What is meant by criticality in nuclear reactors?

• When a reactor’s neutron population remains steady from one generation to the next (creating as many new neutrons as are lost), the fission chain reaction is self-sustaining and the reactor's condition is referred to as "critical".
• Criticality is a nuclear term that refers to the balance of neutrons in the system. When the  neutron population remains constant, this means there is a perfect balance between production rate and loss rate, and the nuclear system is said to be “critical.”
• The power of a reactor is directly proportional to the neutron population. If there are more neutrons in the system, more fission will take place producing more energy. When a reactor is starting up, the neutron population is increased slowly in a controlled manner, so that more neutrons are produced than are lost, and the nuclear reactor becomes supercritical. This allows the neutron population to increase and more power to be produced. When the desired power level is achieved, the nuclear reactor is placed into a critical configuration to keep the neutron population and power constant.  Finally, during shutdown, the reactor is placed in a subcritical configuration so that the neutron population and power decreases.  Therefore, when a reactor is said to have “gone critical,” it actually means it is in a stable configuration producing a constant power.

What is re-criticality in nuclear reactors?

A reactor is maintained critical during normal power operations. In other systems, such as a spent fuel pool, mechanisms are in place to prevent criticality. If such a system still achieves criticality, it is called “re-criticality”. Boron and other materials, which absorb neutrons, are in place to make sure that this re-criticality does not occur. The added neutron absorbers substantially increase the rate of loss of neutrons, to ensure a subcritical system.

Overview

Critical: power output stays constant

Super-critical: power output is increasing

Sub-critical: power output is decreasing, If electricity is not needed, then we need to absorb neutrons to slow down the fission reaction, that is a case of sub criticality