NPP Working 75%: Limitations & Calculation Changes?

[libertad]

Is there any limitation for an NPP to work in 75% of power for a while eg. 3 or 4 months?
If so should be any changes in calculations documents?

 

[Hiddencamper]

The biggest limitation is typically economics. Due to the large O&M cost of maintaining a nuclear plant, it is typically best to operate them at full power.

There may be other limitations based on the specific plant design. For example, in boiling water reactors, typically when operating at less than full power, there are changes to the MCPR penalties. As power decreases the MCPR penalties increase. This penalty is offset by the fact that you are operating at reduced power, however sometimes it may require a change to the control rod sequence in order to ensure that adequate margin to thermal limits remain.

 

[montoyas7940]

If so should be any changes in calculations documents?

I'm not sure what you are asking here. Are you asking about licensed Rx thermal limits?

I can't comment on PWRs but a BWR can run at reduced power indefinitely. In the USA extended power reductions are sometimes required to meet EPA thermal pollution limits.

 

[Astronuc]

Is there any limitation for an NPP to work in 75% of power for a while eg. 3 or 4 months?
If so should be any changes in calculations documents?

Fermi 2 recently spent several months at 60% power, and some PWRs have had reduced power operation. It's usually undesirable economically, but it can be done. The fuel in the core does become deconditioned, or conditioned at the lower power, so power ascension should be done per operating restrictions for fully deconditioned fuel.

It's possible some transients, e.g., certain RIA events might be more severe from reduced power since the equilibrium Xe is lower.

 

[QuantumPion]

Is there any limitation for an NPP to work in 75% of power for a while eg. 3 or 4 months?
If so should be any changes in calculations documents?

Yes. A reactor designed to operate at full power which is than subsequently operated at reduced power for significant period of time will have differences in isotopic production (due to lower fuel temperature) and axial burnup (due to different moderator temperature profile) which will affect the core models and possibly safety analyses.

 

[libertad]

The biggest limitation is typically economics. Due to the large O&M cost of maintaining a nuclear plant, it is typically best to operate them at full power.

There may be other limitations based on the specific plant design. For example, in boiling water reactors, typically when operating at less than full power, there are changes to the MCPR penalties. As power decreases the MCPR penalties increase. This penalty is offset by the fact that you are operating at reduced power, however sometimes it may require a change to the control rod sequence in order to ensure that adequate margin to thermal limits remain.

for the case consider a PWR design

 

[libertad]

I'm not sure what you are asking here. Are you asking about licensed Rx thermal limits?

I can't comment on PWRs but a BWR can run at reduced power indefinitely. In the USA extended power reductions are sometimes required to meet EPA thermal pollution limits.

consider it as PWR. about calculations documents I mean Safety Justification reports and Album of neutronics and physics parameters of core.

 

[QuantumPion]

consider it as PWR. about calculations documents I mean Safety Justification reports and Album of neutronics and physics parameters of core.

The "album of neutronics and physical parameters" would definitely have to be reevaluated to operate at 75% power for months. As I mentioned, this is due to differences in isotopics generated due to the lower fuel temperatures as well as differences in burnup distribution due to different power shape. Our plant has a curve which estimates the reactivity difference vs. number of lost effective full power days and goes out to 30 EFPD. More than that and the estimates are too rough and an actual analysis is required. Note all of this is assuming the plant was designed to operate at full power. It is possible e.g. a load follow plant could be designed to operate at reduced power ahead of time.

Whether safety analysis has to be updated is hard to say but it is possible. For example, a spent fuel pool criticality analysis might have some assumptions about fuel depletion conditions which might not be met.