The amount of negative reactivity by which the reactor is, or would be, subcritical after a full insertion of rods, conservatively assuming the most reactive rod fails to insert. Technical specifications set a minimum, typically around 1 to 2 percent delta-k over k, and operators verify it during startups, boron changes and cooldowns, since cooling the moderator or decaying xenon can erode it.
Before we cool down past 400 degrees, verify shutdown margin with the new boron sample.
Also heard as
- SDM
Related terms
Scram
The rapid automatic or manual insertion of all control rods to shut a reactor down in seconds.
Control rod
A movable rod or blade of neutron-absorbing material used to control reactor power and shut the reactor down.
Reactivity
A measure of how far a reactor is from criticality, positive when power will rise and negative when it will fall.
LCO
Limiting condition for operation: a technical specification requirement for equipment or conditions needed to run the plant.