1.3.1 General Core Design

[1] Features of PWR Core

PWRs operate in an indirect cycle in which the primary coolant heated and pressurized in the reactor core is cooled through steam generators and pumped under high pressure into the reactor core again. The steam generated in the secondary loop is fed through a turbine which drives an electrical generator. The coolant as a neutron moderator is pressurized to about 15.5 MPa in the pressurizer and forced to circulate to cool the core with a single-phase flow with hardly any occurrence of boiling. Therefore, the moderator density variation in the core is small. The boric acid as a neutron absorber is dissolved into the moderator.

Figure 3.30 shows a cross-sectional view and structures of a PWR core which has an arrangement of 193 fuel assemblies with fuel rods of 17 x 17 type array. Each fuel assembly consists of 264 fuel rods, 24 control rod guide tubes (guide thimbles), and 1 core instrumentation tube (guide thimble). The control rod guide tubes are used for insertion of control rod clusters and burnable poison rods.

[2] Basic Criteria of Core Design

The main target of the reactor core design is same as that of BWR core design, to improve cost performance while securing reactor safety. The security of reactor shutdown capability, reactivity insertion limit, self-controllability, fuel failure prevention, power distribution restriction, and stability are all required for safety. Table 3.11 introduces the basic criteria to be considered in the PWR core design.