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PostPosted: Mar 22, 2012 3:22 pm 
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Thanks veromies, welcome to the forum. Sure, it's all about the phase angle of the feedback in relation to the disturbance. As the phase angle approaches 180 degrees the system response will tend to drive the system towards instability, so if the the negative feedbacks arrive with a phase angle of 180 degrees they become positive feedbacks in effect.

I've always been good at tuning real control loops in a power station environment, but was I was never that good at the mathematical analysis of control systems, but it is a very powerful and effective branch of mathematics and very relevant to these designs.


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PostPosted: Mar 23, 2012 3:18 am 
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veromies wrote:
Lindsay wrote:
I think that I get your point, but your analogy (or my understanding of it) is somewhat different to a MSR where there are a number of strong to very strong self-regulating factors that provide negative feedback to the system:
- The first being Doppler broadening of fertile material that is usually in the core as part of a homogeneous fuel mix, which increases neutron absorptions in fertile;
- The second being reduced atomic density at higher temperatures; and
- The big hammer being the fuel salt expansion moving fissile out of the critical geometry of the core into non-critical geometry.


Negative feedbacks are not enough to prove that a reactor is stable.

For example, consider a standard BWR with negative doppler coefficient and negative coolant void fraction coefficient. Assume a fission rate increase, which increases local temperature immediately, thereby reducing reactivity fission rate. On a longer timescale the heat is transferred to the coolant, so that the void fraction increases. The increased void fraction decreases reactivity further, which reducese the fission rate, which reduces fuel temperature. The reduced fuel temperature then increases reactivity and fission rate and the cycle continues. If the time scales and reactivity coefficients happen to be bad the cyclic behavior might not be damped but allow the oscillations to grow in magnitude.

And I think they had such oscillations described above in Swedish BWRs, so it is not only theoretical.

Therefore, negative coefficients: good, but only a starting point to dynamic studies.


Interesting remarks. Though I think what is missing for molten salt reactors is the very strong negative density coefficient, and the lack of voiding potential during operation of the reactor, due to the extremely high boiling point over the normal operating temperature. If you have those things you can get much more negative in the total coefficient than UO2 fuel (which has very little density coefficient) and water coolant (which operates close to voiding temperatures even in normal operation).


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PostPosted: Mar 26, 2012 1:36 pm 
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Cyril R wrote:
Interesting remarks. Though I think what is missing for molten salt reactors is the very strong negative density coefficient, and the lack of voiding potential during operation of the reactor, due to the extremely high boiling point over the normal operating temperature. If you have those things you can get much more negative in the total coefficient than UO2 fuel (which has very little density coefficient) and water coolant (which operates close to voiding temperatures even in normal operation).


LFTR surely is quite different to LWRs in stability area. I don't expect LFTR to be unstable, but I really do wish that stability issues during normal and postulated accident scenaries will be checked before finding them out empirically. That would be bad PR for LFTR.

Lindsay wrote:
Thanks veromies, welcome to the forum. Sure, it's all about the phase angle of the feedback in relation to the disturbance. As the phase angle approaches 180 degrees the system response will tend to drive the system towards instability, so if the the negative feedbacks arrive with a phase angle of 180 degrees they become positive feedbacks in effect.

I've always been good at tuning real control loops in a power station environment, but was I was never that good at the mathematical analysis of control systems, but it is a very powerful and effective branch of mathematics and very relevant to these designs.


Thanks for the welcome.

For LFTR, somebody has to show that the system can be made stable and then somebody has to actually make it stable. I can't contribute to either part without studying first, which allows me to speculate on things :)

But I think also that the thread has gone a bit offtopic lately.


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