In order to control how much power is generated in a nuclear reactor, the number of neutrons available to cause nuclei to fission must be controlled. Control rods which absorb neutrons must be inserted into or withdrawn from the core in order to control the chain reaction.
Less than half of the neutron flux in a nuclear reactor is produced by the emission of neutrons during the fissioning of uranium or plutonium. These neutrons are referred to as prompt neutrons. More than half of the neutron flux comes from the decay of radioactive isotopes that are the products of fission. These neutrons are called delayed neutrons. The half-life of the isotopes produced by fission range from milliseconds to minutes. In order to control the chain reaction, enough of the prompt neutrons must be absorbed by the control rods so that the critical mass necessary for the chain reaction will include the neutrons produced by the delayed neutrons. Otherwise, the reactor would go critical and melt down immediately.
Neutron moderators can slow down the fast neutrons released by fission and convert them into thermal neutrons. Thermal neutrons are more likely to trigger more fission than fast neutrons. So increasing the moderation will increase the power output and lowering the moderation will lower the power output.
In some reactors, the coolant functions as the neutron monitor so the power output can be increased by heating the coolant to make it less dense and less able to slow down the reaction. In other reactors, the coolant absorbs the neutrons and slows down the reaction. Heating the coolant will make it less dense and reduce its ability to absorb the neutrons so the power output will decrease.
In emergency situations when the reaction must be stopped as quickly as possible or scrammed in nuclear terminology, a neutron absorber such as boric acid is injected into the reactor core automatically. If the automatic systems fail, there are manual backup systems that can inject the neutron absorber.
In most types of reactors, there is a problem with the buildup of a radioactive isotope, Xenon-135 which is produced in the fission process. Xenon-135 absorbs neutrons slowing down or halting the chain reaction. If the reaction can be held at a high enough level, the xenon-135 can be destroyed as fast as it is produced. It is not always possible to keep the power output high enough and xenon-135 accumulation can be a serious problem.
Iodine-135 is also produced by the fission process. It has a half-life of seven hours and will quickly decay into xenon-135. After a reactor is shut down, the iodine-135 will continue to decay into xenon-135. This xenon-135 will make it difficult to restart the reactor for a few days until it is transmuted into non-radioactive xenon-136 with the absorption of an extra neutron in each nucleus. The decline in the neutron absorption by the transmuting xenon-135 requires the control rods to be inserted deeper into the core to absorb more neutrons.
Controlling a nuclear reactor is not a simple process. Each reactor design has its benefits and problems. Nuclear power generation is definitely a work in progress.