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PostPosted: Feb 11, 2010 5:49 pm 
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Lars wrote:
If the graphite is exposed to the fuel salt then expansion beyond the original size is a concern. Namely, some of the Xe would tend to get trapped inside the graphite rather than get sparged out and would steal neutrons. Not an issue if the graphite has a metal cladding but then we get back to concerns over damage to the wall - though without the strength requirement so we can be more aggressive.



Yes, I`ve wondered if Xenon trapping is the only thing that ORNL was really worried about in terms of pebbles beyond the tradition fast fluence limit. ORNL would certainly have wanted to minimize xenon since it will lower your breeding ratio. Today though I think we can spare the neutrons so the question of how long a pebble can last is still an interesting one. Perhaps it is of of secondary concern though considering that it should be much easier to replace pebbles than logs. Pebbles do have issues of their own though and might represent a bigger R&D program than we realize. For a DMSR design I`ve really been liking pebbles for awhile now but a good old fashioned huge version of the MSRE that doesn`t need to replace graphite like ORNL proposed has a lot to admire as well. I used to cringe at the thought of how big the DMSR cores might be but when you really compare them to older graphite designs or even just a PWR vessel they really aren`t that big.

David L.


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PostPosted: Feb 11, 2010 6:21 pm 
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If it is only trapping Xe then I could imagine tolerating the neutron loss. This would allow pebbles for the main body of the fuel salt and a thick carbon reflector before the wall. We might even get back to a single fluid design this way. It starts to look like a minor variant on the MSBR design. ORNL was pretty satisfied that the wall was going to be OK in this circumstance (with the possible exception of Te). We won't get as soft a spectrum as ORNL since the packing density of the balls is much lower than the logs of ORNL's design. If I recall right they also worried about the fluid resistance of flowing across the balls increasing the pumping requirements.

The increase core volume also significantly increases the strength requirements of the wall and explains why they had a 5cm thick wall. I'd have to go back to college physics but I think the stress is proportional to (volume) ^ (2/3).


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PostPosted: Feb 11, 2010 7:26 pm 
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Quote:
In one two fluid design, the pebbles are filled with blanket salt.


The blanket pebbles should be composed of the same elements as the blanket salt: a mixture of lithium, beryllium, and thorium but formulated into a solid compound. I favor a blanket pebble made of a well blended mixture of Th4D15 and Li2BeD4 as opposed to carbon. This pebble formulation should have a favorable void coefficient behavior since its chemical composition is so similar to the blanket salt. Any loss of reactor coolant should not cause reactivity problems.

If the reactor salt is used as a coolant so if it drains or even just gets hotter and less dense, moderation performed by the pebbles remain unchanged.

A robust SiC pebble covering enclosing both the seed and blanket pebble should keep the density of the pebble material constant even if the pressure inside the pebble elevates with increasing temperature.

SiC is highly resistant to deuterium penetration. The SiC covering should keep the deuterium enclosed with little or no deuterium leakage.

Remember, circulating pebbles are not a new concept and have been perfected in the PBMR design.

The moderating stopping power of deuterium base pebbles will be much greater than graphite pebbles (pebble for pebble). This will lead to a smaller reactor design since fewer deuterium base pebbles can supply the same moderation power as a much larger number of graphite pebbles. When all things are said and done, the relative cost for deuterium blanket pebbles might turn out to be the same.

The Per Peterson pebble handling design allows for a “two fluid like” design approach without a wall.

You want to use pebbles in both the core and blanket zones. This will keep damage from corrosion and delayed neutrons low in both the reactor and the heat exchangers.

Alpha particles are not very penetrating. They will not make it out of the confines of the core seed pebble.

With circulating pebbles, pebble inspection for damage is done. This mitigates all kinds of problems involved with moderator lifetimes and possible damage situations.

Hastalloy N cladding should only be used on the outer reactor walls to prevent corrosion caused by fluoride coolant salt. You don’t care if this cladding absorbs neutrons there; these outbound thermal neutrons are heading out of the reactor anyway.

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PostPosted: Feb 11, 2010 11:57 pm 
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Lars wrote:
If it is only trapping Xe then I could imagine tolerating the neutron loss. This would allow pebbles for the main body of the fuel salt and a thick carbon reflector before the wall. We might even get back to a single fluid design this way. It starts to look like a minor variant on the MSBR design. ORNL was pretty satisfied that the wall was going to be OK in this circumstance (with the possible exception of Te). We won't get as soft a spectrum as ORNL since the packing density of the balls is much lower than the logs of ORNL's design. If I recall right they also worried about the fluid resistance of flowing across the balls increasing the pumping requirements.

The increase core volume also significantly increases the strength requirements of the wall and explains why they had a 5cm thick wall. I'd have to go back to college physics but I think the stress is proportional to (volume) ^ (2/3).


Yes a pebble only core (i.e. Single Fluid) is pretty interesting on its own. You can actually have just as soft a spectrum as a graphite log design just by having a roughly 1/3 fissile density in the salt so that the ratio of fissile to graphite is about the same (i.e a pebble bed has 3 times the salt fraction 39% vs 13% or so). Of course your neutron losses to the salt are going to be almost 3 times as high and you only save a little on graphite. In a DMSR design this is no big deal, if you are trying to break even with a Single Fluid pebble bed then it does get harder. The ORNL work on pebbles is a bit confusing, some early studies looked amazing but later ones didn`t fair as well. Also, in a pebble bed of even high power density you still have a good healthy volume of salt to give you a nice big thermal inertia. I have a paper recently accepted for Nuclear Engineering and Design in which I try to explore some pebble bed concepts (among others). Still waiting to hear when it will go to print. I should post it, I think their rules allow it but hard to tell with the legal jargon sometimes.

Yes, certainly the pumping power will go up, but I think the pressure drop in the heat exchanger is always so much larger that a few more psi through the core probably won`t matter much. I think it was about 15 psi through the core and 150 psi through the heat exchangers for the MSBR (memory likely off here).

David L.


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PostPosted: Feb 12, 2010 3:53 am 
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Graphite not only suffers radiation damage but also catches fire. There have been serious fire accidents at Windscale and Chernobyl. BeO is nearly as good a moderator and does not catch fire.
http://en.wikipedia.org/wiki/Beryllium_oxide
Quote
Sintered beryllium oxide, which is very stable, has ceramic characteristics. Beryllium oxide is used for rocket engines, catalysts, semiconductors, moderators of atomic reactors, and neutron reflectors.
If you really want a thermal LFTR, it is better to move from graphite to Beryllia.
It could be in the shape of blocks or pebbles, clad or otherwise.


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PostPosted: Feb 12, 2010 8:32 am 
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jagdish wrote:
Graphite not only suffers radiation damage but also catches fire. There have been serious fire accidents at Windscale and Chernobyl. BeO is nearly as good a moderator and does not catch fire.
http://en.wikipedia.org/wiki/Beryllium_oxide
Quote
Sintered beryllium oxide, which is very stable, has ceramic characteristics. Beryllium oxide is used for rocket engines, catalysts, semiconductors, moderators of atomic reactors, and neutron reflectors.
If you really want a thermal LFTR, it is better to move from graphite to Beryllia.
It could be in the shape of blocks or pebbles, clad or otherwise.


All the cladding will make a thermal spectrum inefficient. Could be great for epithermal and faster though, but then you can do that with non-solid moderated reactors as well.

I wonder if we can't use Be4B (using 11-B) or other beryllium boride directly in contact with the melt. I haven't been able to find out what it does in contact with FLiBeU/LiFTh. I think it will not like an alkaline environment like most borides, other than that it could be highly resistant to direct contact with fuel salt.


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PostPosted: Feb 12, 2010 9:15 am 
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jagdish wrote:
Graphite not only suffers radiation damage but also catches fire. There have been serious fire accidents at Windscale and Chernobyl. BeO is nearly as good a moderator and does not catch fire.
http://en.wikipedia.org/wiki/Beryllium_oxide
Quote
Sintered beryllium oxide, which is very stable, has ceramic characteristics. Beryllium oxide is used for rocket engines, catalysts, semiconductors, moderators of atomic reactors, and neutron reflectors.
If you really want a thermal LFTR, it is better to move from graphite to Beryllia.
It could be in the shape of blocks or pebbles, clad or otherwise.


Graphite is much, much safer when you don`t have any explosive potential in or around the core. Windscale had the Wiegner energy problem (not an issue above 200C) and Chernobyl had water in core to give a huge steam explosion and water vapor to catalyze graphite combustion (and no containment building!). For a graphite Molten Salt Reactor with no water at all in the main containment and no other potential source of explosion it is extremely difficult to imagine how you`d ever get a fire started. Even the worst prompt criticality event imaginable might flash some salt to vapor and burst the vessel but no way there would be enough force to burst your two containments. Another thing as well is that especially for pebbles, we have the option to add pyrolytic coatings that make it virtually impossible to burn.

As mentioned already, BeO can`t be in direct contact with the salt so not a viable graphite replacement. In some designs we can indeed get rid of graphite altogether but I view that as more of an operational advantage (no need to replace) than really a safety issue.

David L.


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PostPosted: Feb 13, 2010 5:57 am 
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What is the lifetime of a BeO moderated when kept away from the fluoride melt? Some people say the neutron doubling effect is helpful. But, Be-9 (n,2n) leaves unstable Be-8, which decays almost instantly into helium nuclei. So one would have those nuclei and free oxygen gas to deal with, right?

I assume the process is slow, given the low x section of Be-9, and with BeO blocks always being slightly porous no matter how well they are manufactured, the BeO could last decades?


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PostPosted: Feb 13, 2010 11:38 am 
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If the BeO is clad and placed in the center of the core then the question really is how long will the cladding last.
I suspect this will be measured in a few months - so you would have to think along the lines of a center control rod that is fat and can be easily replaced - like in a 4-6 hour shift.

If the BeO is on the outer edge of the reactor then you have a moderator on the outside and fast neutrons on the inside. This will cause a much bigger fraction of your fissions to happen at the outer edge of the reactor. Generally that is bad for your neutron economy as more of them will escape the reactor then. Perhaps the (n,2n) effect can overcome this loss? Again though, it will be the cladding or exterior wall life that is the key question.


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PostPosted: Feb 13, 2010 11:55 am 
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Lars wrote:
If the BeO is on the outer edge of the reactor then you have a moderator on the outside and fast neutrons on the inside. This will cause a much bigger fraction of your fissions to happen at the outer edge of the reactor. Generally that is bad for your neutron economy as more of them will escape the reactor then. Perhaps the (n,2n) effect can overcome this loss? Again though, it will be the cladding or exterior wall life that is the key question.

This is the design researched at UFLA for the UF4/KF vapour core reactor, with MHD power conversion, in a Rankine-type cycle (ie. includes vapour condenser).
In this case, since the fuel is a relatively low-density medium, the moderator arrangement does not "cause a much bigger fraction of your fissions to happen at the outer edge of the reactor."


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PostPosted: Feb 13, 2010 12:16 pm 
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Lars wrote:
If the BeO is clad and placed in the center of the core then the question really is how long will the cladding last.
I suspect this will be measured in a few months - so you would have to think along the lines of a center control rod that is fat and can be easily replaced - like in a 4-6 hour shift.

If the BeO is on the outer edge of the reactor then you have a moderator on the outside and fast neutrons on the inside. This will cause a much bigger fraction of your fissions to happen at the outer edge of the reactor. Generally that is bad for your neutron economy as more of them will escape the reactor then. Perhaps the (n,2n) effect can overcome this loss? Again though, it will be the cladding or exterior wall life that is the key question.


The 'cladding' I had in mind is actually replaceable piping, either like Jaro's siphon design but with a BeO 'calandria', or a traditional pumped core with piping inserted with very loose tolerances. A very basic improvement over the ARE. The pipes are easily replaced, but the BeO moderator isn't.

It seems to me that if the (n,2n) effect is significant the moderator loss will be significant as well. The ARE didn't run long enough to determine BeO life, right? What kind of degradation did they see over the short operation time?


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