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PostPosted: May 17, 2011 3:29 am 
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Non-volatile coolants is a good idea. Molten salt is a good example. It avoids LOCA incidents.
Fluid fuels is another good idea. It helps in purging of volatile fission products like Xe and leads to neutron economy.
Irradiation of thorium and burning of U233 in situ is another good idea, if you are short of uranium. It may also be possible with a thermal spectrum but that involves a moderator with its own problems. Also there is enough uranium to go around for centuries in some countries including the US, Russia, France and Canada. But a uranium breeder-burner will require fast spectrum reactors.
Mixing up of all these ingredients in a LFTR creates the best discussions on nuclear energy for long periods and enough information to pick or choose your power source.
Molten salt cooled reactor could be fast or thermal spectrum. In fast reactors you have to check compatibility with reactor vessel and fuel. In a thermal reactor, a moderator also gets involved.


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PostPosted: May 17, 2011 6:02 am 
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Cyril R wrote:
Seperation of uranium would be very difficult without 'major plant engineering' in this molten salt cooled reactor.

OK, I guess that's the argument of IFR proponents.... Not sure how well that would fly internationally.

Cyril R wrote:
....while SNF candu fuel is less weapons grade, candus can shuffle fuel around, so you can just pretend that some fuel is defect after low burnup, pull it out and get the good plutonium out.

Yes, this would work in theory - but not in practice, for several reasons.
For one thing, the robotic fueling machines work quite slowly (due to the mechanisms required for interfacing with high-pressure end-fittings, etc.), and are quite busy as things are, just shuffling fuel around the core in order to maintain reactivity with NU fuel: You might be able to do low burnup shuffling on a few of the 380 fuel channels, but not anywhere close to the whole core.
The Pu content per bundle is quite tiny, so that wouldn't be a very productive method.
Worse, the fuel is U-oxide, which is far more difficult to reprocess than either metal or liquid fluoride fuel.
Then, if you don't use the reprocessing U-waste to re-fabricate new fuel bundles (again, much easier with metal fuel than UO2 sintered & ground pellets), you end up using suspiciously large quantities of fresh U.
Even if you re-fabricated reprocessed U, it would still be quite radioactive -- and standard CANDUs are not equipped to load radioactive fuel: the SNF discharge path is different from the fresh fuel loading path....


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PostPosted: May 17, 2011 8:12 am 
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jaro wrote:
Cyril R wrote:
Seperation of uranium would be very difficult without 'major plant engineering' in this molten salt cooled reactor.

OK, I guess that's the argument of IFR proponents.... Not sure how well that would fly internationally.

Cyril R wrote:
....while SNF candu fuel is less weapons grade, candus can shuffle fuel around, so you can just pretend that some fuel is defect after low burnup, pull it out and get the good plutonium out.

Yes, this would work in theory - but not in practice, for several reasons.
For one thing, the robotic fueling machines work quite slowly (due to the mechanisms required for interfacing with high-pressure end-fittings, etc.), and are quite busy as things are, just shuffling fuel around the core in order to maintain reactivity with NU fuel: You might be able to do low burnup shuffling on a few of the 380 fuel channels, but not anywhere close to the whole core.
The Pu content per bundle is quite tiny, so that wouldn't be a very productive method.
Worse, the fuel is U-oxide, which is far more difficult to reprocess than either metal or liquid fluoride fuel.
Then, if you don't use the reprocessing U-waste to re-fabricate new fuel bundles (again, much easier with metal fuel than UO2 sintered & ground pellets), you end up using suspiciously large quantities of fresh U.
Even if you re-fabricated reprocessed U, it would still be quite radioactive -- and standard CANDUs are not equipped to load radioactive fuel: the SNF discharge path is different from the fresh fuel loading path....


Actualy PUREX is one of the most established and well known processes for removing plutonium from oxide fuels. Its designed from the ground up to get clean easy plutonium for weapons.

Oxides in general are well proven established forms for processing, uranium mining operations, conversion and deconversion facilities do it all the time.

Whereas the waste boil off from the liquid metal still would leave actinides behind together; it wouldn’t be possible to separate U from Th using the still because the tungsten would simply melt (!).

The fact that everything happens inside a hot cell means diversion of any fuel material will be near impossible.

A number of fission products also stay with the still bottom melt, making it very radioactive. Plus there’s U232 and daughters, and quite a bit of Pu238 as well.

Since the molten salt cooled reactor probably operates at best as isobreeder, fuel removal would also require makeup fissile feed. Highly unusual for this reactor, far more unusual than once through CANDUs that normally use lots of fresh fuel.

These things make wholesale transfer of fuel for weapons rather implausible.


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PostPosted: May 17, 2011 7:42 pm 
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Cyril R wrote:
Actualy PUREX is one of the most established and well known processes for removing plutonium from oxide fuels. Its designed from the ground up to get clean easy plutonium for weapons.

If you read a bit about the history of weapons development and proliferation, you will find that all the nations that developed plutonium-based weapons - without exception - did so by using production reactors running on U-metal fuel -- that includes India (but not CANDU).

While PUREX may be well known & established for commercial SNF reprocessing, it is far more complex than U-metal processing and, as mentioned earlier, it is ridiculously expensive when you factor in the costs of re-fabricating oxide fuel after just a very short fuel burn in a reactor.
Indeed, this is precisely why IFR advocates want metal fuel -- it makes the closed fuel cycle (near-)economical, where the oxide version can never be....

Cyril R wrote:
Since the molten salt cooled reactor probably operates at best as isobreeder, fuel removal would also require makeup fissile feed. Highly unusual for this reactor, far more unusual than once through CANDUs that normally use lots of fresh fuel.

These things make wholesale transfer of fuel for weapons rather implausible.

OK, I can buy that.
But one of the big selling points of MSRs, if I'm not mistaken, is the faster global deployment rate relative to SFRs -- due to a combination of low starting fissile load AND a fairly decent breeding ratio: With a perfect isobreeder, we would forever be dependent on U enrichment plants -- which are seen as THE worst proliferation risk.....


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PostPosted: May 17, 2011 7:51 pm 
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jaro wrote:
But one of the big selling points of MSRs, if I'm not mistaken, is the faster global deployment rate relative to SFRs -- due to a combination of low starting fissile load AND a fairly decent breeding ratio: With a perfect isobreeder, we would forever be dependent on U enrichment plants -- which are seen as THE worst proliferation risk.....


With a perfect isobreeder we would be forever dependent on U enrichment plants to expand the nuclear capacity but once we have finished the build out we are no longer dependent on them. If one can start a LFTR using LEU then we need 2-3G years worth of enrichment services for an LWR to start an equal sized LFTR that then needs no further enrichment. Assuming there exists sufficient enrichment to operate 400 GWe of LWRs (not quite true yet but there are plants under construction) we could start something like 130 GWe of new LFTRs each year. Within 80 years you have 10,000 GWe running and likely have met the demand for the world. At that point there is no further need for enrichment and we can close off the worst proliferation risk completely.

If MSRs were actual breeders the proliferation risk would continue forever.


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PostPosted: May 18, 2011 1:08 pm 
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The faster deployment rate is almost all in the lower fissile requirement for thermal/epithermal breeders as compared to fast breeders. If you need say 3000 kg fissile startup and have breeding ratio of 1.06 and burn 800 kg fissile/year then you make very roughly 50 kg excess fissile per year. It'd take 60 years to start up a new reactor of this type. I think isobreeding is easier on the design - longer burnup, more parasitic losses tolerated, better proliferation characteristics, (ie less hassle).

We'll have at least 5,000 and probably over 10,000 tonnes of spent nuclear fuel plutonium in the future for startup without mined uranium plus enrichment service. Gen II and Gen III will keep running along, making more transuranics gunk to use.

If we assume this salt cooled design uses 5 tonnes per GWe then you get at least a TWe, possibly two, of startup capacity. If you use a mixed blanket with 50% LEU startup this leverages things up to 4-8 TWe. Ambitious enough for me!


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PostPosted: May 20, 2011 8:20 am 
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More info on SiC cladding:

http://mydocs.epri.com/docs/CorporateDo ... rticle.pdf

Also notice the work on center void channels bored in the fuel pellets; 10% void space hole in the center greatly improved fuel performance. Metallic thorium fuel wouldn't need it for thermal conducitivity but it could help remove volatiles and gasses from the fuel and reduce fuel internal stresses.


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PostPosted: May 22, 2011 2:24 pm 
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From another thread.....
ondrejch wrote:
Cyril R wrote:
Though to truly take advantage of lower melting point of NaF-BeF2 you'd need a salt-cooled solid fuelled design; the actinides in the salt make it higher melting.

Salt-cooled designs need moderating FLiBe; FNaBe lack of moderation results in positive coolant void reactivity coefficient, which is to be avoided..

I suspect there may be some gray area here.... for one thing, Be is an excellent moderator.

One of the more interesting presentations (IMO) at the Spring 2011 TEA Conference was by Charles Holden - "LFTR: 40MW Pilot Plant Outline" -- which descries a salt-cooled rector using NaF-BeF2.

It appears he wants both coolant AND fuel to be liquid.

The important part though, is that by avoiding FLiBe (with highly enriched Li7), the “deep salt pool design” is economically feasible – and independent of dubious Li7 supply sources.

Not sure why liquid fuel was thought necessary – or how the off-gas piping issue is solved – but certainly looks like an interesting idea….. ("Green Fuel; Yellow Coolant ; Red Hastelloy")


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PostPosted: May 22, 2011 3:24 pm 
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jaro wrote:
From another thread.....


It appears he wants both coolant AND fuel to be liquid.



Eh isn´t this the feature of all true MSR ? :shock:
And are there any Reactors with solid coolants? :?:


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PostPosted: May 23, 2011 7:36 am 
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There would be advantages to liquid fuel in fuel rods/elements like this. For example the off gas is removed easier and more completely, there are no issues with maintaining fuel-cladding contact, fuel integrity is easy (important for very high burnups) and very negative fuel dilatation coefficients can be achieved easily.

There would also be disadvantages, such as hard separation of thorium for reprocessing, and if dump tank type mechanisms are used, fuel pumps must be used (very radioactive nasty environment to make pumps), and lower actinide density and less fast fission bonus especially compared to metal fuel.


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PostPosted: May 23, 2011 11:20 am 
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Cyril R wrote:
There would be advantages to liquid fuel in fuel rods/elements like this. For example the off gas is removed easier and more completely, there are no issues with maintaining fuel-cladding contact, fuel integrity is easy (important for very high burnups) and very negative fuel dilatation coefficients can be achieved easily.

Yes, certainly.
I was wondering whether solid metal bathed in Na or Bi, could not be fairly easily substituted with powdered metal, in a very thick slurry with Na or Bi (since it doesn't get pumped around, but just sits there....)
The fuel-cladding contact, fuel integrity, and very negative fuel dilatation coefficients should all be favourable in that case too...
Not sure about Th, but U is pyrophoric, so producing metal powder would have to be done in an inert cover gas.

Cyril R wrote:
There would also be disadvantages, such as hard separation of thorium for reprocessing, and if dump tank type mechanisms are used, fuel pumps must be used (very radioactive nasty environment to make pumps), and lower actinide density and less fast fission bonus especially compared to metal fuel.

I would add a few other minuses....
Compared to SiC fuel rod tubes, Hastelloy will capture far more neutrons, seriously degrading breeding performance and generating a worse kind of radioactive waste in the end.

Also, if you think about the proposed core geometry a bit, it looks like it would be a real bitch to fabricate, since its all in one piece, with a great many tiny coolant salt channels passing through it.
Whithout having seen details of the design, one can nevertheless guess that the calandria-style core would involve a lot of welding – which would likely be impossible to perform using full-penetration welds (probably a requirement for this structure), nor could one easily do radiography of the welds (another requirement).

Looks to me like there might be two ways of constructing such an assembly – one that’s probably a bit easier to weld, but creates bad fluid-dynamic inlet conditions (at the bottom head of the calandria), or one that’s great fluid-dynamically, but more complex to assemble & weld.

The design using individual SiC fuel rods with plugged bottom ends looks to be a piece of cake, by comparison, and can also operate at much higher temperature (for SC CO2 turbine energy conversion).


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PostPosted: May 24, 2011 2:44 am 
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A far more convenient design, IMHO, would be 20-30mm pebbles, with SiC vapor deposited. They could be packed in wire or perforated metal crates (reusable after annealing) of the size of fuel bundles.
The Calandria design could have half meter crates.
The dimensions of fuel elements will become less critical, or at least tolerances will be more liberal, reducing engineering costs.


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PostPosted: Jun 21, 2011 7:06 am 
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Regarding the filler bond metal choice, it looks like thorium and uranium metal are both a bit soluble in bismuth, but not very much in lead.

http://www.orau.org/ptp/PTP%20Library/l ... orium6.pdf

Most of the bond type fuel has been focused on using sodium, which is also great, but has a fairly high volatility, so for vented fuel it would have to be kept in the fuel rod somehow.


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PostPosted: Jun 21, 2011 8:46 am 
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Looks like bonded fuel is well proven:

http://www.osti.gov/bridge/servlets/pur ... 850370.pdf

Sn-Pb-Bi alloy bonding, compatible with UO2 and the zircaloy, excellent reduction in peak temperatures, looks great. Why isn’t this applied in commercial reactors yet?


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PostPosted: Sep 16, 2011 4:35 am 
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I found a reference to a project that is very similar to what we discussed here: NaF-BeF2 eutectic coolant, SiC fuel rods, hydride fuel.

https://www.ornl.gov/fhr/presentations/Feng.pdf

Though I question the use of hydride fuel. There may be problems with hydriding-dehydriding that could harm fuel longevity and having hydrogen in the fuel rods seems less attractive. Keep it simple, use pure metal fuel. With no graphite in the core the reactor is undermoderated so will have good reactivity coefficients already.

Also, the high pressure CO2 heat exchangers in the pool downcomer is bad design. Put it outside the reactor building with an extra (clean) salt loop in between.


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