13.6. The reactor vessel and internal components for a typical PWR are shown in Fig. 13.1. Control rod drive mechanisms are integral with the removable upper reactor head. Magnetic couplings for these drives are used across the pressure boundary. There are steel pads integral with the coolant nozzles for vessel support. These pads rest on steel base plates atop a structure attached to the concrete foundation. The low-alloy carbon steel vessel is clad on the inside with a minimum thickness of 3 mm of austenitic stainless steel. Neutron shield panels are attached to the core barrel opposite the core corners, where the flux tends to be higher.

General Thermal-Hydraulic Power Thermal 3800 MW Electrical 1300 MW Specific power 33 kW(th)/kg U Power density 102 MW(th)/m3 Coolant Pressure 15.5 MPa(a) (2250 psia) Inlet temp. 293°C (560°F) Outlet temp. 329°C (624°F) Flow rate 18.3 Mg/s (1.45 x 108 lb/ hr) Mass velocity 3.67 Mg/s • m2 (2.7 x 106 lb/hr-ft2) Rod surface heat flux Ave. 0.584 MW/m2 (1.85 x 105 Btu/hr-ft2) Max. 1.46 MW/m2 (4.63 x 105 Btu/hr-ft2) Linear heat rate, ave. 17.5 kW/m (5.33 kW/ft) Steam pressure 7.58 MPa (a) (1100 psia) Core Length 4.17 m (13.7 ft) Diameter 3.37 m (11.1 ft) Fuel Rod, OD 9.5 mm (0.374 in.) Clad thickness 0.57 mm (0.0225 in.) Pellet diameter 8.19 mm (0.3225 in.) Rod lattice pitch 12.6 mm (0.496 in.) Assembly width 214 mm (8.43 in.) Rods per assembly 264 (17 x 17 array) Assemblies 193 Fuel loading, U02 115 x 103 kg (2.54 x 105 lb) Ave. feed enrichment —3.3% Ave. core enrichment —2.8% Burnup 2.85 TJ/kg (33,000 MW • d/t) Control Rod cluster elements 24 per assembly Control assemblies 61 full length, 8 part length

13.7. The coolant water leaving the steam generators flows down the annular region (downcomer) between the vessel wall and the lower core barrel and then upward through the core. Flow holes in the lower core plates are sized to permit a higher coolant flow rate through the center of the core where the power generation is greater than at the periphery. After passing through the core, the coolant enters a common upper plenum and exits through the outlet nozzles to the steam generators.

13.8. Thermocouples entering through the vessel head indicate coolant outlet temperatures from fuel assemblies at selected locations. Movable neutron flux detectors, in guide tubes entering through the bottom of the vessel, can be inserted at various points to determine the power distribution in the core.

13.9. The core contains about 200 assemblies of fuel and control rods; in most of the Westinghouse designs, each assembly consists of a 17 x 17 array. Of the 289 spaces available in an assembly, 264 are occupied by fuel rods; the remaining spaces contain guide tubes (thimbles) for control rods with a central tube available for instrumentation. About one third of the assemblies in the core include control rods; in the other assemblies the guide tubes are partially blocked. The fuel rods in an assembly are sup­ported and separated by grid assemblies at intervals along the length (Fig. 13.2). The top and bottom “nozzles” control the flow of coolant water through the fuel assembly. Because the assemblies are open at the sides, lateral flow of coolant is possible from one assembly to another. This arrangement is in contrast to that in a BWR where the fuel assemblies are enclosed by vertical “channel separators” (§13.32).

13.10. The ratio of hydrogen atoms (in the water) to uranium (in the fuel) is an important parameter in PWR core design [3]. This ratio deter­mines the neutron spectrum which, in turn, affects the extent of resonance capture, the Doppler coefficient, and the fraction of fast-neutron fissions. An increase in the H/U ratio results in decreased resonance capture and hence an increase in reactivity. This means that a lower uranium enrichment is required in the fuel. On the other hand, should the H/U ratio be de­creased, the harder spectrum would result in an increase in the conversion of uranium-238 to plutonium-239. However, with no plutonium being sal­vaged through reprocessing, the present trend is to increase the water/fuel ratio. Present designs have atomic H/U ratios in the range 4.0 to 4.3. This corresponds to a H20/U02 volumetric ratio of about 2. The assemblies listed in Table 13.1 have a water/fuel volumetric ratio of 1.95. One newer design for this 17 x 17 lattice features rods having an outside diameter of 9.144 mm, which would provide a 6 percent increase in the water/fuel volumetric ratio.

13.11. Thermal and hydraulic considerations influence core design. The fuel-rod diameter determines the lattice spacing and thus affects the resist­ance to coolant flow. The selected rod diameter depends on such param­eters as fabrication and cladding costs, desired core power density, and surface heat flux limitations. As the diameter of the fuel rod is decreased, the specific power and power density are increased, assuming a constant volume ratio of water to fuel. The linear heat rate remains constant, but the number of rods per unit core volume is increased as the diameter is decreased. A decrease in fuel-rod diameter, however, also results in an increase in the surface heat flux and thus a closer approach to the DNB

condition (Chapter 9). This factor and the increase in fabrication cost of the larger number of fuel rods set a lower limit to the fuel-rod diameter.