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PostPosted: Sep 05, 2008 6:28 am 
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Axil wrote:
TRISO fuel vendors will eventually test to 80% burnup…. 60 MM Silicon Carbide coated pebbles are very tough!
.....
At 80% burnup, no reprocessing will be done; a once through fuel cycle situation. The small amount of pebble waste will be buried and self protested/encapsulated for 1,000,000 years by the Silicon Carbide coating.

Going the pebble bed route seems like an awfully expensive way to (theoretically) achieve the same thing -- and LESS -- than what a break-even MSR running on LEU (or low-fissile MOX) can do: the difference being that in the latter case, only fission products go to waste storage.

Such an MSR does need to be very efficient in neutron economy however, necessitating on-line processing, careful materials selection, and of course heavy water....


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PostPosted: Sep 05, 2008 9:11 am 
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Such once through systems with no reprocessing also means that you don't get to reclaim any of those rare elements that are produced and are worth lots of money as noted on a chart in another thread. Not trying to be negative about the idea, but it is giving up a valid cost-driving opportunity and incurring the existing costly operation of making 100% perfect solid fuel pellets.


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PostPosted: Sep 05, 2008 10:23 am 
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Another option would be to use pure plutonium in the TRISO's while the salt is only thorium, that way all the waste plutonium could be put to use to create U233 in the salt without any transuranics contaminating the salt. Since the plutonium TRISO's can go so such high burnup(600+ GWd/ton) about 3/4 of the plutonium is consumed in one go.



TRISO fuel comes in three verities as shown in the figure. I would think that the security for the plutonium verities is very high.


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PostPosted: Sep 05, 2008 10:28 am 
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One could add the minor actinides from the LWR waste aswell but it would lower fuel utilisation. No reprocessing would be worthwhile after such a high burnup, unless to lower the long term radiotoxicity.



Note,TRISO radiation damage limit is 8 10exp21.


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PostPosted: Sep 05, 2008 10:47 am 
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What is the heat capacity of molten salts compared to graphite?


In a PBMR, the coolant is helium; in a MSR at startup, the coolant is molten salt. IMHO, molten salt must be a better coolant than helium by orders of magnitude.

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It doesnt seem to hard to design a reactor so that the TRISO fuel never exceeds 1600 degrees in case of low of flow accident?


The design of a reactor where the TRISO fuel never exceeds 1600C degrees in case of loss of coolant accident has been tested in china on their prototype test bed.

This is a worse case failure condition that covers the situation in which all molten salt coolant is lost in both the core and the blanket.

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Let the salt absorb the heat and have some kind of passive cooling of the reactor vessel similar to PBMR. Since the salt has superior heat transfer properties to gas wouldn it easier bring heat away from the pebbles to the vessle walls and allow a larger vessle then what is used in the PBMR?


The pebbles are used only at startup. Because of that, the MSR should be optimized for operations with molten salt only.

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PostPosted: Sep 05, 2008 10:48 am 
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Well, it would certainly be better to convert WGPu (effectively) to U233 in this manner than to form MOX fuel and burn it out once. U-233, once formed, represents essentially unlimited energy in a LFTR.

Axil, I'm slowly coming around to some of the potentials of your idea.


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PostPosted: Sep 05, 2008 11:09 am 
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Axil wrote:

In a PBMR, the coolant is helium; in a MSR at startup, the coolant is molten salt. IMHO, molten salt must be a better coolant than helium by orders of magnitude..


Yes but the heat capacity of helium is irrelevant because it is the massive ammounts of graphite in the pebbled bed(both in the pebbles and the structural graphite) that ensure its safety. If salt has a similary heat capacity it will work in a similar fashion as the structural graphite, even better if the salt can have some degree of passive cooling by natural convection. If it doesnt some other heat sink might be needed.


Axil wrote:
The design of a reactor where the TRISO fuel never exceeds 1600C degrees in case of loss of coolant accident has been tested in china on their prototype test bed.

This is a worse case failure condition that covers the situation in which all molten salt coolant is lost in both the core and the blanket.


Its not neccesarly a worse type of failure condition. In the pbmr the structural graphite plays a large role in keeping the core cool and safe during a loss of coolant accident. The structural materials of the MSR might not be able to take the same heat load as the graphite structural material in the pbmr. If not the structural materials of the msr is also graphite which seems like the logical chooise.

Axil wrote:

The pebbles are used only at startup. Because of that, the MSR should be optimized for operations with molten salt only.


The same passive cooling of the vessle would improve the safety of salt only operations aswell though. More margins while the freeze plug melts etc.


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PostPosted: Sep 05, 2008 11:10 am 
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Going the pebble bed route seems like an awfully expensive way to (theoretically) achieve the same thing -- and LESS -- than what a break-even MSR running on LEU (or low-fissile MOX) can do: the difference being that in the latter case, only fission products go to waste storage.


I have my doubts about using light water LEU. The LEU may negate the drain plug melt down prevention if the core gets to hot because the LEU is the source of the heat. . Light water LEU is untested in this case. Pebbles are tested to be meltdown proof, idiot proof, safe.

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…necessitating on-line processing, careful materials selection, and of course heavy water....


You guys must have a lot of heavy water up there in Canada? :lol:

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PostPosted: Sep 05, 2008 11:17 am 
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Such once through systems with no reprocessing also means that you don't get to reclaim any of those rare elements that are produced and are worth lots of money as noted on a chart in another thread. Not trying to be negative about the idea, but it is giving up a valid cost-driving opportunity and incurring the existing costly operation of making 100% perfect solid fuel pellets.


The big weakness in TRISO fuel as well as LEU fuel is that they are very expensive. All things being equal, the MSR will produce the cheapest power.

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PostPosted: Sep 05, 2008 11:19 am 
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Axil wrote:
Quote:
Going the pebble bed route seems like an awfully expensive way to (theoretically) achieve the same thing -- and LESS -- than what a break-even MSR running on LEU (or low-fissile MOX) can do: the difference being that in the latter case, only fission products go to waste storage.

I have my doubts about using light water LEU. The LEU may negate the drain plug melt down prevention if the core gets to hot because the LEU is the source of the heat. . Light water LEU is untested in this case. Pebbles are tested to be meltdown proof, idiot proof, safe.

Sorry about the confusion -- I meant to say UF4 & PuF3, not LWR oxide fuels (I shouldn't have said "MOX" -- but there doesn't seem to be a recognised acronym for mixed fluorides.... maybe "MIF" ? .....some folks use HMF, for "heavy metal fluorides," but I think that's intended to be more general, including TRUs like the curiums coming out of your high-burn-up pebble bed....


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PostPosted: Sep 05, 2008 11:31 am 
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Axil wrote:
The big weakness in TRISO fuel as well as LEU fuel is that they are very expensive. All things being equal, the MSR will produce the cheapest power.

That remains to be proven....

.....instead of having containment around millions of tiny PB kernels, you need an equally effective containment around the entire MSR system.

But in general, I would agree that TRISO fabrication is very expensive.
That is its main anti-proliferation attraction, since its totally senseless to use such fuel in the very low-burnup mode required for WGPu production.

OTOH, if you stick TRISO fuel in a salt pool that can be easily reprocessed to extract WG fissile materials, then you can get both high burnup from the TRISO and high-quality bomb material.
The proliferator's dream machine.


Last edited by jaro on Sep 05, 2008 11:47 am, edited 1 time in total.

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PostPosted: Sep 05, 2008 11:43 am 
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Seems to me the biggest containment challenge is going to be the gases.
Since we continually extract them there isn't a huge volume of them when planning for an accident.
But it needs to be analysed just how much is in the fuel.

A possibly bigger challenge is the offgas storage system. There we will have a larger store house of gaseous radioactive material.

Containing the salt fuel itself does not seem to be a public exposure risk at all.


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PostPosted: Sep 05, 2008 12:21 pm 
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Yes but the heat capacity of helium is irrelevant because it is the massive ammounts of graphite in the pebbled bed(both in the pebbles and the structural graphite) that ensure its safety. If salt has a similary heat capacity it will work in a similar fashion as the structural graphite, even better if the salt can have some degree of passive cooling by natural convection. If it doesnt some other heat sink might be needed.


Reactor Cavity Cooling

The reactor cavity cooling system (RCCS; see Figure) is a constant flow, water-based cooling system that removes heat from the reactor cavity to protect the concrete walls of the cavity during both normal shutdown and accident conditions. It is comprised of standpipes that
line the inside of the cavity, and is a low-temperature, low pressure system with water temperatures below 30°C during normal active operation and reaching the boiling point only during emergency passive operation. The RCCS can operate in both an active mode by pumping
water through the standpipes, or a passive mode by boiling the water for approximately 72 hours. The passive mode time constraint is yet to be defined.


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PostPosted: Sep 05, 2008 12:26 pm 
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In the pbmr the structural graphite plays a large role in keeping the core cool and safe during a loss of coolant accident. The structural materials of the MSR might not be able to take the same heat load as the graphite structural material in the pbmr. If not the structural materials of the msr is also graphite which seems like the logical chooise.


In the diagram, all reflectors are graphite, only the outer reflector is permanent. I also suggest that the MSR design team shop around for a proven modular pebble bed reactor design (there are several about) as a development template.
.


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PostPosted: Sep 05, 2008 12:42 pm 
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OTOH, if you stick TRISO fuel in a salt pool that can be easily reprocessed to extract WG fissile materials, then you can get both high burnup from the TRISO and high-quality bomb material.
The proliferator's dream machine.


The trust of this TRISO/Molten salt startup plan suggested here is to avoid any proliferation venerable material in the molten salt. The core and blanket only contain U233/U232, and only inextricable amounts of PU(***). All PU is inside the TRISO fuel protected from proliferation by graphite.

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