Modern Westinghouse type plants generally have four loops but earlier smaller plants were produced with two or three loops. These included many plants produced by companies such as Framatome (now part of Areva NP) and Mitsubishi under licence. These plants have one steam generator and reactor coolant pump per loop.

Two other US companies developed PWR designs: Combustion Engineering (CE — whose design is now owned by Westinghouse) and Babcock and Wilcox (B&W). In each case their designs have two steam generators and four pumps; i. e. the plants have four cold legs and two hot legs. The Combustion Engineering NSSS is shown in Fig. 10.4 and the B&W NSSS in Fig. 10.5.

The Soviet Union developed its own range of PWRs known as VVERs (Veda-Vodyanoi Energetichesky Reaktor). The smaller reactors, VVER 440s, have six loops each with a pump, loop isolation valves and (horizontal) steam generator. The plant can operate with a loop isolated. The larger, more modern VVER 1000 has four loops, still with horizontal steam generators, but dispenses with loop isolation valves.