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PostPosted: Oct 25, 2011 10:23 am 
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Which gets back to the fundamental question. Is it acceptable to evolve HEU in the core? A second question is that of reprocessing - in the scheme above you would also have HEU isolated from the fission products and the radiation of the reactor itself. This clearly would be a problem for any nation that has signed the non-proliferation treaty.


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PostPosted: Nov 03, 2011 4:27 pm 
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Lars wrote:
Which gets back to the fundamental question. Is it acceptable to evolve HEU in the core? A second question is that of reprocessing - in the scheme above you would also have HEU isolated from the fission products and the radiation of the reactor itself. This clearly would be a problem for any nation that has signed the non-proliferation treaty.

I don't know the requirements of the NPT, but you make a good point. We can denature in the blanket with U238, but then we are producing lightly roasted U238/Pu239 along with our U233, not what you want if you are trying to reduce the attraction of your breeding process from a proliferation perspective.

I don't know if the following approach would be acceptable under the requisite treaties, but I would advocate that as soon as U233 is extracted from a breeding blanket (separation from FP's and U232 daughters) or any form of fuel reprocessing that it is immediately denatured with reprocessed U from SNF or DU in accordance with the accepted ratio U238 => (6 x U233 + 4 x U235). So in practical terms the U233 would only be in two different conditions, 1 inside a core being irradiated or 2 in storage as denatured LEU.

While it means that there is an opportunity for diversion at the point of reprocessing, the window of opportunity would be very small and should be relatively easy to safe-guard. Once denatured the U233 would be unattractive as weapons material. I don't know of any MSR design that cannot startup on LEU, while the U238 is a drag on the system, it's one that we should be able to live with for civilian power reactors.


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PostPosted: Nov 03, 2011 6:40 pm 
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The fastish French design would not be happy with a startup that was 13% 233U and 87% 238U but I think the graphite based reactors could be made critical that way. Not sure one could get to iso-breeder though with that startup fuel.


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PostPosted: Nov 03, 2011 7:10 pm 
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Lars wrote:
The fastish French design would not be happy with a startup that was 13% 233U and 87% 238U

I think that depends on just how "fastish" the French MSR is:
If the neutron spectrum is close to an SFR (quite likely, since SFRs contain lots of O16), then even U235 LEU will work down to about 11% -- consequently 13% 233U and 87% 238U should be no problem at all.


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PostPosted: Nov 03, 2011 7:53 pm 
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I should be more clear. Yes it will be critical and can generate power. But I think that it will have trouble achieving iso-breeding.
There isn't much room left to add thorium and maintain criticallity and I don't think the French spectrum is fast enough to be iso-breeding on 238U/239Pu.


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PostPosted: Nov 03, 2011 10:36 pm 
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As I understand it Lars, thermal is fine and fast is fine, but if you are stuck in the epithermal molasses, then denatured fuel will be a distinct disadvantage, but aside from MOSEL what designs deliberately target the epithermal spectrum?


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PostPosted: Nov 03, 2011 11:36 pm 
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Indians have created 233U for research by irradiating thorium in research reactors. They also used some thorium bundles in newly started PHWRs. Now for power reactors they plan to irradiate thorium in the blanket of PFBR and subsequent fast reactors.
They have also designed fuel for the thorium burning AHWR as Th-19.75% MOX. None of the AHWR designs is even close to being an iso-breeder.
The easiest way to get an initial stock of 233U would be to use bundles with separate Th and 19.75% LEU pins in an LWR or PHWR power reactor and to treat it as an R&D reactor. The pins may be reprocessed separately.


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PostPosted: Nov 04, 2011 11:02 am 
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Lindsay wrote:
As I understand it Lars, thermal is fine and fast is fine, but if you are stuck in the epithermal molasses, then denatured fuel will be a distinct disadvantage, but aside from MOSEL what designs deliberately target the epithermal spectrum?

Thermal is fine but won't get you to iso-breeding using 238U. Still a nice design but you must retain enrichment services over the long haul. Thermal with a mix of Th and 238U is a serious contender for the early LFTRs though.

Fast but using fluoride salts means we have relatively few MeV neutrons so very little 238U fission bonus due to the inelastic scattering of fluorine very rapidly bringing the fast neutrons down to the 100keV range. This is the French approach. I suspect it would have a hard time transitioning to iso-breeding with 20% LEU as its startup.

Using a chloride salt eliminates this problem (but brings on more R&D expense and more schedule to a working reactor).


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PostPosted: Nov 04, 2011 3:28 pm 
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Lindsay wrote:
As I understand it Lars, thermal is fine and fast is fine, but if you are stuck in the epithermal molasses, then denatured fuel will be a distinct disadvantage, but aside from MOSEL what designs deliberately target the epithermal spectrum?



Just to keep in mind, there has been basically no MSR design that you could really call thermal (like you could a CANDU or LWR). Even the designs with lots of graphite are still what we'd call epithermal and there is just small peaks the neutron flux at the thermal end of the spectrum unlike what one gets with light or heavy water.

David L.


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PostPosted: Nov 04, 2011 9:58 pm 
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Thanks guys, you are quite right of course, the epithermal part of the spectrum is very hard to get away from and when we talk about fast or thermal spectrum at best we can normally only talk about fastish e.g French TMSR-NM and thermalish e.g. MSBR, only when we get to the MCFR can we talk about Fast, the rest of the time talk of fast and slow should include a very unscientific 'ish' on the end.
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PostPosted: Nov 04, 2011 9:59 pm 
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Even our friendly very thermal CANDU seems to have quite a number of fast and epithermal neutrons also.
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PostPosted: Nov 05, 2011 10:59 am 
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Lindsay wrote:
Even our friendly very thermal CANDU seems to have quite a number of fast and epithermal neutrons also.

Yes, of course it has them Lindsay -- there is no other way to get from fast to thermal !

BUT the point is that in a CANDU lattice these epithermal neutrons spend their time in non-absorbing heavy water.
This can only be done in a reactor core lattice with fairly widely spaced fuel channels -- i.e. a heterogeneous reactor, with tendency towards bi-modal spectrum.
Optimisation requires further refinements:
Early CANDU designs, such as the small NPD demonstration reactor at Rolphton, near Chalk River, tended to use fuel bundles with only a few, very thick fuel pins.
While this increases the bimodal effect and fertile fuel conversion ratio (U-Pu cycle), in a solid fuel reactor it also badly reduces the volumetric power rating (to limit damage at the center of the fuel pellets), leading to uneconomical power plants.

http://canteach.candu.org/imagelib/37000-fuel/fig005_NPD_7_long.jpg

Since that time, the emphasis has been much more on power production economics than on breeding, so all subsequent designs use many more pins in each fuel bundle, combined with hundreds of small fuel channels, in order to maximise heat transfer.

http://canteach.candu.org/imagelib/37000-fuel/fig116_Bruce_37_el_&_hands.jpg

Fortunately, with MSRs we can "have our cake and eat it too", since the fluid fuel does not impose such strict limits on volumetric power rating, while still allowing a core with relatively few large fuel channels, rather than hundreds of small ones, with water coolant circulating inside.


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PostPosted: Nov 05, 2011 11:10 am 
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The high fuel centerline temperature is only for oxide fuel... carbide, nitride and metal fuel does much better even with very thick fuel rods and a bigger power density.

The other solution to this problem with a CANDU is to use non-moderating coolant, as you can have thin fuel pins and still get a very bi-modal spectrum. I've been wondering why a lead cooled CANDU never got any serious consideration.


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PostPosted: Nov 05, 2011 12:35 pm 
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Cyril R wrote:
The other solution to this problem with a CANDU is to use non-moderating coolant, as you can have thin fuel pins and still get a very bi-modal spectrum. I've been wondering why a lead cooled CANDU never got any serious consideration.

I guess my general observation on this question would be that Canada simply couldn't afford a great deal of experimentation with alternative designs.

More specifically, the Candu design with two fuelling machines and horizontal fuel channels already has issues with sagging zirconium alloy pressure tubes suspended inside the annulus of the calandria tubes -- due to the sheer dead weight of the fuel (plus coolant).

IMO, lead coolant idea would only make sense in a calandria with vertical fuel channels.
Although this has been tried (with light water coolant, not very successfully), the lead version would probably have been considered as simply too extreme a change.
One problem with the vertical calandria is that you only have a single fuelling machine (on top), so its difficult to use the short 3-foot fuel bundles, that can be shuffled around for optimum burnup and neutron flux profile.
The vertical calandria versions typically substitute full-length fuel assemblies for the short bundles, with resulting drop in burnup efficiency.
I believe the Indian AHWR does this too.

With liquid fuel, we get rid of those fuelling machines, and the relatively small number of piping connections lets you orient the calandria whichever way you like -- based on other considerations.....


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PostPosted: Nov 05, 2011 4:43 pm 
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There was an interesting pressure tube design developed some time ago which used yttria stabilized zirconia internal insulation to allow the pressure tube to operate in cold pressurized condition (being cooled externally by the heavy water moderator), greatly increasing the strength of the pressure tube for a given pressure tube thickness. This would allow lead coolant without a large increase in pressure tube thickness.

There are many such promising developments, but they don't seem to make it in any real product. Of course considering the fact that even small changes to existing reactors take 40 years to get into a product, it is not so strange.

And it looks far worse for radical designs such as liquid fuel reactors (could easily take 100 years at this rate).


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