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PostPosted: Sep 21, 2012 7:41 am 
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Cyril R wrote:
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Do any of the fission products form carbides?


Yes, but the ones that do, have more stable fluorides, so won't form carbides. Cesium, the most notable graphite penetrator that isn't in fluoride form, does not form a stable carbide. The only minor elemental group that could become carbide is the lanthanide group. But these are typically not volatile in elemental form either so they won't go out, carbide or not. Because lanthanides form stable fluorides, there shouldn't be much in the graphite.

The kinetic energy as they are emitted from a fission reaction is probably enough to embed some them deep enough in the graphite so their immediate environment is dominated by carbon, not fluorides. It's probably a minor effect, but it might become significant if the graphite can be "healed" again and again.


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PostPosted: Sep 21, 2012 1:11 pm 
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Owen T wrote:
Cyril R wrote:
Quote:
Do any of the fission products form carbides?


Yes, but the ones that do, have more stable fluorides, so won't form carbides. Cesium, the most notable graphite penetrator that isn't in fluoride form, does not form a stable carbide. The only minor elemental group that could become carbide is the lanthanide group. But these are typically not volatile in elemental form either so they won't go out, carbide or not. Because lanthanides form stable fluorides, there shouldn't be much in the graphite.

The kinetic energy as they are emitted from a fission reaction is probably enough to embed some them deep enough in the graphite so their immediate environment is dominated by carbon, not fluorides. It's probably a minor effect, but it might become significant if the graphite can be "healed" again and again.


Yes, I realised this. Just don't think it matters whether the FP is carbide or elemental. FPs of concern would be lanthanides and zirconium metal. Most have very high boiling points whether elemental or carbide. They aren't going to leave it by boiloff in the oven. I guess this means that the oven has to handle rather radioactive graphite logs.


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PostPosted: Sep 21, 2012 1:26 pm 
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Yes the logs will be radioactive. The fission products that get embedded only penetrate a very short distance into the graphite (don't recall precisely but my vague recollection is a fraction of one millimeter). I don't expect that the fission products that get embedded will be a significant problem - even for neutron absorption. If it is one could envision milling off a bit of the surface.

There was also discussion of a surface sealing operation to reduce the penetration rate of xenon and krypton so perhaps this added material would compensate for the milling losses.


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PostPosted: Sep 21, 2012 3:20 pm 
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Our design is on a four year cycle.
The 700 kg logs wll sit for nearly four years before being re-processed
at which point radioactivity per log will be about 20 watts
based on the ORNL 4541 and 7207 numbers.
This should be handle-able.

If we have to do the full graphitization then we are talking
about baking period as long as a month.
We need to do 60 logs year. This will keep
a five log furnace busy full time.
(This is worse case. It may take a much shorter time
to re-establish the lattice.)

But if we bought new graphite we would need the same furnace
plus a 28 day carbonization step, plus materials, plus mixing/molding/extrusion.
Plus we have to get rid of the old moderator
which at least in the US, the regulators are going to claim has
to be treated as TRU waste.

The on-site furnace will have to handle the radioactive boil off,
but an MSR plant needs to be equipped to do just this sort
of thing as part of offgas/fuelsalt processing.

IMHO, this should be tested. If it works, it will be economic.


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PostPosted: Sep 22, 2012 12:07 pm 
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I should have added: provided the baked logs
can be handled with minimal protection.


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PostPosted: Sep 22, 2012 3:27 pm 
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20 Watts is handle-able in terms of heat of course. But it's surprisingly bad for your health! Recall that 20 Watts is 20 joules per second, and that 1 Sv = 1 j/kg absorbed. So no manual handling, and a need for thick shielding. It doesn't help that the oven needs to be >2000 Celsius.


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PostPosted: Sep 22, 2012 4:33 pm 
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20 watts is pre-baking.
Key is how much FP the baking removes.

Prior to baking, of course we have to handle the moderator remotely
even if we are just trying to throw it away.
The difference comes afterwards, when we re-assemble.

But the old reflector, etc will still be a little warm.
If the baking gets the old moderator down to near
the same level as the reflector et al,
then re-using the old moderator does not add
to our problems.


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PostPosted: Sep 23, 2012 10:07 pm 
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I see no reason to plan on any manual handling of the recycled graphite logs. Obviously, the disassembly and loading into the ovens has to be handled robotically. Once we have done this I see no reason we should not also handle the removal from the ovens, inspection and reassembly processes robotically as well.


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PostPosted: Sep 23, 2012 10:40 pm 
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You have to widen your search for moderators. Instead of thinking about carbides of fission products, think of Be2C as moderator. It is hard, high melting and crystalline and may have adequate life.
http://en.wikipedia.org/wiki/Beryllium_carbide
It could be put in the required size and shape in a container. Containers/cladding for permanent moderator is less effort and cost compared to that for fuel.


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PostPosted: Sep 24, 2012 1:50 am 
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A container for the moderator will have the same challenges as the bare graphite. Namely, it is in the middle of the neutron flux and will suffer neutron damage. The best material ORNL found to withstand the temperature and fluoride salts was Hastalloy. They also had a strength requirement which would not be a requirement for a moderator container so that might open up some extra materials. The Hastalloy though would not withstand anything remotely like the neutron flux in the middle of the reactor.


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PostPosted: Sep 24, 2012 2:57 am 
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Lars wrote:
A container for the moderator will have the same challenges as the bare graphite. Namely, it is in the middle of the neutron flux and will suffer neutron damage. The best material ORNL found to withstand the temperature and fluoride salts was Hastalloy. They also had a strength requirement which would not be a requirement for a moderator container so that might open up some extra materials. The Hastalloy though would not withstand anything remotely like the neutron flux in the middle of the reactor.


In the days of the ORNL MSRE/MSBR program there were no composites developed yet. Today we have carbon-carbon and silicon carbon/carbon composites. These are highly resitant to radiation especially at elevated temperatures. This does change the development of a modern MSR (LFTR). For example the AHTR work uses extensive carbon carbon composite structures right in the middle of the core.


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PostPosted: Sep 24, 2012 7:56 pm 
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I presume a carbon/carbon structure will have the same neutron exposure lifetime as the graphite moderator. So this is fine if you assume you are replacing the container periodically.


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PostPosted: Sep 24, 2012 8:18 pm 
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The problem with carbon-carbon as a container material is that it leaks.
The vendors Ive talked to treat it as a porous material,
and actually use Darcy's Law to estimate the leakage rate.
From memory, they were assuming 0.5 mD which is a tight petroleum reservoir
but not a primary loop container.


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PostPosted: Sep 24, 2012 10:51 pm 
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Crystalline carbides like Be2C have to be only physically contained. There is no absorption of gases as in graphite. Minor porosity will not have any adverse effect. They are closer to diamond than graphite in this respect.
It may be possible to sinter it with Be metal and clad it in SiC.


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PostPosted: Sep 25, 2012 8:34 am 
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djw1 wrote:
The problem with carbon-carbon as a container material is that it leaks.
The vendors Ive talked to treat it as a porous material,
and actually use Darcy's Law to estimate the leakage rate.
From memory, they were assuming 0.5 mD which is a tight petroleum reservoir
but not a primary loop container.


It wouldn't be used as container for the primary loop, but as core internals. With beryllium metal, tight containment is not needed. The salt won't react much with beryllium since it is already kept in reducing state. As long as there is not gross flow over large surface area of bare beryllium moderator, it will be fine. It's ok if some salt wicks in, though from a waste/handling perspective it should be kept as low as possible, as the salt contains actinides and fission products.

Possibly the beryllium blocks could be coated with pyrocarbon, so that any salt contact is limited even if the container leaks.

If carbon-carbon does not work for a container, multiplex silicon carbide should. Silicon carbide doesn't swell much like graphite.


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