13.5. A description of the PWR system starts with a summary of the significant design specifications as listed in Table 13.1. As an aid in com­paring the systems described in this chapter, a consistent format has been followed which indicates general specifications, as well as values for the core, fuel, thermal-hydraulic, and control features. Specifications for dif­ferent nuclear steam supply system (NSSS) models offered by the same manufacturer vary somewhat. Those listed in Table 13.1 are for a four — coolant-loop Westinghouse model, as described in a reference safety analy­sis report submitted to the NRC in 1974 [2]. This was about the end of the period when new reactors were ordered. Many currently operating Westinghouse four-loop PWRs were ordered somewhat earlier and have a core 12 ft in length to provide a thermal output of about 3400 MW. However, the 3800-MW version, which operates at the maximum power that the NRC will authorize, takes advantage of economy-of-scale savings. Such savings arise from the principle that fixed and operating expenses scale up as only a weak function of size. Therefore, if the plant is base loaded, the generating cost, on a unit energy basis, would be less for the larger plant.