Generic PWR design

The PWR is the most commonly used reactor for electricity generation worldwide and is based upon preliminary concepts pioneered by the US Navy. In this concept, water remains liquid throughout the entire primary loop (i.e. no boiling in the core during normal operation). This ensures that minimal radiation shielding is needed to protect staff and personnel. FIG. 1 shows a typical, full PWR system.

These systems consist of a primary and a secondary fluid system. The primary system has water flowing directly through the nuclear reactor core past the fuel rods, where it is heated to the design hot leg temperature (typically around 329 ˚C). This hot coolant is maintained at a pressure of around 15.5 MPa to prevent boiling. The hot coolant is then run through a steam generator where secondary water at approximately 6.9 MPa is boiled to steam. This steam is sent to the turbine system, after which it is condensed and pumped back into the steam generator. The primary loop water, after boiling the secondary water and being cooled to 292 ˚C, is then sent through the reactor coolant pumps back into the reactor pressure vessel.

More detailed parameters can be found in Table 1 [2].

FIG. 1. Typical PWR system [3].

TABLE 1. BASIC OPERATING PARAMETERS OF A GENERIC PWR

CHARACTERSTIC VALUE CHARACTERISTIC VALUE

POWER REACTOR PRESSURE VESSEL

Thermal output 3800 MW Inside diameter 4.4 m

Electrical output 1300 MW Total height 13.6 m

Efficiency 0.34 Wall thickness 22.0 cm

CORE FUEL

Length 4.17 m Cylindrical fuel pellets UO2

Diameter 3.37 m Pellet diameter 8.19 mm

Specific power 33 kW/kg(U) Rod outer diameter 9.5 mm

Power density 102 kW/L Zircaloy clad thickness 0.57 mm

Av. Linear heat rate 17.5 kW/m Rod lattice pitch 12.6 mm

Rod surface heat flux Rods/assembly (17 × 17) 264

Average 0.584 MW/m² Assembly width 21.4 cm

Maximum 1.46 MW/m² Fuel assemblies in core 193

REACTOR COOLANT SYSTEM Fuel loading 115 × 10³ kg

Operating pressure 15.5 MPa (2250 psia) Initial enrichment % U-235 1.5/2.4/2.95

Inlet temperature 292 °C Equil. Enrichment % U-235 3.2

Water flow to vessel 65.9 × 10⁶ kg/h Discharge fuel burnup 33 GWd/Tu

STEAM GENERATOR REACTIVITY CONTROL

Number 4 No. control rod assemblies 68

Outlet steam pressure 1000 psia Shape Rod cluster

Outlet steam temp. 284 °C Absorber rods per assembly 24

Steam flow at outlet 1.91 × 10⁶ kg/h Neutron absorber Ag–In–Cd

Soluble poison shim Boric acid H3BO3

The three major components of the primary system are the reactor pressure vessel (RPV), the pressurizer, and the steam generators. A typical PWR RPV is illustrated in FIG. 2. The RPV has several water inlet nozzles, where coolant water from the reactor coolant pumps enters the outer annular region of the vessel. This water flows downward through the vessel until it reaches the bottom of the RPV where it turns 180˚ to flow past the core support assembly. This water flows past the fuel rods, where it is heated by fission energy, and then exits the core via the upper fuel assembly alignment plate. This hot water then enters the outlet plenum, where it is funnelled into water outlet nozzles and directed to steam generators. Note that for the PWR, the control rods are inserted through the top of the core, driven by the control rod drive mechanisms outside the RPV. The instrumentation tubes, however, are inserted into the bottom of the core through the bottom of the RPV. One of the challenges facing new reactor construction is the lack of current expertise in forging large high pressure single forging vessels, as is required in current designs for new PWRs.

The reactor core, found at the heart of the reactor pressure vessel, for this common design is comprised of 193 fuel assemblies arranged in a cylindrical pattern on top of the core bottom support plate. These assemblies, sketched in FIG. 3, contain 264 fuel rods arranged in a square 17 × 17 pattern, held in place by top and bottom nozzles and a series of grid spacers. The fuel rods slide into place within the top and bottom nozzles and are held by mechanical spring mechanisms located in the spacer grids along their length. In each fuel assembly, there are locations where fuel rods are replaced by control rod guide tubes. These guide tubes serve as a sleeve where control rods can be raised and lowered through the core to achieve the desired neutron absorption.

FIG. 2. Schematic of a PWR reactor pressure vessel, with key components labelled [4].

The steam generator component (illustrated in FIG. 3) primarily serves to provide sufficient heat exchange to facilitate boiling of the low pressure secondary water using the high pressure primary water. The primary water enters the steam generator in the bottom where it enters one leg of U shaped tubes and flows upward through the tube and back down the other leg where it exits the steam generator. As a note, the steam generator tubes are U shaped in order to prevent buckling stresses upon thermal expansion of the tubes. The secondary coolant flows on the shell side of the steam generator, and after undergoing phase transition, flows upwards through moisture separators and through the steam outlet to turbines.

(a) PWR fuel assembly. (b) U tube steam generator.

FIG. 3. Labelled sketches of PWR (a) fuel assembly and (b) U tube steam generator [4].

The primary system arrangement, illustrated in FIG. 4, is typically made up of the following components:

(a) Two, three, or four loops of piping (depending on the PWR design);

(b) Two, three, or four U tube steam generators;

(c) Two, three, or four reactor coolant pumps;

(d) One pressurizer;

(e) One reactor pressure vessel.

Note in FIG. 4 the orientation and arrangement of the steam generators, including the secondary loop inlets. The reactor coolant pumps are located on the cold leg, typically after the steam generator, while the pressurizer is located on one of the hot legs.

The pressurizer serves as the key pressure regulating component in a PWR. There is typically both liquid water and steam inside the pressurizer. Spray nozzles and heater elements are used in the pressurizer to regulate reactor pressure by modifying the amount of steam inside the fixed volume of the pressurizer. For example, if the steam pressure in the reactor decreases, the pressurizer will increase pressure in the system by using the heaters to boil some of the water in the pressurizer into steam. This steam mass increases, in turn increasing the system pressure.

If the reactor pressure increases beyond a setpoint, the spray nozzles spray cool water through the steam to condense it and decrease the total steam mass. This results in a reduction of the total pressure in the system. In this way, the pressurizer component, depicted in FIG. 4, controls pressure inside the PWR primary loop.

Although this lesson has described the systems, structures, and components for a typical PWR, there is very little uniformity from plant to plant. Additionally, there are many different PWR designs that are being developed and licensed for future operation. These designs vary greatly in concept and application, though all share the common features described above.

FIG. 4. Sketches of key components in a two loop PWR [4].