Shutdown Margin

Question 1

Define Shutdown margin.

Answer 1

• SHUTDOWN MARGIN is the amount of reactivity by which the reactor is subcritical or would be subcritical from its present condition assuming all full strength CEAs fully insert with the exception of the highest worth CEA (fully withdrawn), there is no change in part strength position.
  
  • The “all full strength CEAs inserted” graphs in the COLR assume the highest worth rod is stuck out.

Question 2

What does meeting SDM requirements provide to OPS?

Answer 2

SDM requirements provide sufficient reactivity margin to ensure that acceptable fuel design limits will not be exceeded for
normal shutdown and anticipated operational occurrences (AOOs).

Question 3

How is SDM assured during power operations?

Answer 3

During power operation, SDM control is assured by operating with the shutdown CEAs fully withdrawn and the regulating CEAs
above the PDILs (LCO 3.1.7).

Question 4

How is SDM assured during shutdown and refuelign modes?

Answer 4

When the unit is in the shutdown and refueling modes, the SDM requirements are met by
adjusting boron concentration.

Question 5

What is the most limiting condition with regard to SDM for the following:

> 350F

< 350F

Answer 5

≥350°F is based on the Main Steam Line Break (MSLB) at EOL and the
resulting cooldown which adds a significant amount of positive reactivity

At <350°F the limiting accident is a boron dilution.

Question 6

What additional protections does SDM provide in modes 3, 4, or 5?

Answer 6

Inadvertent boron dilution.
● Uncontrolled CEA withdrawal in the source range.
● Startup of an idle RCP cold water accident.
● CEA ejection.

Question 7

What must be done to ensure SDM if more than 1 CEA is stuck with RCTBs closed?

Answer 7

If more than one CEA is stuck with RTCBs closed, a calculation is done to determine the worth of the untrippable CEAs.

Question 8

What is the refueling boron minimum concentration?

Answer 8

Refueling boron concentration is required to be ≥3000 ppm.

Question 9

How does the required SDM change over the course of a shutdown/cooldown

Answer 9

Required SDM in the COLR curves increases from 565°F to 500°F due to positive reactivity inserted during the cooldown
(negative MTC). The required SDM decreases from 500°F to 350°F as the required SDM lowers from 5%Δk/k to 1%Δk/k.
Finally, the required SDM increases over the remaining cooldown due to the negative MTC, as required SDM remains constant
at 1%Δk/k.