This is the function of the moderator, which may be heavy water, light water, or graphite
Neutrons are slowed down in a reactor to increase the likelihood of them causing fission reactions in nuclear fuel. Slower neutrons are more easily absorbed by the fuel, increasing the overall efficiency of the reactor. This process is achieved through a moderator, such as water or graphite, which helps reduce the speed of the neutrons.
The reflector in a nuclear reactor helps to reflect neutrons back into the reactor core, increasing the chances of nuclear reactions occurring. The reactor core is where the nuclear reactions take place, generating heat that is used to produce electricity.
Nuclear fission can be slowed by inserting control rods, such as boron or cadmium, into the reactor core. These control rods absorb neutrons, reducing the number available to initiate fission reactions and thus slowing down the rate of fission in the reactor.
Control rods are used in a reactor core to absorb neutrons and regulate the rate of fission reactions. By adjusting the positioning of the control rods, operators can control the power output of the reactor and ensure safe operation.
The layer of lead around the core of a nuclear reactor is known as the reflector. It helps to reflect neutrons back into the core, increasing the number available for fission reactions. This contributes to the overall efficiency and effectiveness of the reactor.
By inserting the control rods which absorb neutrons using boron, cadmium, or other material with a large neutron capture crosssection. If the reactor should begin to run out of control the SCRAM system will suddenly insert large amounts of neutron absorbing material, instantly stopping the neutron chain reaction.
In a nuclear reactor, materials such as water or graphite are commonly used as neutron moderators to slow down fast-moving neutrons to speeds where they are more likely to induce fission in uranium or plutonium atoms. Slowed down neutrons are termed thermal neutrons and are key to sustaining a nuclear chain reaction.
Control rods are devices that absorb neutrons and are used to control the speed of a fission reactor. By adjusting the position of the control rods within the reactor core, operators can regulate the rate of the nuclear chain reaction and manage the reactor's power output.
Lowering control rods in a nuclear reactor will result in the absorption of neutrons, which decreases the rate of fission reactions happening in the reactor core. This leads to a decrease in heat production and ultimately reduces the power output of the reactor.
The reason a nuclear chain reaction occurs is that one of the by products of a single reaction: the neutrons can start reactions of their own. So how do you stop a chain reaction? Stop these neutrons from causing any more reactions. This is done in nuclear reactors by placing control rods into the reactor core. These control rods are made of elements that can absorb the high energy neutrons without reacting themselves. Therefore, by moving these rods in and out of the reactor, the rate of reaction in a nuclear reactor can be easily controlled.
I think you must mean "criticality", which is a term applied to nuclear reactors meaning the neutron flux in the active core remains at a steady level, the actual level can vary from a low level equivalent to a few watts of power up to the reactor maximum allowable. At criticality the number of neutrons being produced every second by fission is just balanced by the number being captured in further fuel nuclei, in other core materials, the moderator itself, and escaping at the core boundaries.
A moderator in a nuclear reactor slows down fast-moving neutrons to increase the likelihood of fission reactions. By reducing the speed of neutrons, the moderator helps maintain a chain reaction within the reactor core. This process helps control the nuclear reaction and sustain the reactor's power output.