Pressurizer (nuclear power)

Put another way, the coolant must remain in the liquid state at all times, especially in the reactor vessel.

One is providing a place to monitor water level in the reactor coolant system.

The other secondary function is to provide a "cushion" for sudden pressure changes in the reactor coolant system.

The upper portion of the pressurizer is specifically designed to NOT contain liquid coolant and a reading of full on the level instrumentation allows for that upper portion to not contain liquid coolant.

This steam bubble provides a cushion for pressure changes in the reactor coolant system and the operators ensure that the pressurizer maintains this steam bubble at all times during operations.

This would mean that a sudden pressure change can provide a hammer effect to the entire reactor coolant system.