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 Post subject: Modified 2 Fluid Reactor
PostPosted: Jan 28, 2014 5:18 am 
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Given that in a thermal spectrum reactor, the U-Pu cycle produces about 1.9n per n absorbed,and the Th-U cycle produces about 2.3, would it be feasible to design a modified two fluid reactor where U238 is added to the fuel salt and the blanket is still Th? The U238/Th ratio would be tuned to assure the overall breeding results in a sustainment reactor. This would allow the Liquid Fluoride Thorium Uranium Reactor (LFTUR) to burn up the accumulated SNF and depleted U stores while still using a thermal spectrum reactor.

Thoughts?

Since there would be no thorium in the fuel salt, the reprocessing would be much easier.

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PostPosted: Jan 28, 2014 11:03 am 
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Starting from a traditional two fluid with no 238U in the core you get a breeding gain of 5-10%. Each capture in 238U rather than 232Th will cost you 0.4neutrons so very crudely you might afford 5/.4 to 10/.4 or 12% to 25% of your neutrons to go to 238U rather than 232Th and still keep your reactor at isobreeding. On the high side then you might burn 250kg of 238U per GWe year.

In a single year a 1GWe LWR produces around 20 tonnes of 238U in the spent fuel so best case it will take you 80 years of running your modified LFTR to burn off one year of spent fuel uranium from the LWR.

To create the fuel for running an LWR for one year you needed to provide around 1 tonne 235U. This came from around 200 tonnes of natural uranium. So for every GWe-yr from the LWR you have around 180 tonnes of depleted uranium. Best case (which is likely optimistic by a factor of 2-4) it will take you 720 years to burn off the depleted uranium.

In the meantime, the electric cars and windmills are mining rare earths and leaving thorium in the mine tailings and these are not being burned.

So I claim that burning all the fertile material that we dredge up from the earth for other purposes is simply not possible. We are free to pick between uranium and thorium fertile material as best suits the reactor design. Either way we can help in a minor way with the "radioactive wastes" generated by other processes but in no way can we really solve either problem.

238U is not a problem - just put it back into the mines whence it came. The depleted uranium is actually less radioactive than the stuff we took out of the ground. It is dangerous only if you eat it (don't use it to make pretty red paint for your dinner plates like we used to do) or if the decay product gas (radon) gets concentrated and breathed. So put it back into the mines and don't go there and everything is fine.

If you want to spend effort (not sure it is worth it) you might consider enriching the spent fuel uranium to remove almost all of the 238U and gain the roughly 1% 235U, along with 236U and tidbits of 234U and putting that into your reactor. This would be better fuel. Another choice would be to send the LWR spent fuel uranium to a CANDU reactor to extract a bit more energy out of it and avoid some uranium mining activity.


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PostPosted: Jan 28, 2014 11:56 am 
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[/quote]
Lars wrote:
Starting from a traditional two fluid with no 238U in the core you get a breeding gain of 5-10%. Each capture in 238U rather than 232Th will cost you 0.4neutrons so very crudely you might afford 5/.4 to 10/.4 or 12% to 25% of your neutrons to go to 238U rather than 232Th and still keep your reactor at isobreeding. On the high side then you might burn 250kg of 238U per GWe year.

In a single year a 1GWe LWR produces around 20 tonnes of 238U in the spent fuel so best case it will take you 80 years of running your modified LFTR to burn off one year of spent fuel uranium from the LWR.
Or 80 GW of LFTUR for every GW of PWR. Thanks, that was not TOO far off from my guess.
Lars wrote:
To create the fuel for running an LWR for one year you needed to provide around 1 tonne 235U.
Or bred Pu239 from the SNF which is the point in this case.
Lars wrote:
This came from around 200 tonnes of natural uranium.
No, it came from the 20T of SNF.
Lars wrote:
So for every GWe-yr from the LWR you have around 180 tonnes of depleted uranium. Best case (which is likely optimistic by a factor of 2-4) it will take you 720 years to burn off the depleted uranium.
Which is what I'm trying to stop. Use LFTURs instead of LWR.
Lars wrote:
In the meantime, the electric cars and windmills are mining rare earths and leaving thorium in the mine tailings and these are not being burned.
Yup, tht is why the LFTURs.
Lars wrote:
So I claim that burning all the fertile material that we dredge up from the earth for other purposes is simply not possible. We are free to pick between uranium and thorium fertile material as best suits the reactor design. Either way we can help in a minor way with the "radioactive wastes" generated by other processes but in no way can we really solve either problem.

238U is not a problem - just put it back into the mines whence it came.
In this case I will agree unless it turns out the burn ratio is closer to 1 than guesstimated above.
Lars wrote:
The depleted uranium is actually less radioactive than the stuff we took out of the ground. It is dangerous only if you eat it (don't use it to make pretty red paint for your dinner plates like we used to do) or if the decay product gas (radon) gets concentrated and breathed. So put it back into the mines and don't go there and everything is fine.

If you want to spend effort (not sure it is worth it) you might consider enriching the spent fuel uranium to remove almost all of the 238U and gain the roughly 1% 235U, along with 236U and tidbits of 234U and putting that into your reactor. This would be better fuel. Another choice would be to send the LWR spent fuel uranium to a CANDU reactor to extract a bit more energy out of it and avoid some uranium mining activity.
Actually, the idea is to use the easily concentrated Pu fractions to charge the LFTUR in the first place and burn the U fractions along with fresh Th during its lifetime of operation. Because of the ~.7% left over U235 in the SNF, the fraction assigned to the U238 might actually be a tad higher. Hmmm, more to think about. Thanks.

Anyone else?

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PostPosted: Jan 28, 2014 12:02 pm 
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Absolutely LFTRs can serve a useful function to eliminate Pu from the SNF. This is a problem where the scale is about right. Large scale deployment of LFTRs can startup with SNF/Pu will roughly consume the expected amount of SNF/Pu.


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PostPosted: Jan 28, 2014 12:17 pm 
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Lars wrote:
Absolutely LFTRs can serve a useful function to eliminate Pu from the SNF. This is a problem where the scale is about right. Large scale deployment of LFTRs can startup with SNF/Pu will roughly consume the expected amount of SNF/Pu.
Yup. And at sustainment mode, a good bit of the left over SNF/U too. Hmm. With laser enrichment, maybe the lower/more active U isotopes can be enriched from the U238 and be consumed at a higher that 1:80 ratio.

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PostPosted: Jan 28, 2014 12:34 pm 
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KitemanSA wrote:
Lars wrote:
Absolutely LFTRs can serve a useful function to eliminate Pu from the SNF. This is a problem where the scale is about right. Large scale deployment of LFTRs can startup with SNF/Pu will roughly consume the expected amount of SNF/Pu.
Yup. And at sustainment mode, a good bit of the left over SNF/U too. Hmm. With laser enrichment, maybe the lower/more active U isotopes can be enriched from the U238 and be consumed at a higher that 1:80 ratio.

Even with normal centrifuge you could enrich to LEU limits meaning around a 20:1 concentration so that the ratio is closer to 1:4, further since now it is mostly fissile (though with a significant amount of 236U) your neutron consumption is lower so you could go even further. Note though that the 236U takes a lot of neutrons to burn so you will likely end up burning for a while then having to eventually toss your uranium or re-enrich it to get rid of the excess 236U. I suspect a CANDU will be able to tolerate excess 236U better than a LFTR since a CANDU is extremely thermal.


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PostPosted: Jan 28, 2014 12:41 pm 
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Apparently the overenrichment factor for CANDU to deal with 236U contamination is 2.5% (of the 236U concentration in extra 235U) instead of 25% for LWRs.

So a material with ~1% U-236 only needs to be overenriched by 0.025% instead of 0.25%, which is far more practical.


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PostPosted: Feb 08, 2014 6:31 pm 
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What kind of ratio would be needed to startup a LFTR with LWR SNF (without removing U-238 from SNF) + some reactor grade plutonium ? Would 90% SNF + 10% reactor grade plutonium do ?

Perhaps a better question would be how much SNF + U233 ratio would startup a LFTR.

The big argument on IFR is they don't need new fuel, just SNF + depleted uranium (add plutonium to the mix optionally). If LFTRs could be started by taking in all SNF, doing the fission products removal, it essentially kills any pro IFR arguments, except that their development is much further along.

The idea is we don't need nuclear fuel enrichment of any kind (even for startup).

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