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PostPosted: Sep 19, 2012 6:15 am 
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EnergyUser wrote:
No good. At around 1200 degrees C fluoride salt will start to evaporate. This is way hotter than how we want to operate the reactor.


The idea is to shut down the reactor every 2-20 years (depending on power density), then remove some graphite, maybe wash/boil the fission products out of the first millimeter, then put it in a high temperature oven for a while. The actual reactor would operate much colder, 650-800 Celsius probably depending on the salt and the design. Hopefully the graphite will shrink again at high temperatures without a neutron flux.

Since this can be done with existing, used nuclear graphite (whose activity is relatively low), and we only need a high temp oven, this seems to me as something that can be tested for a low budget.


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PostPosted: Sep 19, 2012 6:15 am 
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EnergyUser wrote:
No good. At around 1200 degrees C fluoride salt will start to evaporate. This is way hotter than how we want to operate the reactor.


The idea is to shut down the reactor every 2-20 years (depending on power density), then remove some graphite, maybe wash/boil the fission products out of the first millimeter, then put it in a high temperature oven for a while. The actual reactor would operate much colder, 650-800 Celsius probably depending on the salt and the design. Hopefully the graphite will shrink again at high temperatures without a neutron flux.

Since this can be done with existing, used nuclear graphite (whose activity is relatively low), and we only need a high temp oven, this seems to me as something that can be tested for a low budget.


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PostPosted: Sep 19, 2012 2:04 pm 
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The Wiki article on FLiBe says it boils at 1430. Would that be high enough to anneal the graphite?

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PostPosted: Sep 19, 2012 5:00 pm 
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No. When they made the nuclear grade graphite the temperatures were around 2000C. But it doesn't matter. As Cyril points out, the concept is to have two sets of graphite moderators. Once every four years (timed to align with servicing the turbines and generators) you swap them. In between times, you take the graphite logs, one at a time and bake them at high temperature.

Would simply baking them be sufficient to restore them to the proper size? Seems like an experiment that could be run - perhaps using old graphite from a shutdown reactor in the UK or expose graphite in the fast flux test reactor(?) and then do the experiments.


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PostPosted: Sep 19, 2012 8:40 pm 
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Well, there is "anneal" and there is "re-form". If we could heat each graphite column up to ~1450 in-situ, could the column's life be extended? If so, be how much? Suppose that you could get a year. Then each year you heat it in-situ and never have to shut down. I mean, why can't the core continue on while overhauling each TG off-line in series?

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PostPosted: Sep 20, 2012 10:29 am 
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Lars wrote:
Would simply baking them be sufficient to restore them to the proper size? Seems like an experiment that could be run - perhaps using old graphite from a shutdown reactor in the UK or expose graphite in the fast flux test reactor(?) and then do the experiments.

Perhaps the bulk of the graphite can be restored to essentially its original state an unlimited number of times by re-heating. But even if that is the case, the outer layers will still accumulate increasing amounts of embedded fission products that have slammed into it at relativistic speeds. This will result in increased neutron absorption and may also affect other properties of this outer layer such as brittleness and porosity.


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PostPosted: Sep 20, 2012 10:37 am 
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A lot of those fission products should boil off at 2500C.


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PostPosted: Sep 20, 2012 11:15 am 
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Wouldn't the FPs in a heated graphite column transit into a cooler salt bath and stay there?

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PostPosted: Sep 20, 2012 11:20 am 
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Owen T wrote:
Lars wrote:
Would simply baking them be sufficient to restore them to the proper size? Seems like an experiment that could be run - perhaps using old graphite from a shutdown reactor in the UK or expose graphite in the fast flux test reactor(?) and then do the experiments.

Perhaps the bulk of the graphite can be restored to essentially its original state an unlimited number of times by re-heating. But even if that is the case, the outer layers will still accumulate increasing amounts of embedded fission products that have slammed into it at relativistic speeds. This will result in increased neutron absorption and may also affect other properties of this outer layer such as brittleness and porosity.


Like DJW pointed out, almost all of that will be boiled off in the oven that is >2000 Celsius. Cs-137, the biggest bugger due to its mobile parent, is on the fly for sure. All we need is a decent oven gas filter system. Since this is a closed system, that would be a simple closed loop recirc filter.

If this really works - as in shrinking back the graphite - then it seems feasible. Re-coating would likely be needed for sealing the graphite, but that's not complicated (all you need's a small vacuum oven with hydrocarbon gas feeder).

Kiteman - there's no place for a salt bath in the oven. This definately must be a dry oven, considering the temperatures needed.


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PostPosted: Sep 20, 2012 3:48 pm 
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While I applaud these innovative ideas for extending the working life of reactor graphite, I get the impression that it will be expensive and is unlikely to be economic against the alternative of periodic replacement.


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PostPosted: Sep 20, 2012 4:41 pm 
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Lindsay wrote:
While I applaud these innovative ideas for extending the working life of reactor graphite, I get the impression that it will be expensive and is unlikely to be economic against the alternative of periodic replacement.


ORNL said graphite replacement costs about 0.04 cents per kWh, so yes.

But some people, certainly not me, are worried about the PR problems of graphite waste. If that is true though, having a way to recycle the graphite would help.


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PostPosted: Sep 20, 2012 6:17 pm 
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Guys,

Our 1 GWe design goes through 50 tons of moderator a year at steady state,
New graphite will cost at least $20/kg and god knows how much to dispose of.
Where I come from, a milion dollars per year is real money,

IF this works, it means we only have to put the old graphite thru the last step
in the moderator production process. IF it works, it should save money.

Jack


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PostPosted: Sep 20, 2012 8:49 pm 
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djw1 wrote:
Guys,

Our 1 GWe design goes through 50 tons of moderator a year at steady state,
New graphite will cost at least $20/kg and god knows how much to dispose of.
Where I come from, a milion dollars per year is real money,

IF this works, it means we only have to put the old graphite thru the last step
in the moderator production process. IF it works, it should save money.

Jack

A million dollars per year is not that significant for a 1 GWe reactor. I don't expect that recycling the graphite will be cost competitive with fresh graphite. But if the additional cost is modest it would be worthwhile for the PR value in the US and Japan.


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PostPosted: Sep 21, 2012 2:58 am 
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Cyril R wrote:
Owen T wrote:
Lars wrote:
Would simply baking them be sufficient to restore them to the proper size? Seems like an experiment that could be run - perhaps using old graphite from a shutdown reactor in the UK or expose graphite in the fast flux test reactor(?) and then do the experiments.

Perhaps the bulk of the graphite can be restored to essentially its original state an unlimited number of times by re-heating. But even if that is the case, the outer layers will still accumulate increasing amounts of embedded fission products that have slammed into it at relativistic speeds. This will result in increased neutron absorption and may also affect other properties of this outer layer such as brittleness and porosity.

Like DJW pointed out, almost all of that will be boiled off in the oven that is >2000 Celsius. Cs-137, the biggest bugger due to its mobile parent, is on the fly for sure. All we need is a decent oven gas filter system. Since this is a closed system, that would be a simple closed loop recirc filter.

Do any of the fission products form carbides?


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PostPosted: Sep 21, 2012 7:13 am 
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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.


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