Maximum scram reactivity capability

[Syed Alam]

I have found in a literature that "For our purposes, all control rods in PWR are assumed fully out during normal core operation, which provides the maximum scram reactivity capability at all times."

--What does it mean by "maximum scram reactivity capability"?

--Has there any conservative benefit of calculation in PWR if all rods are assumed to be withdrawn?

Thanks!

 

[Hiddencamper]

Recall that PWRs utilize boron and control rods for controlling reactivity.

At any given point in core life, if a scram is required all the control rods must drop in and achieve a required shutdown margin. This means there is a minimum required negative reactivity which the control rods must be able to insert to meet your shutdown margin requirements.

The greater length of control rod inserted in the core, the lower the scram capability is of your control rods. If you are at 100% power with no control rod insertion compared to 100% power with 10% rod insertion you’ll have less negative reactivity available for the scram because those rods are already in.

The tech specs for a pwr specify rod insertion limits, which are maximum allowed insertions for a given power level, to ensure you have sufficient reactivity to meet your shutdown margin requirements. If you go out of those limits you typically need to utilize boron to recover.

Running with all rods out provides the maximum shutdown reactivity capability which is one of several reasons why it is desirable.