Source: Westinghouse Electric.

Source: Westinghouse Electric.

FIGURE 1 Flow diagram for pressurized water reactor coolant system (Westinghouse Electric)

All of the pumps and vital equipment associated with pipe rupture (that is, the emergency core cooling system) is supplied with diesel generator electrical power backup. If needed, the diesel generators will accept the various pump loads sequentially at intervals of a few seconds until all needed equipment is on line.

Other pumps serve other systems. Spent fuel pit pumps provide the necessary cooling of the fuel elements that have been removed from the reactor. Resin beds, which are a part of the water purification system, are flushed to a storage tank by spent resin sluicing pumps. An evaporator package, partly for removing boron from the primary water, is supplied by recycle evaporator feed pumps. Chilled water pumps supply the boron thermal regeneration system. Similarly, other pumps, some not listed in Table 1, support auxiliary systems.

BWR Plants In boiling water reactor (BWR) plants, active boiling takes place in the nuclear core and steam is piped to the turbogenerator (Figure 5). Typical reactor water conditions are 1000 lb/in2 gauge (6.895 MPa) and 550°F (288°C). In the United States, the plant is usually arranged with a low-leakage containment vessel completely surrounding a dry well and a pressure-suppression pool (Figure 6). The containment vessel is a cylindrical steel or concrete structure with an ellipsoidal dome and a flat bottom supported by a reinforced concrete mat. The containment forms a security barrier and prevents the escape of radioactive products to the atmosphere if an accident should occur.

Table 2 shows the principal pumps used in BWR plants together with significant characteristic data.

To assure a high reactor flow rate and to avoid local areas of core overheating, internal jet pumps have been used in all but the earliest U.S. BWR plants. These jet pumps are driven by large-volume, medium-head recirculation pumps. Variable flow rate is achieved either by flow control valves or by variable-speed motors driven by motor generator sets. The latest designs employ a flow control valve. The use of jet pumps decreases the size of the external loop piping and pumps and provides a core reflood capability in the event of pipe rupture. The main recirculation pumps are not required for emergency cooling. In

FIGURE 2 PWR reactor vessel (Westinghouse Electric)


FIGURE 2 PWR reactor vessel (Westinghouse Electric)

normal operation, the recirculation pumps operate in conjunction with the jet pumps, which are wholly contained in the reactor vessel. The purpose of the jet pumps is to increase the flow from the recirculation pumps at reduced head for reactor cooling. The jet pumps have no moving parts. In addition to their normal service, they also play a role in the natural circulation of the reactor water during emergency cooling.

Several subsystems operate in support of the recirculation system, and each contains one or more pumps. Reactor water cleanup pumps are used in a filter-demineralizer system to remove particulate and dissolved impurities from the reactor coolant. This system also removes excess water from the reactor. The control rods are operated hydraulically with water pressure provided by the control rod drive pumps. The pumps are located in an auxiliary building, and the fluid is piped to the control rod drive units, which are posi-

FIGURE 3 Flow diagram for chemical and volume control system (Westinghouse Electric)

tioned directly under the reactor vessel. For those components that require constant cooling, such as the recirculation pump motors and other equipment located in the containment, auxiliary, fuel, or radwaste buildings, closed cooling water pumps furnish the necessary flow. These pumps are located in an auxiliary building to permit ready access for servicing if needed. The system is closed so it can be isolated from an ultimate, usually raw water, heat sink, such as a river, lake, or ocean.

To cool the fuel stored under water in the fuel building and the water in the upper containment, separate fuel pool cooling pumps are provided in an independent system.

The emergency core cooling system is in reality an array of subsystems providing the necessary features, including redundancy, to protect the core in case of a significant malfunction. The high-pressure core spray system uses a vertical high-pressure core spray pump, motor-driven but backed by a diesel generator in event of loss of electric power. This pump, a single unit, provides the initial response when a small pipe breaks or an equivalent malfunction occurs. Should this system be inadequate to maintain reactor water level, the reactor vessel is automatically depressurized and the low-pressure core spray pumps supply additional capacity. As an added safeguard, the RHR pumps are used in a secondary-mode operation to inject cooling water directly into the reactor vessel. If steam should enter the containment region, the RHR pumps operate in another mode—as containment spray pumps—and are manually operated to condense the steam and thus reduce any potential pressure buildup in the containment. The RHR pumps function when needed to limit the temperature of the water in the suppression pool. The turbine-driven reactor core isolation cooling pumps, in a redundant and independent system, inject cool water into the reactor vessel. The standby liquid control system pumps inject boron solution into the reactor for alternative shutdown.

FIGURE 4 Flow diagram for safety injection system (Westinghouse Electric)
FIGURE 5 Boiling water direct cycle reactor system (General Electric)

Additional Equipment In addition to these systems, a number of support systems exist, most of which require some form of pumping. Without attempting to describe their systems, there are, for example, pumps for feedwater, condensate, chilled water, booster service, condenser service, demineralized water transfer, condensate transfer, dry-well drain, containment drain, concentrated borated water tank, water leg, precoat, radwaste, and sample station.

FIGURE 6 BWR containment and reactor vessel, showing one of two external recirculation pumps (General Electric)
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