Main Coolant Pumps

The term main coolant pumps as used here includes both the recirculation pumps in BWR plants and the reactor coolant pumps in the primary systems of PWR plants. As shown typically in Figures 7, 8, 9, and 10, main coolant pumps use a vertical shaft with the impeller at the bottom and a drive motor coupled to the top end of the shaft.

Bearings Main coolant pumps are usually single-bearing units, although one manufacturer provides a second guide-and-thrust oil-lubricated bearing just below the coupling. Within the pump, hydrodynamic water-lubricated bearings are conventionally used (one supplier using a hydrostatic type). The hydrodynamic bearing generally consists of a hardened sleeve journal shrunk on the shaft and a carbon-lined bearing with some type of self-aligning feature. Bearing diametral clearances are approximately 1.5 mils per inch of bearing diameter (0.0015 mm per millimeter). This rather large clearance is desirable because of thermal transient conditions. Where carbon bearings are used, a cooling mechanism must be provided because the carbon is not suited to long exposure in a hot environment. Many methods are available to accomplish this.

With bearings adequately sized and operating in clean water, virtually trouble-free performance can be expected even with relatively frequent starts and stops.

The hydrostatic, or pressurized, bearing is used where it is not convenient or desirable to provide bearing cooling, for the hydrostatic bearing can be designed to operate in reactor temperature water. Usually it will be larger than its hydrodynamic equivalent and somewhat more sensitive to starting because of the metal-to-metal rubbing that occurs until rotative speed has built up a small pressure to provide a water film clearance. At normal operating speed, however, a hydrostatic bearing will have a significantly larger lubricating film than a hydrodynamic bearing and can tolerate larger particulate matter without wear.

An exception to the previous discussion occurs in both German and Swedish BWR designs, where the recirculation pumps are inverted and inserted directly into the bottom

TABLE 2 Typical nuclear pump parameters in BWR plants

Pump

(m3/h)a

Head, ft (m)

Design pressure, lb/in2

(MPa)

(kW)a

Shaft

Length or height, including driver, in (mm)

Speed (nominal), rpm

Notes

Recirculation coolant

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Survival Treasure

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