This 2D cross-section shows a pressurized-water reactor core feeding a steam generator and turbine-generator set. Control rod depth sets the neutron multiplication factor k — pull the rods out and k rises above 1, driving an exponentially growing chain reaction and rising core temperature, while inserting them past the equilibrium point drops k below 1 and shuts the reaction down. Coolant flow rate determines how fast that heat is carried out of the core; balance rod depth against flow to hold a steady k ≈ 1 and a stable power level, or push the core into overheating and watch the safety status flip to warning as core temperature climbs.