NUCLEAR REACTOR COOLANT PUMP IMPELLER/SHAFT ASSEMBLY

Abstract: A nuclear reactor coolant pump including an impeller (64) and impeller mounting bolt (68) with a plurality of passages formed in the impeller mounting bolt adjacent a portion of the shaft in such a manner that the coolant is effectively pumped through the passageways to heat the shaft to a temperature approximately equal to that of the impeller, thus creating thermal balance of the pump's component parts and providing improved pump effi­ ciency.

Description: This invention relates to a pump and, more particularly, to a thermally balanced nuclear reactor coolant pump impeller/shaft assembly. As described in commonly owned U.S. Patent Nos. 4,427,620 and 4,434,132, and as shown in the simplified, schematic view of Fig. 1 therein, a typical nuclear reactor, power supply apparatus 10 generally includes a nuclear reactor 12 and steam generators 14 and 16. The apparatus 10 also includes separate loops 18 and 20 for supplying coolant to the nuclear reactor 12. Each loop 18 and 20 includes the primary tubes 22 of the corresponding steam generator 14 and 16. Each loop 18 and 20 further includes a "hot" leg 24 through which hot coolant emanating from the nuclear reactor 12 is conducted to the inlet plenum 26 of each steam generator 14 and 16. While in the primary tubes 22, the hot coolant is in heat-exchange relationship with the feedwater supplied to each steam generator 14 and 16. Each loop 18 and 20 also includes a "cold" leg 28 through which the coolant, whose temperature has been reduced by the heat exchange, is returned to the nuclear reactor 12 from the outlet plenum 30 of each steam generator 14 and 16. Finally, each loop 18 and 20 includes a coolant pump 32 for driving the coolant therethrough. Since the coolant pumps 32 are instrumental in cooling the nuclear reactor 12, the most efficient design and trouble-free operation of the coolant pumps 32 is greatly desired. As shown in greater detail in Figs. 2 and 3 herein, a conventional coolant pump 32 generally includes a housing or casing 34 and a solid shaft 36 located axially of the housing 34 for rotation therein. A thermal sleeve 38 (omitted from Fig. 4) surrounds a portion of the shaft 36. An impeller key 40 is interposed between the shaft 36 and an impeller or rotor 46 (Fig. 3). The impeller key 40 acts as a "backup" measure such that, if the impeller 46 becomes loosened from the shaft 36, the impeller key 40 prevents rotation of the impeller 46 relative to the shaft 36. More particularly, the impeller 46 is the rotating member of the coolant pump 32 and generally includes a central, cylindrical hub 49, to which is attached a plurality of blades or fins 51 (Fig. 2). The impeller 46 is heated during installation about the shaft 36 to create a relatively tight tapered fit therebetween. The tapered mating area between the impeller 46 and the shaft 36 is indicated by reference numeral 47 (fig. 3). After the impeller 46 is installed on the shaft 36, a lockbolt 44 is placed through a central hole 45 formed in an impeller nut 42 and screws into the shaft 36 via right-hand threads 52 (Fig. 3). The impeller nut 42 also acts as a "backup" measure should the impeller 46 become loosened from its tapered fit with the shaft 36. That is, the impeller nut 42 is intended to prevent sudden expansion and backing off of the impeller 46 relative to the shaft 36. The lockbolt 44 and the impeller nut 42 are then welded together to ensure that the impeller nut 42 will not come off and that the impeller 46 is fixedly attached to the shaft 36. The coolant pump 32, of course, also includes a fluid inlet 48 and a fluid outlet 50. During operation of the pump 32, coolant enters the fluid inlet 48, circulates across the outer surface of the impeller 46 in the direction indicated by arrow "A", and exits via the fluid outlet 50. In addition, an auxiliary flow is created through a conduit 53 formed in the impeller 46, in the direction indicated by arrow "B" (Fig. 3). The temperature at the impeller 46 during operation of the coolant pump 32 is about 290°C. On the other hand, due to a required conventional heat exchanger (not shown), which is also a part of the coolant pump 32, the temperature at the thermal sleeve 38 is about 50-66°C. The shaft 36 is similarly at a much lower temperature than the impeller 46. Accordingly, a large thermal imbalance results between the impeller 46 and the solid shaft 36 during operation. This imbalance can cause the impeller 46 to expand relative to the cooler shaft 36, thus reducing the interface pressure between the impeller 46 and the shaft 36. As a result, the impeller 46 could loosen from its tapered fit with the shaft 36. If this occurs, the friction between these components, which previously carried the loads, is lost and unwanted stress is placed on the impeller key 40, the impeller nut 42 and the lockbolt 44. It is the principal object of the present invention to provide a pump with an impeller/shaft assembly, wherein the tapered fit of the impeller relative to the shaft is reliably maintained. With this object in view, the present invention resides in a nuclear reactor coolant circulating pump, comprising a casing having an inlet and an outlet, a shaft rotatably supported in the casing, an impeller mounted onto said shaft and disposed within said casing and an impeller mounting bolt threaded into a central opening in the shaft characterized in that said bolt and shaft have passageways for directing coolant through said shaft so as to maintain the temperature of said shaft essentially at the temperature of said impeller. The invention will become more readily apparent from the following description thereof on the basis of the attached drawings, wherein, Fig. 1 is a schematic view of a conventional nuclear reactor, power supply apparatus, including coolant pumps; Fig. 2 is an elevational, cut-away, right side view of a conventional, nuclear reactor coolant pump; Fig. 3 is a cross-sectional, right side view of a conventional, nuclear reactor coolant pump impeller/shaft assembly; and Fig. 4 is a cross-sectional, right side view of the nuclear reactor coolant pump impeller/shaft assembly according to the present invention, illustrating particularly the flow of coolant through the plurality of conduits formed in the impeller nut and the passageway formed in the impeller nut. In contrast to the conventional nuclear reactor coolant pump impeller/shaft assembly described above, and as shown in Fig. 4, the nuclear reactor coolant pump 60 of the present invention generally includes in a housing, which is unchanged from the conventional housing 34 shown in Fig. 2, a shaft 64 located axially of the housing for rotation therein. The shaft 64 also includes a thermal sleeve, which is omitted from Fig. 4 for purposes of clarity in describing the present invention. An impeller key 66 is interposed between the shaft 64 and an impeller or rotor 72. The impeller 72 generally includes a central, cylindrical hub 73 to which is attached a plurality of blades or fins (not shown). The impeller 72 is also heated during installation to create a relatively tight tapered fit on the shaft 64. The tapered mating area between the impeller 72 and the shaft 64 is indicated by reference numeral 75. An impeller mounting bolt 68 and a lockbolt 70 are used to fixedly attach the impeller 72 to the shaft 64. More particularly, the impeller bolt 68 has left-hand threads 69 which mate with the left-hand threads 71 formed in the shaft 64. The lockbolt 70 is inserted through a hole 77 formed centrally in the impeller bolt 68 and anchors in the shaft 64 via right-hand threads 79. The lockbolt 70 and the impeller bult 68 are then welded together after assembly to secure the impeller nut 68 and the impeller 72, as discussed above. The impeller bolt 68 includes at least one passageway 78 formed therein. Preferably, the at least one passageway 78 is formed in hole 77 between lockbolt 70 and impeller bult 68 and extends into contact with the exterior of the shaft 64 about the exterior of the impeller bult 68. The preferred number of the at least one passageway 78 is two; however, other numbers of passageways 78 are contemplated by the present invention. Alternatively, a portion of the at least one passageway 78 can be formed internally of the impeller bult 68, as indicated by reference numeral 78′ in Fig. 4. In addition, at least one other passageway 78″ can be formed, for example, skew to the passageway 78′. A first plurality of conduits 80 is formed in a first area 82 of the impeller mounting bolt 68 and is in fluid communication with an end of the passageway 78. The preferred number of the first plurality of conduits 80 is six, although the present invention contemplates any practicable number of conduits. The preferred diameter of each of the first plurality of conduits is approximately 1.3 cm. The impeller mounting bolt 68 also includes a second plurality of conduits 84 formed in a second area 86 thereof in fluid communication with the other end of the passageway 78. The preferred number of the second plurality of conduits 84 is four, although any practicable number can be used. The preferred diameter of each of the second plurality of conduits is 1.9 cm. The first plurality of conduits 80, passageway 78, and second plurality of conduits 84 must be of a sufficient size so that flow resistance in the form of friction between the water and conduits 80, passageway 78, and conduits 84 can be overcome. Proper sizing of the above-referenced passages guarantees a large enough flow to ensure that heat transfer from the water to the shaft interior is virtually independent of any external influences. An important consideration in choosing the number and location of passageways 78 is effecting balance in the pump 60 during rotation of the shaft 64 and the impeller 72. Accordingly, equally spaced passageways 78 formed in opposing relation in the impeller mounting bolt 68 are preferred. The same consideration regarding "balance" is pertinent to formation of the first and second plurality of conduits 80 and 84, respectively, in the impeller mounting bolt 68. The pump 60, of course, also includes a fluid inlet 74 and a fluid outlet 76. Coolant enters the fluid inlet 74 and circulates around the impeller 72 as indicated by arrow "C". In addition, an auxiliary flow, as indicated by arrow "D", is created through the impeller 72. In addition, because of: (1) the difference in the diameters and numbers between the first plurality of conduits 80 and the second plurality of conduits 84; (2) the fact that the pump 60 contains a continuous supply of fluid coolant, i.e., there are no voids or air gaps in the pump 60; and (3) the relatively long distance the fluid coolant must travel through the passageway(s) 78, 78′ and 78″, a pumping action is effectively created. More particularly, coolant is effectively pumped out of the larger, second plurality of conduits 84, through the path indicated by arrow "E" within the passageway(s) 78, 78′ and 78″ of the impeller nut 68, and in through the first plurality of conduits 80 from the main flow of coolant indicated by arrow "C". It is to be understood that the above-described pumping action moving the coolant through the combination of conduits 80 and 84, and the passageways(s) 78, 78′, 78″ is exclusive of and is unaffected by the traditional pumping action of the coolant pump 60 across the impeller 72. The temperature of the impeller 72 during operation of the coolant pump 60 of the present invention is again about 260°C. On the other hand, due to the pumping of coolant through the passageway(s) 78, 78′ and 78″ formed in the impeller mounting bolt 68 and into contact with the shaft 64, the temperature of the shaft 64 is made close or equal to that of the impeller 72. Accordingly, no thermal imbalance exists between the impeller 72 and the shaft 64. Therefore, the potential for expansion of the impeller 72 relative to the cooler shaft 64 and the possible loosening of the impeller 72 from its tapered fit with the shaft 64 is eliminated. Finally, the traditional, additional stress upon the impeller key, the impeller nut and the lockbolt caused by loosening is eliminated. Accordingly, by providing a fluid passageway into contact with the shaft, the impeller/shaft assembly compliance is improved so that thermal transients (generally impeller/shaft temperature differentials) have a reduced impact on impeller/shaft interface pressure relative to the conventional impeller/solid shaft assembly with the same torque capacity at 21°C as shown in the following Tables I and II. In addition, stress levels on the impeller hub 73 and shaft 64 are reduced according to the present invention, as shown in the following Table III. The present invention can also be used to improve an existing conventional, coolant pump impeller/shaft assembly without major modifications. That is, material can be removed from a conventional solid shaft 36 and a new impeller mounting bolt 68 and lockbolt 70 can be installed as taught by the present invention. Impeller/shaft interference fit can then be increased to bring the 21°C torque capacity up to the original value (before modification) without an increase in impeller hub stress levels. Hot torque capacity will also increase significantly. The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention and the appended claims and their equivalents.

Claims: 1. A nuclear reactor coolant circulating pump, comprising a casing (34) having an inlet (74) and an outlet (76), a shaft (64) rotatably supported in the casing, an impeller (72) mounted onto said shaft (64) and disposed within said casing and an impeller mounting bolt (68) threaded into a central opening in the shaft (64) characterized in that said bolt (68) and shaft (64) have passageways (78, 80) for directing coolant through said shaft (68) so as to maintain the temperature of said shaft (64) essentially at the temperature of said impeller (72). 2. A pump as claimed in claim 1, characterized in that a portion of the passageways (78, 80) is formed in the external surface of the impeller bolt (68) in contact with a portion of the internal surface of the shaft opening. 3. A pump as claimed in claim 1 or 2, characterized in that said bolt (68) has inlet means (80) near its center for directing coolant to said passageways (78), said bolt (68) further having radial outlets (84) adapted to provide for pumping action for pumping the coolant through said passageways (78). 4. A pump as claimed in any of claims 1 to 3, characterized in that said mounting bolt (68) has a central bore (77) and a locking bolt extending through said bolt (77) in spaced relationship from the wall of said bore (77) so as to form part of said passageways (78) between said locking bolt (70) and the walls of said bore (77). 5. A pump as claimed in claim 4, characterized in that said mounting bolt and said locking bolt, both being threaded into said shaft (64), have opposite type thread- one left-hand the other right-hand- and are welded together after being firmly mounted onto said shaft (64).

Inventors: JENKINS, LEONARD STANLEY
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: G21C15
IPC codes: F04D29, G21C15, F04D7, F01D5