinides just don't stand a chance in that environmant which is also part of the appeal.

Regarding using reactor grade Pu as startup charges or fuel addition for thermal converter designs, I favour deeply thermal Th/U233 or LEU/Th/U233 designs using little or no Pu and using very fast MCFR cores for burning the trash including Pu and breeding U233 to support the thermal cores. In order to give the thermal cores the highest possible proliferation resistance I favour avoiding Pu use or production where possible. LEU works well enough to get going and well enough to run DMSR for 30 years, we can do a lot without having to put Pu into thermal cores. That vision does rely somewhat on having access to breeding cores to rebalance the U233/U238 ratio in the thermal cores.

The nice thing is that there is not one right answer, with MSR technology there are many ways of sensibly meeting the overall objectives. This is just one man's vision of how it could be done. You guys all have your own.


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PostPosted: Jul 22, 2011 3:47 pm 
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Lindsay,
if you are really interested in fast chlorides there is also in the docs section an interesting old British (Winfrith) paper about a 6000 MWth/2500 MWe fast chloride breeder, I think it can worth the read
http://www.energyfromthorium.com/pdf/AEEW-R956.pdf


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PostPosted: Jul 22, 2011 6:57 pm 
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Thanks Alex I do have that one, but I will go back and read it again. If you come across any other MCFR papers, please let me know as I am 'really interested in fast chlorides'.

Cheers


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