Doppler coefficient

[Rennhack]

Another name used for the fuel temperature coefficient of reactivity.

[Rad Sponge]

Another name used for the fuel temperature coefficient of reactivity.

I am reading about this right now, as I type.

So was this ever part of the Navy Reactor Principles discussion or did it just get "N/A"d? Did I miss something?

[cincinnatinuke]

We never really discussed FUEL Temperature Coefficients if memory serves me correctly.  I could be wrong.  We always seemed to talk of MODERATOR Temperature Coefficients in the Navy.

Here is a link if you want a quickie explanation:

http://www.tpub.com/content/doe/h1019v2/css/h1019v2_50.htm

[Rad Sponge]

Thanks man, that explains it a little better than my current text.

[Fermi2]

Due to the Enrichment of the Fuel and it's design Doppler wasn't very prevalent in Naval reactors. In truth the Doppler coefficient in civilian reactors isn't hugely high compared with other coefficients however it's always there and as soon as fuel starts heating up it starts feedback. This is important as in most cases there is a significant time constant before the moderator sees any heatup and reacts. That's why on a BWR there's a time delay on the power to flow trips. A commercial PWR doesn't need the time constants because it's Power to Flow trips are based on number of Pumps Running as a flat out comparison with Power.

If you have any questions about Doppler just ask. Self Shielding and Energy Boxes are quite the concept. Doppler gets more negative as the core ages due to the buildup of Pu and it gets less negative as the core heats up because the doppler bands lower however there are more of them due to self shielding going away. It always remains negative.

It effects the resonance escape probability.

Mike

[xobxdoc]

This reminds me of the battle of wits in " The Princess Bride"

[cincinnatinuke]

Due to the Enrichment of the Fuel and it's design Doppler wasn't very prevalent in Naval reactors. In truth the Doppler coefficient in civilian reactors isn't hugely high compared with other coefficients however it's always there and as soon as fuel starts heating up it starts feedback. This is important as in most cases there is a significant time constant before the moderator sees any heatup and reacts. That's why on a BWR there's a time delay on the power to flow trips. A commercial PWR doesn't need the time constants because it's Power to Flow trips are based on number of Pumps Running as a flat out comparison with Power.

If you have any questions about Doppler just ask. Self Shielding and Energy Boxes are quite the concept. Doppler gets more negative as the core ages due to the buildup of Pu and it gets less negative as the core heats up because the doppler bands lower however there are more of them due to self shielding going away. It always remains negative.

It effects the resonance escape probability.

Mike

There you go Jason.  BZ's first sentence confirms that and along with that link which discusses the effect being observed in low enrichment RX's.  So we would have/should have never seen that concept. 

Look up resonance absorption and grasp that concept.  Then apply what happens when you heat things up.  Hope things are well.

[Rad Sponge]

Thanks Broad. I have a clearer understanding of the material now.

Now how about a brilliant description of the Doppler Defect?

[tr]

Note that while the Doppler coefficient is small, it is very important in several limiting UFSAR accident analyses.  This is due to its rapid response as compared to the other coefficients (such as moderator temperature).  In the commerical PWR world, events like CEA ejection are mitigated almost completely by Doppler (in a CEA ejection initiated from critical at zero power, power goes from zero to about 2000% to zero in around a second).  The moderator temperature barely has a chance to change before the event is over.

[ddklbl]

So was this ever part of the Navy Reactor Principles discussion or did it just get "N/A"d? Did I miss something?

Chapter 3 of a certain T Manual.  That is a subject, among many, that the Navy is deeming Need-to-Know.  Whether or not RO's and EOOW's need to know is debatable, according to NR.  That is a subject worthy of its own thread.

[Fermi2]

Yeah give me a day or two. I have a couple excellent physical examples I use when teaching Reactor Theory, I just have to figure out how to explain them over the internet!

[Fermi2]

Jason,

What are you using to study doppler right now?

Mike

[Rad Sponge]

I have the GE BWR Fundamentals lesson plan. Its the one given at some point to the ILT class. It reads pretty much like a T manual from the Navy.

[rlbinc]

Thumbrule:

543 DMV

Its not a license plate or how many hours it takes for the Department of Motor Vehicles to change your address on your registration.
-1 x 10^-5 dk/k per degree F_fuel DOPPLER
-1 x 10^-4 dk/k per degree F_moderator MODERATOR
-1 x 10^-3 dk/k per % voids - for all you BWR guys or Thermal Limit violating PWR guys. VOIDS

Doppler is only 10% as strong as alpha T and only 1% as strong as alpha V.
BUT - and a big but (for all you rappers) - Fuel Temperature is in the thousand degree club and rises RAPIDLY on void collapse and other power increasing transients.
Doppler is Fuel Temperature dependent - no TIME is required for conduction of convection (as with alpha T and alpha V) so you get immediate negative reactivity in response to power increase - immediately, quickly, and first. Look for the word "FIRST" in responses on exam questions regarding reactivity and doppler.

Transient Analyses consider this guy on Turbine Trip Without Bypass, as a cladding integrity saver. 

[Fermi2]

Mike's Doppler Made Easy:

This is difficult to explain over the internet as I don't have the drawings I'd usually use to assist in my explanation.

U238 is makes up most of the fuel, it's a strong resonance absorber. Assume the U 238 is at a given temperature. If you look at it's resonance absorption chart/graph you'll see it has peaks, each of these peaks is an energy level that U238 is likely to absorb a neutron. These peaks are NET Energy. By that I mean a combination of the U238 KE and the Tron KE. Example, if the Resonance Peak is at 21 EV if the Tron is moving at 19 EV and the U238 is moving at 2 EV towards the Tron it's likely to get absorbed. The probability of absorbtion is dependent on two items, the net energy AND the time that net energy exists. Think of the second part as the "reaction" time. A physical example, let's assume you and I are playing catch. Then lets assume you have a strong probability of catching my fast ball. If I were to throw a fast ball from two feet away odds are you'd miss it, the time you have to catch the ball is minimal. Now go 90 feet away, you have a larger amount of time therefore your overall probability for catching that fastball is greater.

Now picture yourself on a hill. You have some bowling balls. As you look down the hill you see some ditches that cut so the narrow end is towards you. Lets say these ditches represent Resonance Peaks/Energy levels. Your Bowling balls are Trons. There are other Bowling balls scattered down the hill, these represent the moderator. As you look down the hill you see maybe 10 of these ditches all in a row. (Like looking at a horizon). Your job is to throw the ball past the ditches. After the ditches it is home free and the Ball can hit the pins. Any ball hitting pins comes back to you, the more pins you knock down the more balls you get back.  Also assume anytime the ball hits a ditch the ditch is sufficiently long to stop the ball making it unavailable to hit the pins. At this "energy" level the length of the ditches will never change. I think we can all agree if we threw the ball down the hill there's always a certain given probability the ball can end up in any ditch and be resonantly absorbed.

Now lets heat up the Core/Hill or cause a minor earth tremor. The heat is a respresentation of more KE in the core. The bowling balls on the Hill start moving and the ditches start moving. This represents the greater overall KE in the core. In other words there are more available Energy states than before. For simplicity lets say the average state of the ditches and bowling balls is 5EV. Whats this mean for resonance? Well for one it broadens the ditches. Now if a 16 EV Bowling Ball strikes a 5EV ditch (assuming the ditch is moving towards it) and it stays long enough we'll get an absorption. Also if we have a 26 EV Ball hit a ditch moving away at 5EV the net energy is 21 EV. Another possible Absorption. Whats this do to your ditch? It makes if look broader right? This is Doppler Broadening. While the ditch gets broader it's area can NEVER change so the 21EV portion got shorter. Any Tron or Ball at 21 EV might not get absorbed if it doesn't stay at that energy long enough to stay in the ditch. (Now remember due to the increased energy of the bowling balls that were on the hill the balls you are throwing might get knocked around and hit the 21EV part of the ditch at odd angles.

What's happened so far? Your ditch got broader and is not a 16 to 26 EV Ditch. That means your bowling ball has a possibility of being captured in this ditch at other parts besides the 21EV part. This is called "Off Resonant" Absorption. provided the Tron/Ball stays at that energy long enough.

Has your overall probability of Absorption changed? No, the area under the resonance curve has not changed. HOWEVER resonance absorption is not available to many more bowling ball energies. Look at it like you have more gutters on a bowling ally but they aren't as long.

Now if Doppler Broadening was the only thing at issue here there'd be no Doppler Coefficient because the resonance area hasn't changed. You could always chunk the ball harder and get through the resonance.

There's another affect known as self shielding. The fuel is a cylinder. Think if it as made up of concentric layers of fuel. The row of ditches I described was only the FIRST layer. Remember I said anything getting past that layer or ditches made it to the pins? What would happen if I added a second Layer?

Ok, Self Shielding: At any given energy a Tron will only go so far through the fuel. (Mean Free Path). So at energy level one it can only get through layer one. Now go to the second situation above. You'll have more neutrons moving around at higher energies, along with more U238 moving around. The first layer of ditches no longer "shields" the other layers. If I throw the ball faster I might get through the first layer, but after that I can hit another of those moderator bowling balls and get trapped in a second layer of ditches. Think of it this way, whenever power goes up, the Trons have more energy but another layer of ditches pops up and more bowling balls start showing up on the hill.. Given the multitudes of bowling balls and layers the number of balls captured by the ditches has to go up.

This is the Doppler coefficient. It always exists and is always negative. It becomes less negative as core temperature rises simply because at higher temperature levels the ditches might get extremely broad but they have a lot less depth so trons don't spend a lot of time at any energy level and eventually the whole pellet is near a thermal equilibrium so there's no more rows of ditches to confront the Trons.

Now I believe Jason is at a BWR so lets run through this quickly on a normal BWR Power Rise using Core Flow.

1: Core Flow Increases this creates a denser moderator, more "stationary" Bowling balls exist on the hill.

2: Do to increased moderation Trons spend less time at any given energy and resonance absorption goes down. Also Thermal Utilization goes up more fission/bowling pins being knocked down.

3: Uranium is a poor conductor of energy so fuel temperature goes or the hill starts shaking. The ditches start getting broader and more rows of ditches are available because the first row of ditches no longer shields other rows of ditches. Extra Bowling Balls start getting absorbed in the ditches. This starts turning the amount of Bowling Balls that make it to the pins, less pins start getting knocked down AND less Bowling balls are getting returned to you.

4: About 6 or so seconds later the cladding temperature starts rising heating the Water turning it into steam. This creates voids, making the moderator less dense or the stationary bowling balls get farther apart. The moving bowling balls are less likely to hit a stationary ball so they spend more time at energies that ditches can steal balls. Resonance absorption goes up.

5: Eventually a new steady state is reached.

As you can see, Doppler happens IMMEDIATELY and given the rise in Uranium Temperature it can start turning power relatively quickly.

I hope this helps!


Mike

[PapaBear765]

Power School definitely never mentioned any of the weird reactivities, nothing beyond what they wanted us to know for reactivity balance equations.  I guess the rest of the types of reactivity were in the T-9 (?) because my LPO on the boat one time gave a lecture and talked about power reactivity, pressure reactivity, and doppler.  I can't remember which one, doppler or power, but he said that it's the cause for temperature to decrease at a flank bell.

[Fermi2]

Naval Plants don't really have a Doppler Coefficient at all.

The reason Naval Plants don't really discuss Power Defect is the only predominant coefficient you have is Alpha T.

Mike

[Fermi2]

Thanks!! I try, I used to teach this stuff.

By the way, I noticed I put some NOTS where I meant NOW in that long post. If the sentence doesn't make sense chunk a NOW instead of a NOT.

LAST NIGHT OF NIGHTS. FINALLY!

Mike

[LaFeet]

Thanks Mikey... I relearned something... now I got all those OLD T Manuals starting to surface... guess Ill drink a few TQ shots and try to put them back to sleep.

BTW  -  OUTSTANDING explanation....dude, I wanna have you as my teach.