Control rods are inserted into the reactor core to absorb neutrons, thereby reducing the neutron population and slowing down the fission chain reaction. This allows for precise control of the reactor's power output and helps maintain safe and stable operation.

Reactivity is a measure of the neutron production rate in a nuclear reactor. It is determined by the balance between neutron production and neutron loss. Positive reactivity indicates an increasing neutron population and potential for a power increase, while negative reactivity indicates a decreasing neutron population and potential for a power decrease.

Fuel enrichment is not a method used to control reactivity during reactor operation. It is a process performed prior to reactor startup to adjust the concentration of fissile isotopes in the reactor fuel.

The moderator in a nuclear reactor is a material used to slow down neutrons produced by fission. By reducing the neutron energy, the moderator increases the probability of neutron absorption by fissile isotopes, thereby sustaining the chain reaction and controlling the reactor's power output.

An increase in fuel temperature leads to increased neutron production due to higher fission rates. This positive reactivity feedback mechanism can result in a power increase if not properly controlled.

The scram system is a safety mechanism designed to rapidly insert control rods into the reactor core, causing a sudden shutdown of the reactor. This is typically initiated in response to abnormal conditions or emergencies to prevent uncontrolled power increases and potential accidents.

Criticality refers to the condition in which a nuclear reactor is self-sustaining, meaning that the neutron production rate is exactly balanced by the neutron loss rate. At criticality, the reactor operates at a constant power level without the need for external control.

A decrease in moderator density leads to decreased neutron moderation and increased neutron leakage from the reactor core. This negative reactivity feedback mechanism results in a power decrease if not compensated for.

The control room in a nuclear power plant is the central location from which operators monitor and control all aspects of reactor operation. It houses various instruments, displays, and controls that allow operators to adjust control rod positions, regulate coolant flow, and respond to abnormal conditions.

The containment building in a nuclear power plant is a robust structure designed to prevent the release of radioactive materials into the environment in the event of an accident. It typically consists of multiple layers of concrete and steel and is equipped with various safety systems to mitigate the consequences of potential incidents.

Digital control is not a type of nuclear reactor control system. Nuclear reactor control systems typically employ analog or automatic control mechanisms.

Startup refers to the process of gradually increasing the power level of a nuclear reactor from a shutdown condition to a desired operating level. This is typically done by carefully withdrawing control rods and adjusting other reactor parameters.

Coolant is not a component of a nuclear reactor control system. It is a substance used to remove heat from the reactor core and prevent overheating.

The emergency core cooling system (ECCS) is a safety system designed to provide backup cooling to the reactor core in the event of a loss of normal cooling. It typically consists of multiple redundant systems that can inject water or other coolants into the reactor core to prevent overheating and potential damage.

Shutdown refers to the process of gradually decreasing the power level of a nuclear reactor from an operating level to a shutdown condition. This is typically done by inserting control rods and adjusting other reactor parameters.