Energy From Thorium Discussion Forum

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PostPosted: Apr 30, 2013 12:57 pm 
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CANDU has a slight fuel cycle cost advantage because fuel fabrication is much cheaper than enrichment, and it uses a bit less mined uranium per kWh, doesn't waste any mined uranium in enrichment tails, and has no conversion and deconversion cost since UF6 form for enrichment isn't needed. Granted the fuel cycle cost advantage is small - higher burnup in CANDU reduces both fuel cycle cost and natural uranium usage, even if slightly enriched uranium is needed for that.


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PostPosted: Apr 30, 2013 2:29 pm 
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Nathan2go wrote:
David,
Thanks for the info in uranium resources. I guess that explains why breeders have not caught on.

But I am still perplexed by the comparison to water cooled reactors. Since CANDUs have similar neutronic efficiency to IMSRs, shouldn't they have similar resource utilization when used with recycling? For LWRs at least, as I understand it, recycling only boosts the energy delivered from the uranium about 20%.



Nathan,

Cyril covered the most important aspect, that while CANDU's have a good conversion ratio, they end up throwing away most of the Pu they produced, whereas in a converter MSR like the IMSR you just keep any fissile produced (U233 or Pu) in the reactor to burn off. Another big difference is that MSR let Xenon come out (very neutron hungry) and don't have any internal structure. A CANDU sounds great with heavy water but there is still massive amounts of zirconium in it.

David LeBlanc


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PostPosted: Apr 30, 2013 11:26 pm 
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Ok, I guess I've lost interest in HWRs again. To avoid discarding Pu, recycling is required. In order for recycling to be economical, much higher burnup is needed. To get higher burnup, higher fuel enrichment and lower moderation are needed (and higher fissile load). With lower moderation, we might as well use regular light water, since the neutrons won't be hitting it much anyway. The Shippingport Light Water Breeder experiment showed that LWRs can be neutronically efficient (zirconium, xenon, and all), but such cycles are not cost competitive with once-thru.

So Cyril,
Did you try cooling the concrete using air cooling? I supposed we need a water pool anyway for used fuel canisters to sit in following refueling operations.

It is interesting though that you are proposing (if I understand correctly) a cool cell rather than a oven, like the old ORNL work. I suppose they had a loop design (with lots of plumbing outside of the vessel that needed to stay warm, unlike the modern integral designs with internal plumbing).

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PostPosted: May 01, 2013 12:29 am 
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Air cooling is possible, but results in higher concrete temperatures and higher risk of tritium release. It is also a lot less compact.

The cell is indeed a "cool hot cell" but the vessel is insulated on the inside with cans of frozen fluoride salt eutectic. This keeps heat losses during normal operation very low, but drastically increases heat losses beyond the melting point of the frozen fluoride salt (eg, LiF-NaF eutectic). This is basically a temperature switch insulation that protects against freezing in normal operation and later on in long duration station blackouts or pump failures.

I should add that one of the advantages of this approach is a relatively cold vessel during normal operation, <400 degree Celsius is feasible. This makes creep much less of an issue, so standard stainless steels can be used in stead of expensive Hastelloy.


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PostPosted: May 01, 2013 5:53 am 
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Quote:
I supposed we need a water pool anyway for used fuel canisters to sit in following refueling operations.


Dry casks can be used to store the spent fuel salt. Currently I'm thinking of steel casks where spent fuel salt passes into continuously. They would be double walled and passively air cooled.

The offgas tank is more tricky. It makes lots of heat in a few kg of gas, though a small reactor (60 MWt in this case) has little trouble. Still, the offgas tank likely needs its own cavity, similar to the reactor vessel, with passive natural circulation water cooling in the concrete liner. To improve efficiency it could have a forced salt cooling coil in it, that normally transmits heat productively. When that system fails the solid salt can insulation idea would be put into use passively, radiating heat to the steel plate that conducts heat to the pipe inside, which conducts to the water in it.


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PostPosted: May 01, 2013 8:19 pm 
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Nathan2go wrote:
To get higher burnup, higher fuel enrichment and lower moderation are needed (and higher fissile load).
Burnup in NU is limited by fission product buildup.
Remove the FP's and the sky is the limit.


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PostPosted: May 01, 2013 9:05 pm 
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jaro wrote:
Burnup in NU is limited by fission product buildup.
Remove the FP's and the sky is the limit.

Let me guess: all we need is reactor that is heavy water moderated and molten salt fueled. Of course to cheaply process the salt, there can't be any thorium in it to hamper rare-earth removal.

Do you have an estimate of the conversion ratio?

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PostPosted: May 02, 2013 12:48 am 
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With metallic thorium radial and axial blanket, and a suitably enriched uranium core in a Candu PHWR, you could have a U-233 producer and possibly a breeder. Change the thorium at 1.2-1.4% conversion and and before any substantial burning in situ and electro-refine it. Small bundles make it very convenient.
You could use U-233 produced as the fissile feed in the core or in a thorium MSR, if and when developed.


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PostPosted: May 02, 2013 12:36 pm 
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Nathan2go wrote:
Do you have an estimate of the conversion ratio?
Usually CR trades off against power density: How much is each worth to you ?

The nice thing about liquid fuels is that they let you get over the usual heat transfer limitations: You can boost CR by having nice fat fuel channels, without worrying that you will fry the crap out of solid ceramic pellets.


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PostPosted: May 02, 2013 1:14 pm 
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Dr. LeBlanc is not concerned about conversion ratio with this design. The burnup is higher than LWRs so a low conversion ratio can be compensated.

With uranium only (no thorium), there is less of a penalty going for higher power densities, as there are no longer problems with captures on protactinium. There is even a slight advantage in actinide fission with higher power density as some TRUs are good fuels that fast decay into poor fuels, so higher power density means less decay to poorer fuels, in stead more fissioning of the good stuff.

Fission products accumulation could hurt more in a high power density core, but there's just fuel salt and graphite in the core, no masses of zirconium, stainless steel grids, Inconel and such that solid fuel reactors have.

Too bad we can't discuss too many details of the design here on the forum.


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PostPosted: May 02, 2013 9:04 pm 
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jagdish wrote:
With metallic thorium radial and axial blanket, and a suitably enriched uranium core in a Candu PHWR, you could have a U-233 producer and possibly a breeder.

Yes, making the blanket of metallic thorium does sound like a good workaround for the high difficulty/cost of recycling thorium oxide (though adding easily an removed metal blanket to a reactor could be seen as a proliferation issue - that's how weapons grade Pu used to be made). Oxide seems to be the preferred fuel form for water cooled reactors though; does the low power in the blanket make metal acceptable?

I wonder if a metallic thorium blanket could be used with a salt-cooled (or salt fueled) reactor, perhaps with re-usable graphite or silicon-carbide cladding (get the thorium out by melting rather than chopping). The salt-cooled reactors (like SmAHTR) can use frequent fuel shuffling or blanket-replacement since they need not cool-off before opening the vessel to refuel.

The Candu would still have the issue of low burn-up in the fuel (due to fission product build-up). If you simply increased enrichment and added fuel shuffling, that would help a bit (but would reduce conversion ratio). Better to also reduce moderation, so that the fission products' neutron absorption decreases. This also reduces the performance margin over LWRs, since the harder spectrum presumably causes lower losses to light water.


Last edited by Nathan2go on May 03, 2013 2:05 pm, edited 1 time in total.

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PostPosted: May 02, 2013 11:42 pm 
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Metallic thorium would be easier to process by electrolysis.
Lower moderation could be achieved by lead cooling in the tubes. Heavy water could continue as main moderator in the drum.
In case of salt cooling, you have to be sure that thorium does not go into solution. A passive ThO2 layer is water resistant.


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PostPosted: May 10, 2013 12:41 pm 
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Sherrill Greene of SmAHTR gave positive feedback on iMSR.

http://sustainableenergytoday.blogspot. ... r.html?m=1


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PostPosted: Mar 01, 2014 9:56 am 
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David,

At every discussion of nuclear energy, in youtube, slashdot, I'm now mentioning your DMSR design plus FLiBe's LFTR concept, along with your 8 year to production quote.

There's way too many cards stacked against anything molten salt, so whatever I can (as in calling attention to your efforts), I will.

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