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PostPosted: May 07, 2011 9:22 am 
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Cyril R wrote:
...the idea of a molten salt cooled reactor with traditional fuel rods can really work. Advantage is that metal fuels can be used with a very high HM loading compared to TRISO, and potentially much cheaper fuel fabrication, while getting similar high fuel failure temperatures that the TRISO have as major advantage. Also with no graphite to cause positive coolant void other coolants become available such as NaF-BeF2 with a melting point of just 340 degrees Celcius, a lower vicosity and lower cost/better availability compared to FLiBe.

This would be a much higher power density core compared to the AHTR - probably more similar to PWRs or higher. And lots of fast fission in the thorium-plutonium thick metal fuel.

Excellent concept Cyril ! ....I like it very much !

I suspect that the inside of the SiC fuel pins might need some sort of thin buffer layer to absorb the metallic fuel's thermal and radiation/FP-induced expansion, while efficiently transmitting heat to the SiC sheath -- since the material likely can't strain (expand) with the fuel without cracking, as much as an ordinary Zr or SS sheath would be able to (as in the case of the Integral Fast Reactor - IFR).

This buffer layer approach is used in the TRISO fuel kernels of the PB-AHTR design -- where the UO2 fuel itslf is likely more stable against expansion than U-metal.
Venting of the fuel pins only reduces internal pressure due to gaseous fission product (FP) buidup, but it doesn't do much to reduce the fuel expansion.

Also, if you’re going to be using Th in this thing, then you’ll probably need a good deal of out-of-core fuel buffering, to allow for Pa233 decay.
This frequent fuel shuffling might be difficult, if you have vented fuel pins connected to off-gas piping & valves….


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PostPosted: May 07, 2011 12:28 pm 
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I was thinking about very loose tolerance fuel with a high heat transfer metal as liquid to buffer expansion. The IFR design uses this trick with a little sodium slug that melts and takes up the space between the fuel and the cladding. If we want something a little less reactive, bismuth can also work, its a bit low on thermal conductivity compared to sodium, but much better than the helium fill that oxide fuelled designs use. Bismuth also has a lower vapor pressure. There would be a need for some active carbon as a 'cigarette filter' to retain volatile condensables such as iodine and cesium while venting non-condensables such as xenon and krypton.

Fuel shuffling will be harder in this design. Just do it only once a year and accept more losses to Pa. Fast fission makes up a bit with U234 and such.


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PostPosted: May 07, 2011 1:12 pm 
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Cyril R wrote:
I was thinking about very loose tolerance fuel with a high heat transfer metal as liquid to buffer expansion. The IFR design uses this trick with a little sodium slug that melts and takes up the space between the fuel and the cladding.

Yeah…. That should do it !

Cyril R wrote:
There would be a need for some active carbon as a 'cigarette filter' to retain volatile condensables such as iodine and cesium while venting non-condensables such as xenon and krypton.
Fuel shuffling will be harder in this design. Just do it only once a year and accept more losses to Pa. Fast fission makes up a bit with U234 and such.

Actually, fuel shuffling could be easier – if you don’t have off-gas piping connections.
If you’re going to put an active carbon 'cigarette filter' on each fuel pin, then you could just leave the tops open, to vent to the reactor hot cell, which would then deal with the non-condensable gases independently.
I like it better by the minute !


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PostPosted: May 07, 2011 2:31 pm 
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Hmm yes if everything is at atmospheric (actually 'pool') pressure then the non active portion of the fuel rods could simply be open to the hot cell at the top. We still have to find a way to seal the bottom of the fuel rods, though.


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PostPosted: May 08, 2011 6:09 am 
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jaro wrote:
Cyril R wrote:
...the idea of a molten salt cooled reactor with traditional fuel rods can really work. Advantage is that metal fuels can be used with a very high HM loading compared to TRISO, and potentially much cheaper fuel fabrication, while getting similar high fuel failure temperatures that the TRISO have as major advantage. Also with no graphite to cause positive coolant void other coolants become available such as NaF-BeF2 with a melting point of just 340 degrees Celcius, a lower vicosity and lower cost/better availability compared to FLiBe.

This would be a much higher power density core compared to the AHTR - probably more similar to PWRs or higher. And lots of fast fission in the thorium-plutonium thick metal fuel.

Excellent concept Cyril ! ....I like it very much !

I suspect that the inside of the SiC fuel pins might need some sort of thin buffer layer to absorb the metallic fuel's thermal and radiation/FP-induced expansion, while efficiently transmitting heat to the SiC sheath -- since the material likely can't strain (expand) with the fuel without cracking, as much as an ordinary Zr or SS sheath would be able to (as in the case of the Integral Fast Reactor - IFR).


This buffer layer approach is used in the TRISO fuel kernels of the PB-AHTR design -- where the UO2 fuel itslf is likely more stable against expansion than U-metal.
Venting of the fuel pins only reduces internal pressure due to gaseous fission product (FP) buidup, but it doesn't do much to reduce the fuel expansion.

Also, if you’re going to be using Th in this thing, then you’ll probably need a good deal of out-of-core fuel buffering, to allow for Pa233 decay.
This frequent fuel shuffling might be difficult, if you have vented fuel pins connected to off-gas piping & valves….

A thorium casing enclosing 20% LEU, a variant of Indian AHWR fuel design is quite suitable for the purpose as metallic thorium has a MP close to that of Zirconium and a much higher thermal conductivity. SiC or other coating will be required compatible with the coolant.
Water functions as both moderator and coolant. BeO or Be2C bundles in a pattern with the fuel will be an excellent arrangement in a non-volatile non-moderating coolant bath.
If you have a Reactor Grade Plutonium in the thorium casing, you could have a fast spectrum, no need of a moderator and a bigger choice of coolants. My favorites being SnF2 and Al-Mg eutectic. Just match the coating to the coolant.


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PostPosted: May 08, 2011 7:52 am 
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Cyril R wrote:
We still have to find a way to seal the bottom of the fuel rods, though.

I don't know how the SiC tubes are manufactured, but it seems to me that it shouldn't be difficult to leave one end with a short section that doesn't have a hole in it.....


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PostPosted: May 08, 2011 11:31 pm 
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Quote from http://en.wikipedia.org/wiki/Silicon_carbide
Quote:
Pure silicon carbide can also be prepared by the thermal decomposition of a polymer, poly(methylsilyne), under an inert atmosphere at low temperatures. Relative to the CVD process, the pyrolysis method is advantageous because the polymer can be formed into various shapes prior to thermalization into the ceramic.[18][19][20][21]

It should be possible to coat the fuel with the polymer and pyrolyze it. Vapor deposition is also feasible.
Thorium casing is more conductive (54W/m/K) than Zirconium (22.6W/m/K). If you are not using natural Uranium, you can put all the fissile inside the thorium casing, which also acts like fertile part of nuclear fuel.
SiC can probably be dissolved in the HF like glass as SiF4 and CF4 are both gases. Should be quite easy to create escape holes.


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PostPosted: May 10, 2011 7:00 am 
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Lots of nice details regarding thorium:

http://www.ornl.gov/info/reports/1965/3445601336962.pdf

The thermal conductivity of thorium metal increases with temperature! A very nice safety feature. 45 wmk at 650 Celcius. Transformation only occurs at 1400 Celcius, very nice. Thorium metal looks really great for fuel!


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PostPosted: May 10, 2011 8:30 am 
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INL study on vented SiC fuel for high temperature gas cooled reactor design:

http://www.inl.gov/technicalpublication ... 074897.pdf

Now that will get much better with molten salt coolant & thorium metal fuel. Far better heat transfer.


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PostPosted: May 11, 2011 2:48 am 
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I've been thinking about how to avoid sealing the bottom of the tube. I wasn't thinking clearly, because obviously the solution is to not have a bottom at all - by using U shaped fuel rods with both ends open at the top.

Here's a picture of a SiC u-tube running red hot:

Image

8)

The Aircraft Reactor Experiment also used U shaped fuel tubes, but upside down:

http://moltensalt.org/references/static ... Design.pdf


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PostPosted: May 11, 2011 5:30 am 
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....seems to me that if a SiC U-tube can be manufactured, then a straight one with a plugged end should be considerably easier to do.

The other thing to consider is the top end, where you must have some sort of feature that allows a grappling device to get a firm hold -- for loading/unloading & shuffling operations.
The rods are liable to be quite heavy, when filled with Th/U metal.
The other option is to use a "basket" holding several fuel rods -- somewhat like ordinary LWR fuel assemblies or, more likely, like the AHTR plate fuel assembly design by ORNL (Jess Gehin) -- only with round rods instead of plates.


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PostPosted: May 11, 2011 6:41 am 
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There's no need to reinvent the wheel, PWRs seem to have got tie plates, spacers and such covered nicely. With the greater stiffness of SiC and fewer rods, plus low pressure operation, things should be much easier to do in a molten salt cooled fuel rod version. In fact refuelling and reshuffling fuel is one of the reasons I'm interested in molten salt cooled fuel rod designs.

In addition to the PWR experience there are new studies specific to molten salt cooled solid fuel refuelling, here is an excellent overview by Charles Forsberg from the document repository:

http://www.energyfromthorium.com/pdf/OR ... 006-92.pdf


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PostPosted: May 11, 2011 10:33 am 
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Yes, the Forsberg document is a good overview.
Especially the two appendices: They illustrate very well the great complexity of fuel handling in sealed reactors -- with all sorts of rotating plugs, port valves, etc., etc.
Makes your head spin !

OTOH, if you have vented fuel, with the reactor top open to the hot cell, then its a totally different ball of wax.
More like the fuel handling on small teaching & research water pool reactors, or also like a permanently open LWR vessel. Except that everything must be done remotely/automatically.

In recent years, France's CEA has been developing some very interesting robotic and master/slave systems qualified for high-radiation areas -- they plan to use more of these in their reprocessing plants.
Similar devices will also be needed eventually for the maintenance of the ITER fusion reactor.
I think this type of thing would be more appropriate for your AHTR than the old-style monster machines in the Forsberg report....


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PostPosted: May 11, 2011 11:18 am 
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I was thinking that the radiation wouldn't be too bad (for robotics at least, its still hot cell level due to krypton & xenon plus activated fluorine/sodium) because there can be a layer of coolant over the active core (which also allows plenty of non-active fuel rod length for carbon filter plugs at the top). A meter of molten fluoride should be quite a decent radiation shield. Probably grab over 99% of gammas and neutrons?

How will the krypton and xenon be removed? Can we make a closed system in the hot cell that centrifuges the hot cell argon gas to seperate out krypton and xenon, and then bottle that?


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PostPosted: May 11, 2011 12:53 pm 
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Cyril R wrote:
...there can be a layer of coolant over the active core (which also allows plenty of non-active fuel rod length for carbon filter plugs at the top). A meter of molten fluoride should be quite a decent radiation shield.

IMO you would need a lot more than a one-meter layer of coolant over the active core: you would need the equivalent of a rod height (when you pull it out) PLUS that extra meter.
This is how LWR refueling is done: the reactor pit is flooded -- including the trench leading to the SNF pool, all to the same level -- such that when a fuel assembly is pulled out and moved sideways, it always stays submerged by several meters of shielding water....


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