queston about xenon oscillation

[jinggang]

Dear all:

     In PWR, from my experience it seems more readily for the core to happen xenon oscillation at the end of cycle compared to at the beginning of cycle. what's the physics explanation for this phenomenon?
     Thank you all.

[HydroDave63]

something about more xenon, and less fuel and neutrons?

[alphadude]

http://www.tpub.com/content/doe/h1019v2/css/h1019v2_62.htm

some info on xenon



www.energy.kth.se/courses/4A1627/Material2005/PhysicsPart/03%20Neutron%20Poisons%20Rev%200.pdf -


this one is even better.

[landlubber]

Lots of good physics in the second link (which all nukes must love, even if they pretend not to )

However, the original question was essentially,"why is the effect more pronounced as fuel is depleted?"

HydroDave is on the right track: The concentration of fuel in the core goes down over time, but the concentration of Xe-135 does not go down as fast (see slide 18 for the equilibrium equation. Thermal flux goes up as fuel is depleted to maintain constant power) That makes the reactivity due to Xe-135 grow as fuel is depleted. And that makes the magnitude of the oscillations larger.  Now my head hurts.

[Fermi2]

None of you are correct or even in the ball park.

The reason is at BOL you have a Flux and XE Peak at the bottom of the core. So when you partially insert a rod you're dealing with a XEnon Burnout in one portion of the core and increase in the other.

As the Commercial Core ages two Flux peaks are formed. One is near the bottom, one near the top and they are not closely coupled, in other words your reactor is actually behaving as two reactors working loosely in conjunction with each other instead of  one very closely coupled reactor. The tighter a Reactor is coupled the more an action in one section affects another thus it's easier to control. With EOL having two flux peaks you essentially set up two different sets of Xenon Oscillations nwhich feed into each other so the effect is roughly quadrupled.

Having the higher flux at EOL really has nothing to do with it.
Fuel depletion doesn't either, it's the flux profile that matters, not the actual flux or fuel concentration in and of itself.

Mike

[jinggang]

I think Mike's explanation is reasonable.  For PWR, the xenon concentration at full power will decrease versus burnup although the flux has a little increase with burnup.
thank you all.

[Fermi2]

My explanation isn't reasonable, it's the only correct one given.

IIRC Xenon Concentration at EOL at Full Power is greater than at BOL because more Iodine is being produced due to the increase in Full Power Flux.

Oscillations are due to the location of the flux peaks which have a different profile at EOL than BOL.

Mike

[rlbinc]

Add AFD to the discussion and MY head will hurt, too.
God must love BWRs, look at what He puts PWR guys through.

[jinggang]

I found some other explanation on xenon oscillation in a paper "The Influence of Xenon-135 on Reactor Operation" written by Paul L. Roggenkamp
"The oscillations can be damped, in which case each succeeding cycle diminishes, or undamped, in which case the amplitude of each succeeding cycle increases. The dividing line or threshold in terms of power density) for undamped oscillations depends on several factors that are characteristics of a particular reactor charge.Characteristics that tend to lower the threshold are: lower fuel loading (e. g., end of fuel cycle vs.beginning); greater flatness of the power distribution(e. g., greater number of fuel assemblies operating at the same power); and smaller negative temperature coefficients of reactivity.Because of the large size, high power density, and great degree of power distribution flatness,most reactor charges that have been operated at SRS are susceptible to xenon oscillations despite their negative temperature coefficients of reactivity."

One question trouble me also: "why the wave with loosely coupled peaks is more readily to oscillate than tight coupled peaks " or "why the wave with two peak at the both end is more readily to oscillate than cosine wave"? Is there any hankbook or website analysing these issues?

[Fermi2]

I think I explained that already. Please read again and what you just discussed has nothing to do with EOL vs BOL.

Read about coupling and you'll understand.

rlbinc LOL Darn right. It's nice having a flux peak that moves uniformly as the core ages and voids are a wonderful thing. ON the other hand a loss of feedwater heating or a recirc pump trip while operating in the MELL/MEOD region can induce azimuthal power oscillations so I guess it's 6 of one, half dozen of the other.

Mike

[flamatrix99]

Add AFD to the discussion and MY head will hurt, too.
God must love BWRs, look at what He puts PWR guys through.

I am in BWR HLO class now. I am orginally from a PWR and I would go back in a heart beat. Too many interlocks and isolations. and only moving one rod at a time. wow. 

[Fermi2]

They're both fun machines to Operate. Boilers are easier to move around but you're right, there are SO many interlocks and isolations. I rather like the idea of Phase A and Phase B liker the Westingouse PWR uses. They both have their advantages, one thing I also liked about the BWR, once the Control Rods are in you never have to worry about Shutdown Margin anymore.

Wait till you get to the ATWS EOP and you have to figure out how to get rods in!

Mike