pressuriser

A pressuriser, sometimes spelled pressurizer in American English, is a component of the primary coolant system in many pressurized water reactors (PWRs). Its main function is to maintain the reactor coolant pressure within the high- pressure range necessary to prevent boiling of the primary loop water as heat is added in the reactor core.

It is a dedicated vessel connected to the hot leg of the primary loop. Inside it lies a pool of liquid water and a separate steam space, with a cushion of non-condensable gas above the liquid, typically nitrogen. The water level is controlled by feedwater and spray lines; electric heaters in the steam space raise pressure by generating more steam, while a spray system injects water into the steam space to condense steam and lower pressure. Pressure is monitored and controlled by balancing heater power, spray rate, and relief valves designed to vent excess pressure.

Functionally, the pressuriser acts as a buffer for thermal expansion and load changes, helping keep the primary circuit at a relatively constant pressure. This arrangement prevents boiling in the reactor core and supports stable heat transfer between the fuel and the coolant. Safety features, including relief valves and automatic controls, protect against overpressure and contribute to the stable, single-phase operation of the primary coolant loop.