During a routine design review of a commercial pressurized water reactor (PWR), safety engineers inspect the configuration of the core components to ensure stable and safe operation.

Component W consists of materials with a high neutron absorption cross-section (such as boron or cadmium). These can be dynamically raised or lowered to regulate the thermal power output of the reactor or to initiate an emergency shutdown (scram).

Component X is a medium surrounding the fuel assemblies. It reduces the kinetic energy of fast-moving fission neutrons through elastic collisions, bringing them to thermal speeds so they have a much higher probability of inducing further fission in uranium-235.