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PostPosted: Nov 27, 2010 8:29 am 
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The molten salt cooled reactor concepts all use TRISO fuel. Why are there no traditional fuel rod molten salt cooled designs out there? Since that’s what current commercial nuclear power is all about, it seems a bit strange. Fuel rods is something the industry would be much more comfortable with than TRISO.
Here is one option:

Coolant: FLiBe
Spectrum: LWR or even a bit faster
Fuel cladding: Hastelloy N (possibly with copper or carbon coatings for extra protection if necessary)
Fuel: UF4/UF3 (need a lot of UF3 since exact online fuel redox control is difficult). ThF4 optional, more attractive if CR=1 can be achieved (but may not be attractive due to frequent processing requirement).
The fuel would be allowed to melt to allow good heat transfer. This also acts as an overheating safety feature.
Power cycle: supercitical steam (off the shelf coal plant tech).

Advantages:
- Clean coolant salt with full redox control possible; minimal fission products in coolant, minimal corrosion, easier licensing of the coolant.
- Higher temperature operation means 30-45% more electrical power for the same PWR thermal power density.
- Low pressure. Thin pressure vessel.
- Easy natural circulation emergency decay heat cooling (high delta T possible during emergency).
- Commonly used fuel rods, use existing business case for fuel fabrication revenues.
- Thicker fuel rods allow lower fuel fabrication costs (and compensate for lower U density of uranium fluorides).
- Low differential pressure between the fuel and coolant
- Allows easy migration of gaseous fission products to dedicated out-of-flux bulb attached to the fuel rods (kudos to Iain!)
- Easy reprocessing (fluorinate to get all U and Np out)
- Fuel damage may be easy to avoid.
- High burnup (combining above).

Disadvantages:
- May be hard to license new fluoride fuel (especially the melting part would be bad). If this is the case, standard UO2 pellets is the conservative alternative. Uranium –thorium metal alloy is a more innovative alternative.
- No online removal of salt seeking fission products
- Effects of (partial) melting in the fuel could lead to serious fuel heterogeneity problems (plutonium fluoride might concentrate somewhere?).


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PostPosted: Nov 27, 2010 8:51 am 
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In my opinion, salt-cooled, solid-fueled reactors are the worst of both worlds. The crappy fuel cycle of a solid-fueled reactor, the high-temperature circulation issues of molten-salt. Plus you have to work hard to avoid positive void coefficients (which limits you to only FLiBe). So just go all the way to a true MSR.


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PostPosted: Nov 27, 2010 10:05 am 
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Kirk Sorensen wrote:
In my opinion, salt-cooled, solid-fueled reactors are the worst of both worlds. The crappy fuel cycle of a solid-fueled reactor, the high-temperature circulation issues of molten-salt. Plus you have to work hard to avoid positive void coefficients (which limits you to only FLiBe). So just go all the way to a true MSR.


Yes, agree, but the point would be that high temperature may be cheaper, more efficient, and easier than high pressure, plus not having the entire coolant loop intensely radioactive could be a big pro in getting started.

There might be liquid fuel in the fuel rods though. Actinide fluorides. There may be problems with fuel heterogeneity but on the other hand, melting could be a nice overheating safety feature (plus molten fluorides are less dense which helps lower reactivity). I guess there might be some strange zone refining type processes going on, which I find fascinating, but no doubt regulators would put up some serious frowning.


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PostPosted: Nov 27, 2010 11:14 am 
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Cyril R wrote:
I guess there might be some strange zone refining type processes going on, which I find fascinating, but no doubt regulators would put up some serious frowning.


There is already some fuel restructuring going on in a standard PWR, it gets even worse in a high temperature system such as SFR. So the regulatory bodies will most likely ask for a good knowledge of in-pile behavior. Taken into account that new fuels (carbides, nitrides, ...) are hindered by that, I doubt that a UF3/UF4 fuel will make any chance...

_________________
Liking All Nuclear Systems, But Looking At Them Through Dark And Critical Glasses.


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PostPosted: Nov 27, 2010 12:10 pm 
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Well, UO2 is always available, if we need to be conservative. Still get a higher efficiency, low pressure reactor. Adding BeO thermal bridging technology recently developed, would be welcome, with the higher coolant temperatures.


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PostPosted: Nov 27, 2010 12:29 pm 
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Cyril R wrote:
Coolant: FLiBe
Spectrum: LWR or even a bit faster
Fuel cladding: Hastelloy N

LWRs use zirconium cladding for a good reason: with thousands of fuel pins in the core, that's a lot of metal, parasitically absorbing precious neutrons.
Zr will minimise that issue -- Hastelloy will blow it up to intolerable levels.
Only fast reactors can have steel cladding, without impacting the neutron economy excessively.


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PostPosted: Nov 27, 2010 1:38 pm 
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jaro wrote:
Cyril R wrote:
Coolant: FLiBe
Spectrum: LWR or even a bit faster
Fuel cladding: Hastelloy N

LWRs use zirconium cladding for a good reason: with thousands of fuel pins in the core, that's a lot of metal, parasitically absorbing precious neutrons.
Zr will minimise that issue -- Hastelloy will blow it up to intolerable levels.
Only fast reactors can have steel cladding, without impacting the neutron economy excessively.


Its important if you want to have isobreeding, but if not then only a minor increase in uranium consumption seems like a minor penalty to bear. The reactor will be epithermal or even fastish, not thermal like CANDU. LWRs have used stainless steel cladding in the past. PWRs have greater mechanical stresses in the cladding from the big pressure differential of the fuel with the coolant, and higher flow speeds leading to FAC. A molten salt cooled version can have the same pressure in the coolant as in the fuel plus considerably lower flow speeds. That helps a lot for the cladding. It can be thin. Higher temperature in the cladding is definately a downside though, but here it also helps that the cladding is thinner (better heat exchange).


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PostPosted: Nov 27, 2010 2:12 pm 
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Fission generates a lot of gas. Would you try to contain the gases within the fuel rods resulting in high pressure or would you design the fuel rods to release the gases into the coolant salt making the coolant salt radioactive (but also opening the possibility of He sparging)?


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PostPosted: Nov 27, 2010 2:25 pm 
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Lars wrote:
Fission generates a lot of gas. Would you try to contain the gases within the fuel rods resulting in high pressure or would you design the fuel rods to release the gases into the coolant salt making the coolant salt radioactive (but also opening the possibility of He sparging)?


I can't think of a conservative method to sparge the gasses like that. Some kind of solonoid might be attempted but that defeats the purpose of using conservative design and not putting fission products in the coolant. Tritium we might live with since it gets grabbed in the solar salt. But the noble gasses and their decay products? Might as well go for an MSR then, like Kirk said!

Iain had some calculations that there could be a bulb or other extension on the fuel rods and that it would take most noble gasses out of flux. That sounds good enough. We might appreciate an extra atmosphere of backpressure - the coolant has to be pushed through the rods which will involve a few atmospheres most likely. If the fuel is allowed to be actinide fluorides, it will auto-sparge even better. Big noble gas molecules ain't soluble in fluoride melts. But maybe we start out with UO2 since it fits the conservative design.


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PostPosted: Nov 27, 2010 7:35 pm 
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Metal clad fuels would be a step backward, since metal cladding loses its structural integrity at relatively low temperatures.

An important advantage of LFTR/AHTR is the capability to use graphite as well as carbon/carbon and SiC composites as structural materials in the core. The temperatures needed to cause thermal damage to these materials are well above the boiling temperature of fluoride salts (>1400°C) and thus it is impossible heat these materials to temperatures that could cause damages during any transients or accidents. The temperature limits for LFTR/AHTR is instead determined by the structural materials used for the primary loop pressure boundary. For currently available materials this allows LFTR/AHTR to operate in the temperature range from 600°C to 700°C (e.g., Hastelloy N, 316 SS, etc.).

All reactors that use metallic fuel cladding and structures must run at significantly lower temperatures than LFTR/AHTR and thus will have lower thermal efficiencies. Their operating temperatures are determined by damage thresholds for these metallic materials. For example, IFR core outlet temperatures are limited to 510°C by the formation of eutectics between uranium in the fuel and iron in the cladding. With an inlet temperature of 363°C, this results in heat being delivered to power conversion at an average temperature over 200°C lower than LFTR/AHTR.


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PostPosted: Nov 27, 2010 9:07 pm 
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Cyril R,

I appreciate your imagination.


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PostPosted: Nov 27, 2010 9:53 pm 
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Dr. Peterson,

Good to see you back on the forum.


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PostPosted: Nov 28, 2010 3:19 am 
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Per Peterson wrote:
Metal clad fuels would be a step backward, since metal cladding loses its structural integrity at relatively low temperatures.

An important advantage of LFTR/AHTR is the capability to use graphite as well as carbon/carbon and SiC composites as structural materials in the core. The temperatures needed to cause thermal damage to these materials are well above the boiling temperature of fluoride salts (>1400°C) and thus it is impossible heat these materials to temperatures that could cause damages during any transients or accidents. The temperature limits for LFTR/AHTR is instead determined by the structural materials used for the primary loop pressure boundary. For currently available materials this allows LFTR/AHTR to operate in the temperature range from 600°C to 700°C (e.g., Hastelloy N, 316 SS, etc.).

All reactors that use metallic fuel cladding and structures must run at significantly lower temperatures than LFTR/AHTR and thus will have lower thermal efficiencies. Their operating temperatures are determined by damage thresholds for these metallic materials. For example, IFR core outlet temperatures are limited to 510°C by the formation of eutectics between uranium in the fuel and iron in the cladding. With an inlet temperature of 363°C, this results in heat being delivered to power conversion at an average temperature over 200°C lower than LFTR/AHTR.


Thanks Dr. Peterson. We're all quite familiar with your AHTR and can see advantages there, though perhaps it would also be an advantage to use the existing industrial base & experience for fuel rod fabrication. While TRISO is proven fuel, it has almost zero industrial base compared to fuel rod fabrication. Certainly with a low pressure coolant, a much thinner cladding with reduced peak clad temps can be used? SiC cladding was proposed some time ago for fuel rod fabrication, but perhaps this is too brittle for this application.

How does the cost of TRISO fuel fabrication compare to traditional fuel rod fabrication?


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PostPosted: Nov 28, 2010 8:07 pm 
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Cyril R wrote:
Thanks Dr. Peterson. We're all quite familiar with your AHTR and can see advantages there, though perhaps it would also be an advantage to use the existing industrial base & experience for fuel rod fabrication. While TRISO is proven fuel, it has almost zero industrial base compared to fuel rod fabrication. Certainly with a low pressure coolant, a much thinner cladding with reduced peak clad temps can be used? SiC cladding was proposed some time ago for fuel rod fabrication, but perhaps this is too brittle for this application.

How does the cost of TRISO fuel fabrication compare to traditional fuel rod fabrication?


Good point. B&W has now scaled the U.S. TRISO fabrication technology to commercial scale, so the complete domestic capacity to fabricate, test, qualify and manufacture TRISO fuel now exists, in addition to the Chinese and the Japanese capabilities. Fabrication costs are uncertain, still, due to the lack of an established commercial market, but we know that TRISO is more expensive to fabricate, per unit of heavy metal, than LWR LEU fuel. On the other hand, AHTRs consume 40% less natural uranium and 20% less enrichment than LWRs, which compensates some.

But today the combined cost of fuel and waste management contribute less than 10% to the levelized cost of electricity (LCOE) from a new ALWR (about 8 cents/kWhr LCOE). Most of this is the cost to pay off the mortgage for constructing the reactor. Arguably, the most important near and intermediate term goal is to bring down construction costs while maintaining the high availability (>90%) that ALWRs achieve. Further improvement in ALWR technology and construction methods are clearly possible in the coming decade. Beyond this, the major characteristics of fluoride salt cooled/fueled reactors, which we've discussed in previous posts, provide probably the best opportunity for further substantial reduction in LCOE.

Making nuclear electricity abundant and affordable, while continuing to meet high standards for safety, security, and environmental protection, is an important goal. In the longer term the sustainable use of fertile materials (thorium and/or uranium) can become important, but today the biggest hurdle to expanding the role of nuclear energy is the LCOE.


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PostPosted: Dec 04, 2010 10:50 pm 
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Per Peterson wrote:
... AHTRs consume 40% less natural uranium and 20% less enrichment than LWRs, which compensates some. ...

Dr. Peterson, this is better than what was stated in your review slides from 2009 (30% less natural uranium, I think?).

Is this a thorium cycle result perhaps?


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