Low Power Density Reactor Core

The advantages of the AP600's low power density are achieved by making the core larger than conventional 600 MWe designs. As a result, the number of fuel assemblies is increased -from 121 to 145, so that many of the important nuclear and thermal parameters are improved by 25 to 30 percent over those of a standard plant of the same power rating. This results in lower fuel enrichments, less reliance on burnable absorbers, longer achievable fuel cycles, and an increase of more than 15 percent in departure from nucleate boiling (DNB) and loss-of-coolant accident (LOCA) margins. A larger gas plenum has been incorporated into the fuel to allow higher burnup.

Improved Overall Reactor Design

The combination of the radial reflector, the low power density core, and optimized fuel assemblies results in a 20-percent fuel cycle cost savings compared to a standard PWR design of the same power rating. The core design allows 18-month refueling cycles to be achieved with an 85-percent capacity factor (approximately 466 effective full power days per cycle) and requires no burnable absorbers other than for the first cycle of operation.

Gray Rods For Load Follow Without Boron

Another core design feature of the AP600 is the use of reduced worth control rods, which are termed "gray" rods, to achieve daily load follow without requiring daily changes in the soluble boron concentration. While the same control rods are used in existing PWRs for load follow, it is also necessary for these plants to process thousands of gallons of water per day in order to change the soluble boron concentration sufficiently to achieve a daily load follow schedule. The use of gray rods eliminates the need for processing the primary coolant on a daily basis and greatly simplifies operations using the boron systems.

With the exception of the neutron absorber materials used, the design of the gray rod assembly is identical to that of a normal control rod assembly. Thus, the design of gray rods, the fuel assembly, the reactor vessel, and reactor internals are all based on existing Westinghouse PWR technology.