question about PWR control strategy and history thereof 1

[patrick19]

In normal power operation, PWR light water reactors are apparently usually controlled by varying poison level (boron/boric acid) in the coolant/moderator water - not by moving the control rods, which are only used for startup and shutdown. (The coolant flow rate at power does not change, suggesting that a constant-average-temperature strategy is usually used, so that power demand would lower the primary-coolant inlet temperature, making the core more reactive and increasing power.)

Why is this? I have some guesses as to why this might be (like, preventing step-function reactivity insertions), but are there any references out there which point to how this design choice evolved?

Thanks...

 

[electricpete]

From steady state to steady state at power, steam demand controls power, reactivity controls temperature.

Completely withdrawing rods allows for more uniform fuel burnout. If rods were partially inserted, we have in that area a problem of increased power density/depletion below, and unused fuel at the top.

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[electricpete]

I guess I forgot to say that normal operating mode is usually with all rods withdrawn and primary temperature controlled by changing boron concentration. Controlling reactivity with rods would require inserting rods below the top.

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