Patent Number: 043893686
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a nuclear reactor 10 including a nuclear reactor pressure vessel 20 surrounded by biological shielding 34 and having an active core or fuel region 24 therein. Core 24 is supported in the reactor pressure vessel 20 in a well-known manner by core support barrel 22 (not shown). The reactor coolant is circulated through the cold leg 16 by the reactor coolant pump 14. As best seen in FIG. 2, the cold coolant enters the reactor pressure vessel 20 and impinges upon the core support barrel 22. The flow of the coolant is then deflected downwardly to pass through the annular region 21 between the core support barrel 22 and the reactor pressure vessel 20 to the lower portions of the reactor pressure vessel where it is deflected upwardly to pass to the interior of the reactor core support barrel. Once on the interior of the reactor core support barrel, the coolant flows upwardly through the fuel assemblies (not shown) of the reactor core 24 and subsequently passes into the reactor plenum 25 immediately above the reactor core 24. From here the coolant is again deflected to pass out of the reactor pressure vessel 20 and into hot leg 18 by which means it is delivered to steam generator 12. During its passage through the steam generator 12, the coolant is cooled in a well-known manner by transferring its heat content to the secondary coolant system. After being cooled in the steam generator 12, the primary coolant is recirculated by the reactor coolant pump 14 and the cycle is repeated. Also shown in FIGS. 1 and 2 is a portion of the emergency core cooling system which includes storage tank 28, check valve 36 and delivery pipe 26. Storage tank 28 contains a large quantity of highly borated water. Check valve 36 is designed to permit the passage of the borated water contained in tank 28 to the primary coolant system by means of pipe 26 when the pressure in the primary coolant system drops below a predetermined pressure. Such a pressure drop occurs with a loss of coolant accident or LOCA. The borated water is then injected into the primary coolant system at penetration 27 in the cold leg 16. The borated emergency coolant is injected under a high pressure so that the coolant is caused to flow through the cold leg 16 into the reactor vessel 20, and down through the annulus 21 between the reactor vessel 20 and the core support barrel 22 to reflood the reactor core 24 from the bottom. As best shown in FIG. 2, a coolant pump 14 comprises a pump impeller 40 mounted by means of a shaft 41 in a pump housing 42 which is interposed in the cold leg 16 of the reactor system. The pump shaft 41 is mechanically coupled to the drive shaft 45 of an electric motor 46. In the prior art the coupling between the pump shaft 41 and the drive shaft 45 comprised a pair of plates or flanges each rigidly fixed to a different one of the shafts 41 and 45. The flanges were then bolted together in order to provide a rigid mechanical coupling between the shafts 41 and 45 and yet allow the shafts to be disconnected so that the pump housing 42 could be removed from the system to allow repair of the impeller 40 and pump seals on the impeller shaft 41 as necessary. Also, in the prior art, the motor 46 was provided with a fly-wheel represented generally at 48 to maintain rotation of the motor 46 and pump impeller 40 in the event of a power failure. According to this invention, the mechanical coupling between the pump shaft 41 and the drive shaft 45 is provided by a unidirectional drive means adapted to enable the pump impeller 40 to rotate at a higher rotational speed than the rotational speed of the motor 46 in the pumping direction only. One preferred embodiment of the unidirectional drive means according to this invention is shown in FIGS. 3 and 4 with rotation in the clockwise direction corresponding to the pumping direction. Referring to FIG. 3, a ratchet block 50 is rigidly fixed to the free end of the pump shaft 41. According to this embodiment of the invention, a pair of semicircular cam surfaces 51 are provided on the surface of the cam block 50 facing the drive shaft 45. Each of the cam surfaces 51 terminates in an abutment surface or ratchet tooth 52. Similarly, according to this embodiment of the invention, one or more ratchet arms 54 are mounted on a mounting block 55 by pivot means 56 at one end of the arm 54. The mounting block 55 is rigidly fixed to the free end of the drive shaft 45. The free end of each ratchet arm 54 is provided with an abutment surface 58 adapted to engage the abutment surface or ratchet tooth 52 of the ratchet block 50. In operation, the ratchet block 50 and mounting block 55 as shown in FIG. 3, would be brought into close spaced relationship to each other with the axes of the pump shaft 41 and the drive shaft 45 in coaxial alignment. Relative rotational movement of the drive shaft 45 about its axis in a clockwise direction with respect to the drive shaft 41 would force the surfaces 58 at the free ends of the ratchet arms 54 into abutment with the ratchet teeth 52 of the ratchet block 50 causing the pump shaft 41 to rotate in a clockwise direction with the drive shaft 45. If the drive shaft 45 should cease to rotate in a clockwise direction, the pump shaft 41 can freely continue to rotate in a clockwise direction with the cam arms 54 riding on the cam surfaces 51 and ratcheting over the ratchet teeth 52. Similarly, the pump shaft 41 may rotate in a clockwise direction at a higher rotational rate than the rate of clockwise rotation of the drive shaft 45. However, the pump shaft 41 cannot rotate in a counterclockwise direction with respect to the drive shaft 45. In the event of a loss of coolant accident or LOCA, due to a leak in the cold leg of the system between the pump 14 and the reactor pressure vessel, a high rate of flow of fluid through the pump housing 42 will occur tending to drive the impeller 40 of the pump 14 at a very high rate of speed. According to the teaching of this invention, the impeller 40 of the pump 14 will be permitted to free-wheel with respect to the motor 46 and flywheel 48. Thus, there will be no tendency to drive the motor 46 and flywheel 48 at an excessive rotational speed and yet the flow of fluid through the system will not be impeded. Furthermore, the flow of fluid in the normal direction proper for facilitating the introduction of borated water under emergency conditions will be enhanced. It is believed that those skilled in the art will make obvious modifications in the specific embodiment of this invention as shown in the drawing without departing from the scope of the following claims. Any number of ratchet teeth and ratchet arms may be used. Spring-loaded ratchet teeth or ratcheting means of any type capable of handling the forces involved can be used. Furthermore, other unidirectional drive means such as unidirectional bendix or unidirectional fluid drives may be used, although neither would be as efficient as the preferred ratchet drive. In addition, a unidirectional fluid drive would not be as effective as the preferred ratchet drive in enhancing the flow of fluid in the normal direction under emergency conditions.