Prediction of reactor End of cycle

[libertad]

There is a technical document in reactor operation which contains neutronic calculations to predict reactor situation in different power lever and effective power days. Operators use this document which usually called Album of Reactor Core Neutronic to operate the reactor based on that. This document can also predict the date of reactor end of cycle using diminishing of Boron concentration until reaching to zero in the core but in many cases this prediction is not exactly the same with actual date; that is, approaching to end of cycle during reactor operation based on Boric Acid consumption is occurred sooner or later in comparison with calculation.
I want to know the main reasons of this discrepancy.

 

[Astronuc]

There is a technical document in reactor operation which contains neutronic calculations to predict reactor situation in different power lever and effective power days. Operators use this document which usually called Album of Reactor Core Neutronic to operate the reactor based on that. This document can also predict the date of reactor end of cycle using diminishing of Boron concentration until reaching to zero in the core but in many cases this prediction is not exactly the same with actual date; that is, approaching to end of cycle during reactor operation based on Boric Acid consumption is occurred sooner or later in comparison with calculation.
I want to know the main reasons of this discrepancy.

The discrepancy may have to do with unplanned or unanticipated power reductions or shutdowns, and some uncertainties related to core design and operation. Very few reactor cycles occur as planned.

 

[QuantumPion]

There is a technical document in reactor operation which contains neutronic calculations to predict reactor situation in different power lever and effective power days. Operators use this document which usually called Album of Reactor Core Neutronic to operate the reactor based on that. This document can also predict the date of reactor end of cycle using diminishing of Boron concentration until reaching to zero in the core but in many cases this prediction is not exactly the same with actual date; that is, approaching to end of cycle during reactor operation based on Boric Acid consumption is occurred sooner or later in comparison with calculation.
I want to know the main reasons of this discrepancy.

Possible sources of differences between modeled and predicted critical boron concentration:

a) Reactor power is different than assumed, leading to incorrect assumed fuel burnup. This could happen due to differences between gross electrical output and gross thermal power. These differences can be hard to track down due to having many interconnected systems, each with their own uncertainties.

b) Manufacturing differences of fuel - fuel loading or burnable poison loading could be manufactured different from what is assumed in the core model.

c) B10 depletion uncertainties - unknown boration or dilution sources altering B10 content of the reactor coolant system can lead to incorrect boron concentration.

d) Computer model bias - problems in computer model due to a variety of sources, could be cross-section error, computer code deficiencies/modeling assumptions, or differences between model and reality.

 

[QuantumPion]

The discrepancy may have to do with unplanned or unanticipated power reductions or shutdowns, and some uncertainties related to core design and operation. Very few reactor cycles occur as planned.

Power reductions or shutdowns do not affect critical boron concentration prediction. Critical boron is a measure of overall core reactivity and is a function of burnup. Start ups and shutdowns should have no affect on critical boron concentration (as long as B10 is correctly accounted for).

edit: to clarify - there are reactivity effects of decay products which can change the critical boron concentration of a core that is shut down - but they do not affect the total core reactivity vs burnup which is what I believe the OP was inquiring about.

 

[Astronuc]

Power reductions or shutdowns do not affect critical boron concentration prediction. Critical boron is a measure of overall core reactivity and is a function of burnup. Start ups and shutdowns should have no affect on critical boron concentration (as long as B10 is correctly accounted for).

edit: to clarify - there are reactivity effects of decay products which can change the critical boron concentration of a core that is shut down - but they do not affect the total core reactivity vs burnup which is what I believe the OP was inquiring about.

I was thinking about the calendar date as opposed to EFPD/EFPH or cycle burnup. Extended shutdowns do change the isotopic vector of the core as short-lived radionuclides decay (and that can change the parasitic absorption).

The codes are much better now than they were 10, 20, . . . years ago.