In usual practice, both a moderator and control rods are necessary to operate a nuclear chain reaction safely for the purpose of energy production. Cite the function of each and explain why both are necessary.

The process at the heart of a nuclear reactor's operation is the nuclear chain reaction. During this reaction, atoms of a fissile material such as Uranium-235 are split into smaller parts, a process known as fission. When a nucleus of this material is struck by a neutron, it becomes unstable and divides, releasing a significant amount of energy, but more importantly, more neutrons. These newly released neutrons can, in turn, collide with other fissile atoms, causing them to split and continue the cycle.

This process can potentially escalate quickly, enabling a large amount of energy to be released in a very short period, which is the principle behind both nuclear reactors and nuclear weapons. However, for energy production, the goal is to manage this reaction to maintain a controlled, steady state of energy release. This is achieved through the utilization of moderators and control rods, which balance the reaction to avoid the extremes of fizzling out or escalating uncontrollably.

At the center of a nuclear reactor's design is the nuclear reactor moderator, which plays a key role in maintaining a sustained nuclear chain reaction. As the neutrons produced from fission are moving at high speeds, they have a lower probability of causing further fission in other fissile atoms. The function of the moderator is to slow down these fast-moving neutrons to thermal speeds, where they are referred to as thermal neutrons. Thermal neutrons are more likely to be captured by other fissile nuclei, maintaining the chain reaction at a rate conducive to steady power production.

Common materials used as moderators include water, heavy water, and graphite. They are chosen for their ability to slow neutrons effectively without capturing them. By incorporating a moderator into the reactor design, engineers ensure that the chain reaction can be both initiated and sustained during operation.

While the moderator is crucial for sustaining the chain reaction, control rods serve an equally essential function in the nuclear reactor's operation: controlling the reaction's intensity. These rods are made of materials like boron and cadmium that are very good at absorbing neutrons. By absorbing neutrons, control rods reduce the number of neutrons available to cause further fission reactions.

Control rods can be inserted deeper into or withdrawn from the reactor core to adjust the neutron flux, which directly influences the rate of the chain reaction. This adjustability is vital to maintain the reactor at the correct power level, preventing the reactor from becoming overheated or shutting down the reaction entirely if necessary, such as during an emergency stop. Their precise operation is what ensures a nuclear reactor can produce energy reliably and safely, highlighting the necessity of control rods alongside moderators for the health and safety of nuclear energy production.