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PostPosted: Mar 04, 2010 5:06 pm 
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Recently in these pages I've been hearing about 19-F's ability to slow down fission neutrons more effectively than is implied by just its mass, because it turns neutron kinetic energy into gamma ray energy. Does this make it almost as good a moderator as carbon? Could a Stagg Field demo have been capable of divergence if its bricks had been PTFE rather than carbon?

(How fire can be domesticated)


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PostPosted: Mar 04, 2010 5:51 pm 
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Interesting question !

Note that for fast neutrons, the F19 inelastic x-section is only about 1/4 to 1/2 of the elastic x-section (ie. 2 to 4 barns elastic, vs. 1 barn inelastic), so a majority of neutrons will still get the usual (poor) moderating effect from F19 -- albeit with very little absorbtion, since even in the resonance region, the (n,g) x-sections for F19 are only in the millibarn range.
One can nevertheless see a significant effect in the French calculations, due to pretty extreme F19 loading in the LiF/ThF4 salt.

As for a CP-1 type of pile using PTFE instead of graphite bricks, the moderation effectiveness would be inferior, because of the low atomic density of carbon in PTFE, relative to graphite.

So my guess is that even if it did manage to get to critical on NU, it would have to be at a larger overall pile size.


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PostPosted: Mar 04, 2010 7:32 pm 
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Here's a comparison of C12 and F19 :

Image


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PostPosted: Mar 04, 2010 10:07 pm 
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Jaro,

Each elastic collision with F-19 causes a neutron to lose something like 9.5% of it's energy.

If a neutron starts out at, say, 1.5 MeV, it will take about 27 collisions for it to come down o 100 KeV.

A single inelastic collision will take it down to 100 KeV.

So, my guess is that in pure fluorine, nearly every neutron takes an inelastic collision to get down to 100 KeV, but on average about 2 to 4 elastic collisions first.

Assuming that fluorine's capture cross section is not vastly larger than carbons, fluorine would appear to be a superior moderator to graphite in the region above 100 KeV.


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PostPosted: Mar 04, 2010 10:39 pm 
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Yes, I think you're right.
iain wrote:
fluorine would appear to be a superior moderator to graphite in the region above 100 KeV.

Maybe.
But CP-1 relied on very thermal neutrons, far below 100 keV.
For that you still need an effective moderator like carbon.
So PTFE bricks would likely not have been a good substitute for graphite in CP-1.


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PostPosted: Mar 06, 2010 9:50 pm 
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iain wrote:
A single inelastic collision will take it down to 100 KeV.

Not according to the formula in Lamarsh ! (p.450)

From my post in another thread today,
Quote:
inelastic neutron scattering only has a significant effect on neutron energy when the target is a heavy nucleus.
Thus for an inelastic collision of a 2MeV neutron with a U238 nucleus, the neutron energy drops to 0.6MeV on average (using the formula E' = 6.4* sqrt(E/A), where A is the mass number).
For F19, the result of an inelastic neutron collision is about the same as an elastic one, for energies below about 2.5MeV ! (for the more extreme case of a 6MeV neutron, it drops to about 3.6MeV)


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PostPosted: Mar 07, 2010 4:18 am 
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A Teflon block has as many C12 nuclei as it has F19. Should it not be a very good moderator except for thermal/radiolytic stability? Polymers have a tendency to break up at higher temperatures. Dissociation products could be liquid or gas. In some of my posts I have asked similar questions about liquid per-fluorocarbons like Krytox. Aim in those questions was to get to a less volatile non-inflammable coolant as the liquid coolants tend to double up as moderators. It may be useful to have a separate coolant other than liquid salts.
In another thread I have suggested a fluid reactor of 233UO2F2 solution in water (or heavy water, if necessary) on 05 Mar10. It can work only at a lower temperature but shall be convenient for disposal of Samarium in addition to gas neutron poisons.
viewtopic.php?f=7&t=745&start=105


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PostPosted: Mar 07, 2010 12:47 pm 
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jagdish wrote:
A Teflon block has as many C12 nuclei as it has F19...


No, half as many. It is [-CF2]n, one-third carbon and two-thirds fluorine.

There are some interesting, fairly durable oxocarbon materials, for instance, ethylenetetracarboxylic dianhydride, Image. It is said to be a pale yellow oil.

The energy cost of repairing such molecules has to be compared with the fission energy release they catalyse before they get broken.

(How fire can be domesticated)


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PostPosted: Mar 07, 2010 12:48 pm 
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jaro wrote:
Thus for an inelastic collision of a 2MeV neutron with a U238 nucleus, the neutron energy drops to 0.6MeV on average (using the formula E' = 6.4* sqrt(E/A), where A is the mass number).
For F19, the result of an inelastic neutron collision is about the same as an elastic one, for energies below about 2.5MeV ! (for the more extreme case of a 6MeV neutron, it drops to about 3.6MeV)


Okay, this explains it. Thank you.

-Iain


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PostPosted: Mar 07, 2010 5:32 pm 
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iain wrote:
Okay, this explains it. Thank you.

-Iain

Well, it doesn't explain where your 100keV figure came from.

That really threw me for a while -- I assumed you had a reliable number !

...and David's comments on this didn't help either.

I wish you guys would say so, when you're guessing :|

Anyway, as noted in my post in the other thread, the picture of an HW-MSR with UF4/UF3 fuel channels looks somewhat different now.....

Too bad the HETERO code conversion to Java, with fluoride fuel data, never worked out -- that may have helped us take some of the guesswork out.... (although it too requires input of correct data -- or else its "garbage in, garbage out.." .....it would have been particularly useful for preliminary fuel channel sizing estimation)


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PostPosted: Mar 08, 2010 2:41 am 
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Post moved.

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The old Zenith slogan: The quality goes in before the name goes on.


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PostPosted: Mar 09, 2010 5:14 am 
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GRLCowan wrote:
jagdish wrote:
A Teflon block has as many C12 nuclei as it has F19...

No, half as many. It is [-CF2]n, one-third carbon and two-thirds fluorine.

I stand corrected about Teflon. However perfluorocarbons, especially the liquid ones need to be studied. They wont catch fire and selected ones will be less volatile resulting in lower pressure reactor vessels.


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PostPosted: Mar 09, 2010 2:37 pm 
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Jagdish-
Whiteshell reactor-1 (WR-1) was an organic liquid cooled, heavy water moderated design that ran well for 20 years. I've started to try to model it to see what improvements might be made.

W. B. Lewis promoted the design for a high burnup thorium/U-233 cycle (Valubreeder) and AECL demonstrated the fuel and reprocessing technology. By the time the work was done, uranium prices had declined and the work was put on the shelf. It still looks good to me:

Coolant Temperature & Pressures Temperature:
Inlet 280oC - 400oC
Outlet 320oC - 425oC

Pressure:
Inlet Header 315 psig (2.15 MPa)
Outlet Header 160 psig (1.1 MPa)

with extremely low radioactivity transport due to the lack of corrosion in the organic liquids.


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