Patent Number: 051184664
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactor coolant system pumps and, more particularly, is concerned with an improved reactor coolant pump having an internal self-cooling arrangement. 2. Description of the Prior Art In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and reactor coolant pumps. In one version of the reactor coolant system used in a nuclear power plant, the reactor coolant pumps are high inertia pumps hermetically sealed and mounted to the one steam generator in the respective coolant loop. Each pump has an outer casing, a central axially extending rotor rotatably mounted at its opposite ends by upper and lower bearings, and a canned motor located about the pump rotor between the upper and lower bearings. The motor includes a rotor section mounted for rotation on the pump rotor and a stator stationarily mounted to the casing about the rotor section. An impeller mounted at one end of the pump rotor rotates therewith and draws reactor coolant water axially through a central inlet nozzle in the pump casing and discharges the water tangentially through an outlet nozzle in the pump casing. The temperature of the reactor coolant water is typically in the range of from approximately 500.degree. to 600.degree. F. which is too hot to also use to cool the motor and bearings of the pump. Thus, a heat removal arrangement separate from, and which does not employ, the reactor coolant water has been utilized in the prior art. One heat removal arrangement includes an annular hollow jacket surrounding the motor, a set of coils contained in the jacket and surrounding the motor, and other sets of coils located adjacent the upper and lower bearings. The multiple sets of coils are connected in flow communication so as to define a closed path for circulation of an internal coolant fluid therein for cooling the bearings and motor. The annular jacket of the heat removal arrangement has an inlet and outlet connected in flow communication with an external source of a secondary coolant fluid which can then flow through the jacket over the set of coils contained therein. The secondary coolant fluid is typically at a temperature much lower than the temperature of the internal coolant fluid circulating about the closed path such that the heat carried by the internal coolant fluid gained from cooling the motor and bearings is readily transferred to the secondary coolant fluid through the one set of coils in the jacket. Use of the above-described heat removal arrangement of the prior art is necessary in reactor cooling systems where the temperature of the reactor coolant water is too high to also be useful in cooling the pump motor and bearings. A drawback of this prior art heat removal arrangement, however, is that it does increase the complexity of the pump. SUMMARY OF THE INVENTION The present invention provides an improved reactor coolant pump having an internal self-cooling arrangement designed to avoid the aforementioned drawback. The self-cooling arrangement of the present invention employs reactor coolant water from the main flowstream to cool the pump motor and bearings. The reactor coolant water from the main flowstream, and thus the self-cooling arrangement of the invention, can be used in those situations where the temperature of the reactor coolant water entering the pump is below approximately 200.degree. F. Reactor coolant water at that temperature circulated by the self-cooling arrangement of the improved pump can readily remove motor heat generated by electrical losses and bearing heat generated by friction, eliminating the need for use of an external secondary coolant fluid and a separate internal closed path coolant fluid. Accordingly, the present invention is directed to a pump for pumping a fluid. The pump comprises: (a) a casing defining an inlet for receiving a fluid, an outlet for discharging the fluid, and a passage interconnecting the inlet and the outlet through which the fluid can flow in a main stream from the inlet to the outlet; (b) a central rotatable rotor having an end disposed adjacent the annular passage of the casing; (c) at least one bearing rotatably mounting the rotor adjacent to the end thereof to the casing; (d) a motor disposed about the rotor and adjacent the bearing and being operable for rotatably driving the central rotor; (e) means mounted to the end of the rotor in communication with the annular passage and the flow of fluid therethrough and being rotatable with the rotor for creating a lower pressure at the inlet of the casing than at the outlet thereof for drawing fluid into the casing through the inlet thereof and discharging fluid from the casing through the outlet thereof after flow of the fluid in the main stream through the annular passage; and (f) a self-cooling arrangement defining a fluid flow loop in flow communication with the annular passage and in heat transfer relationship with the bearing and motor and being operable for diverting only a fraction of the fluid from and back to the main stream through the annular passage to cool the bearings and motor. More particularly, the fluid flow loop is composed of outer and inner annular loop portions. The outer loop portion extends generally coaxial with, but is located farther radially outwardly from, the central rotor than is the inner loop portion. The fluid flow loop also includes a plurality of entry and exit ports which open respectively into and from the outer and inner loop portions. The entry and exit ports are defined in flow communication with the annular passage. Particularly, the entry ports are located downstream of the exit ports and thus at points of greater pressure in the main stream of the fluid through the annular passage. Further, the self-cooling arrangement includes foreign particle deflectors provided with respect to the fluid flow loop so as to minimize passage of particles into the fluid flow loop and to collect those particles which do enter the loop at a desired location along the loop. These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.