The work is aimed at analysing the impact of moderators on the ability to sustain a chain reaction within a nuclear reactor. A pressurized water reactor (PWR) was analysed for the three most common types of moderation: ordinary water, heavy water, and graphite. The objective of the analysis is to demonstrate how the choice of moderator affects the safety and performance of nuclear power plants. The method used for determining the self-sustainability of the chain reaction is through the criticality factor of a reactor of finite dimensions.