Why is water kept under high pressure in a pressurized-water reactor?

In a pressurized-water reactor (PWR), water is kept under high pressure primarily to prevent it from boiling while it circulates through the reactor core.12

Because the boiling point of a liquid increases as the surrounding pressure increases, maintaining high pressure allows the water to reach the high temperatures necessary to efficiently remove heat from the fuel without undergoing a phase change into steam. This pressurized water, known as the primary loop, acts as both a coolant and a moderator, which is essential for sustaining the nuclear fission chain reaction.1234

By keeping the water in a liquid state, the system can effectively transport thermal energy to a steam generator. There, the heat is transferred to a separate, lower-pressure secondary loop, which then produces the steam required to drive the plant's turbines. This design also provides a safety benefit by keeping potentially radioactive primary water contained within a closed loop, separate from the steam used to power the turbine.12