Energy From Thorium Discussion Forum

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PostPosted: Mar 27, 2014 2:07 pm 
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jagdish wrote:
You may stand a chance with Beryllium oxide or carbide instead of graphite, if you can keep it solid. I do not claim to be an expert and am open to correction.


Nope, I tried both of those unclad. Couldn't get a critical solution.


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PostPosted: Mar 27, 2014 2:08 pm 
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Kurt Sellner wrote:
Kirk Sorensen wrote:
KitemanSA wrote:
Dumb question, and sorry if it has been answered here, but can a MSR start up on NU?


I did this calculation about seven years ago. Yes, but only with D2O moderation. It's not possible with graphite.


When I first saw that I thought, "OK, problem solved." Now that I thought about it more that sounds like an engineering nightmare.


It's worse than that. You'd better have a channel material for the D2O that is infinitely thin and invisible to neutrons, but limits the heat transfer from the salt to the D2O. I don't think such a thing exists.


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PostPosted: Mar 27, 2014 8:52 pm 
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Couldn't you just use an aerogel lagged tube made of some suitable material (be it zircalloy or graphite or what not) and just accept that the moderation heat from the neutrons will be lost at 100 Celsius in the boiling heavy water moderator?
Efficiency like that is not a big issue considering the core is so compact even if you 'throw away' that energy - not that it would actually be thrown away thanks to feedwater heaters and what not.


Last edited by E Ireland on Mar 28, 2014 7:32 am, edited 1 time in total.

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PostPosted: Mar 27, 2014 10:10 pm 
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' I did this calculation about seven years ago. Yes, but only with D2O moderation. It's not possible with graphite.'
The absorption cross-section of deuterium is 0.000519. Radiogenic lead, 208Pb, is lower, at 0.00048 barns. The scattering cross-section for 208Pb is higher than deuterium, 11.34 compared to 7.64. If you could find the traditional zero-width, zero neutron absorption material to separate the two, could you run a natural uranium/Flibe reactor with molten lead as moderator and coolant?


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PostPosted: Mar 27, 2014 11:24 pm 
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Depends also on what sort of fluid fuel: You could have the best possible moderator, but it just won't work with a shoot* fuel.
Standard FLiBe carrier is probably not that great - especially if Li7 is < 99.999% pure.
Carrier-free UF4/UF3 would be better for NU.
Molten NU-Si eutectic even better.
Definitely no Thorium. That would poison the system for sure.


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PostPosted: Mar 28, 2014 1:54 am 
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First off consider the very high probability that I have no clue whatsoever on building nuclear reactors.

E Ireland wrote:
Couldn't you just use an aerogel lagged tube made of some suitable material (be it zircalloy or graphite or what not) and just except that the moderation heat from the neutrons will be lost at 100 Celsius in the boiling heavy water moderator?


I would think that boiling the moderator would be bad. Steam would be little better than a vacuum as a moderator. The D2O has to stay liquid.

Keeping the D2O moderator liquid means finding a suitable combination of temperature and pressure. Aerogel has no real structure to it, dropping a paper clip on it will leave a dent. There would have to be metal on both sides of the aerogel to hold it up, that adds thickness when there is none to spare. Assuming that we can keep everything at atmospheric pressure the pipe would still have to hold up to the pressure of the fuel salt, it's low but not nothing. Assuming atmospheric pressure you have 1000C salt on one side of your aerogel and 100C D2O on the other. How thick would your aerogel have to be to keep that temperature gradient? I have no clue on the insulating properties of aerogel but that kind of temperature difference seems quite extreme.

Now lets consider the other extreme, we pressurize the D2O so it stays liquid at 1000C, that's a temperature I could not find on any water phase diagrams. It's probably something like several THOUSAND atmospheres. I suppose you could keep the pipe wall between your fuel salt and moderator paper thin by pressurizing the molten salt but would think finding anyone willing to do so would be quite difficult.

Again, maybe I know nothing. From what I think I know this just sounds like an engineering nightmare.

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PostPosted: Mar 28, 2014 7:33 am 
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BWRs work despite the moderator boiling.

I don't see any particular reason that the heavy water boiling would necessarily be catastrophic.


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PostPosted: Mar 28, 2014 12:45 pm 
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E Ireland wrote:
BWRs work despite the moderator boiling.


In BWRs the fuel is enriched, enriched enough that it uses light water. CANDU uses natural uranium and heavy water as moderator/coolant, the coolant is pressurized to keep it from boiling.

I assume the issue is also a matter of coolant efficiency, steam does not cool nearly as effectively as water. Too much steam and you don't have enough coolant.

I'll admit I know nothing, perhaps you are correct.

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PostPosted: Mar 28, 2014 12:58 pm 
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There was a boiling CANDU at Gentilly though, it didn't work particularily well due to Xenon losses - which are not a problem with an MSR.


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PostPosted: Mar 30, 2014 12:32 am 
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Natural uranium MSR appears to be difficult. So you use reactor grade plutonium/U-233 from chemical reprocessing for required fissile feed/enrichment. MSR fuel is a chemical broth in any case.
As the chemical composition can be adjusted, you could give importance to cost economy keeping neutron economy in the background. The things you could do are:-
1. Use FNaBe or FNaZr avoiding very high enrichment 7Li.
2. Use a common water tube boiler configuration. Boiler tubes are normally designed for high steam pressure.
3. Fill fuel salt in fuel space without provision for air.
4. Use a closed circuit of water as a moderator-coolant. The moderation can be adjusted but under-moderated core should be expected. The spectrum could be thermal near the tubes and faster further away. Avoid graphite waste.
5. Confine fuel to the boiler reactor vessel and remove Xe periodically. The fuel could be reprocessed as and when required.
The fertile could be uranium, thorium or a mixture as in a DMSR. You could start with a waste burner and graduate to a thorium MSR.


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PostPosted: Mar 30, 2014 2:36 am 
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Would a D2O Aqueous Homogeneous Reactor run on NU?

Oops, next time look at Wikipedia FIRST.

Quote:
A heavy water aqueous homogeneous reactor can achieve criticality (turn on) with natural uranium dissolved as uranium sulfate.

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PostPosted: Mar 31, 2014 2:53 am 
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The real problem with Aqueous reactors is corrosion. An aqueous homogeneous reactor in a vessel with sacrificial thorium liner could be an interesting idea! The uranium/U-233 would be in solution as uranyl sulfate or fluoride.
Water could boil off as steam and be returned after condensation. Xe could be removed on condensation and Samarium/ gadolinium precipitate as sulfate or fluoride and be flushed off periodically.


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PostPosted: Mar 31, 2014 8:34 am 
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Me and Cyril R actually ran with that idea a while back - it seems intriguing if it could be made to work.
Primarily the problem was plutonium plating out during the startup with enriched uranium fuel though.


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PostPosted: Mar 31, 2014 10:25 am 
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An oxide layer would grow on metallic thorium naturally and plutonium will not be reduced to metal. The U-233, on the other hand, will dissolve when formed. Needs a practical test.


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PostPosted: Mar 31, 2014 10:33 am 
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If you all want to discuss aqueous homogeneous reactor design there are plenty of other threads for that.


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