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PostPosted: Aug 21, 2008 11:31 am 
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Kirk Sorensen wrote:
David wrote:
Another point and I am not sure if it applies for both natural uranium and Cl-37 is that you get a particularly nasty induced radioactive element, CL 36 I believe.


With isotopically-enriched Cl-37 we can make that Cl-36 problem go away, along with the generation of 35S from an (n,p) reaction in Cl-35. That sulfur can cause corrosion.



Thanks Kirk, I thought you'd probably remember the details better. Those reasons alone might mean isotopic enrichment is needed regardless of how good one could do with natural uranium on breeding ratio.


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PostPosted: Aug 21, 2008 11:38 am 
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Kirk Sorensen wrote:
David wrote:
Another point and I am not sure if it applies for both natural uranium and Cl-37 is that you get a particularly nasty induced radioactive element, CL 36 I believe.


With isotopically-enriched Cl-37 we can make that Cl-36 problem go away, along with the generation of 35S from an (n,p) reaction in Cl-35. That sulfur can cause corrosion.

With the large quantities of chlorides envisioned, that Cl-37 enrichment (or Cl-35 depletion) would have to be pretty high grade, if you wish to avoid the types of "legacy" problems created by trace amounts of Cl-36 (T½ = 310,000 y) in graphite moderator from decommissioned first-generation nukes.
I suspect this may well be *the* reason why chloride reactors never got past paper studies....

Quote:
NUCLEONICS WEEK JULY 31, 2008
Review panel prods Andra to bury
graphite waste in deep repository

Andra should consider disposing of France’s graphite waste
from nuclear facilities in a deep-geologic repository along
with the high-level waste and transuranic waste now planned
for deep disposal, the National Scientific Evaluation
Committee said in its 2008 annual report, published in June.
It said that approach would surely increase the size and
cost of a deep repository but that it would be “useful to have
a study on the consequences and extra cost” of burying
graphite wastes deep underground, rather than in a “subsurface”
facility. Andra’s reference solution for disposal of lowlevel
long-lived wastes, a category that includes graphite
wastes and radium-bearing wastes, has been a repository
about 15 meters (49.2 feet) deep.
France’s environment ministry in June asked Andra, the
French radwaste management agency, to seek candidate
communities to host a planned repository for long-lived,
low-level waste (NW, 12 June, 10).
The shallow-burial concept is sufficient to prevent radon
daughter elements from coming to the surface, the waste
review committee said, meaning it can be used for radiumbearing
wastes. But the committee, known in French as the
CNE, said shallow burial “doesn’t work well” for graphite
waste because of the latter’s inventory of chlorine-36, an
extremely mobile long-lived isotope that requires confinement
for “at least 300,000 years.” Erosion over that period
could reduce the thickness of the layer covering a shallow
repository and lead to exposures higher than acceptable in a
repository safety case, the committee said.
The CNE said it calculated that in order to guarantee that
Cl-36 doesn’t pose a radiological risk, a graphite waste repository
should be at least 100 meters underground and within
a clay layer some 100 meters thick.
Burying the country’s 100,000 cubic meters of low-level,
long-lived graphite waste alongside high-level waste and
long-lived medium-level (transuranic) waste in a 500-meter deep
repository would require doubling of the area planned
for medium-level waste in current repository designs, translating
into a 33% increase in the repository’s total storage
area, the committee said. But the scientists said that disposing
of graphite waste is “more complex than it seems, even
if they are low-level, long-lived wastes,” a category known as
FAVL in France.
Radium-bearing wastes and graphite wastes both belong
to the FAVL waste category for which no final disposal route
has been mapped out up to now. France’s 2006 nuclear
waste program act calls for Andra to put a FAVL repository
in operation by 2013, a schedule the CNE last year called
“quasi-impossible.”
Since then, the government has asked Andra to propose
a new schedule, something the CNE said would give time
to acquire data underpinning a scientific safety case for the
new facility.
Choosing sites
When it announced the beginning of the search for sites
in June, the ministry said the goal is now to choose a site at
the end of 2010, so that an application to build the repository
could be submitted at the end of 2013, with operation
targeted for 2019.
Andra last month launched an appeal for candidates in
3,115 communities throughout France seen to have favorable
geology for a FAVL waste repository. Andra aims to propose
two or three sites at the end of this year, but with no
obligation for candidate communities to commit definitively.
A commitment to host the potential facility would be
requested in 2010, Andra spokeswoman Jacqueline Eymard
said last month.
Radium-bearing wastes stem from various origins, from
early radium experiments to production of rare earth elements,
catalytic converters and electronic components.
The graphite wastes come mainly from graphite-moderated
gas-cooled reactors, or GCRs, operated by Electricite de
France and the Commissariat a l’Energie Atomique.

The reactors, all decommissioned, cannot be dismantled
until there is a place to put the waste. EDF a few years ago
committed to dismantling its GCRs on a fast track. That
would also allow clearing of sites for potential new reactors.
Earlier studies envisioned incinerating the graphite from fuel
sleeves and reactor stacks, but the safety case for that was
found to be too difficult because it would release long-lived
carbon-14 into the atmosphere.
According to the CNE, the inventory of radium-bearing
wastes to be placed in the FAVL repository is about 35,000
cubic meters, with a projected activity in about a century of
roughly 1 megabecquerel per kilogram of waste.
Graphite wastes, on the other hand, are estimated at
100,000 cubic meters; they also have an activity of roughly
1 MBq/kg, from Cl-36. Even though C-14 is 500 times more
radioactive, the CNE said, it is Cl-36 that poses the problem
in a repository because of its extreme mobility in the biosphere.

Andra’s 2005 dossier on deep disposal of high-level
waste in clay “clearly showed that Cl-36 is one of the major
contributors to the radiological impact,” the CNE said.
At an industry seminar in 2004, Andra said it was studying
a subsurface repository in clay but had the alternative of
building it in an abandoned rock quarry or mine several
hundred meters deep (NW, 11 Nov. ‘04, 13). At the time,
EDF said it was essential that a graphite repository be operating
by 2008-2009 so it could begin dismantling the GCRs.
According to an informal inventory compiled by the CEA
in 2004, France had he fourth-largest inventory of graphite
waste in the world. By far the largest quantity, about 81,000
mt, was in the UK; the US had about 55,000 mt,
Russia
about 50,000 mt, and France about 23,000 mt. Much smaller
inventories were held in Ukraine, Lithuania, Italy, Spain,
Germany, Japan, China and North Korea, all of which have
operated graphite-moderated reactors,
the CEA said.
-Ann MacLachlan, Paris

FYI, the chlorine in graphite stems from a purification process during graphite manufacture.


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PostPosted: Aug 21, 2008 11:45 am 
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jaro wrote:
I suspect this may well be *the* reason why chloride reactors never got past paper studies....


I highly, highly doubt it. Those studies of 310,000 yr Cl-36 (which probably poses essentially no threat) are probably driven by the political need to go burn some of the $25B in the Yucca Mtn waste fund under the guise of protecting public health.


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PostPosted: Aug 23, 2008 4:35 am 
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If Cl-36 is a manageable hazard, then wouldn't it be easier to use unenriched, natural chlorine, and separate out the corrosive sulfur species (SF4?) during reprocessing? It's an ordinary chemical separation rather than an isotopic one.

Quote:
We should also have an acronym dictionary on the forums sticked somewhere. If we don't have one we should make one. Example:


Funny - I just ran into such a list, as appendix B of this NEA study. Maybe another study has a more comprehensive list.


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PostPosted: Aug 23, 2008 7:08 am 
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Kirk Sorensen wrote:
Those studies of 310,000 yr Cl-36 (which probably poses essentially no threat) are probably driven by the political need to go burn some of the $25B in the Yucca Mtn waste fund under the guise of protecting public health.

So.... I guess one could argue this for TRUs as well -- especially since they don't have such extreme mobility in the biosphere, right ?
....in which case, no need for chloride MSRs to burn TRUs :shock:


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PostPosted: Aug 24, 2008 4:43 pm 
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After the above discussions, an 80 year active disposal effort using LFTRs looks positively sane.

Fluorides aren't usually very soluble.


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PostPosted: Aug 24, 2008 4:58 pm 
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rgvandewalker wrote:
Fluorides aren't usually very soluble.

The important difference is that, upon absorbing a neutron, F-19 becomes F-20, which has a half-life of 11 seconds, turning into stable Neon-20, by way of a beta decay.


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PostPosted: Aug 26, 2008 2:41 am 
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From discussions on my understanding, it appears that isotope separation is either essential or desirable for a liquid chloride reactor. How shall it work if Th/U233/Pu239 fuel for fast reactors is in form of metallic balls with a suitable coolant passing through the voids? (as in a Pebble Bed Reactor.) Solubility of heavy metals in fluoride salts is limited (which in any case act as moderators) and chlorides need a totally new process of separation of Cl37, which may be ready for research but not yet for development. Reprocessing of metallic fuel shall have to be done in batches electrolytically like copper refining.


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PostPosted: Feb 14, 2010 8:49 am 
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Chlorides reactors have a lot of development issues ahead of them, so the first question we have to ask ourselves is whether fluorides can be fast enough to do their task. It is possible to make fluoride reactors really fast, by going for uranium fluoride only. UF4 or UF4/UF3 eutectic, possibly with trace BeF2 for eutectic maintenance (beryllium reducing UF4 to UF3 to the eutectic level). Jaro prefers to use only fuel salt, mostly because of bi modal neutron spectrum advantages, though it could also be used in a very fast fluoride reactor.

The problem is that this requires very high temperatures, probably >1000 degrees C. So special metals and composites will be needed that are mostly untested for this application. Chlorides can operate at very low temperatures. On the other hand, they are typically less stable so more corrosive, so maybe we can't use the proven hastalloy N, which puts us in the unproven materials category there as well.

So, as I see it, if we require a fast reactor, the question is between the feasibility and performance of a very high temperature fast LFTR versus a moderate easier temperature but with unproven chlorides. Since both designs have serious materials issues, but chlorides are less proven, less stable, and have lower heat capacities, we may want to investigate the very high temperature fluoride fast version first...


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PostPosted: Feb 14, 2010 10:18 am 
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Cyril R wrote:
we may want to investigate the very high temperature fluoride fast version first... special metals and composites will be needed that are mostly untested for this application

One claim that appears in various references is that high temperature fluoride fuel problems occur, where non-fuel salt is benign -- particularly with reference to graphite and C-SiC (either in-core or as HX equipment).

Apparently the difference is due to the great many different species of fission product fluorides that build up in the system -- a number of which are not very stable, causing all sorts of undesirable chemistry with the wall materials (although at very low reaction rates).

So, if we aggressively remove fission products, we will continuously have a pure fluoride salt -- in this case UF4-UF3 (-ThF4) eutectic, with small amounts of PuF3 and PaF4 (both of which are very stable).
Problem solved.

High temperature by itself is not a show-stopping materials issue -- many comercial smelters operate at twice the required temperature (UF4-UF3 eutectic m.p. ~865C (1138K); operating range ~965C - 1065C)


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PostPosted: Feb 14, 2010 11:06 am 
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jaro wrote:
Cyril R wrote:
we may want to investigate the very high temperature fluoride fast version first... special metals and composites will be needed that are mostly untested for this application

One claim that appears in various references is that high temperature fluoride fuel problems occur, where non-fuel salt is benign -- particularly with reference to graphite and C-SiC (either in-core or as HX equipment).

Apparently the difference is due to the great many different species of fission product fluorides that build up in the system -- a number of which are not very stable, causing all sorts of undesirable chemistry with the wall materials (although at very low reaction rates).

So, if we aggressively remove fission products, we will continuously have a pure fluoride salt -- in this case UF4-UF3 (-ThF4) eutectic, with small amounts of PuF3 and PaF4 (both of which are very stable).
Problem solved.

High temperature by itself is not a show-stopping materials issue -- many comercial smelters operate at twice the required temperature (UF4-UF3 eutectic m.p. ~865C (1138K); operating range ~965C - 1065C)


Yes, aluminium smelters operate at very high temps, and use electrolysis on top of that! Though they use graphite lined baths, right? Do they use the carbon composite for any sort of plumbing?

If you continuously process very quickly, the concentration of fission products would always be low enough for it to be a manageable problem, so you may be right. I hope you are. As you've mentioned before, assuming pure two fluid, one could also add some non moderating very stable salt that has a reasonably low melting point, maybe ZrF4, to make a ternary UF4 UF3 ZrF4, mostly UF4, some UF3 and some ZrF4 to suppress melting temperature.


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PostPosted: Feb 14, 2010 11:25 am 
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Cyril R wrote:
....one could also add some non moderating very stable salt that has a reasonably low melting point, maybe ZrF4, to make a ternary UF4 UF3 ZrF4, mostly UF4, some UF3 and some ZrF4 to suppress melting temperature.

I figure ThF4 might do the same trick -- only I haven't been able to find any ternary phase diagrams for this combo....
In the LiF-ThF4 and NaF-ThF4 binary systems, ~23% ThF4 lowers m.p. by 300 C.
In other binary systems, ThF4 typically has much less of an effect, and usually at much smaller concentrations, so its anyone's guess...


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PostPosted: Feb 14, 2010 11:37 am 
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jaro wrote:
Cyril R wrote:
....one could also add some non moderating very stable salt that has a reasonably low melting point, maybe ZrF4, to make a ternary UF4 UF3 ZrF4, mostly UF4, some UF3 and some ZrF4 to suppress melting temperature.

I figure ThF4 might do the same trick -- only I haven't been able to find any ternary phase diagrams for this combo....
In the LiF-ThF4 and NaF-ThF4 binary systems, ~23% ThF4 lowers m.p. by 300 C.
In other binary systems, ThF4 typically has much less of an effect, and usually at much smaller concentrations, so its anyone's guess...


Well I'm thinking pure two fluid here, though I was wondering whether fast LFTRs have to be single fluid, due to lack of strong negative blanket void coeffcient, because of the unmoderated blanket. It might be safe enough with a highly absorbing outer vessel wall? Also, did you look at the fuel void coefficients for UF4/UF eutectic?


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PostPosted: Feb 14, 2010 12:44 pm 
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Cyril R wrote:
did you look at the fuel void coefficients for UF4/UF3 eutectic?

There are different levels of looking at this question -- all the way up to running neutronics codes.

For a qualitative appreciation, we need to look at a more global picture:

The minimum U enrichment for a practical fast reactor is about 12% U235.

Heavy water moderated thermal reactors, by contrast, can operate down to a fraction of 1% U235 minimum enrichment.

A bi-modal thermal-fast reactor can be designed to have the same low fuel enrichment minimum as the thermal reactor -- the only difference is that the fuel channels must be very fat and free of moderator, so that some of the extra fissile breeding effect of fast reactors is realised.
The UF4-UF3 eutectic fuel mix can be used to achieve this, by minimising light nuclides in the fuel channels (it is difficult to do this with solid fuel bundles, for a number of reasons, among them the limited coolant fluid choices, as well as the difficulty of cooling the outer rows of fuel bundle pins in fat fuel channels, because that's where most of the fission & heat production will result..... ).

But its plainly clear that a core designed for criticality with 1% fissile load, thanks to D2O, will not remain critical if the moderator starts voiding (boiling or leaking out of the calandria).

As for voiding of fuel inside fuel channels, that is actually the normal shutdown method: the reactor can only remain critical with all fuel channels filled.
There may be second-order effects, like boiling of the fuel salt causing briefly increasing reactivity due to increased penetration of thermal neutrons into the interior of the fat fuel channels.
But boiling can also be used in the design to shut down the reactor, if the fuel circulation is made to depend on a vacuum siphon: vapour from boiling fuel will un-prime the siphon just as surely as opening a valve to ambient air.....


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PostPosted: Feb 14, 2010 2:55 pm 
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So, with an efficient bi modal reactor, do you think there is not much of a case for a few synergetic fast reactors? If you start up on near natural U-235/U238 concentration, then there's not much point in breeding U-233 with fast reactors, whether chloride or fluoride. What about transuranics burning from existing spent nuclear fuel?

Also, with thorium in the fuel salt, the reprocessing tends to be more expensive and complicated - will this be a problem if you're going to rely heavily on continuous reprocessing?


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