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aVary depending on reactor power rating.

Source: General Electric.

aVary depending on reactor power rating.

Source: General Electric.

FIGURE 7 Main coolant pump (Flowserve Corporation)

periphery of the reactor vessel and use wet winding motors. There are no jet pumps. Flow is varied by changing motor speed with solid-state power supplies, one for each recirculation pump.

Seals Pump seals are invariably of the pressure-balanced type because of the high pressures involved. Several suppliers use pressure breakdown techniques to distribute the pressure either equally or in some desired proportion between two or more seals in series (Figure 11). The technique is analogous to that of a potentiometer, where particular voltages may be obtained by selecting the proper point along an electrical resistance. Also, as with a potentiometer, the flow through the primary resistance path must exceed the tapoff flow in order to maintain the system stability. For pressures in PWR systems, three series seals are frequently used (with each seal taking one-third of the overall pressure), and with lower-pressure BWRs, two series seals are usually sufficient. A margin of safety is built into the systems such that pump operation can be continued even if one of the series seals fails.

In addition, a safety seal may also be installed as a backup. At least one manufacturer uses a high-pressure ceramic seal with a carbon seal backup followed by a vapor seal. Main coolant pump seals are usually supplied with reactor-grade water through a seal injection system. The advantage of using injection water is that it can be temperature-controlled and filtered. Most pumps, however, can be operated without it for at least reasonable time periods.

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