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PostPosted: Aug 14, 2010 10:46 am 
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Cyril R wrote:
Elastic scattering isn't the issue, its inelastic. Chlorine is very low, while fluorine, iodine and bromine are all very significant. This means your neutrons will quickly lose their fast spectrum and accumulate likely somewhere in bad resonance regions. To get the fullest from a fast spectrum you need a homogeneous very fast spectrum, because you want to get to the eta and fast fission ‘thresholds’.


Comparing fast neutron inelastric scattering x-sections for various nuclides, one quickly finds that the dominant one is U238, while F19 is not that different from Na23 :

Image


Since SFRs contain a good deal of stainless steel – besides the Na23 coolant – a comparison with Fe and Ni is also appropriate.
Again, not a lot of difference :

Image

Finally, a look at some of the alternative liquid metal coolants, Bi and Pb :

Image

.....even with a gas-cooled fast reactor, one would still have the inelastic scattering from U238 (plus alloying elements).


Cyril R wrote:
There are actually medical treatment moderators employing fluorine (IIRC Teflon) to quickly get out of the MeV region.

This appears to be a reference to BNCT (Boron Neutron Capture Therapy).
The ideal neutron flux in this instance is epithermal : the patented “Fluental” material is not a moderator – rather, it is commonly referred to as a filter, because the nuclide mass of the elements in the material are such that BOTH fast and thermal neutrons are largely eliminated from the outlet stream.
In the early years of BNCT development, only thermal neutrons were used, as they came out from a pool-type (water moderated) research reactor.
This had the unfortunate consequence of burning the patients’ skin, while delivering an inadequate neutron flux to the boron-laced tumour – with predictably poor therapeutic results.
Water moderated research reactors are still the mainstay of BNCT research (with one exception), only today they replace the water between the reactor core and the hole in the pool wall with Fluental – so as to AVOID over-moderating the neutron flux : it is not “to quickly get out of the MeV region”.


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PostPosted: Aug 14, 2010 11:17 am 
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Lars wrote:
Since 37Cl(n,2n) reaction isn't in any of the NNDC databases I have to presume that the reaction is extremely rare so I don't think I'd shy away from the chloride based reactor for that reason.

It has a very high threshold -- no relevance to fission reactors :

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PostPosted: Aug 14, 2010 11:26 am 
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Jaro, where are you getting this from? The NNDC/sigma pulls me an empty plot for inelastic scattering of U238, in the US/Russian and Chinese databases. Confusingly it sometimes gives 'non-elastic' and other databases give 'inelastic'. What's the difference?

I'm pretty sure fluorine as an atom won't get you beyond a fastish spectrum. It isn't a good moderator at all below 100 keV, and nearly useless below 10 keV, because you completely rely on elastic scattering and fluorine isn't that light an element.

I also wonder how the inelastic/non-elastic figures deal with the weight of an atom. If U238 is so high then that's odd, and may be explained by the large size of the uranium atom. This inelastic scattering thing is very confusing. But, if you're right then that is good news since you can have a very fast spectrum while sticking with the proven fluoride chemistry.


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PostPosted: Aug 14, 2010 11:39 am 
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Lars wrote:
I don't think the fission products will alter the melting/boiling point of your liquid significantly since even after 60 years they will still be a minor portion of the salt (roughly 300 kg/yr * 60 yrs or 1.8 tonnes while the fissile portion will need to be on the order of 10 tonnes and the fertile around 80 tonnes and then I presume you have some dilutant beyond this).


Good to hear! Of course many of the salt seeker chlorides are low melting, and actually quite volatile.

Quote:
I would still be concerned that UCl4 wants to evaporate on you. This seems to be troublesome to me.


If I understand this issue correctly then it is strongly reduced by the dilutant if the dilutant is low volatility. NaCl looks great. In about 1:1 ratio NaCl:UCl4 the stuff melts around 370 degrees C.

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The other area to watch for is controlability. Since your spectrum is fast your prompt neutron lifetime is very short. Since the spectrum is very fast the proportion of fissions that happen with slow neutrons presumably is very small so likely you have almost no negative temperature coefficient contribution from resonance. I'm guessing any mechanical control system will be too slow for this style of reactor. If the generational lifetime is shorter (or even about the same) as the time for the salt to expand out of the core region then where does the control for this reactor come from? In particular, in the safety studies for LFTR the extreme (on paper only) test is to suddenly stop all the fuel pumps. Did Taube do safety studies of the chloride reactor? What is the control mechanism?


I think Taube used a 'thermal column'to burn fission product waste, and he might have used this as a fission and resonance control since it was in the middle of the reactor. Taube also said the salt dilatation coefficient was sufficiently negative to control the reactor. The chloride salt he used did have higher thermal expansion coefficient than the FLiBe. Hey, you're the nuclear engineer, you tell me! 8)


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PostPosted: Aug 14, 2010 12:51 pm 
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Cyril R wrote:
Jaro, where are you getting this from? The NNDC/sigma pulls me an empty plot for inelastic scattering of U238.....

I used the NNDC/ENDF facility for this series.
The description looks pretty much the same – not sure what differences there may be....
http://www.nndc.bnl.gov/exfor/endf11.jsp

Cyril R wrote:
Confusingly it sometimes gives 'non-elastic' and other databases give 'inelastic'. What's the difference?

In NNDC/ENDF the parameter MT#3 “(Z,NON)” is defined as “Nonelastic neutron cross section” – there is no other MT# for “inelastic”.

Cyril R wrote:
I'm pretty sure fluorine as an atom won't get you beyond a fastish spectrum. It isn't a good moderator at all below 100 keV, and nearly useless below 10 keV, because you completely rely on elastic scattering and fluorine isn't that light an element.

Agree.
Seems to be about the same as sodium, all else being equal.
SFRs have been designed with a breeding ratio as high as 1.57 (Westinghouse)

Cyril R wrote:
I also wonder how the inelastic/non-elastic figures deal with the weight of an atom. If U238 is so high then that's odd, and may be explained by the large size of the uranium atom. This inelastic scattering thing is very confusing. But, if you're right then that is good news since you can have a very fast spectrum while sticking with the proven fluoride chemistry.

Interesting question.
Pls. Let me know if you ever find out.
According to current theory, the protons & neutrons in a nucleus have shell structure similar to that of electrons in atoms.
I’m thinking that if the outer shell of a heavy nucleus like U238 has a low-density, then perhaps it makes for a better “cushion” for incoming neutrons to be slowed down in (like a p–n collision) – in contrast to a hit on the more densely-populated inner shells, which would result in an elastic collision.
But why should the neutron energy make a difference ?


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PostPosted: Aug 14, 2010 1:18 pm 
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Cyril R wrote:
Taube also said the salt dilatation coefficient was sufficiently negative to control the reactor.

Yes.
The other factor one must also take into account is that while a fast reactor prompt fission burst can send peak power sky high, the burst pulse half-width is extremely short, so the total energy output (area integral of the pulse, or simply peak height multiplied by half-width) is typically far smaller and less damaging than a similarly high pulse in a thermal spectrum reactor: fast reactor bursts are over in microseconds, thermal ones in milliseconds to seconds.


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PostPosted: Aug 14, 2010 2:53 pm 
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Lars wrote:
I don't think the fission products will alter the melting/boiling point of your liquid significantly since even after 60 years they will still be a minor portion of the salt (roughly 300 kg/yr * 60 yrs or 1.8 tonnes while the fissile portion will need to be on the order of 10 tonnes and the fertile around 80 tonnes and then I presume you have some dilutant beyond this)
Er... typo? 300 kg/yr * 60 years = 18,000 kg = 18 Te. Probably still OK, but it wouldn't hurt to have a cold spot/solids trap somewhere for a small portion of the flow to guard against plating things onto the cold end of the heat exchanger.


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PostPosted: Aug 14, 2010 3:00 pm 
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Yikes. Not a typo - a thinko (: 18 tonnes might well be enough to alter the salt physical properties.


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PostPosted: Aug 14, 2010 5:18 pm 
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Lars wrote:
Since 37Cl(n,2n) reaction isn't in any of the NNDC databases I have to presume that the reaction is extremely rare so I don't think I'd shy away from the chloride based reactor for that reason.



On the other hand, one should not expect that one is going to have a 100% isotopic Cl-37 system.

I would expect that any isotopic separation is going to do something like gas diffusion. Note that Cl2 gas is not going to be pretty, since there is a distribution of Cl-35-Cl-37 dimers as well as Cl-35 dimers and Cl-37 dimers, the latter being the rarest form. That's not going to be very pretty, I think, from an economic standpoint. Invariably there is going to be an isotopic purity well below 99%.

One could make a gas like methyl chloride and decompose it or HCl gas, if one is willing to risk the complication of exchange reactions in the latter, not only within the gas, but also in the diffusion apparatus materials. Still not very pretty.

Commercially available Cl-37 is generally less than 98% pure.

Again, I still say, why bother?

The main feature of a chloride reactor that strikes me is - not to be mean - is a lack of imagination.


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PostPosted: Aug 14, 2010 6:53 pm 
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NNadir wrote:
Lars wrote:
Since 37Cl(n,2n) reaction isn't in any of the NNDC databases I have to presume that the reaction is extremely rare so I don't think I'd shy away from the chloride based reactor for that reason.



On the other hand, one should not expect that one is going to have a 100% isotopic Cl-37 system.

I would expect that any isotopic separation is going to do something like gas diffusion. Note that Cl2 gas is not going to be pretty, since there is a distribution of Cl-35-Cl-37 dimers as well as Cl-35 dimers and Cl-37 dimers, the latter being the rarest form. That's not going to be very pretty, I think, from an economic standpoint. Invariably there is going to be an isotopic purity well below 99%.

One could make a gas like methyl chloride and decompose it or HCl gas, if one is willing to risk the complication of exchange reactions in the latter, not only within the gas, but also in the diffusion apparatus materials. Still not very pretty.

Commercially available Cl-37 is generally less than 98% pure.

Again, I still say, why bother?

The main feature of a chloride reactor that strikes me is - not to be mean - is a lack of imagination.


Am I a reactor dreaming that I am a man? Or a man dreaming that I am a rea-- ... you know.

What would be more imaginative?

(How fire can be domesticated)


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PostPosted: Aug 15, 2010 9:00 am 
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From Lamarsh, p.50 :
Quote:
Inelastic Scattering
This process does not occur unless the neutron has sufficient energy to place the target nucleus in its first excited state. As a result, sigma(i) is zero up to some threshold energy.
Generally speaking, the energy at which the first excited state is found decreases with increasing mass number, and, as a consequence, sigma(i) is nonzero over a larger energy region for the heavier nuclei than for the lighter nuclei. The threshold for inelastic scattering is 4.80 MeV for C12, while it is only 44 keV for U238. At energies well above threshold, sigma(i) is roughly equal to sigma(s).
.....for F19 the threshold is about 120 keV, for Na23 450 keV.
(note that in the NNDC graph below, the C12 threshold matches perfectly Lamarsh's texbook example..... except for his somewhat liberal use of the word "zero" in "sigma(i) is zero up to some threshold energy")

Inelastic neutron scattering only has a significant effect on neutron energy when the target is a heavy nucleus :

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: Aug 16, 2010 10:36 am 
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Jaro, thanks for digging this stuff up. But, if what you say is true, then there are some important conclusions which we have omitted:

1. Fast reactors with lots of U238 can be fast but not extremely fast (the type of fast you want to really take advantage of fast fission of fertiles). This suggests that if you use higher enrichment than NU or SEU you will get better neutron budgets. This may also explain why fast reactor engineers have often wanted to use very high enrichment, since the faster spectrum by itself doesn't strictly justify more than say LEU (20% enrichment).

2. Inelastic scattering for fluorine may be similar to sodium for neutron 'babies' (just born @ 2-3 MeV) but will be much worse than sodium below 1 MeV, where you want to stay to take advantage of fast fission, especially for thorium fuelled designs since you can fission U234 and Pa-233 (ie less requirement for Pa seperation).

If we combine 1. and 2. the prospects of a big fast fission bonus in your HW-MSR are a bit weakened. While we expected 2. to be the case, I was surprised to hear about U238 inelastic scattering. I learn something new every day!


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PostPosted: Aug 16, 2010 11:23 am 
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Cyril R wrote:
2. Inelastic scattering for fluorine may be similar to sodium for neutron 'babies' (just born @ 2-3 MeV) but will be much worse than sodium below 1 MeV, where you want to stay to take advantage of fast fission......

What do you mean by "much worse" ?
As noted above, for light nuclides like F-19, the result of an inelastic collision is about the same as an elastic one -- we don't care whether we have more or less of either.
Besides which, if you look carefully at the graph, the F-19(N,NON) x-section hump, relative to Na-23(N,NON) is between about 0.1 to 0.6 MeV.

There is NO U238 fission below about 1.4 MeV, so once the neutrons slow below that (and below ~0.5 MeV for the even-numbered Pu nuclides), we want to get them past the epithermal resonances as fast as possible......
Similarly, eta for U238 only begins approaching the magnitude of U235 & 233 at about 2 MeV & higher (Pu239 being ~1.3 x higher).

The fast fission threshold for Pa233 is virtually identical to U238, while U234 is a bit better -- falling below 0.1 barn at about 0.3 MeV.


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PostPosted: Aug 16, 2010 11:46 am 
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jaro wrote:
Cyril R wrote:
2. Inelastic scattering for fluorine may be similar to sodium for neutron 'babies' (just born @ 2-3 MeV) but will be much worse than sodium below 1 MeV, where you want to stay to take advantage of fast fission......

What do you mean by "much worse" ?
As noted above, for light nuclides like F-19, the result of an inelastic collision is about the same as an elastic one -- we don't care whether we have more or less of either.
Besides which, if you look carefully at the graph, the F-19(N,NON) x-section hump, relative to Na-23(N,NON) is between about 0.1 to 0.6 MeV.

There is NO U238 fission below about 1.4 MeV, so once the neutrons slow below that (and below ~0.5 MeV for the even-numbered Pu nuclides), we want to get them past the epithermal resonances as fast as possible......
Similarly, eta for U238 only begins approaching the magnitude of U235 & 233 at about 2 MeV & higher (Pu239 being ~1.3 x higher).

The fast fission threshold for Pa233 is virtually identical to U238, while U234 is a bit better -- falling below 0.1 barn at about 0.3 MeV.


Yeah. For your design it won't matter much, as it has little or no thorium. Its nice for a pure fast Th-U233 cycle though, or even if you add thorium to your design and balance with TRU fluorides. In any case, there is still the average number of neutrons per fission benefit - eta is significantly higher for Pu at higher energies. And the combined fast fission bonus of Pu240, Pu242, Am241 and Am 243 must be quite nice too, especially with their eta increasing as well at MeV energies...


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PostPosted: Aug 16, 2010 2:21 pm 
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GRLCowan wrote:
NNadir wrote:
Lars wrote:
Since 37Cl(n,2n) reaction isn't in any of the NNDC databases I have to presume that the reaction is extremely rare so I don't think I'd shy away from the chloride based reactor for that reason.



On the other hand, one should not expect that one is going to have a 100% isotopic Cl-37 system.

I would expect that any isotopic separation is going to do something like gas diffusion. Note that Cl2 gas is not going to be pretty, since there is a distribution of Cl-35-Cl-37 dimers as well as Cl-35 dimers and Cl-37 dimers, the latter being the rarest form. That's not going to be very pretty, I think, from an economic standpoint. Invariably there is going to be an isotopic purity well below 99%.

One could make a gas like methyl chloride and decompose it or HCl gas, if one is willing to risk the complication of exchange reactions in the latter, not only within the gas, but also in the diffusion apparatus materials. Still not very pretty.

Commercially available Cl-37 is generally less than 98% pure.

Again, I still say, why bother?

The main feature of a chloride reactor that strikes me is - not to be mean - is a lack of imagination.


Am I a reactor dreaming that I am a man? Or a man dreaming that I am a rea-- ... you know.

What would be more imaginative?

(How fire can be domesticated)


Speaking only for myself, I am a man dreaming that I am a reactor.

:lol: :lol: :lol: :lol: :lol:


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