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PostPosted: Aug 25, 2008 5:00 pm 
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David,

Because of the almost universally viewed opinion that TRISO fuel is very proliferation resistant, its use in “the design” will make your reactor more likely to be deployed in third world countries. The logistics of highly enriched light water reactor fuel use in the third world might be politically difficult to sell. The reprocessing of light water reactor fuel during the operation of the reactor might also be difficult to sell if the reactor is deployed in the third world.

Yes, the place to put the TRISO fuel is in the core. Automatic TRISO fuel handling is designed and its modification to work in a liquid salt coolant environment is small, and as such, automatic fuel handling is a small development risk.

TRISO fuel will open up the possibility of using thorium fuel exclusively in the reactor both in the core and the blanket, and afford the possibility to use inexpensive thorium salt in the design.

Please allow me to change my thinking to address the design with TRISO fuel, graphite pebbles and liquid salt mixed in the core.


Automatic graphite/fuel pebble handling buys higher availability and reduced maintenance costs associated with periodic shutdown for graphite replacement. It also minimizes human radiation exposure.

Precise Control

On-the-fly fuel handling also allows for the maintenance of the nuclear reaction to operate right at criticality.

In fact, on-line TRISO refueling capability provides a mode of operation that greatly decreases the likelihood that the power plant will be co-opted for weapons production.

By allowing the addition or deduction of fuel only as needed to maintain criticality, the reactor can operate with a very small amount of excess reactivity.

Precise reactor control on a daily timeframe is a good selling point and provides a sense of safety.



U238 denaturing in the core salt is not necessary. Waste production is cleaner in the political sense and therefore more sellable.

Removal of U232/U233 from the core, if required, can be compensated for by the addition of a precise amount of TRISO fuel to maintain criticality.

Less Maintenance


PU239 is minimal in the waste which is a great anti proliferation selling point.

If U232/U233 is removed from the core, it can be compensated for by the addition of a precise amount of TRISO fuel to maintain criticality.


In fact, U233 removal from the core can be postponed indefinitely which increases proliferation resistance.

Here again, transfer of Uranium from the blanket is inherently denatured, proliferation resistant and can be precisely adjusted through TRISO fuel removal.


Shutdown

After a short amount of operating time, the TRISO fuel load will be subcritical since some amount of U233 will contribute to the reaction. The removal of the core salt will stop the reaction to support reactor shutdown. In an emergency, nuclear poison can be added to the salt or a control rod can be used to stop the reaction.

Also, after shutdown, blanket salt cooling can take core heat away if necessary.

I realize that this set of ideas is a departure from your thinking but I offer them in the hope that something useful can be used in the design effort.

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PostPosted: Aug 25, 2008 6:21 pm 
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Axil wrote:
Because of the almost universally viewed opinion that TRISO fuel is very proliferation resistant, its use in “the design” will make your reactor more likely to be deployed in third world countries.


TRISO fuel may be proliferation resistant, but it is nearly impossible to reprocess and will limit the ultimate utilization of thorium significantly.


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PostPosted: Aug 25, 2008 6:44 pm 
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Also, the fuel kernels in TRISO are UO2 -- for which standard reprocessing techniques exist, so probably not as proliferation resistant as is often claimed.
The main problem is getting at the UO2 through the layers of graphite (easy) and silicon carbide (hard).
But with enough determination, clever proliferators can probably manage that feat.....


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PostPosted: Aug 25, 2008 7:50 pm 
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Hi Kirk,

From what I have read from General Atomics, TRISO fuel is hard to recycle but not impossible. That is one of the many reasons why it is so expensive; and that is why the thorium liquid fuel reactor is so much more desirable. I don’t agree that TRISO fuel will limit the ultimate utilization of thorium fuel significantly because of overwhelming cost advantage of thorium fuel; unless I misunderstand what you mean.

In politics, facts are less important than perception. All the big nuclear companies are currently selling the TRISO perception. And from what I can see, it’s effective. If the liquid thorium reactor can come close to the proliferation perception of TRISO fuel, then the battle is won for its world wide use. The use of light water reactor fuel and its online reprocessing may be a weak spot here in that perception.

After all, I am just proposing a very short startup period where TRISO fuel is only used to boot strap the thorium nuclear reaction in agreement with David’s plan. For the other 30 - 60 years of operation, just mostly thorium fuel is used. In addition, minimization of the lifetime reactor cost is what will sell in the end to the utilities, all other things being equal.

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PostPosted: Aug 25, 2008 8:08 pm 
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Hi Axil, I've been reading a document on the use of thorium in high-temperature reactors, much like the TRISO fuel you favor, and it doesn't come down very favorably for thorium. I'm planning to write a rebuttal/expansion to the paper showing how liquid fluoride thorium can alter their conclusions.

But there are many good points in the papers. The more costly the fabrication process for thorium fuel is, the more costly and difficult reprocessing is, the less fuel can be recovered---all of these undermine the case for using thorium in the first place, and you don't have to have too many of them to undercut the case completely.

TRISO fuel, despite its durability, can't go to the burnup limits of fluoride fuel (which are essentially unlimited). Thus, there's no way to get a complete "burn" of thorium in a solid-fueled reactor, and the whole utilization/waste/economic case will be built on the ease and economy of fuel reprocessing. This is a pretty bad story for thorium oxide fuel in general, and really really bad for TRISO type thorium fuels.

Here's the paper I was referring to:

http://www.torium.se/Backgrounder.htm

(link is called : Industry's evaluation of thorium in existing uranium water reactors - AREVA)


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PostPosted: Aug 25, 2008 8:43 pm 
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Quote:
After all, I am just proposing a very short startup period where TRISO fuel is only used to boot strap the thorium nuclear reaction in agreement with David’s plan. For the other 30 - 60 years of operation, just mostly thorium fuel is used. In addition, minimization of the lifetime reactor cost is what will sell in the end to the utilities, all other things being equal.


Axil,

Yes, that mode of operation would have benefit. If the TRISO is only used as a way to burn LEU (Low Enriched Uranium) to boot strap to a pure Th-U233 cycle in the core and blanket salts. It would probably take a more fissile than other startup methods, but LEU is much easier to ship around. You just burn out enough TRISO pebbles until there is enough U233 in the core salt to continue and eventually all pebbles are just graphite (with maybe a Silicon Carbide coating for fire prevention). Probably no attempt at the difficult TRISO reprocessing but this leaves some nasty transuranics wastes in the pebbles though. I have proposed another way to startup on LEU but if a design was going to use graphite pebbles anyway, this would certainly be a possible way to start up on LEU.

The advantages seem to be an even safer and publicly acceptable way to ship out startup fissile material but the main drawbacks are you produce a lot of transuranic wastes that are very hard to process out (for the first few years anyhow). As well dealing with decay heat of the startup TRISO pebbles adds some complication.

David L.


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PostPosted: Sep 04, 2008 2:10 pm 
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Preliminaries:

Quote:
Probably no attempt at the difficult TRISO reprocessing but this leaves some nasty transuranics wastes in the pebbles though.


The “difficult TRISO reprocessing” is part of the TRISO fuel cycle and as such is part of the very high cost of TRISO fuel. The firm(s) that sell TRISO fuel claim/will have reprocessing capability. IMHO, the “difficult TRISO reprocessing” is not a technical concern of the MSR design team, but it is a cost concern. Since the TRISO fuel will only be used at MSR startup, it is only a small fraction of the cost structure of the MSR operational lifecycle.

Quote:
As well dealing with decay heat of the startup TRISO pebbles adds some complication.


As a design guideline, the TRISO fuel should not be removed from the MSR until the fuel has reached burn-up to eliminate this latent heat problem.

Design Suggestions:

IMHO, 60 MM Silicon Carbide coated graphite pebbles (SCCGP) are the optimum moderator for the MSR as follows:

• If these SCCGP are designed identically to the TRISO fuel pellets but without the 7 grams nuclear fuel present, it will be highly resistant to radiation damage/swelling since the TRISO fuel has been designed to eliminate this mode of moderator failure by having expansion space reserved inside the pebble for gases produced by the nuclear burn process.

• One or more TRISO fuel vendors can be easily contracted to build SCCGP which will keep the cost of custom construction to a minimum.

• The US government will have extensively tested TRISO fuel for radiation failure. This will lend to very high confidence on the part of the MSR certification testers that the SCCGP are a absolutely reliable nuclear technology.

• All the TRISO fuel handling equipment will be compatible with the SCCGP which will reduce the design complexity and cost of the MSR consistent with the module design and construction principles that I believe are important to a successful MSR project conclusion.

These modules include pebble insertion extraction modules, TRISO fuel/SCCGP inspection and storage modules, and control automation which will make TRSIO fuel and SCCGP handling automatic thereby avoiding human radiation exposure/dangers.

Reference:

Quote:
Re: Pure Th-U233 cycle Versus Denatured Operation


The use of TRSIO fuel in the core of the MSR has a unique advantage in regards to “Pure Th-U233 cycle Versus Denatured Operation” in that uranium based fuel can be used to start the operation of the MSR and no nasty transuranics wastes are left after startup, thus having been removed by automatic TRSIO fuel handing procedures. This keeps the Thorium fuel as well as its waste byproduct pure. Furthermore, any amount of TRISO fuel can be added or removed at any time to keep criticality at optimum levels without nasty transuranics wastes production.

These nasty transuranics wastes are proliferation protected by the inherent character of TRISO fuel.

TRSIO fuel is third world use capable, since it’s reprocessing to extract weapons material is harder than to extract material from raw ore. This is especially true if 80% burn up TRISO fuel is used.

Re: Reprocessing

Thorium fuel reprocessing of the molten salt can be done at all times without redesign or modification in the most simplified fashion in any situation as follows:

At any percentage of TRISO fuel habitation in the core from 0% to 100%

Here again TRISO fuel can be added/removed at any time without any impact on molten fuel reprocessing.

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PostPosted: Sep 04, 2008 7:32 pm 
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Axil,

Some very good points but a couple areas that I think you need to look into deeper. One is in terms of dealing with decay heat which will be a challenge if you try to have fuel and/or fertile in the graphite balls. Decay heat removal is integral to reactor safety since a reactor continues to produce heat at 7% of full power at shutdown and continues to be over or near 1% for several days (this is from all the radioactivity, not any fissions). A molten salt reactor benefits from the ability to dump the salt to special tanks set up to deal with this decay heat. If however, there is fuel that has been fissioning within pebbles, these pebbles will continue to generate copious amounts of heat and risk surpassing even their very high temperature limits. A pebble bed reactor deals with this by being small in dimension to give up heat by conduction or having fail safe gas cooling. It would be very nice to avoid these limitations. As I`ve said, I`ve also looked into various ways to have a combination of molten salt fuel and some fertile or fissile in graphite but dealing with decay heat is always what I have been most concerned about. I actually have a few things in my recent patent regarding this but I guess I`ve been still keeping it a bit under wraps...

Another area is in terms of graphite lifetime. The damage we worry about has nothing to do with gas build up but dimensional change brought on by fast neutron damage. Pebbles are of course easier to replace than blocks but they will certainly need replacing unless we keep the power density very low (i.e. very large cores).

I am sure that pebble bed people will claim that recycling is possible but from all accounts I`ve heard, it is incredibly difficult. Yes, as you point out it may only be needed for the first few years of operation but having two completely different reprocessing needs is a drawback.

It is certainly an interesting idea though, and I will keep it in mind, especially since pebbles are about the best way I see to have graphite in the core (but likely better to have none at all).

David L.


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PostPosted: Sep 04, 2008 8:12 pm 
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I'm not crazy about the idea, but I do see the advantage of "starting" the reactor on 20EU pebbles, and then withdrawing them after enough U233 has been bred from thorium.


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PostPosted: Sep 04, 2008 8:40 pm 
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Quote:
Some very good points but a couple areas that I think you need to look into deeper. One is in terms of dealing with decay heat which will be a challenge if you try to have fuel and/or fertile in the graphite balls.


Attachments:
File comment: TRISO fuel has been tested for coolant failure in terms of decay heat and has proved to be core melt down proof.
Naturelly Safe Fuel.jpg
Naturelly Safe Fuel.jpg [ 60.39 KiB | Viewed 1224 times ]

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PostPosted: Sep 04, 2008 8:49 pm 
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Decay heat removal is integral to reactor safety since a reactor continues to produce heat at 7% of full power at shutdown and continues to be over or near 1% for several days (this is from all the radioactivity, not any fissions).


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File comment: Decay heat removal is sufficient to reduce the heat in the reactor even is the blanket is drained and only a cavity remains.
Pebble bed temperature profile.jpg
Pebble bed temperature profile.jpg [ 100.4 KiB | Viewed 1223 times ]

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PostPosted: Sep 04, 2008 8:51 pm 
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Quote:
Another area is in terms of graphite lifetime. The damage we worry about has nothing to do with gas build up but dimensional change brought on by fast neutron damage.


Dimensional change inside the 60 MM Silicon Carbide coated graphite pebbles (SCCGP) is precisely what I was taking about.

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PostPosted: Sep 04, 2008 9:07 pm 
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Pebbles are of course easier to replace than blocks but they will certainly need replacing unless we keep the power density very low (i.e. very large cores).


Recent testing of SCCGP/TRISO fuel shows that they hold up to long duration high intensity radiation testing to 9% burnup ……soon to be 20% burnup.

TRISO fuel vendors will eventually test to 80% burnup…. 60 MM Silicon Carbide coated pebbles are very tough!

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PostPosted: Sep 04, 2008 9:25 pm 
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Yes, as you point out it may only be needed for the first few years of operation but having two completely different reprocessing needs is a drawback.


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.

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PostPosted: Sep 05, 2008 2:32 am 
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David wrote:
Quote:
After all, I am just proposing a very short startup period where TRISO fuel is only used to boot strap the thorium nuclear reaction in agreement with David’s plan. For the other 30 - 60 years of operation, just mostly thorium fuel is used. In addition, minimization of the lifetime reactor cost is what will sell in the end to the utilities, all other things being equal.


Axil,

Yes, that mode of operation would have benefit. If the TRISO is only used as a way to burn LEU (Low Enriched Uranium) to boot strap to a pure Th-U233 cycle in the core and blanket salts. It would probably take a more fissile than other startup methods, but LEU is much easier to ship around. You just burn out enough TRISO pebbles until there is enough U233 in the core salt to continue and eventually all pebbles are just graphite (with maybe a Silicon Carbide coating for fire prevention). Probably no attempt at the difficult TRISO reprocessing but this leaves some nasty transuranics wastes in the pebbles though. I have proposed another way to startup on LEU but if a design was going to use graphite pebbles anyway, this would certainly be a possible way to start up on LEU. .


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. 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.

What is the heat capacity of molten salts compared to graphite? 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? 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?


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