The Pressurised Water Reactor (PWR) is the most popular choice for commercial nuclear power plants around the world. Heat produced at its reactor core is extracted by two adjoining closed circuits, with the primary reactor cooling circuit removing reactor heat by water to a heat exchanger, at which the heat is picked up by the adjacent secondary circuit to raise steam to drive a steam turbine that in turn drives an electrical generator.

For plant efficiency, the secondary circuit operates with steam under pressure and a temperature well above ambient. Heat transfer at the heat exchanger dictates that the reactor cooling circuit operates at an even higher temperature and therefore pressure (typically 15.5 MPa and 310°C on average), so that an operating reactor is immersed in liquid water all the time.

The reactor cooling circuit is sealed at ambient conditions after the nuclear reactor is loaded with uranium fuel. The pressure and temperature in the cooling circuit are then raised to normal operating conditions.

The path for raising pressure and temperature in the reactor cooling circuit, from cold to reactor criticality then power operation, is confined within a narrow band in the pressure-temperature diagramme for the reactor cooling water. Too low a pressure (or too high a temperature) will risk encroaching upon the saturation line of water, thus risking vapour formation in the reactor core and reducing local cooling effectiveness. Too high a pressure (or too low a temperature) will raise the temperature gradient and therefore stress the connecting line between the reactor cooling circuit and the pressuriser vessel which contains a bubble of steam to regulate the pressure of the circuit.

Recent advances in computing power and knowledge in engineering and materials have allowed studying reactor core behaviour in more detail to better define the design margin for the saturation line, and adopting material that may better withstand the temperature gradient and fluctuation in the connecting line to the pressuriser. It then permits widening the envelope for raising pressure and temperature of the reactor cooling system, which contributes towards improved operating safety and flexibility of the current PWR designs.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.