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PostPosted: Dec 01, 2013 11:54 am 
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If the new fission/(capture+fission) ratio is less than the prior fission/(capture+fission) then the reactivity would go down. In your specific example if the fission/(capture+fission) < 1/2 then the reactivity increases if it is > 1/2 then the reactivity decreases.


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PostPosted: Dec 01, 2013 4:48 pm 
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Lars wrote:
If the new fission/(capture+fission) ratio is less than the prior fission/(capture+fission) then the reactivity would go down. In your specific example if the fission/(capture+fission) < 1/2 then the reactivity increases if it is > 1/2 then the reactivity decreases.


Thanks Lars. Using Fab's figures for Pu239 @ 0.1 and 0.3 eV, this gives a ratio of 0.68 @ 0.1 eV and 0.6 @ 0.3 eV.

So a reduction of reactivity on fission resonance spectrum shift for Pu239 would result.


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PostPosted: Dec 01, 2013 5:26 pm 
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I have trouble understanding why increasing the temperature of the graphite causes an increase in reactivity. It seems like it should have two effects:
1) the graphite will take up more volume so it should force some of the fuel salt out - which I presume would reduce reactivity.
2) the graphite is hotter so the neutrons don't get as thermal - which would also reduce the reactivity.

And I guess there is a third effect - the percentage of the core that is occupied by graphite will go up. Which during design tradeoffs would have the effect of additional moderation and increasing reactivity. I can't quite figure out what the effect is though when there are the same number of graphite atoms.

All this to say, I don't understand what drives an increase in reactivity when the graphite heats up.


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PostPosted: Dec 01, 2013 5:52 pm 
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Lars, as we're discussing here, there appears to be a resonance fission effect in U233. The effect nr. 2 that you mention, hotter graphite meaning less moderation, results in a slight reduction in moderation so that in a thermal reactor where many neutrons are below the resonance speed, more neutrons would be pushed up to the fission resonance, increasing U233 fission.

The effect does not appear in Pu239, rather the opposite happens as the capture resonance seems to dominate (not the case with U233).

U235 has no significant fission resonance near thermal region.

All things considered, thermal U/Th cycle has increase, thermal U/Pu cycle has decrease of reactivity upon graphite heatup.


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PostPosted: Dec 01, 2013 8:15 pm 
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Lars wrote:
I have trouble understanding why increasing the temperature of the graphite causes an increase in reactivity. It seems like it should have two effects:
1) the graphite will take up more volume so it should force some of the fuel salt out - which I presume would reduce reactivity.
2) the graphite is hotter so the neutrons don't get as thermal - which would also reduce the reactivity.

And I guess there is a third effect - the percentage of the core that is occupied by graphite will go up. Which during design tradeoffs would have the effect of additional moderation and increasing reactivity. I can't quite figure out what the effect is though when there are the same number of graphite atoms.

All this to say, I don't understand what drives an increase in reactivity when the graphite heats up.


I think this 3rd item might be significant. Another way to think of the percentage of graphite in the core going up, is to look at it as an increase in the thickness of the graphite, which results in more moderation as the temperature goes up (or a higher probability of a graphite interaction). Therefore, with more moderation, the cross section of fission goes up more than the negative effect of the fuel getting squeezed out of the core. I believe the French paper (EU EVOL project) doing sensitivities on graphite fractions implied this was the reason for the graphite positive reactivity effect, but I would need to look up the report to be sure. For instance, if the Grenoble work included 232Th in the core, then that would make a huge difference.


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PostPosted: Dec 01, 2013 8:41 pm 
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Lars wrote:
I have trouble understanding why increasing the temperature of the graphite causes an increase in reactivity.


This is because the temperature of scattering nuclei will affect the scattering process-----the thermal neutron scattering of moderator, the real scattering cross section is affected by material structure and temperature. As for graphite, temperature rise cause the atom vibrating stronger, and then the spectrum more hardened, but the result might be different for other moderators such as water. There is no business of density here we discuss.


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PostPosted: Dec 02, 2013 1:42 am 
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longwei1221 wrote:
This is because the temperature of scattering nuclei will affect the scattering process-----the thermal neutron scattering of moderator, the real scattering cross section is affected by material structure and temperature. As for graphite, temperature rise cause the atom vibrating stronger, and then the spectrum more hardened, but the result might be different for other moderators such as water. There is no business of density here we discuss.


Won't this result in neutrons being less thermal and more in the resonance zone. If so, won't that decrease reactivity?


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PostPosted: Dec 02, 2013 3:20 am 
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Lars wrote:
longwei1221 wrote:
This is because the temperature of scattering nuclei will affect the scattering process-----the thermal neutron scattering of moderator, the real scattering cross section is affected by material structure and temperature. As for graphite, temperature rise cause the atom vibrating stronger, and then the spectrum more hardened, but the result might be different for other moderators such as water. There is no business of density here we discuss.


Won't this result in neutrons being less thermal and more in the resonance zone. If so, won't that decrease reactivity?


Yes, more resonance, but for U233/Th cycle, that means fission resonance that dominates. In U/Pu, capture resonance dominate. So an increase of reactivity for U233/Th, a decrease for U/Pu.


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