Axial xenon distribution in a PWR

[ulriksvensson]

Hi all. I discovered a thing the other day that I've known for a long time but never really thought about.

If power distribution is shifted towards the bottom of the core the xenon concentration is shifted towards the top of the core. This is equilibrium conditions, i.e. only very small axial oscillations. Can someone explain this? If you raise power, xenon (equilibrium) concentration goes up. Why doesn't the axial xenon distribution have the same shape as the power distribution at equilibrium?

The difference between the "axial xenon offset" and axial offset vary some with burnup, but overall they are "opposite" in shape. Also, in coast down the tend to meet, suggesting a power level where the axial xenon distribution gets the same shape as axial power distribution.

Thanks in advance.

//Ulrik

 

[rmattila]

Fast-to-thermal flux ratio is one thing that has an effect in that direction: at the top of the core the average neutron velocities are faster; thus less neutrons are captured by Xenon.

Never done any quantitative analysis of the magnitude of that effect, though,

 

[ulriksvensson]

Yeah, that's probably it. Should've thought of that. It also explains the phenomenon at coast down when T-in is lowered.

 

[Astronuc]

Fuel temperature is slightly hotter in the top of the core, and moderator density is slightly lower in the top than the bottom of the core. Typical Tin ~ 285-293 C, and exit is something like 320-330 C depending on flow rates and power density.

AO is also affected by burnable absorber distribution. Some plants may have a slight axial offset on the IFBA loading. I haven't heard that for Gad though.

I have had some experience with looking at the Xe swings at BOC. The magnitude is affected by RCCA program and ascension rate. I believe there is also an effect from delta AO between EOC and BOC, as well as outage length.

 

[QuantumPion]

Fuel temperature is slightly hotter in the top of the core, and moderator density is slightly lower in the top than the bottom of the core. Typical Tin ~ 285-293 C, and exit is something like 320-330 C depending on flow rates and power density.

AO is also affected by burnable absorber distribution. Some plants may have a slight axial offset on the IFBA loading. I haven't heard that for Gad though.

I have had some experience with looking at the Xe swings at BOC. The magnitude is affected by RCCA program and ascension rate. I believe there is also an effect from delta AO between EOC and BOC, as well as outage length.

I believe what the OP was inquiring was how can you have a xenon offset different from your axial offset in equilibrium conditions (i.e. steady state, no xenon transient). I agree with rmattila's answer - the reason is that xenon is more sensitive to a harder neutron spectrum than the fuel is. At the top of the core, you have a greater production of xenon compared to absorption, and thus a balanced positive xenon offset.