Energy From Thorium Discussion Forum

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PostPosted: Dec 02, 2010 1:53 pm 
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Lars wrote:
I'm not clear on the idea yet - here is what I understand.
Single fluid, chloride salt, fed with SNF/TRUs.
Producing 233U so presumably lots of thorium in the fuel salt.
Denatured - so enough 238U to denature the produced 233U.
Fast spectrum.


For criticality we need to be sure that for every fission one of the generated neutrons causes a fission. This limits the ratio of fertile to fissile (too much fertile and you absorb too many neutrons in the fertile not leaving enough for the next generation of fission).

SNF/TRU is not all fissile so that absorbs much of your neutron budget and means you don't get to put in bunches of fertile.

Denaturing the 233U means we need 7x as much 238U as 233U.
If there is much 233U compared to 239Pu then you will not have any neutron budget left to add the thorium.

So, I'm thinking such a machine would need to pull out the uranium on a pretty rapid basis (say a 6 month time to process all the salt). This isn't hard, but when you pull out the uranium you will pull out both the 238U and the 233U. So you will need to periodically add 238U to keep the reactor denatured.

The operators of such a machine could be easily replace the 238U addition with thorium and thus produce 233U (+232U). Or they could just take the SNF/Pu. So the denaturing feature here isn't really for the risks associated with operating the machine itself but rather to reduce the risks during shipment from this machine to other machines.


The chloride DMSR doesn't process at all online. This can be done because the U238 is only to make sure the initial U233 remains as LEU until it accumulates significant U232 plus is surrounded by lots and lots of fission products. This makes it similar to LWR, which as you've argued, have nearly pure Pu239 at the beginning of life. Actually its better because the LWR Pu239 has no gamma spewing daughter products from a nearby Pu isotope, plus the chloride DMSR has poor quality Pu as startup which means Pu239 is also denatured, and LWRs swap and shuffle their fuel many times during their lifetime.

My guess is we won't need more than a few hundred kg U238 as initial charge. After a few months the fission product plus U232 offer better self protection levels than LWRs. There is also no need to ship enriched top up fuel during the reactor lifetime, unlike LWRs. Indeed there is no need for any more enrichment at all, since we even start up on reactor TRUs rather than enriched uranium.

Poor TRU fissile quality, as can be the case for older SNF TRU, is only an economical problem as you just put more into the reactor at startup to compensate. In a very fast spectrum all the TRU will just burn down to tiny amounts (probably less than 1% of startup load - impressive for a no processing machine!)

Considering all these things, it seems quite obvious to me that the total proliferation protection of the chloride once through machine is much better than LWRs. In the process a lot of spiked U233 is made, which can startup LFTRs or other well thermalized MSRs.


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PostPosted: Dec 02, 2010 1:59 pm 
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As a final note, U238 may not be needed at all even at startup, since no means to extract U233 is available online. IMHO it makes it just as implausible as LWR Pu239 proliferation.


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PostPosted: Dec 02, 2010 3:04 pm 
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uranium is much easier to separate form fission products than plutonium is so there is a bit of a difference there.


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PostPosted: Dec 02, 2010 3:42 pm 
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Lars wrote:
uranium is much easier to separate form fission products than plutonium is so there is a bit of a difference there.


That's why some initial U238 may have to be added in, until the U232 builds up. However, I can't think of a way to remove the U233 from tons of salt when there is no equipment in the hot cell to do so. Anyone with that capability would not have much difficulty in making their own small Pu reactor so I don't see the added proliferation risk here. There is also no enriched fuel addition required after startup. So from the proliferation angle it might be acceptable to start this particular reactor type without denaturing with U238.

Couple other things I found out from the French TMSR work. One was the more negative global coefficient in faster spectra. Even without doppler the value is considerably negative. Speeds are not much mentioned but thermal and kinetic inertia is probably good enough for these things (things like a decent salt volume and high inertia pumps).

One other thing is the other isotopes are also present. There would be quite a bit of U234 in the uranium since Pa is left in just like ORNL's DMSR. I never saw that mentioned but this may further help the proliferation resistance. If the initial denaturant U238 is from SNF U then there will also be some U235 and U236. We've got the whole uranium family here, U232 U233 U234 U235 U236 U238. Good luck enriching that for weapons.


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PostPosted: Dec 02, 2010 3:43 pm 
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Nice comparison with different spectra here:

http://hal.archives-ouvertes.fr/docs/00 ... t_IAEA.pdf


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PostPosted: Dec 02, 2010 6:15 pm 
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Cyril R wrote:
Couple other things I found out from the French TMSR work. One was the more negative global coefficient in faster spectra. Even without doppler the value is considerably negative. Speeds are not much mentioned but thermal and kinetic inertia is probably good enough for these things (things like a decent salt volume and high inertia pumps).

If the control path (or contributor to the negative coefficient) is much slower than the neutron generation time then it does not help control the reactor and you should not count it as part of your negative coefficient. The nice thing about doppler is that it is much faster than any neutrons. Thermal expansion of the salt is fast enough for a thermal spectrum. I think it is fast enough for an epithermal spectrum. But it may not be fast enough for a truly hard spectrum like the chloride salts. I am not sure and this would need study by someone really interested in the chloride reactor.

Quote:
One other thing is the other isotopes are also present. There would be quite a bit of U234 in the uranium since Pa is left in just like ORNL's DMSR. I never saw that mentioned but this may further help the proliferation resistance. If the initial denaturant U238 is from SNF U then there will also be some U235 and U236. We've got the whole uranium family here, U232 U233 U234 U235 U236 U238. Good luck enriching that for weapons.

The u234 primarily comes from 233U that captured rather than fissioned.
Yes there is the whole collection of uranium (actually not much 238U unless you add it) but in a thorium fed machine the uranium is around 60% fissile - which would not need enriching to use in a weapon. Enough 232U will make it impractical to build a weapon due to its decay chain gamma emissions.


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PostPosted: Dec 02, 2010 6:32 pm 
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Lars wrote:
For criticality we need to be sure that for every fission one of the generated neutrons causes a fission. This limits the ratio of fertile to fissile (too much fertile and you absorb too many neutrons in the fertile not leaving enough for the next generation of fission).

SNF/TRU is not all fissile so that absorbs much of your neutron budget and means you don't get to put in bunches of fertile.

Denaturing the 233U means we need 7x as much 238U as 233U.
If there is much 233U compared to 239Pu then you will not have any neutron budget left to add the thorium.

So, I'm thinking such a machine would need to pull out the uranium on a pretty rapid basis (say a 6 month time to process all the salt). This isn't hard, but when you pull out the uranium you will pull out both the 238U and the 233U. So you will need to periodically add 238U to keep the reactor denatured.

The operators of such a machine could be easily replace the 238U addition with thorium and thus produce 233U (+232U). Or they could just take the SNF/Pu. So the denaturing feature here isn't really for the risks associated with operating the machine itself but rather to reduce the risks during shipment from this machine to other machines.

I wouldn't like to suggest that I understand the issues around core neutronics, but I note the following in relation to the Rebus 3700 concept design:
- Minimal fuel salt reprocessing (35L/day) is enough to stop solubility issues for the FP's, while those FP's that remain act as a handbrake limiting excess reactively that might otherwise result from excess neutrons;
- In a similar fashion fertile additions are required to avoid excess reactivity and replace fissile burned.

So one way or another, some combination of FP processing, fertile additions and fissile extraction seem very likely over time. Failure to do that may result in significant swings in reactivity over time. Sorry no answers only more questions.


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PostPosted: Dec 03, 2010 4:33 am 
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Lars, according to the French work, the fast fluoride TMSR has similar negative Doppler as their thermal fluoride TMSR. We recently discussed the energy dumped into the fuel salt from reactivity excursion that is too fast. Its very small! The strong negative global coefficients will force reactivity back after that.

http://hal.archives-ouvertes.fr/docs/00 ... t_IAEA.pdf

Re weapons. Has anyone ever made a weapon from 60% U233?


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PostPosted: Dec 03, 2010 10:49 am 
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To my knowledge, u233 + plutonium was used for one weapons test. It kind of worked but the yield was significantly below the plan.


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PostPosted: Dec 03, 2010 2:23 pm 
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Most any grade plutonium will be better than U234. It has lots of fissile whereas U234 is only fissionable (albeit a very good fissionable material given its reasonable fast fission threshold), and it can't sustain a chain reaction on itself. This was in one of the chloride documents, where it was suggested that too high a U234/U233 ratio would choke the reactor...


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PostPosted: Dec 04, 2010 2:18 am 
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Cyril R wrote:
However, I can't think of a way to remove the U233 from tons of salt when there is no equipment in the hot cell to do so. Anyone with that capability would not have much difficulty in making their own small Pu reactor so I don't see the added proliferation risk here.

You could treat it with Chlorine to convert it to a volatile hexachloride. Zirconium has a higher evaporation temperature.


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PostPosted: Dec 04, 2010 4:26 am 
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jagdish wrote:
Cyril R wrote:
However, I can't think of a way to remove the U233 from tons of salt when there is no equipment in the hot cell to do so. Anyone with that capability would not have much difficulty in making their own small Pu reactor so I don't see the added proliferation risk here.

You could treat it with Chlorine to convert it to a volatile hexachloride. Zirconium has a higher evaporation temperature.


There is no equipment in the hot cell to do so. If attempted you might just be disappointed at the volatility overlap of the UClx series. Even vacuum distillation wouldn't yield that clean a fissile product. Chlorides are different here than fluorides.


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