To shutdown a nuclear reactor by insertion of control rods, it is necessary to have a reactor shutdown margin which leads to subcriticality (e. g., >1.0 % Ak/k) with the exception of the most reactive control rod being stuck in the full-out position from the core (one rod stuck). For the reactor shutdown margin, burnable poison is partially mixed with fuel pellets to suppress the initial reactivity of fuel, and as well, a sufficient control rod worth is secured. The excess reactivity of core is restrained and controlled by both the control rods and burnable poison.

The worth per control rod is restricted below a constant value to secure safety against accidental withdrawal of control rods. The maximum reac­tivity insertion rate due to withdrawal of control rods is also limited to supervise and control the withdrawal safely.