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PostPosted: May 25, 2008 3:31 am 
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Paper calculating reactivity lost due to delayed nuetron precursers poppng out their respective neutron when outside of core.

http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/5.21_121.pdf


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PostPosted: Jun 11, 2008 11:52 am 
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Kirk with a significant percentage of the delayed neutron precursers giving up thier neutron outside the core do you have any calcs on how much that changes the effective delayed neutron fraction? This is a pretty critical parameter for any thermal or epi-thermal reactor from a reactor control prespective. From the rudimentary code trials I have done so far it appears that you can change the delayed neutron fraction simply by speeding up or slowing down the fuel fluid flow rate through the core. Slow it down and you cause more delayed neutrons to be emitted in the core. Speed it up and you start losing the longer lived precurser groups. Precurser groups 3 and 4 for example. Very wierd to me. I have a PWR/LWR solid fuel background so it is quite strange to be able to vary this value which is normally a function of the fuel isotopes and amount of fissile breeding that has occurred. But perhaps this is another advantage we can take advantage of? Maybe we should be thinking seriously about some type of fluid delay mechanism to have some way of controlling where in the primary fluid loop these precursers emit their neutrons. My thoughts of a variable speed pump could be more useful than I thought.

For the non-nuclear trained the value of the effective delayed neutron fraction determines how fast reactor power can change. A larger number means fission power changes more slowly and the reactor is easier to control. A smaller number means the opposite. I am concerned that with a fluid fuel reactor the delayed neutrons will be born outside the core in a large percentage of the time and contribute nothing to slow down the reactor power rate of change. Put another way, if you have to maintain criticality without these delayed neutrons, you will be that much closer to criitcality on prompt neutrons alone, a very unsettling prospect because they are emitted almost instantly and so power can build on itself very very fast. Reactor power can change faster than a reactor where the delayed neutron precursers all stay in the core all the time and you need them to maintain criticality.


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PostPosted: Jun 11, 2008 11:56 am 
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USPWR_RO wrote:
Kirk with a significant percentage of the delayed neutron precursers giving up thier neutron outside the core do you have any calcs on how much that changes the effective delayed neutron fraction?


From what I've read you multiply the delayed neutron fraction by the in-core fraction of the core loop to get an effective delayed neutron fraction. All the more reason to keep the out-of-core delayed neutron fraction low, or to come up with some scheme where the delayed neutron "communicate" neutronically with the rest of the reactor, perhaps by using a primary heat exchanger very close to the core.

Not that I think we should necessarily do this, but from a controls perspective, if you have very prompt reactivity feedback mechanisms in your reactor design, then theoretically the reactor would be self-controlling even in a superprompt critical scenario, like a TRIGA reactor can do. LWRs don't have reactivity feedback that's that fast, but I think a well-designed two-fluid LFTR could.


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PostPosted: Jun 11, 2008 12:36 pm 
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Just to clear that up in my mind:
Negative reactivity feedback in a reactor that gets it by the moderator, requires first that heat energy needs to flow from the heat generating fuel to the material that provides the negative mechanism. This heat-flow mechanism acts as a low-pass filter, letting only the low frequency components of the heat energy fluctuations through. Therefore the regulation mechanism cannot react to high frequency components, even very large pulses, of the heat flow. Which means the avoidance of high frequency components (like through fast prompt reactivity changes) has to be avoided at all costs.
In contrast a LFTR does not have an intervening low pass filter in its regulating mechanism. The source itself provides the negative feedback. The frequency limit there is then the speed of sound in the molten salt, as that is the maximum speed at which density changes can be communicated through the regulating material (the fuel).

Am I correct?


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PostPosted: Jun 11, 2008 1:01 pm 
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Bob,

Yes, those are very good points that it is not only the fraction of fuel salt out of core that is important but how fast the fuel goes through the core (to keep the "briefly" delayed neutrons in the core).
In general ORNL mainly was concerned with the fraction of salt out of core. The rule of thumb seemed to be not to go much beyond half the salt out of core. The group in France typically assumes 1/3 out of core. It would be nice to not lose any delayed neutrons but as Kirk points out, the virtually instant and strong negative reactivity coefficients of most MSR designs really helps in this respect.

David L.


P.S. Yes Klaus, that is a good way to sum it up.


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PostPosted: Jun 11, 2008 1:17 pm 
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I thought of a good analogy for reactivity feedback. When I was a kid my neighbors had a mean dog that liked to chase me. After my folks complained to them, they put the dog in the front yard on a line that didn't quite stretch from the door to where the sidewalk was.

So the dog would sit there in the shade of the porch until I came along on the sidewalk heading past the neighbors to my friend's house, and that dog would see me and start take off running to me at top speed. But the stupid dog would run out of line before it made it to the sidewalk and would get a vicious yank back from the now-taut line.

I used to take perverse pleasure in watching that nasty little dog reach the end of its line after building up all that forward momentum, only to have it quickly canceled out around its neck. The dumb dog never figured out not to do it.

Anyway.....

You could think of a prompt critical reactor with strong prompt reactivity feedback like that. If the reactor was "turned off" so to speak, with shutdown rods inserted, and sitting there with a neutron flux of 10^9 or something, then you removed the shutdown rods and the reactor went prompt critical and started increasing power (and flux) exponentially, from 10^9 to 10^10 to 10^11 and so forth--each of those flux settings being of such low power that temperature didn't change any.

But once the flux level reaches the level (like 10^15) where it makes appreciable thermal energy, reactivity feedback kicks in, and like my neighbor's dog, all of a sudden its forward "momentum" is arrested very quickly through reactivity feedback and the power stays constant at the heat removal rate, and the reactor sits there at "hot critical" waiting for you to start removing significant amounts of power.

Of course you wouldn't want to do this in an LWR because if you had the reactor in a prompt critical configuration it's likely that you could melt the fuel elements before the heat had a chance to cause reactivity feedback--if the primary reactivity feedback mechanism WASN'T in the fuel but in the moderator. But what Bob's told us intrigues me, because it sounds like the the main reactivity feedback mechanism in the LWR IS the Doppler in the fuel, which should be pretty prompt. Bob, can you help me understand this better?


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PostPosted: Jun 11, 2008 1:25 pm 
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Klaus Allmendinger wrote:
Negative reactivity feedback in a reactor that gets it by the moderator, requires first that heat energy needs to flow from the heat generating fuel to the material that provides the negative mechanism.

Just need to add the fuel density effect, which increases neutron leak rate (and probability of fission).
In the extreme, vapour bubbles in the fuel salt will make for a very leaky core, shutting down the reaction.


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PostPosted: Jun 11, 2008 2:29 pm 
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Totally blue sky stuff, probably does not belong here, but I have this (maybe crazy) notion:

Gaseous fuel (UF6 with 100% U233) has I assume too low a density at atmospheric pressure to become prompt critical. But could prompt criticality be achieved by introducing a strong ultrasonic standing wave field?


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PostPosted: Jun 11, 2008 2:54 pm 
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The difference in multiplication constant between critical (k-effective = 1.0000) and prompt critical (k-effective = 1.0064 for U235 I think) is really small, so its probably safe to say "can a critical arrangement be imagined for such-and-such an idea..."


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PostPosted: Jun 11, 2008 3:26 pm 
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Klaus Allmendinger wrote:
Totally blue sky stuff, probably does not belong here, but I have this (maybe crazy) notion:

Gaseous fuel (UF6 with 100% U233) has I assume too low a density at atmospheric pressure to become prompt critical. But could prompt criticality be achieved by introducing a strong ultrasonic standing wave field?


Gaseous reactors on UF6 have been thought about all the way back to the 50s at least (and continually rediscovered). I believe Florida State University still has people looking into them. It is surprisingly easy to get gaseous UF6 to reach criticality but the big problem is the highly corrosive nature of it at any useful temperature to get energy out.


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PostPosted: Jul 05, 2008 11:28 am 
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Speaking of delayed neutrons, does anyone know of a definitive value for the delayed neutron fraction of a U-233 thermal fission? I've seen the values that the data libraries tend to use (a little over 270pcm), but when you go digging into the actual data the evaluations for nu-d seem to be bi-modal. One group of values that yield that <300pcm values for beta, and another set of values for nu-d that are about 50% higher. That's a huge spread.

Brady and England have a good paper on this, and their conclusions seem to be to use the higher values, not the ones chosen for the data libraries.

Anyone have any insights into this one?

Thanks,

-Gary


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PostPosted: Jun 10, 2009 12:00 am 
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Thank you for this thread. I understand the necessity of delayed neutrons for stable operations. I begin to understand their source, but let me ask this to make it clearer. After a neutron is absorbed by U233, are the delayed neutrons from the fission of U233 (is the fission delayed), or are they the slightly delayed neutron emissions from the daughter products?

USPWR_RO wrote:
http://typhoon.jaea.go.jp/icnc2003/Proceeding/paper/5.21_121.pdf

They Typhoon write up and others seems to say they are from the daughter products.

This is my first post.
Rob

_________________
It is good to be splitting atoms again on the weekend. :wink:


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PostPosted: Jun 10, 2009 1:10 am 
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Delayed neutrons are associated with the beta decay of the fission products. Indeed, after prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission occurs. (wikipedia)

For 233U the delayed neutron fraction is 0.0026. In other words 99.74% of the neutrons are prompt and only 0.26% are delayed.

In an LWR the delayed neutron fraction dramatically helps with controlling the reactor.

In an MSR it makes much less difference. There are two dominate feedback mechanisms should the reactor output more power than you are extracting (ie. the temperature of the fuel is increasing). The fastest feedback mechanism is Doppler - the thermal motion of the fertile (Th232 and U238) in the fuel salt makes these atoms absorb more neutrons relative to the fissile. As long as the neutron spectrum includes a significant percentage of the neutron captures in the portion of the neutron spectrum that includes the resonance interval and you have fertile in the fuel you will have this control mechanism (some MSRs do not have fertile in the fuel). The Doppler effect is very fast (femto-seconds) - much faster than the prompt neutron generation.

The second term is the expansion of the fuel salt, decreasing the density and hence increasing the neutrons that escape the core and get to the blanket. The second effect is much slower (a few milliseconds). This is slower than the prompt neutron generation but still fast enough to control the reactor. This second effect is much more pronounced for reactors with no fertile in the fuel.


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PostPosted: Jun 10, 2009 9:09 pm 
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Lars wrote:
The second term is the expansion of the fuel salt, decreasing the density and hence increasing the neutrons that escape the core and get to the blanket. The second effect is much slower (a few milliseconds).


The expansion wave travels by the speed of sound of the medium. On the other hand, I guess that during a reactivity spike transient the heating would occur over a larger area simultaneously. This of course depends on the transient origin.

The most obvious transient origin for a MSR with a pumped core is the loss of pumping, which means that the delayed neutrons which were "swept" away by the flowing core salt, remain inside the core, increasing the reactivity. So delayed neutrons are inherent source of instability for a MSR. On the other hand the early experiments shown that overall temperature coefficient of reactivity is strongly negative, such that one can use the reactor to follow load.


Last edited by ondrejch on Jun 11, 2009 2:11 am, edited 1 time in total.

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PostPosted: Jun 11, 2009 12:26 am 
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Yes the odd thing is that the most stressful safety test is the loss of pumping and the resultant reactivity gain due to the recovered delayed neutrons. We'd have been better off with no delayed neutrons. I think the French overdid the stress test in that we can't stop the pump instantly so the injection of extra reactivity will take a significant amount of time. The good news is that the reactor behaved well even with the extreme stress test they did.


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