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PostPosted: Feb 15, 2013 4:23 am 
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Kinetics, dynamics and neutron noise in Molten Salt Reactors
Imre Pázsit
Chalmers University of Technology Department of Nuclear Engineering
CMSNT-13
9 - 11 January 2013 • Mumbai, India
http://moltensaltindia.org/wp-content/u ... Pazsit.pdf

From the conclusions slide:
Quote:
• The dynamic response of an MSR deviates in certain aspects quite markedly from that of traditional systems
• .....In general, noise amplitudes will be higher and a more coupled (less space-dependent) response of the the core is envisaged
• In addition, new types of disturbances or phenomena can be expected, such as the increased significance of propagating perturbations.


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PostPosted: Feb 15, 2013 7:04 am 
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The question is whether increased noise amplitude is a problem. If the frequency is shorter and the amplitude higher, the situation will improve, because, in reality the materials in a MSR core all have nonzero heat capacity. In fact the heat capacity of the fuel is much larger in a MSR than in a BWR, and reactivity changes are far more gradual compared to what happens in a BWR, where a one degree difference makes a huge sudden impact on the reactivity. MSR fuel has a huge resistance to high and low frequency thermal cycling compared to any solid fuel. Free floating graphite moderator elements are also thermally robust and won’t suffer thermal shock.

Various simplified 1d and 2d models predict all sorts of short term instabilities in LWRs, that are not present in actual experiments done, due to nonzero heat capacity of the fuel and cladding.


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PostPosted: Feb 15, 2013 10:46 am 
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Heat capacity is one thing.
In MSRs we get the new phenomenon of gas bubbles - mostly sparging helium, or just fission products (Kr, Xe, I, etc.).
This tends to help propagate instabilities, because even low intensity pressure waves can collapse the bubbles, causing significant density changes, which means changes in reactivity: the net effect is waves of enhanced neutron flux bouncing around the core, driving oscillating sound waves - subjecting the reactor vessel to vibrations that could be damaging in the long run.
Undoubtedly, this much more likely in moderator-free MSRs, such as the French TMSR or Taube's chloride salt fast reactor, due to the absence of flow channels to constrain propagation of instabilities.
The sausage-style "tube-in-tube" concept would be even more likely to support oscillations, only in a more simple axial manner, rather than the complex 3D version in low aspect ratio vessels.....


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PostPosted: Feb 15, 2013 4:06 pm 
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Has something like this been observed in the MSRE? If helium sparging adds to the problem, could alternatives help like the one proposed in this thread:
viewtopic.php?f=4&t=3320&hilit=sparging


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PostPosted: Feb 15, 2013 7:53 pm 
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Jaro, have you ever looked at the mathematical analysis of the stability of control systems or the behaviour of spring, mass, damper assemblies?


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PostPosted: Feb 15, 2013 10:02 pm 
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Lindsay wrote:
Jaro, have you ever looked at the mathematical analysis of the stability of control systems or the behaviour of spring, mass, damper assemblies?
Yes Lindsay, I believe I still have my textbook from that course... As I recall, I didn't particularly enjoy it at the time 8)


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PostPosted: Feb 15, 2013 10:17 pm 
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Burghard wrote:
Has something like this been observed in the MSRE? If helium sparging adds to the problem, could alternatives help like the one proposed in this thread:
viewtopic.php?f=4&t=3320&hilit=sparging

I don't believe it has been observed in the MSRE - not surprising, since it was a low power reactor, with lots of graphite moderator and hundreds of small fuel channels (ie. geometry similar to a solid fuel reactor, with hundreds of well-constrained fuel rods...)
I do like the alternatives to helium sparging.
Again, for a larger, high power reactor using loads of helium might turn out to be impractical - on top of possibly contributing to instability.
I really think that this is an important issue that people like folks in Grenoble, working on the TMSR design, need to have a good look at (so far, they have only looked at gross reactor transients, but not local instabilities within the core, including potentially damaging oscillations....)


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PostPosted: Feb 15, 2013 11:13 pm 
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jaro wrote:
Lindsay wrote:
Jaro, have you ever looked at the mathematical analysis of the stability of control systems or the behaviour of spring, mass, damper assemblies?
Yes Lindsay, I believe I still have my textbook from that course... As I recall, I didn't particularly enjoy it at the time 8)

Me neither, I never did find the complex number math easy, but one thing I recall is how most of these systems can be reduced to transfer functions and then one can explore the parameters that create instabilities or stable operation. So hopefully people who are good at such things can explore the envelope and tell the rest of us what not to do.


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PostPosted: Feb 15, 2013 11:27 pm 
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A couple of observations here, I presume that all gas sparging and bubble extraction occurs out of the core. I presume that no one is proposing to bubble through the active core, therefore any gas bubbles would be extremely small presumably. Small bubbles => small effects, yes?

A second thought, even if one has discrete flow channels the rapid diffusion of neutrons throughout the core has a tendency to couple everything together doesn't it? So the ripple effect of a bubble in one flow channel will affect the local flux and power in other channels nearby in a similar way that a single bubble passing through a graphite free core will have a sphere of influence.


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PostPosted: Feb 15, 2013 11:54 pm 
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Lindsay wrote:
A couple of observations here, I presume that all gas sparging and bubble extraction occurs out of the core. I presume that no one is proposing to bubble through the active core, therefore any gas bubbles would be extremely small presumably. Small bubbles => small effects, yes?

A second thought, even if one has discrete flow channels the rapid diffusion of neutrons throughout the core has a tendency to couple everything together doesn't it? So the ripple effect of a bubble in one flow channel will affect the local flux and power in other channels nearby in a similar way that a single bubble passing through a graphite free core will have a sphere of influence.

No. The salt flow where the bubbles get extracted is a side-stream of around 10% of the total flow. The injected helium makes trips around the loop before being extracted. In the core approximately 0.5% of the volume is helium bubbles. If I recall correctly the choice of 0.5% was made because ORNL wanted to minimize the neutrons collected by the xenon and hence wanted to maximize its removal. They thought there might be issues going above 1% bubbles so they settled on 0.5%.

Seems to me that the reactor is pretty self-regulating. If you have a local surplus in reactivity then you will also get a local surplus in heat generation and thus reduce the reactivity in that local area. I'd be more concerned about this with a fast reactor simply because the neutron generation time is so much shorter. It makes control more difficult.


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PostPosted: Feb 16, 2013 8:20 am 
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jaro wrote:
Heat capacity is one thing.
In MSRs we get the new phenomenon of gas bubbles - mostly sparging helium, or just fission products (Kr, Xe, I, etc.).
This tends to help propagate instabilities, because even low intensity pressure waves can collapse the bubbles, causing significant density changes, which means changes in reactivity: the net effect is waves of enhanced neutron flux bouncing around the core, driving oscillating sound waves - subjecting the reactor vessel to vibrations that could be damaging in the long run


The phenomenon you're describing is called Density Waves Oscillations, DWO. It is seen in BWRs because small changes in temperature can cause saturated water to massively change in density (>90% locally).

That's a world of difference from a 0.5% bubble collapse of MSR helium bubbles.

Futhermore, in BWRs, the moderator (coolant) coefficient dominates all others. This will not be the case in a molten salt reactor where the coefficients are expected to be in a similar ballpark.

Finally, I'll note that the thermal response time between fuel and coolant in MSRs is nearly instant, whereas in BWRs with thermally sluggish oxide fuel - would be a rather good insulator in fact, if it weren't radioactive - there's a large delay between fuel and coolant response. This means the BWR has low frequency oscillations where more damaging energy can accumulate.


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PostPosted: Feb 16, 2013 11:55 am 
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Cyril R wrote:
....In BWRs, the moderator (coolant) coefficient dominates all others. This will not be the case in a molten salt reactor where the coefficients are expected to be in a similar ballpark.

Finally, I'll note that the thermal response time between fuel and coolant in MSRs is nearly instant, whereas in BWRs with thermally sluggish oxide fuel - would be a rather good insulator in fact, if it weren't radioactive - there's a large delay between fuel and coolant response. This means the BWR has low frequency oscillations where more damaging energy can accumulate.
On the other hand, a graphite-free TMSR or chloride MSR has a much shorter average neutron lifetime than your typical BWR: exponential power ramp is a lot quicker, in response to any reactivity changes.
Looking forward to any sort of realistic analysis - as opposed to blanket dismissal of potentially troublesome instability operation modes.
Got any ?
Thanks.


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PostPosted: Feb 16, 2013 1:35 pm 
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jaro wrote:
Cyril R wrote:
....In BWRs, the moderator (coolant) coefficient dominates all others. This will not be the case in a molten salt reactor where the coefficients are expected to be in a similar ballpark.

Finally, I'll note that the thermal response time between fuel and coolant in MSRs is nearly instant, whereas in BWRs with thermally sluggish oxide fuel - would be a rather good insulator in fact, if it weren't radioactive - there's a large delay between fuel and coolant response. This means the BWR has low frequency oscillations where more damaging energy can accumulate.
On the other hand, a graphite-free TMSR or chloride MSR has a much shorter average neutron lifetime than your typical BWR: exponential power ramp is a lot quicker, in response to any reactivity changes.
Looking forward to any sort of realistic analysis - as opposed to blanket dismissal of potentially troublesome instability operation modes.
Got any ?
Thanks.


I'm sorry, I didn't mean to be dismissive, rather I think it important to have a mechanistic undertanding of the processes and differences involved. Perhaps it's my lack of formal training in nuclear physics, but just ranting on about the processes helps me understand much.

The issue of oscillations is certainly complex. But we need to also look at existing reactor systems and their experience. Even PWR and BWR differ much so that oscillations seen in BWR are not seen in PWRs with sufficient subcooling margin. MSR is basically more similar to PWR, except you get a 700 degree Celsius margin to boiling rather than a 15 degree Celsius margin, and smaller % density changes per degree Celsius. Even with that small margin for a PWR, it gets to avoid most of the oscillations that are observed in BWRs.

In practise, the oscillations in BWRs were never found to be damaging. They were found to be rather usefully self-flattening the power profile of the core.

The viscosity of the molten salt, and it's great weight (several times that of pressurized hot water) will also be beneficial as it absorbs energy from pressure waves.

Agreed that it is different for fast spectra MSRs. But I don't find these very convincing for a first build, certainly not the French design that lacks a blanket in one dimension. I'd like to see a start with a simple graphite moderated converter reactor. With lots of little channels, laminar flow. Turbulent just results in pressure drop and is not necessary for heat transfer as the coolant is integrated with the fuel. A simple, orificed, laminar flow and well thermalized spectrum would be perfect.


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PostPosted: Feb 16, 2013 2:49 pm 
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And if there is no sparge gas? See the other topic.

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PostPosted: Feb 16, 2013 3:26 pm 
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KitemanSA wrote:
And if there is no sparge gas? See the other topic.


Even with the sparge gas, it's still only 0.5%. This inherently limits the transient or oscillations. If all of it were to disappear instantly, you get 0.5% more fuel per liter. But also 0.5% more fertile. Net effect should be very small.


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