Energy From Thorium Discussion Forum

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 Post subject: solid Fluoride Fuels ?
PostPosted: Dec 12, 2013 10:50 am 
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If working with ThO2 is so difficult, why not try making solid fuel with ThF4 and PuF3? It would certainly allow for simple reprocessing, no?

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PostPosted: Dec 12, 2013 6:19 pm 
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It is certainly possible.

Question is whether it works with zircalloys. If a new cladding is needed then that puts it off a while.


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PostPosted: Dec 12, 2013 8:06 pm 
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Cyril R wrote:
It is certainly possible.

Question is whether it works with zircalloys. If a new cladding is needed then that puts it off a while.
How about graphene lined zirk?

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PostPosted: Dec 13, 2013 12:53 am 
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Thorium metal has a melting point close to that of Zirconium. It is also passivized by an oxide layer. Why do we need a cladding for metallic thorium? More so for half meter Candu bundles. Coolant in a closed circuit can be periodically distilled.


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PostPosted: Dec 13, 2013 4:29 am 
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Because the oxide layer will undergo breeding and fission just like everything else made of thorium in the reactor.

You need a layer of some material that isn't going to disintegrate and blast the fission products into the coolant.


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PostPosted: Dec 13, 2013 6:21 am 
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Thorium metal fuel is attractive though. It doesn't swell like uranium fuel, so you don't have to add zirconium to the fuel itself to control swelling. It is compatible with zircalloy. It has a high density, far more thorium per liter of fuel rod than thorium fluoride. And it has a very high thermal conductivity so it operates cold (close to cooling temperature). Thorium fluoride has poor thermal conductivity, it will likely melt in the center, doesn't happen with thorium metal.

Thorium metal is easy to reprocess. Not being hampered by any specific chemistry training or in depth knowledge, I can speculate that it should be easy to distill the fission products out, almost all are a lot more volatile than thorium, uranium, and plutonium.


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PostPosted: Dec 13, 2013 6:52 am 
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If you can actually make thorium metal fuel that would seriously improve the odds of using it in LWRs.
For instance an allow of 20% enriched uranium and thorium in a 20-80 ratio to give effective ~4% Uranium 'enrichment' in fresh fuel.

But 20-80 U-Th alloy probably does swell significantly, even if getting thorium metal to dissolve in nitric acid is far simpler than thorium dioxide, meaning you can probably put it through a standard PUREX plant if all else fails (not that you would want to but its still useful).


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PostPosted: Dec 13, 2013 6:30 pm 
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Th metal fuel with 20% U metal would swell less than Zr stabilized U metal fuel, or U-Si fuel.

Some swelling can be accomodated, by having a liquid metal bond in between the cladding and fuel. Pb/Bi/Sn eutectics are attractive.


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PostPosted: Dec 15, 2013 9:38 am 
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You could have the uranium or RG plutonium as oxide to give a Ceramic-Metal fuel. Ceramic intrusion may further stabilize the thorium metal.


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PostPosted: Dec 15, 2013 10:48 am 
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jagdish wrote:
You could have the uranium or RG plutonium as oxide to give a Ceramic-Metal fuel. Ceramic intrusion may further stabilize the thorium metal.


What made you think the oxygen stays on the plutonium, rather than on thorium and fission products?


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PostPosted: Dec 18, 2013 3:10 am 
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Once trapped in thorium metal, Pu will stay in place even if reduced to metal. There is a ThO2 stable passive layer on surface in any case.


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PostPosted: Dec 18, 2013 6:11 am 
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It appears that an 80Th-20U alloy would have a density of roughly 13, compared to about 10 for Uranium Dioxide.
This means you could afford to add significant 'diluent' in the form of zirconium to reduce the swelling issues if there are any.
Although the Th has a far larger cross section for capture than 238U, it is still so small that I expect neutronics would be generally comparable.

It appeasr the fuel could be reprocessed by simple fluoridation to obtain uranium hexafluoride which would be sent directly to re-enrichment, the remainder being the thorium, the zirconium cladding/diluent and the fission products/plutonium.
I am pretty sure that 95% of the bred fuel would be 233U but that is a simplistic view of neutron capture so it would need to be modelled properly, suffice to say that most of the benefits of reprocessing could be obtained without actually doing any aqueous or other liquid phase processes.

Whatever you do with the fluorides left afterwards is not really important but would probably be comparable to the intermediate storage of fluoride waste salt from MSRs.

The output uranium hexafluoride would be blended with fresh material that had been enriched to the same enrichment (to avoid losing the effective SWUs invested in the output material) to dilute the 233U and 232U with 235U to keep neutronics and gamma fluxes close to once through fuel before being enriched to the final 20% enrichment.

I would wonder how high the effective burnup per tonne of 235U input would be using such a cycle with an ESBWR.


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PostPosted: Dec 18, 2013 8:36 am 
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Why fluorination? It changes the chemical state, we don't want fluoride that corrodes zircalloy, and metal based processes seem far simpler and can keep all actinides including Th, Pu, U, Pa, Am and Cm. You could simply vacuum distill the fuel pellets. Not even melting them, just run them hot in vacuum and most fission products are on the fly. If possible a higher temperature with melting would be even better as it can remove more of the fission products.


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PostPosted: Dec 18, 2013 8:46 am 
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Cyril R wrote:
Why fluorination? It changes the chemical state, we don't want fluoride that corrodes zircalloy, and metal based processes seem far simpler and can keep all actinides including Th, Pu, U, Pa, Am and Cm. You could simply vacuum distill the fuel pellets. Not even melting them, just run them hot in vacuum and most fission products are on the fly. If possible a higher temperature with melting would be even better as it can remove more of the fission products.


Why don't we want to use something that corrodes zircallloy?
The fuel pins would presumably be Lightbridge style cast jobs with the zircalloy clad cast onto the outside (or with the fuel cast inside it, not sure in this case?).

This way we can just fluorinate whole fuel rods, skipping the decladding step.
The density changes from fluorination will break the rod into a fine powder, releasing the uranium hexafluoride as a gas, which can then be rectified and seperated from the gaseous fission products (xenon, krypton and iodine).
Everything else can be kept as a metal or as a fluoride in the waste material which then goes off to storage with potentially no further processing/handling at all.

Pa losses would be fairly minor if we cool the rods for a few months and the Pu/Am losses are probably not going to be significant since AIUI 232Th capture will dominate over 238U capture.
If you really want you can vacuum distill the remaining material later to recover things like Zirconium that might have some recycle value, but otherwise I see little reason to touch it.
The only element worth anything in the spent fuel is the uranium as it has had massive amoutns of SWUs (and effective SWUs from 233U breeding) invested in it.


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PostPosted: Dec 18, 2013 9:27 am 
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Too slow. With cladding intact it takes too long for the darn thing to fluorinate. You want maximum reaction rates. That cladding is tenacious. Designed to be so. Also cladding is too big to insert into a practical fluorinator. Even without the cladding the fuel wouldn't fluorinate easily without melting it or cutting it up to tiny bits. There would be a layer of solid fluoride (ThF4) that inhibits further fluorination. Grinding up or melting is needed in any case. If you're avoiding radioactive dusting, then melting must be used. If you use melting, you might as well use distillation in stead of fluorination.

If you fluorinate, you lose the thorium, plutonium, amercium, curium. Bad. You can try to recover plutonium, but it will only come out after your fluorinator has mostly fluorinated itself. You can't recover am/cm/th even with that aggressive fluorination.

Decladding should be a simple operation. Just cut off the (only lowly radioactive) tops, then push the fuel pellets/extruded rods out mechanically. A machine could be designed to push out all fuel rods mechanically in one go. Then melt the fuel and distill of the fission products.


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