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PostPosted: Apr 11, 2013 10:55 am 
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Cyril R wrote:
jaro wrote:
Here's a hint on how they plan to keep the zirconium hydride from decomposing:

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[0124]
In the modeled case, the fuel-salt mixture flows both inside and outside the annular rod. In examples in which the fuel salt flows on the outside and a different, non-radioactive coolant on the inside of each rod, the purpose of the non-radioactive coolant would be to keep the annular rod from overheating. Such an approach could be used if the annular rod were made of a material that could not be allowed to get hotter than a certain maximum temperature.

....hopefully the coolant is heavy water :P


So you're back into serious safety territory with loss of coolant, hydrogen evolution, steam pressurization accidents. Vbad.

Personally, I'd get rid of the zirconium hydride and just go with D2O, ambient pressure....


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PostPosted: Apr 11, 2013 12:08 pm 
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The thread is about what their patent application actually says, not what you might wish it said.


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PostPosted: Apr 13, 2013 8:54 am 
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I'd like to comment on the subtopic of salts and what is salt-like in this context.
As you learned in high school chemistry, closed-valence-shell atoms are fundamentally inert, the best example being the rare gases. In the salts, the transfer of a valence electron closes the valence shells of both constituents. But, unlike the rare gases, these closed-shell systems are charged. The significance of this picture is that the salts are a collection of charged marbles. The liquid is held together not by directional, quantum-mechanical forces, but bu classical Coulomb forces attempting to screen the ionic charges. Thus, when a molten-salt ion is hit and displaced by a neutron, the liquid just rearranges itself -- just as if it were hit by another ion. So, unlike the atoms in solid metal cladding, no permanent damage is done by the neutron.

Art Williams


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PostPosted: Apr 13, 2013 12:31 pm 
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Am I correct is assuming that unclad Zr hydride
will go to the fluoride in a fluoride salt and create some HF
or the choride ina chloride salt and creat some HCl?


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PostPosted: Apr 13, 2013 4:07 pm 
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To the best of my knowledge, clad or unclad hydrides will dissociate at MSR temperatures.


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PostPosted: Apr 14, 2013 11:31 pm 
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That leaves fast MSR as the only option to burn the SNF.


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PostPosted: Apr 15, 2013 9:20 am 
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jagdish wrote:
That leaves fast MSR as the only option to burn the SNF.

What, exactly, is the limiting factor in burning SNF-derived TRU mixture in a thermal reactor? If no breeding is attempted (trying to maximize fuel burned rather than minimize it) there are quite a lot of extra neutrons to be wasted on unproductive reactions such as even-numbered nuclides, absorption in reactor materials etc. What becomes the limiting factor? What burnup is achievable?


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PostPosted: Apr 15, 2013 9:38 am 
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Owen T wrote:
jagdish wrote:
That leaves fast MSR as the only option to burn the SNF.

What, exactly, is the limiting factor in burning SNF-derived TRU mixture in a thermal reactor? If no breeding is attempted (trying to maximize fuel burned rather than minimize it) there are quite a lot of extra neutrons to be wasted on unproductive reactions such as even-numbered nuclides, absorption in reactor materials etc. What becomes the limiting factor? What burnup is achievable?

The limiting factor is the accumulation of TRU isotopes which have an energy threshold for fission that is higher than the energy of delayed neutrons: It's as if delayed neutrons didn't exist at all. The reactor becomes uncontrollable.
With fast neutron reactors, these isotopes get burned off and don't accumulate.
Hence the controllability afforded by delayed neutrons remains in effect.


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PostPosted: Apr 15, 2013 11:40 pm 
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jaro wrote:
...
The limiting factor is the accumulation of TRU isotopes which have an energy threshold for fission that is higher than the energy of delayed neutrons: It's as if delayed neutrons didn't exist at all. The reactor becomes uncontrollable.
With fast neutron reactors, these isotopes get burned off and don't accumulate.
Hence the controllability afforded by delayed neutrons remains in effect.


So adding some thorium helps by breeding just enough fissile material that does "see" these delayed neutrons to make them adequately controllable? IIUC, fast reactors are already less controllable than conventional thermal reactors, anyway.

What then becomes the limiting factor?


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PostPosted: Apr 16, 2013 2:01 am 
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Delayed neutrons are important for solid fuelled reactors. For fluid fuel reactors with instant fast negative terms (faster than delayed neutrons time constant) it's not clear that it is all that vital.

The solubility of trifluorides (TRUs all occur as trifluorides) is limited in fluoride salts. This becomes a problem at some point for most reactors, though lower power density reactors can run a long time before this becomes an issue.


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PostPosted: Apr 16, 2013 2:09 am 
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Cyril R wrote:
Delayed neutrons are important for solid fuelled reactors. For fluid fuel reactors with instant fast negative terms (faster than delayed neutrons time constant) it's not clear that it is all that vital.

The solubility of trifluorides (TRUs all occur as trifluorides) is limited in fluoride salts. This becomes a problem at some point for most reactors, though lower power density reactors can run a long time before this becomes an issue.


Thermal reactors need much lower fissile loading so solubility should be less limiting. I realize that this is all just qualitative generalizations rather than actual quantitative analysis but it seems that the "you need fast neutrons to burn TRUs" claim is not so clear cut.


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PostPosted: Apr 16, 2013 2:15 am 
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Owen T wrote:
Cyril R wrote:
Delayed neutrons are important for solid fuelled reactors. For fluid fuel reactors with instant fast negative terms (faster than delayed neutrons time constant) it's not clear that it is all that vital.

The solubility of trifluorides (TRUs all occur as trifluorides) is limited in fluoride salts. This becomes a problem at some point for most reactors, though lower power density reactors can run a long time before this becomes an issue.


Thermal reactors need much lower fissile loading so solubility should be less limiting. I realize that this is all just qualitative generalizations rather than actual quantitative analysis but it seems that the "you need fast neutrons to burn TRUs" claim is not so clear cut.


That's true. In many ways, TRUs are more attractive in thermal reactors, especially if breeding isn't necessary. A thorium DMSR with TRU feed is an effective destroyer of TRUs whilst producing high quality U233 that can be recovered for future LFTRs (yet safeguarded by U232 and fission products in the meanwhile).


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PostPosted: Apr 16, 2013 4:54 am 
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Cyril R wrote:
For fluid fuel reactors with instant fast negative terms (faster than delayed neutrons time constant)....
Well that's it, isn't it: When the delayed neutrons don't function, because the fuel doesn't react with neutrons below ~0.7MeV, then the applicable time constant is that of prompt neutrons, which is on the order of a few milliseconds for an epithermal reactor: That is "instant fast", just like the negative terms.
I doubt that any nuclear regulatory agency would want to bet on which of the two wins out.
In practical terms, any power ramping in such a system is likely to lead to nasty oscillations, at best.

Incidentally, in metal-fuel fast reactors the negative terms have been demonstrated (EBR II, HPRR, etc.) to be at least as fast as in MSRs. (In the case of HPRR, power oscillations were noted. Power oscillations are also typical of past accidents involving aqueous fissile solutions - thermal spectrum - such as the 1999 Tokaimura accident).
That was with LEU fuel, which includes a significant delayed neutron contribution from U238, which has an anomalously high delayed neutron fraction - about six times that of U233 and seven times that of Pu239.


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PostPosted: Apr 16, 2013 9:02 am 
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jaro wrote:
Cyril R wrote:
For fluid fuel reactors with instant fast negative terms (faster than delayed neutrons time constant)....
Well that's it, isn't it: When the delayed neutrons don't function, because the fuel doesn't react with neutrons below ~0.7MeV, then the applicable time constant is that of prompt neutrons, which is on the order of a few milliseconds for an epithermal reactor: That is "instant fast", just like the negative terms.


Doppler is faster. Salt dilatation is faster too isn't it?

Quote:
I doubt that any nuclear regulatory agency would want to bet on which of the two wins out.


It appears a fairly simple calculation of amplitude and constants.

Quote:
In practical terms, any power ramping in such a system is likely to lead to nasty oscillations, at best.


I'm not so sure about this. I'm not a nuclear engineer but have studied vibration damage on machinery in the past. The system without delayed neutrons might be more prone to oscillations, but they would appear to be also of smaller amplitude (simply due to the elimination of a large term that is out of sync in terms of time constants with the prompt fission process). In my experience, the cases where vibration damage was an issue were always the cases with large amplitudes. Purely speculating, I'd expect high frequency short amplitude oscillations which would not be damaging. I've cleaned gemstones with ultrasonic cleaning, it's a great technique that doesn't damage the stone (with some exceptions of really brittle materials that are unsuitable for ultrasonic cleaning).

I should also point out that when you get down to it, delayed neutrons are only a few hundred percent milliRho. Solid fuel reactors typically have thousands of percent milliRho excess reactivity. The molten fuel reactor greatly reduces the excess reactivity (maybe 10-100x). So on net reactivity we still stand out ahead even with zero delayed neutron worth.


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PostPosted: Apr 16, 2013 11:08 am 
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The topic is the patent, not reactor design discussions. There's other threads for that.


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