Role of the control rods during the core cycle

[wronski11]

Dear all,

I would like to learn more about what happens to CRs during the reactor cycle. At the beginning of the cycle large excess reactivity is present in the core and has to be compensated with burnable absorbers (lumped in rods and WABA coating), chemical shim and CRs. There are several things I am not sure about. I presume that CR are just gradually extracted from the core as the cycle progresses. But is it rally as simple as that? I heard that some rods remain in the core throughout the entire cycle. In my opinion this is done to counter some sort of excess reactivity that remains even towards the end of the cycle. (Things are even more confusing, since in a PWR different CR are present i.e. half- length and shut down rods. In BWR only one type of absorber crosses are present.) Eventually the absorber in the rods will start to burn-out. So I guess that CR are exchanged to prevent swelling.
Therefore, I would like to ask what happens to a CR during the cycle.

 

[Hiddencamper]

It depends on the plant design.

In PWRs its popular to have the control rods fully removed at full power and to use boron for bulk temperature and reactivity control. The control rods are only used for Tave control during load changes, until the xenon transient is over and boron concentration is adjusted. As PWR plants deplete their HER, bulk average reactor coolant temperature will slowly drop, and dilution of boron will be used for maintaining temperature in band.

For BWRs, rods can be used for axial flux shaping and bulk reactivity control. In general, BWRs use gadolinium and core/fuel design to eliminate the need for axial control. The rule of thumb is a rod in the bottom 1/3rd of the core is only for axial control, and in the upper 1/3rd for power control, with rods in the middle depending on core design and burn up. Because of gadolinium shaping, normally we operate with no shallow rods in the core (bottom 1/3rd of core...remember BWR rods go in the bottom)

In a BWR, we always have some rods deep in the core until coast down. Every couple months you perform a sequence exchange on your control rods, or a rod pattern adjustment, so that the fuel is evenly burned up and to minimize bowing and shadow effects. Moving deep control rods will raise and lower the rod line, and reactor power is then controlled with Recirculation flow. While there is sufficient hot excess reactivity, the target control rod pattern is set, and is only adjusted for sequences exchanges. When you deplete your HER, you start using rod patterns with less deep rods in the core, and notch rods out for rodline maintenance.

At the end of cycle a BWR will have all rods out. Usually Feedwater temperature is also reduced to help raise reactivity for the last part of the cycle until the outage.

 

[wronski11]

It depends on the plant design.

Thank you for the answer. It helped me greatly in improving my knowledge. As far as I understand, control rods are not the primary meaning for excess reactivity control. There are more for regulating the power shape and level? Unfortunately,I don't understand what is the meaning of the term rod line and rod line maintenance.
I am also very curious how exactly Xenon oscillations are dampened. After time Doppler and void reactivity (especially in BWR) will stop them, but what is the role of the CRs in this problem. I imagine this to be a computer adjusting the rod insertion level and position to counteract the resulting power peaking.

 

[Hiddencamper]

For a PWR, xenon oscillations can be an issue. There is a measurement called "Delta-I" which is the difference in iodine (a xenon precursor) in the core. There are allowable limits on delta-I, and operators have to take action to maintain delta-I in band and minimize its effects during power maneuvers.

Using boron and turbine load to adjust reactivity has very little effect on delta-I and is the preferred way to adjust power and reactivity. However if a rapid power change is needed, the control rods may be left in automatic and allowed to respond to temperature program changes. The iodine instability that occurs will need to be suppressed by the operators. There are different techniques for inserting and withdrawing control rods at specific points in the oscillation to forcibly dampen the oscillation. This is done manually, but a computer program can be used to assist in the appropriate timing and amount of adjustment needed. Once the instability is suppressed, boron is used and rods are removed as they are able.

For bwrs, there are no concerns with spatial xenon oscillations. They are entirely self suppressing thanks to the void effect. We pay no attention to xenon oscillations. We do look at the localized effects on Linear heat generation, MCPR, and preconditioning margin, as you may have a localized xenon related effect during power Ascension and ramping.

Typical BWRs don't have computer control over their rods. The new ESBWR does and some of the gen 3 plants with fine motion control rod drive have some load following characteristics. But the majority of bwrs have locked piston drive units which can only move in 6 inch increments, a single rod or a gang (up to 4) at a time. We will run prediction models on the core monitoring computer to assist in figuring out what we need to do on the fly for power and shape requirements, but for the most part the target rod patterns are determined by the core designers before the fuel is even made.

As for "rod line", reactivity in a BWR is controlled through either control rods or core flow. The rod line is a linear line that shows how power will move with changes in core flow. At the 80% rod line, that means at 100% core flow I will be at 80% power. Withdrawing control rods will raise rod line. At the 105% rod line, that means if I were at 100% core flow I would be at 105% power. As your available reactivity drops, rod line lowers, and when necessary, control rods will be withdrawn to raise for like back up.

The closest a typical BWR has to automatic reactivity control is with the core flow control system. The later BWRs had an integrated control system that worked with the generator load controller and the steam pressure regulator to allow for rapid load changes with flow only. No US plant that I know of uses these features anymore, and the automatic load following features were never installed in the first place.

 

[wronski11]

Thank you again for the information. Reading your replies to my question, I came to the conclusions
1) PWRs operate without CRs at full power and CRs are not often used - in contrast to BWR
2) CRs in PWR are not used for excess reactivity control under normal operation
3) Control of the BWRs is more complicated, CRs are inserted for longer periods and are used for reactivity control.

 

[Hiddencamper]

Thank you again for the information. Reading your replies to my question, I came to the conclusions
1) PWRs operate without CRs at full power and CRs are not often used - in contrast to BWR
2) CRs in PWR are not used for excess reactivity control under normal operation
3) Control of the BWRs is more complicated, CRs are inserted for longer periods and are used for reactivity control.

That's correct!

 

[wronski11]

Thanks once again for the information. I was actually confused, because in an exam question I was asked whether CRs can be used for reactivity control. I wrote that CRs are fore fine tuning the power and absorbers (Boron and lumped absorbers) are for homogeneous reactivity suppression throughout the core.