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 Post subject: Re: MSFR
PostPosted: Jan 26, 2011 11:38 am 
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What I'm wondering about this design is if you are going to use a hard (i.e. fast) neutron spectrum, why would you use thorium?

U-Pu gives considerably greater neutron yield per fission, and need to process 2 fewer actinides (no Th, and also no Pa). You could also more effectively use UF3-UF4 redox to limit corrosion.

The only downsides I could see are more higher actinides at end-of-life, and (possibly?) lower proliferation resistance.


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 Post subject: Re: MSFR
PostPosted: Jan 26, 2011 12:17 pm 
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Mike wrote:
What I'm wondering about this design is if you are going to use a hard (i.e. fast) neutron spectrum, why would you use thorium?

I suspect that MSFR is a misnomer - with that much LiF in the salt.

.....could be intentional, since just about everything else in GenIV is "fast" :lol:


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 Post subject: Re: MSFR
PostPosted: Jan 26, 2011 12:43 pm 
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Mike wrote:
What I'm wondering about this design is if you are going to use a hard (i.e. fast) neutron spectrum, why would you use thorium?
U-Pu gives considerably greater neutron yield per fission.....
The spectrum isn't quite fast enough to run on U/Pu cycle. Neutrons/fission for Pu goes up very steeply at the high end, a reactor running on U/Pu needs to have most neitrons above 1 MeV. No fluoride salt reactor can do that (except possibly jaro's carrier free version), it requires chloride salts, for which no prototype reactor has ever been built.

The MSFR design is 'nearly fast', lots of 100 keV neutrons, but not enough MeV ones. It will incinerate actinide waste quite effectively, but only 233U fission gives enough neutrons for sustainable opperation without fissile feed.


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 Post subject: Re: MSFR
PostPosted: Jan 26, 2011 2:14 pm 
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From the French work it appears the spectrum for the fastest LiF version is getting average neutron energy in the keV region already, and an efficient reactor only needs 40 keV average spectrum to breakeven on breeding.

Sodium solid fuel fast reactors are claiming 1.03 breeding ratio with around 100 keV average spectrum, and the spectrum division doesn't look radically different for a fast fluoride reactor. From the French work it seems that you get lots of MeV neutrons with fastish fluorides or faster sodium-uranium metals, since that's where most neutrons are born.

Swapping LiF out for NaF or NaF-RbF eutectic might be just enough to get to 40 keV average spectrum, which could mean a fluoride reactor could isobreed on U-Pu if it is extremely efficient. Also get higher Pu solubility which is needed for faster fluoride machines.


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 Post subject: Re: MSFR
PostPosted: Jan 26, 2011 2:31 pm 
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Please see this paper for some figures:

http://hal.in2p3.fr/docs/00/03/09/52/PDF/TMSR.pdf

Figure 3, neutron spectrum by channel radius. 10 cm channel, or single channel, the fastest design, looks like a very fast spectrum to me, with very good negative reactivity.

With a single channel there is no graphite in the central part of the core and the need for flux flattening is thereby removed. That means less leakage and lower neutron loading on the barrier or vessel. It might isobreed on U-Pu and will certainly be good on U/Th with 1.5 fluid as the French group envisions it. Slower processing too, with fewer processing losses.


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 Post subject: Re: MSFR
PostPosted: Jan 26, 2011 2:47 pm 
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Found it!

http://hal.in2p3.fr/docs/00/32/64/66/PDF/FP355.pdf

Looks like almost the same as SFR, exept for the region 100 keV-1 MeV where the SFR has more neutrons. Would have been nice to see a NaF or NaF-RbF high heavy metal spectrum.


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 Post subject: Re: MSFR
PostPosted: Feb 03, 2011 3:48 pm 
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Cyril R,
am I wrong or one of the greatest merit of MSRs (particurally for two fluids versions) is the lower fissile start-up vs fast (solid fuel or not) reactors ? If it's so, I don't really understand the need to have a so fast spectrum reactor and thus an higher fissile start up, at a stated reprocessing cost of ~ 100 thousands $ per kg of HM; indeed, I'd focus on MSR versions with fissile start-up of only 200-1000 kg (even with no moderator at all) of uranium-233 or LWR transuranics equivalent or a few tonnes of uranium 235 as low enriched uranium in the "denatured" version DMSR using a mix of thorium and LEU...can you comment it ?


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 Post subject: Re: MSFR
PostPosted: Feb 03, 2011 4:06 pm 
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I believe the French team went down the road of graphite-less reactors to avoid graphite waste. In France it has been declared that graphite waste requires geologic disposal and the volumes are sufficient to drive up the cost of dealing with the waste.

They also wanted to avoid Be to avoid dealing with its poisonous properties. (I don't quite get this one - we already are dealing with 700C, no oxygen, and high radiation so what difference would the idea that Be is poisonous make - no one should go in there ever).

They went fast to achieve higher breeding. The doubling time is lower with the faster spectrum. (Personally, I think we can generate sufficient startup charges using uranium enrichment and that this path is a better choice so I recommend iso-breeders rather than breeders).

You can not go fast, fluoride salts, and 239Pu as you primary fissile. You need lots of fissile for a fast spectrum and the fluoride salts won't dissolve enough plutonium. So you spectrum softens, and as the spectrum softens the percentage of Pu that is fissile goes down. If you want a molten salt, fast spectrum, plutonium machine you need to avoid lithium fluorides. Due to the inelastic capture of fluorine I suspect you need to avoid fluorines altogether.


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 Post subject: Re: MSFR
PostPosted: Feb 03, 2011 6:37 pm 
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I think to understand why the French group are where they are, you have to look at how they got there:-

1)Start where ORNL left off. ORNL put a vast number of highly talented man-hours into the 2-fluid reactor, and gave up. Pursue the 1-fluid option, as ORNL were when they got shut down.

2)Modern nuclear data and computer codes show that the MSBR design, with fluid channels in large graphite blocks, has a slightly positive thermal reactivity coefficient. This is unlicensable. To get -ve coefficients requires either much more graphite (huge core, and too many neutrons lost to the graphite to get to breakeven), or much less. Once you look at 'less', the safety coefficients keep improving all the way down to no graphite at all. This gives a bonus of less waste, no shutdowns to replace graphite, and a simple core. The downside is increased startup fissile requirements as the neutron spectrum gets harder.

3)Once at a graphite-free design, minimising the reactor doubling time for a fixed reprocessing rate pushes the design to a faster spectrum, high fissile inventory reactor, because even though this requires a lot of fissile to start, the breeding rate goes up faster than the inventory, as absorptions into fission products become less likely.

4)Change the name to MSFR.


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 Post subject: Re: MSFR
PostPosted: Feb 03, 2011 8:00 pm 
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Luke wrote:
I think to understand why the French group are where they are, you have to look at how they got there:-

1)Start where ORNL left off. ORNL put a vast number of highly talented man-hours into the 2-fluid reactor, and gave up. Pursue the 1-fluid option, as ORNL were when they got shut down.

2)Modern nuclear data and computer codes show that the MSBR design, with fluid channels in large graphite blocks, has a slightly positive thermal reactivity coefficient. This is unlicensable. To get -ve coefficients requires either much more graphite (huge core, and too many neutrons lost to the graphite to get to breakeven), or much less. Once you look at 'less', the safety coefficients keep improving all the way down to no graphite at all. This gives a bonus of less waste, no shutdowns to replace graphite, and a simple core. The downside is increased startup fissile requirements as the neutron spectrum gets harder.

3)Once at a graphite-free design, minimising the reactor doubling time for a fixed reprocessing rate pushes the design to a faster spectrum, high fissile inventory reactor, because even though this requires a lot of fissile to start, the breeding rate goes up faster than the inventory, as absorptions into fission products become less likely.

4)Change the name to MSFR.

Interesting sequence Luke.

Any idea what happened to the graphite-free two-fluid concept ?


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 Post subject: Re: MSFR
PostPosted: Feb 03, 2011 9:19 pm 
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A pure two fluid concept where there is no fertile in the fuel salt requires half the neutrons to go through the wall. This is a much heavier neutron load than a 1.5 fluid reactor where there is enough fertile in the fuel salt to do most of the breeding inside the fuel salt. In that case the wall is only exposed to 5% of the neutrons. So, it comes down to which problem do you think is easier to solve
a) the materials problem of making a wall that can endure 10x the neutron exposure
b) the chemical problem of separating the thorium from the fission products

They (and I) chose option b). We don't have to do a very good job of it. In fact, initially we likely don't want to do it at all otherwise we create a headache by concentrating the decay heat too much.

I'm pretty sure David did talk to them about his pure two fluid, tube in a tube concept but I don't see any evidence that they are looking into it.


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 Post subject: Re: MSFR
PostPosted: Feb 03, 2011 9:36 pm 
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Makes sense.

Too bad they never looked at alternatives to graphite.

Lars wrote:
I'm pretty sure David did talk to them about his pure two fluid, tube in a tube concept but I don't see any evidence that they are looking into it.

I doubt anyone will do serious analysis of high aspect ratio configurations -- very prone to oscillation (...unless that's what you actually want!).
There was a very good reason why ORNL guys limited their non-graphite two-fluid concepts to spheres !


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 Post subject: Re: MSFR
PostPosted: Feb 04, 2011 4:59 am 
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Alex P wrote:
Cyril R,
am I wrong or one of the greatest merit of MSRs (particurally for two fluids versions) is the lower fissile start-up vs fast (solid fuel or not) reactors ? If it's so, I don't really understand the need to have a so fast spectrum reactor and thus an higher fissile start up, at a stated reprocessing cost of ~ 100 thousands $ per kg of HM; indeed, I'd focus on MSR versions with fissile start-up of only 200-1000 kg (even with no moderator at all) of uranium-233 or LWR transuranics equivalent or a few tonnes of uranium 235 as low enriched uranium in the "denatured" version DMSR using a mix of thorium and LEU...can you comment it ?


There are many ways one can design a molten salt reactor which has a good chance of being effective and low cost. The more thermal designs do look good for mass production but this isn’t necessarily much cheaper or easier than faster designs. I really don’t see how LWR transuranics reprocessing costs of 100 dollars per gram is going to be correct when we do the fluorination/hydrofluorination/liquid metal exchange thing. But keep in mind a fluoride fastish reactor is only a bit faster than LWRs. Think in the ballpark of 5 tonnes fissile startup. Such a reactor could be on thorium but also as LEU started and then going on only U-Pu. U-Pu is a much more familiar cycle and there will be no ‘nuclear suprises’ like neutron budgets and other stuff that we currently only have theoretical models on, no real evidence from practice. Thorium and plutonium are also difficult to reprocess online so one option is to start with no thorium and do not reprocess to avoid the Pu online processing. Fastish spectrum allows no online processing with very high conversion ratios. So no online fuel processing developments required and still lower lifetime fissile cost than LWRs. NaF at reasonably high temperatures has excellent solubility for plutonium fluoride.

There are many options. You cannot do just one thing. Making choices in one thing means compromising others. Since we don’t know how hard different variables in design will weigh, it’s not clear what the best path is to follow. IMHO, any new concept with serious financial backing would be a welcome addition to the arsenal.


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 Post subject: Re: MSFR
PostPosted: Feb 04, 2011 9:56 am 
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The cost of the fissile is significant though. A machine that needs 5 tonnes fissile for startup as LEU20 will need 1200 tonnes natural uranium, $436M for fuel (as UF6), and 958k SWU. By comparison a machine that needs 3 tonnes fissile for startup of 3.6% LEU uses 793 tonnes natural uranium, $325M for fuel (fabricated), and 377k SWUs. So, the extra startup fissile needed for this machine is definitely significant.

We will have to see how complicated and expense the online processing is at the end of the day before we know whether a more thermal design or a faster design is the lower cost option.


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 Post subject: Re: MSFR
PostPosted: Feb 04, 2011 11:33 am 
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So you have 100 or 200 million bucks more startup fissile cost than LWRs. Big deal, a 1 GWe LWR buys roughly 100 million bucks every YEAR worth of new fabricated fuel. Call it a billion every decade. You can beat that business case easily if you don't have those annual fuel costs, even with high interest rates.

The 20% enrichment is possibly a bigger constraint, but more for business/political cowardice than technical reasons. Jaro doesn't have enrichment. There is the cost of the heavy water. 'a few cubic meters' that Jaro mentions in the other thread is an understatement. CANDU-6 uses over 260 cubic meters heavy water just for the moderator. ACR1000 uses 250 cubic meters for the moderator. My guess is Jaro needs at least 100 cubic meters. Maybe 50 million worth of heavy water. Heavy water will be much cheaper though, when its use becomes more widespread over the world. Deuterium isn't rare, and larger plants produce much cheaper.

Either way you can beat LWRs on fissile cost by a big margin.

It seems kind of interesting to build a fastish fluoride design that operates on once-through and U-Pu like LWRs. There wouldn't be the online fuel processing requirements but it would be advancing MSRs. Like ORNLs DMSR but faster and without thorium. The Chinese have their hands in the cookie jar while we continue arguing about how to best access that cookie jar. It will be an empty jar soon.


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