The PWR was originally developed as a submarine propulsion unit. As such it was designed to be small and responsive. Larger versions were developed for surface ships but these became the basis for the commercial PWR. The prototype was the Shippingport reactor (230 MW thermal, 60 MW electrical), which was developed as a joint AEC (US Atomic Energy Commission)/vendor (Westinghouse)/utility (Duquense Light Company) project. Over time other manufacturers have entered the market and there are a number of different designs but the basic concept remains the same. For the purposes of this chapter the modern Westinghouse four — loop design will mainly be used to illustrate the design characteristics.

10.3.1 Reactor Coolant System (RCS)

Figure 10.3 shows a typical Westinghouse four-loop nuclear steam supply system (NSSS). The NSSS consists of a reactor pressure vessel (RPV) containing the reactor core, which is connected by pipework to a number of steam generators, four in this case. Reactor coolant pumps (RCPs) located in the return (cold) legs of the circuit provide the circulation of water through the system. Connected to one of the RPV

outlet (hot) legs is the pressuriser. This is the only part of the circuit where a free surface exists.

The pressure in the circuit is maintained at a level such that boiling is virtually suppressed. (A small amount of nucleate boiling may occur in the top of the core.) This is achieved using the pressuriser, which is partly water filled with a steam ‘bubble’ above it. The pressure of the circuit is thus set by the saturation pressure of the water in the pressuriser, which is higher than the rest of the circuit. The pressure is controlled by means of electrical heaters in the bottom of the pressuriser, which can increase the saturation temperature, and sprays in the top, which spray cooler water from the cold legs into the steam space to reduce the pressure. The pressuriser is therefore at a higher temperature than the rest of the circuit and under normal operation a small spray flow is maintained to ensure a slow outflow of fluid into the circuit to establish a temperature gradient along the pressuriser surge line. The pressuriser is also fitted with power-operated relief valves to control larger pressure variations and safety relief valves to provide pressure protection for the reactor coolant system as a whole.