Internal pump for nuclear reactor

Abstract: A nuclear reactor internal pump has a pump shaft (26) connected to a motor and rotatably passing through the bottom of a nuclear reactor pressure vessel (1), an impeller (23) fitted to a front end portion of the pump shaft (26), a diffuser (54) disposed on the downstream side of the impeller and having an inner cylinder (52) fixed to the bottom of the reactor pressure vessel (1) to surround the pump shaft (26), a purge water passage (C) defined between the pump shaft (26) and the inner cylinder (52) to communicate with the inside of the reactor pressure vessel (1) and a purge water supply device for supplying purge water to reactor pressure vessel through the passage. Further, the nuclear reactor internal pump comprises a clad dropping preventive device (58) for trapping clad coming from the reactor pressure vessel (1) in the passage (C) to prevent the clad from dropping.

Description: The present invention relates to an internal pump adopted for an internal pump type boiling water reactor (hereinafter called (ABWR); Advanced Boiling Water Reactor), and particularly to an internal pump which is so constituted to trap clad that enters the internal pump from a reactor pressure vessel. EP-A1-0 111 024 discloses a nuclear reactor internal pump having a front end adapted to protrude into a nuclear power reactor vessel and a rear end where a motor case adapted to be fitted to the outside of the reactor vessel is located, comprising: a motor contained in the motor case, and a pump shaft connected to the motor and adapted to rotatably pass through the bottom of the pressure vessel; an inner cylinder surrounding the pump shaft; an impeller fitted to the front end portion of the pump shaft; a purge water passage defined between the pump shaft and the inner cylinder adapted to communicate with the inside of the pressure vessel; a diffuser disposed on the purge water downstream side of the impeller; and a purge water inlet for injecting purge water into the motor case and for supplying purge water to the pressure vessel through the water passage ; the purge water serving for cooling the inside of the motor case and for preventing clad from the pressure vessel from entering the motor case. This prior art nuclear reactor internal pump has a stretch tube arranged around and spaced apart from the pump shaft and whose front end fixes the diffuser to the pressure vessel, the purge water supply being connected to the motor case to supply purge water into a space between the pump shaft and the stretch tube and into a space between the pump shaft and the diffuser. The water passage has a narrow portion which increases the flow velocity of the purge water directed to the front end. For diminishing the heat transfer from the pressure vessel to the motor case, the above-indicated prior art internal pump makes an efficient use of the purge water which is originally intended for preventing contaminants in the pressure vessel from coming into the motor case. According to this prior art design labyrinths are formed for improving the cooling efficiency, and the labyrinths are located on the outside of the pressure vessel and would therefore not be suitable for preventing contaminants from the inside of the pressure vessel from entering into the motor case at the outside thereof. In order to facilitate the explanation of the present invention another conventional internal pump will be explained with reference to Figs. 1 to 3. Figure 1 is a vertical section schematically showing the ABWR. A reactor pressure vessel 1 accommodates coolant 2 and a core 3. The core 3 comprises a plurality of fuel assemblies (not shown), control rods 4, etc. In the reactor pressure vessel 1, there are arranged a shroud 5, a core support plate 6 and an upper grid plate 7, to support the core 3. The control rods 4 are driven by a control rod driving mechanism 8. Above the core 3, a steam separator 9 is arranged, and a steam dryer 10 is disposed above the steam separator 9. Below the steam separator 9, a water supply pipe 11 is connected to the reactor pressure vessel 1. The water supply pipe 11 is connected to a water supply sparger 12. Between the shroud 5 and the reactor pressure vessel 1, a downcomer portion 13 is defined. At a lower part of the downcomer portion 13, internal pumps 14 are circumferentially disposed at regular intervals. The internal pumps 14 forcibly circulate the coolant 2 toward the core 3. A reference mark 15 represents main steam piping. Figure 2 shows structure of the internal pump 14. The internal pump 14 comprises a pump section 21 and a motor section 22. The pump section 21 comprises an impeller 23, a diffuser 24 disposed on the discharge side of the impeller 23, a diffuser ring 25, etc. The impeller 23 is connected to the motor section 22 through a pump shaft 26. At a bottom of the reactor pressure vessel 1, nozzles la are arranged to protrude upward. The diffuser 24 is fixed to each nozzle 1a with a stretch tube 27. The motor section 22 comprises a motor case 32 which accommodates a rotor 30 engaging integrally with the pump shaft 26, a stator 31, etc. A front end portion of the motor case 32 is inserted into the nozzle la and welded thereto. Coolant water via piping 33 circulates inside the motor case 32 to prevent the motor section 22 from burning. An auxiliary impeller 34 is fixed to a lower end of the rotor 30. A lower end of the stretch tube 27 engages with a stepped portion formed at an upper part of the motor case 32. A purge water inlet 41 is formed at an upper part of the motor case 32. Through the purge water inlet 41, purge water is supplied into the motor case 32. As also shown in Fig. 3, the purge water supplied into the motor case 32 passes through a space A defined between the pump shaft 26 and the stretch tube 27 and flows into a space B defined between the pump shaft 26 and the diffuser 24. By supplying the purge water in this way, contaminated particles in the reactor pressure vessel 1 are prevented from flowing into the motor case 32. At the same time, the purge water cools an upper part of the motor case 32. Due to such a cooling effect of the purge water, despite a high water temperature of about 300°C in the reactor pressure vessel 1, a temperature inside the motor case 32 is suppressed to about 40°C. As a result, polymeric materials such as a coil insulation material of the stator 31 and a sealing rubber material of a secondary seal are prevented from deteriorating due to the high temperature. However, the above-mentioned conventional structure of the internal pump has several drawbacks. Clad of large specific gravity generated in the reactor pressure vessel 1 may flow into the motor case 32 against the flow of the purge water. To prevent the clad of large specific gravity from entering the motor case 32, the flow rate of the purge water shall be increased. If the flow rate of the purge water is increased, however, the pump shaft 26 and stretch tube 27 are subjected to a great temperature difference, i.e., large thermal stress to deteriorate soundness of the pump shaft 26 and stretch tube 27. In this way, the conventional internal pump structure requires a flow rate of the purge water to be increased to prevent the clad of large specific gravity from entering into the motor case. If the flow rate of purge water is increased, however, the pump shaft and stretch tube are subjected to large thermal stress to deteriorate soundness of the pump shaft and stretch tube. To solve the problems of the conventional internal pumps, an object of the present invention is to provide a reactor internal pump which is able to prevent clad of large specific gravity from entering into the motor case of the pump without increasing the flow rate of purge water. Another object of the present invention is to provide a reactor internal pump which is able to prevent clad from entering into the motor case of the pump even if the flow rate of purge water is small or there is no flow of the purge water. Still another object of the present invention is to provide a reactor internal pump which is able to prevent clad from entering into the motor case of the pump without increasing a flow rate of purge water or even if the flow rate decreases due to clogging of a purge water injection pipe. According to a first aspect of the present invention recited in independent claim 1, clad entering preventive means for preventing clad from entering into the motor case is provided comprising a cover portion formed at a boss portion of the impeller, the cover portion facing toward the rear end and extending in the direction of a rotation radius of the impeller to overlap a front end of the inner cylinder to prevent the clad from entering the passage while allowing the purge water to pass through the passage. The cover portion for covering the front end of the inner cylinder preferably extends radially outwardly from the impeller for covering the front end of the passage. According to a second aspect of the present invention recited in independent claim 3, the velocity increasing means is adapted to increase the local flow velocity at the location of the velocity increasing means to 20 cm/s. Preferably the velocity increasing means comprises a projecting plate provided at the predetermined part of the passage to narrow the predetermined part of the passage compared to the front and the rear ends of the predetermined part. A sectional area of the passage at the predetermined part may be narrowed less than one half of that at the front and the rear ends. According to an additional preferred embodiment the predetermined part is located at a front part of the passage. According to a third aspect of the present invention recited in independent claim 7, clad dropping preventive means is provided comprising a first annular member provided on the pump shaft to protrude in the passage outwardly in the direction of a rotation radius of the impeller and a second annular member provided on the inner cylinder rearwardly of the first annular member to protrude in the passage inwardly in the direction of the rotation radius of the impeller, such that the front edge portions of the first and second annular members overlap in the radial direction. Preferably the clad dropping preventive means comprises a clad discharging means for discharging the trapped clad outside the diffuser. The clad discharging means may comprise through holes each formed through the inner cylinder to connect the inside and the outside of the inner cylinder to each other and having a diameter larger than that of the clad. The through holes may be inclined rearwardly from the inside toward the outside of the inner cylinder; or the through holes may extend radially outward from the inside toward the outside of the inner cylinder. According to a fourth aspect of the present invention recited in independent claim 12, a clad discharging means is provided on the diffuser to discharge clad entered inside the diffuser from the pressure vessel outside the diffuser. Preferably the clad discharging means comprises through holes each formed through an inner cylinder of said diffuser to connect the inside and the outside of the inner cylinder to each other and having a diameter larger than that of the clad. The through holes may be inclined rearwardly from the inside toward the outside of the inner cylinder, or the through holes may extend radially outward from the inside toward the outside of the inner cylinder. In this way, the diffuser is provided with the clad discharging means to discharge clad dropped from the reactor pressure vessel into the diffuser, to the outside of the diffuser. Since the clad is discharged by the clad discharging means to the outside of the diffuser, the clad never enters the motor case of the internal pump. Even if the flow rate of purge water is small, the same discharging action and effect can be achieved. Therefore, unlike the conventional internal pump, the flow rate of purge water is not necessary to be increased in purging the clad of large specific gravity, thus resolving various inconveniences that are caused by increasing the flow rate of purge water. According to a fifth aspect of the present invention recited in independent claim 16, swirling flow generating means is provided for generating a swirling flow of the purge water in the passage caused by the rotation of the pump shaft. The swirling flow generating means may comprise spiral projections formed on the pump shaft. The swirling flow generating means may also comprise vanes formed on the pump shaft. Alternatively the swirling flow generating means may comprise spiral grooves formed on the pump shaft. When the pump shaft is rotated, the fluid filling a space between the pump shaft and the stretch tube swirls around the pump shaft due to friction against the surface of the pump shaft. At this time, spiral projections, vanes or grooves formed on the surface of the pump shaft act as rotary vanes to provide upward driving force to the swirling flow around the pump shaft. Therefore, a flow of the fluid (purge water) is always an upward flow to push clad which tries to enter the space between the pump shaft and the stretch tube from the pressure vessel, back into the pressure vessel. These and other objects, features and advantages of the present invention will be more apparent from the following detailed description of preferred embodiments in conjunction with the accompanying drawings. Fig. 1 is a sectional view schematically showing constitution of a general internal pump type boiling water nuclear reactor; Fig. 2 is a sectional view showing constitution of a conventional internal pump adopted for the reactor of Fig. 1; Fig. 3 is an enlarged sectional view showing part of the internal pump shown in Fig. 2; Fig. 4 is a vertical section showing a reactor internal pump according to a first embodiment of the present invention; Fig. 5 is a vertical section generally showing the internal pump of the first embodiment; Fig. 6 is an enlarged vertical section showing the vicinity of a stretch tube of the internal pump shown in Fig. 5; Fig. 7 is a vertical section showing essential part of a modification of the first embodiment; Fig. 8 is a sectional view showing essential part of a second embodiment of the present invention; Figs. 9 and 10 are graphs for explaining the second embodiment; Fig. 11 is a sectional view showing constitution of an upper part of an internal pump according to a third embodiment of the present invention; Figs. 12 and 13 are sectional views showing the constitution and action of a clad dropping preventive mechanism of the third embodiment; Fig. 14 is a view showing a clad dropping preventive mechanism according to a first modification of the third embodiment; Fig. 15 is a sectional view showing a clad dropping preventive mechanism according to a second modification of the third embodiment; Fig. 16 is a sectional view showing a clad dropping preventive mechanism according to a third modification of the third embodiment; Fig. 17 is a vertical section showing a reactor internal pump according to a fourth embodiment of the present invention; Fig. 18 is a view for explaining a through hole (a clad discharging means) of the fourth embodiment; Fig. 19 is a horizontal section showing distribution of the through holes (the clad discharging means) disposed on the circumference of a diffuser of the fourth embodiment; Figs. 20 to 22 are views for explaining a function of the internal pump of the fourth embodiment; Figs. 23 to 26 are vertical sections and a horizontal section, showing internal pumps according to first and second modifications of the fourth embodiment; Fig. 27 is a vertical section showing an internal pump according to a fifth embodiment of the present invention; Fig. 28 is an enlarged vertical section showing the vicinity of a stretch tube of the internal pump of the fifth embodiment; Fig. 29 is a view for explaining a function of the internal pump of the fifth embodiment; Fig. 30 is a view for explaining a function of an internal pump according to a first modification of the fifth embodiment; and Fig. 31 is a view for explaining a function of an internal pump according to a second modification of the fifth embodiment. The first embodiment of the present invention will be explained with reference to Figs. 4 to 7. Figure 5 shows an internal pump of a wet motor type reactor. A reactor pressure vessel 1 is provided with a motor case 32 which is fitted to the outside of the bottom of the pressure vessel 1. Inside the motor case 32, a vertical pump shaft 26 is rotatably supported. A motor 22 is received in the motor case 32. The motor 22 comprises a rotor 30 fixed to the pump shaft 26, and a stator 31 surrounding the rotor 30. To a lower end of the pump shaft 26, an auxiliary impeller 34 is fitted to stir purge water flown into the motor case 32. An upper end of the pump shaft 26 passes through a shaft hole 50 which is formed through the bottom of the pressure vessel 1, to protrude in the pressure vessel 1. The upper end of the pump shaft 26 protruding in the pressure vessel 1 is fitted to an impeller 23 for circulating cooling water in the pressure vessel 1. Inside the pressure vessel 1, a diffuser 54 is arranged on the bottom of the pressure vessel 1. The diffuser 54 is positioned downstream the impeller 23 and fixed to an upper end portion of a stretch tube 27. The diffuser 54 has an inner cylinder 52 that surrounds the pump shaft 26. The stretch tube 27 surrounds the pump shaft 26 and, under this condition, inserted into the shaft hole 50. A lower end portion of the stretch tube 27 is fitted to the motor case 32. As shown in Fig. 6, a purge water inlet 13 is arranged in the middle of the motor case 32. Through the inlet 13, purge water is injected into the motor case 32 and passed through a passage "A" formed between the pump shaft 26 and the stretch tube 27. The purge water is then passed through a passage "B" formed between the pump shaft 26 and the inner cylinder 52 of the diffuser 54, and flows into the pressure vessel 1. The purge water prevents clad in the pressure vessel 1 from entering the motor case 32 as well as cooling the inside of the motor case 32. A boss portion 56 of the impeller 23 is provided with a cover portion 58. The cover portion 58 covers an upper end of the inner cylinder 52 of the diffuser 54 to prevent clad from entering the passage B from the pressure vessel 1. In more detail, the periphery of an upper end 60 of the inner cylinder 52 of the diffuser 54 is machined to form a thin cylinder as shown in Fig. 4. The cover portion 58 forms a passage C which extends from the inside of the inner cylinder 52, passes over the upper end 60 and reaches the outside of the inner cylinder 52. The cover portion 58 has a cylindrical cover shape formed integrally with the periphery of the boss portion 56 of the impeller 23. The cover portion 58 is located at an outermost position of the inner cylinder 52 of the diffuser 54 to pass over the inner cylinder 52 in the direction of a rotation radius of the impeller 23. The passage C is so sized to pass the purge water through the same. Operation of the embodiment will be described. When the pump is not operating and whether or not the purge water is being supplied, the cover portion 58 covers the upper end 60 of the inner cylinder 52 of the diffuser 54 to cover the passage B. Therefore, clad which descends in the pressure vessel 1 only by its gravity, never passes through the passage C between the cover portion 58 and the inner cylinder 52 to enter the passage B inside the inner cylinder 52. When the pump is operated, the clad will move not only by its gravity but also by the flow of pumped water. In this case also, the cover portion 58 effectively acts as a lid to substantially prevent the clad from entering the passage B. The passage C formed between the inner cylinder 52 of the diffuser 54 and the cover portion 58 is an outlet through which the purge water flows into the pressure vessel 1. The shape of the passage c is complicated due to the shape of the cover portion 58. Therefore, a pressure loss coefficient of the passage C is very. easy to increase compared to the conventional internal pump. If the purge water is supplied, a pressure in the passage B will surely be increased higher than a pressure in the pressure vessel 1, even if a flow rate of the purge water is small. Accordingly, the purge water uniformly flows into the pressure vessel 1 through the passages B and C to prevent clad from flowing from the pressure vessel 1 into the passage B, thus preventing the clad from contaminating the inside of the motor case 32. Figure 7 is a view showing a modification of the passage C. In this modification, the passage C is further complicated with an upper end of the inner cylinder 52 and the cover portion 58. As shown in the figure, two cylindrical portions 62 and 64 are concentrically formed on the upper end of the inner cylinder 52 of the diffuser 54. The cover portion 58 comprises two concentrical cylinder portions 66 and 68 which are located between the inner and outer cylindrical portions 62 and 64 and outside the outer cylindrical portion 64, respectively. The cover portion 58 is formed integrally with the boss portion 56 of the impeller 23. According to this modification, the shape of the passage C defined by the inner cylinder 52 of the diffuser 54 and the cover portion 58 is further complicated compared to the previous embodiment. As a result, the pressure loss coefficient is further increased to provide a greater clad preventive effect. The number of the cylinrical portions 62, 64, 66 and 68 may further be increased. The number of the cylindrical portions 62 and 64 of the inner cylinder 52 and the number of the cylindrical portions 66 and 68 of the cover portion 58 are not necessary to be equal to each other. As described in the above, the first embodiment of the present invention provides the following effect. Since the cover portion formed on the boss portion of the impeller covers the upper end of the purge water passage formed between the pump shaft and the inner cylinder of the diffuser, clad is prevented from entering the motor case from the reactor pressure vessel through the purge water passage even if a flow rate of the purge water is small or even if there is no flow of the purge water. Due to this, the inside of the motor case is prevented from being contaminated by the clad in the reactor pressure vessel. As a result, the reliability and safety of the reactor internal pump are improved. The second embodiment of the present invention will be described with reference to Figs. 8 to 10. According to the conventional reactor internal motor, a sectional area of the purge water passage upper than the stretch tube 27 is wide so that a velocity of the purge water is slow if a flow rate of the purge water is small. Therefore, clad in the reactor easily enters the area. When the pump is stopped, the pump comes down in an axial direction such that a projection 70 contacts with the top of the stretch tube 27. At the same time, the purge water is stopped and, therefore, an upward flow of the purge water is stopped. Then, clad existing around the upper part of the stretch tube 27 may enter into the motor case whenever the pump is stopped. Figures 9 is a graph showing relations between diameters of clad and minimum upward flow velocities that are needed to stop clad from descending. As apparent from the graph, an upward flow velocity of about 20 cm/s, which may differ depending on the diameter of clad, shall be maintained. To satisfy this requirement, the conventional internal pump provides a flow rate of about 40 l/min. Flow velocities at respective sections of the conventional internal pump are shown in Fig. 10. According to the conventional internal pump, the upper the flow passage, the wider the flow passage, i.e., the slower the flow velocity. The conventional technique determines, therefore, a flow rate of the purge water. so as to attain an upward flow velocity of 20 cm/s at an upper end of the passage. The present invention intends to narrow part of the flow passage. Generally, as shown in Fig. 8, plates 72 are provided on the projection 70. Instead of providing the plates 72 on the projection 70, the plates 72 may be provided on the diffuser or they may be provided on both the projection 70 and diffuser. With these arrangements a local flow velocity at the plates 72 is increased to 20 cm/s. Therefore, even if there exists a location where the flow velocity is less than 20 cm/s, clad will never penetrate a region below the projection 70. Figure 10 shows distribution of flow velocities when the plates 72 are provided according to the invention. As shown in the figure, a mean flow velocity of the invention is slower than that of the conventional technique. Therefore, a flow rate of the purge water may be small in the present invention. Since the small flow rate of purge water is sufficient for the present invention, thermal stress to be caused on a pump nozzle due to a cooling effect of the purge water can be minimized. Although the embodiment has arranged the plates 72 in the vicinity of the projection 70 to narrow the flow passage, the plates 72 may be arranged at any position in the middle of the flow passage or they may be located at plural positions. As a means for narrowing the flow passage, not only the plates but any other means may be adopted. Namely, it is possible to increase the projection height of the projection 70 so as to narrow the flow passage. To improve an effect of the means for narrowing the flow passage, it is preferable to locate them as upper as possible. The narrower the flow passage, the higher the flow velocity and the clad preventive effect. However, due to the problem of contact between rotational members and stationary members, a preferable narrowing ratio is 1/2 to 1/4. As described in the above, the second embodiment of the present invention narrows part of a passage between a stretch tube and a rotor and between a diffuser and the rotor to prevent clad from entering a motor case with purge water of small flow rate. The third embodiment of the present invention will be described with reference to Figs. 11 to 16. In the figures, the parts explained before are represented with like numerals to omit their explanations. As shown in Fig. 11, between a pump shaft 26 and a diffuser 24, a clad dropping preventive mechanism 80 is disposed. The clad dropping preventive mechanism 80 comprises an annular member 82 protruding outwardly in a radial direction from the pump shaft 26, and an annular member 84 positioned below the annular member 82 and protruding inwardly in the radial direction from the diffuser 24. A projecting length of the annular member 82 is "p1" while a projecting length of the annular member 84 is "p2." A gap between the pump shaft 26 and the diffuser 24 is "g." These figures have the following relation: p1 + p2 > g Namely, front edge portions of the two annular members 82 and 84 overlap each other in the radial direction. A vertical distance between the two annular members 82 and 84 is "h." Operation of the embodiment will be explained. When the pump is not operating, clad 86 of specific gravity larger than that of purge water will settle on the upper surface of the annular members 82 and 84 as shown in Fig. 12, and the clad 86 is restricted from dropping downward farther. Since the front edge portions of the annular members 82 and 84 overlap each other, the clad 86 is trapped in the whole region in the gap between the pump shaft 26 and the diffuser 24. At this time, a passage for purge water is naturally secured. When the pump is operated, a swirling flow of purge water is caused between the pump shaft 26 and the diffuser 24 so that the clad 86 is pushed outward in the radial direction due to centrifugal force and, as shown in Fig. 13, collected at a base of the annular member 84. In this case also, the clad 86 will not fall downward but they are securely collected by the annular member 84. According to this embodiment, the following effect is provided. The clad dropping preventive mechanism 80 is arranged between the pump shaft 26 and the diffuser 24 to collect the clad 86 dropped from the reactor pressure vessel 1, so that the clad 86 may never enter a motor case 32. Unlike the conventional technique, there is no need to increase a flow rate of the purge water in preventing the clad 86 which has specific gravity larger than that of the purge water. As a result, various problems that may be caused by increasing the flow rate of the purge water will not occur. The clad dropping preventive mechanism 80 of this embodiment comprises the annular members 82 and 84 protruding from the pump shaft 26 and the diffuser 24, respectively. The front edge portions of the two annular members 82 and 84 overlap each other in the radial direction. Therefore, the whole sectional area of the gap between the pump shaft 26 and the diffuser 24 benefits from a clad collecting function of the annular members 82 and 84. With a higher possibility, the clad is prevented from dropping into the motor case. According to this embodiment, a required clad preventive function can be realized by only providing a pair of the projecting annular members 82 and 84 so that this embodiment may be easy to apply for existing facilities. The first modification of the third embodiment will be described with reference to Fig. 14. In this modification, front edge portions of the annular members 82 and 84 of the third embodiment are bent vertically such that the front edge portions come closer to the other annular member. the projecting lengths of bent portions 82a and 82b are q1 and q2 respectively, and a gap between the annular members 82 and 84 is "h." Then, they have the following relation: q1 + q2 > h The first modification can provide the similar effect as that of the third embodiment as well as an effect of preventing the clad 86 from dropping more efficiently. This is realized because the front edge portions of the annular members 82 and 84 are provided with the bent portions 82a and 84a, respectively. Particularly when the clad 86 has specific gravity which is larger than that of the purge water, the clad 86 cannot pass over the bent portion 84a. Therefore, even if the internal pump under operation disturbs a flow of the purge water in the passage B, the clad 86 is surely collected. The front edge portions of the bent portions 82a and 84a are so constituted to overlap each other to improve the clad collecting effect. The second modification of the third embodiment will be explained with reference to Fig. 15. Between front edge portions of the bent portions 82a and 84a and the annular members 84 and 82 of the first modification, there are arranged non-return valves 88 and 90, respectively. The non-return valves 88 and 90 allow a flow (in the direction of arrow marks shown in fig. 15) of purge water but stop a flow of the clad 86 in the opposite direction. The second modification realizes the same action and effect of the first modification as well as an effect of more securely preventing the clad 86 from entering the motor case. The third modification of the third embodiment will be explained with reference to fig. 16. Front edge portions of the bent portions 82 and 84 of the first modification are further bent in a radial direction to form bent portions 82b and 84b which narrow an area of a passage 92. According to the third modification, a flow velocity of purge water is increased to more effectively prevent the clad 86 from dropping into the motor case. According to the second and third modifications, the clad 86 is surely prevented from dropping into the motor case. Therefore, even if a flow of the purge water in the passage B is disturbed not to provide sufficient centifugal force acting on the clad 86, or even if a quantity of the clad 86 is quite large, the clad 86 may be collected effectively. The present invention is not limited to the embodiments described before. Any means can be interposed between the pump shaft and the diffuser to prevent the clad from dropping while securing a passage of the purge water. In the first and second modifications, the bent portion or the non-return valve may be provided for only one of the pump shaft and the diffuser. As described in the above, the reactor internal pump of the third embodiment of the invention can effectively prevent clad from dropping from a reactor pressure vessel into a motor case. Unlike the conventional technique, the third embodiment of the invention does not require a flow rate of purge water to be increased, thus solving various problems caused by increasing the flow rate of the purge water. A reactor internal pump according to the fourth embodiment of the present invention will be explained with reference to Figs. 17 to 23. In the figures, parts similar to those of the previously explained pumps are represented with like numerals. Figure 17 shows a fitting portion of a clad discharging mechanism according to the fourth embodiment. An upper end of a pump shaft 26 protrudes inside a reactor pressure vessel 1 and is fixed to an impeller 23. On the downstream side of the impeller 23, a diffuser 24 is arranged. A lower end of the diffuser 24 is fixed to the reactor pressure vessel 1 with an upper end 27a of a stretch tube 27. Between the pump shaft 26 and the diffuser 24, a passage B for purge water is defined. Where the diffuser 24 is fixed to the upper end 27a of the stretch tube 27, the diffuser 24 is provided with a plurality of small through holes 100. Details of one of the through holes 100 are shown in Fig. 18. An entrance of the through hole 100 on the inner side of the diffuse 24 is located in a region C. The region C exists between a lower limit at a joint between the upper end 27a of the stretch tube 27 and the diffuser 24 and an upper limit which is a horizontal plane contacting with the top of the upper end 27a of the stretch tube 27. an inner diameter of the through hole 100 is in the range of, for instance, 0.1 mm to 5.0 mm. The through hole 100 is formed through a wall of the diffuser 24. The through hole 100 is horizontal or inclined downward from the passage B toward the outer side of the diffuser 24. Figure 19 is a view showing an example of distribution of the through holes 100 on a circumference of the diffuser 24. The number of the through holes 100 may be, for example, 20 or more that are distributed at regular intervals (at an angle of 18° or less for each through hole) on the circumference of the diffuse 24. Operation of the internal pump of the embodiment will be explained. Clad 86 entered the passage B from the reactor pressure vessel 1 has specific gravity larger than that of purge water in the passage B. When the pump is not operating, the clad 86 settles on the upper end 27a of the stretch tube 27 as shown in Fig. 20. When the pump is operated, a circumferential flow of the purge water is generated around the pump shaft 26 in the passage B due to high-speed rotation of the pump shaft 26. As a result, the settled clad 86 is blown toward the diffuser 24 as shown in Fig. 21. Since a pressure in the passage B is higher than that in the reactor pressure vessel 1 outside the diffuser 24, the clad 86 blown toward the diffuser 24 is sucked into the through holes 100 and discharged outside the diffuser 24 as shown in Fig. 22. In this way, the clad 86 entered the passage B is discharged outside the diffuser 24 through the through holes 100 while the pump is rotating. Therefore, the clad 86 never enters a gap A between the stretch tube 27 and the pump shaft 26 or into a motor case 32 located under the gap A. Figure 23 is a view showing the first modification of the fourth embodiment. This modification intends to improve an effect of discharging the clad 86 out of the diffuser 24 while the pump is operating. An annular recess 102 is provided on the diffuser 24 in the vicinity of the entrances of the through holes 100. A diameter "h" of the recess 102 is larger than a diameter "d" of each through hole 100, and the diameter is, for example, 1 mm or larger. Other constitution of the first modification is the same as that of the fourth embodiment. When the pump is operated, the clad 86 is blown toward the diffuser 24 and collected in the recess 102 so that the clad 86 may be discharged more effectively through the through holes 100. Figures 24 shows the second modification of the fourth embodiment. Through holes 100 are positioned on the wall of the diffuser 24 farther above the upper end 27a of the stretch tube 27. An annular recess 102 similar to the one shown in Fig. 23 is formed at entrances of the through holes 100 on the inner side of the diffuser 24. Under the annular recess 102, an annular projection 104 is provided. The annular projection 104 protrudes in the purge water passage B. Above the annular recess 102, an annular projection 106 is provided on the pump shaft 26. The annular projection 106 protrudes in the purge water passage B. A width of the annular projection 104 on the diffuser 24 is "x," while a width of the annular projection 106 on the pump shaft 26 is "y." A width of the purge water passage B is "z." These figures satisfy the following relation: x + y > z A shape of each through hole 100, distribution of the through holes 100 in the circumferential direction and other constitution are the same as those of the first modification of the fourth embodiment. As shown in Fig. 25, clad 86 of large specific gravity which has entered the purge water passage B will settle on the annular projections 104 and 106 while the pump is not operating. When the pump is operated, the settled clad 86 is blown toward the annular projection 104 on the diffuser 24 and collected in the annular recess 102. Then the clad 86 is discharged through the through holes 100. Even though the through holes 100 are not provided in contact with the upper end 27a of the stretch tube 27, a clad discharging capacity equivalent to that realized by the through holes 100 of the fourth embodiment is achieved by the second modification. In addition, the second modification will improve freedom of design. The through holes 100 shown in Fig. 24 may be combined with the through holes 100 provided in the vicinity of the upper end 27a of the stretch tube 27 of the fourth embodiment and of the first modification. Figure 26 shows another modification of the present invention in which the through holes 100 are distributed on the circumferene of the diffuser 24 only on the side of the center of the reactor pressure vessel 1. Except the difference n distribution of the through holes 100 on the circumference of the diffuser 24, the constitution of this modification is the same as that of the fourth embodiment, of the first modification or of the second modification. In Fig. 26, the through holes 100 are arranged at regular intervals only on a half circle portion of the diffuser 24 that faces the reactor core 3. Therefore, the number of the through holes 100 is about a half of that arranged at regular intervals on the whole circumference of the diffuser 24. A pressure difference between a pressure in the passage B and a pressure in the diffuser 24 will be largest on the reactor core side where the velocity of a discharge flow from the pump outside the diffuser 24 is large. Therefore, a clad discharging capacity of the through holes 100 formed through the diffuser 24 will be highest on the reactor core side. Accordingly, if the through holes 100 are disposed only on the half circle portion of the diffuser 24 on the reactor core side, a sufficient clad discharging capacity will be obtained. As apparent from the above explanation, even if a flow rate of purge water is small or even if there is no flow of the purge water, the fourth embodiment of the invention can discharge clad which has specific gravity larger than that of the purge water flowing in the passage, to the outside of the diffuser while the pump is rotating. Accordingly, the clad which is radioactive matter generated in the reactor pressure vessel, will never enter the motor case and never contaminate the inside of the motor case, thereby improving the reliability and safety of the internal pump, An reactor internal pump according to the fifth embodiment of the present invention will be described with reference to Figs. 27 to 31. In the figures, parts which are the same as those explained before are represented with like numerals. Figures 27 shows a wet motor type internal pump according to the fifth embodiment of the invention. From the bottom of a reactor pressure vessel 1, a motor case 32 is suspended. A motor comprising a rotor 30 and a stator 31 is assembled in the motor case 32. Inside the rotor 30, a pump shaft 110 is integrally fixed. An auxiliary impeller 34 is fixed to a lower end of the rotor 30. Figure 28 is a view showing a fitting portion of the internal pump of the embodiment. At an upper end portion of the pump shaft 110 which protrudes inside the reactor pressure vessel 1, there is provided the diffuser 24. The impeller 23 is fixed to the upper end of the pump shaft 110. On the downstream side of the impeller 23, a diffuser 24 is disposed. A lower end portion of the diffuser 24 is fixed to the reactor pressure vessel 1 through a stretch tube 27. Between the pump shaft 26 and the stretch tube 27, a purge water passage is formed. At a lower end of the passage, i.e., in the vicinity of a purge water inlet 41, the pump shaft 26 is provided with several spiral projections 112. Other constitution of this embodiment is the same as that of the conventional internal pump. Operation of the fifth embodiment of the invention will be explained. As shown in Fig. 29, when the pump is operated, a swirling flow of purge water is generated in the passage according to rotation of the pump shaft 26, and the spiral projections 112 generate an upward flow of the purge water. Due to the upward flow of swirling purge water, clad is pushed back into the pressure vessel 1 and will never flow into the motor case 32. Figure 30 is a view showing the first modification of the fifth embodiment of the present invention. Instead of the spiral projections 112, vanes 114 are provided to generate an upward flow. This modification can provide the action and effect equivalent to those of the fifth embodiment. Figure 31 is a view showing the second modification of the fifth embodiment of the present invention. Instead of the spiral projections 112, spiral grooves 116 are provided on the pump shaft 26 to generate an upward flow. This modification can provide the action and effect equivalent to those of the fifth embodiment. The present invention can prevent radioactive clad in the reactor pressure vessel from contaminating the inside of the motor case, thus improving the reliability and safety of the internal pump. As apparent from the above explanation, without increasing a flow rate of purge water or even if the flow rate of purge water is decreased due to clogging of purge water injection piping, the fifth embodiment of the invention can prevent powder-like matter (clad) which has specific gravity larger than that of the purge water, from entering a motor case from a reactor pressure vessel. This action can prevent the radioactive clad in the reactor pressure vessel from contaminating the inside of the motor case, thus improving the reliability and safety of a reactor internal pump.

Claims: Nuclear reactor internal pump having a front end adapted to protrude into a nuclear power reactor vessel (1) and a rear end where a motor case (32) adapted to be fitted to the outside of the reactor vessel (1) is located, comprising: a motor (22) contained in the motor case (32), and a pump shaft (26) connected to the motor (22) and adapted to rotatably pass through the bottom of the pressure vessel (1) ; an inner cylinder (52) surrounding the pump shaft (26); an impeller (23) fitted to the front end portion of the pump shaft (26); a purge water passage (B) defined between the pump shaft (26) and the inner cylinder (52) adapted to communicate with the inside of the pressure vessel (1); a diffuser (54) disposed on the purge water downstream side of the impeller (23); a purge water inlet (41) for injecting purge water into the motor case (32) and for supplying purge water to the pressure vessel (1) through the water passage (B); the purge water serving for cooling the inside of the motor case (32) and for preventing clad from the pressure vessel (1) from entering the motor case (32); characterized by clad entering preventive means (58) for preventing clad from entering into the motor case (32) and comprising a cover portion (58) formed at a boss portion (56) of the impeller (23), the cover portion (58) facing towards the rear end and extending in the direction of a rotation radius of the impeller (23) to overlap a front end of the inner cylinder (52) to prevent the clad from entering the passage (B) while allowing the purge water to pass through the passage. The nuclear reactor internal pump as claimed in claim 1, wherein the cover portion (58) for covering the front end of the inner cylinder (52) extends radially outwardly from the impeller (23) for covering the front end of the passage (B). Nuclear reactor internal pump having a front end adapted to protrude into a nuclear power reactor vessel (1) and a rear end where a motor case (32) adapted to be fitted to the outside of the reactor vessel (1) is located, comprising: a motor (22) contained in the motor case (32), and a pump shaft (26) connected to the motor (22) and adapted to rotatably pass through the bottom of the pressure vessel (1); an inner cylinder (52) surrounding the pump shaft (26); an impeller (23) fitted to the front end portion of the pump shaft (26); a purge water passage (B) defined between the pump shaft (26) and the inner cylinder (52) adapted to communicate with the inside of the pressure vessel (1); a diffuser (54) disposed on the purge water downstream side of the impeller (23); a purge water inlet (41) for injecting purge water into the motor case (32) and for supplying purge water to the pressure vessel (1) through the water passage (B); the purge water serving for cooling the inside of the motor case (32) and for preventing clad from the pressure vessel (1) from entering the motor case (32); velocity increasing means (72) for narrowing the water passage (B) and for increasing the flow velocity of the purge water directed to the front end at a predetermined part of the water passage (B); characterized in that the velocity increasing means (72) is adapted to increase the local flow velocity at the location of the velocity increasing means (72) to 20 cm/s. The nuclear reactor internal pump as claimed in claim 3, wherein the velocity increasing means comprises a projecting plate (72) provided at the predetermined part of the passage (B) to narrow the predetermined part of the passage compared to the front and the rear ends of the predetermined part. The nuclear reactor internal pump as claimed in claims 3 or 4, wherein a sectional area of the passage (B) at the predetermined part is narrowed less than one half of that at the front and the rear ends. The nuclear reactor internal pump as claimed in claims 3, 4 or 5, wherein the predetermined part is located at a front part of the passage (B). Nuclear reactor internal pump having a front end adapted to protrude into a nuclear power reactor vessel (1) and a rear end where a motor case (32) adapted to be fitted to the outside of the reactor vessel (1) is located comprising: a motor (22) contained in the motor case (32), and a pump shaft (26) connected to the motor (22) and adapted to rotatably pass through the bottom of the pressure vessel (1); an inner cylinder (52) surrounding the pump shaft (26); an impeller (23) fitted to the front end portion of the pump shaft (26); a purge water passage (B) defined between the pump shaft (26) and the inner cylinder (52) adapted to communicate with the inside of the pressure vessel (1); a diffuser (54) disposed on the purge water downstream side of the impeller (23); a purge water inlet (41) for injecting purge water into the motor case (32) and for supplying purge water to the pressure vessel (1) through the water passage (B); the purge water serving for cooling the inside of the motor case (32) and for preventing clad from the pressure vessel (1) from entering the motor case (32); characterized by clad dropping preventive means (82,84) for trapping clad in the passage (B) to prevent the clad from dropping, said clad dropping preventive means comprising a first annular member (82) provided on the pump shaft (26,110) to protrude in the passage outwardly in the direction of a rotation radius of the impeller (23), and a second annular member (84) provided on the inner cylinder (52) rearwardly of the first annular member (82) to protrude in the passage inwardly in the direction of the rotation radius of the impeller (23), such that the front edge portions of the first and second annular members (82,84) overlap each other in the radial direction. The nuclear reactor internal pump as claimed in claim 7, wherein said clad dropping preventive means comprises a clad discharging means for discharging the trapped clad outside the diffuser (24, 54). The nuclear reactor internal pump as claimed in claim 8, wherein the clad discharging means comprises through holes (100) each formed through the inner cylinder (52) to connect the inside and the outside of the inner cylinder (52) to each other and having a diameter larger than that of the clad. The nuclear reactor internal pump as claimed in claim 9, wherein the through holes (100) are inclined rearwardly from the inside toward the outside of the inner cylinder (52). The nuclear reactor internal pump as claimed in claim 9, wherein the through holes (100) extend radially outward from the inside toward the outside of the inner cylinder (52). Nuclear reactor internal pump having a front end adapted to protrude into a nuclear power reactor vessel (1) and a rear end where a motor case (32) adapted to be fitted to the outside of the reactor vessel (1) is located, comprising: a motor (22) contained in the motor case (32), and a pump shaft (26) connected to the motor (22) and adapted to rotatably pass through the bottom of the pressure vessel (1); an inner cylinder (52) surrounding the pump shaft (26); an impeller (23) fitted to the front end portion of the pump shaft (26); a purge water passage (B) defined between the pump shaft (26) and the inner cylinder (52) adapted to communicate with the inside of the pressure vessel (1); a diffuser (54) disposed on the purge water downstream side of the impeller (23); a purge water inlet (41) for injecting purge water into the motor case (32) and for supplying purge water to the pressure vessel (1) through the water passage (B); the purge water serving for cooling the inside of the motor case (32) and for preventing clad from the pressure vessel (1) from entering the motor case (32); a stretch tube (27) arranged around and spaced apart from the pump shaft (26) and whose front end fixes the diffuser (54) to the pressure vessel (1); the purge water supply being connected to the motor case (32) to supply purge water into a space between the pump shaft (26) and the stretch tube (27) and into a space between the pump shaft (26) and the diffuser (54); characterized by clad (86) discharging means (100) provided on the diffuser (54) to discharge clad (86) entered inside the diffuser (54) from the pressure vessel (1) outside the diffuser (54). The nuclear reactor internal pump as claimed in claim 12, wherein the clad discharging means comprises through holes (100) each formed through an inner cylinder (52) of said diffuser (24, 54) to connect the inside and the outside of the inner cylinder (52) to each other and having a diameter larger than that of the clad. The nuclear reactor internal pump as claimed in claim 13, wherein the through holes (100) are inclined rearwardly from the inside toward the outside of the inner cylinder (52). The nuclear reactor internal pump as claimed in claim 13, wherein the through holes (100) extend radially outward from the inside toward the outside of the inner cylinder (52). Nuclear reactor internal pump having a front end adapted to protrude into a nuclear power reactor vessel (1) and a rear end where a motor case (32) adapted to be fitted to the outside of the reactor vessel (1) is located, comprising: a motor (22) contained in the motor case (32), and a pump shaft (26) connected to the motor (22) and adapted to rotatably pass through the bottom of the pressure vessel (1); an inner cylinder (52) surrounding the pump shaft (26); an impeller (23) fitted to the front end portion of the pump shaft (26); a purge water passage (B) defined between the pump shaft (26) and the inner cylinder (52) adapted to communicate with the inside of the pressure vessel (1); a diffuser (54) disposed on the purge water downstream side of the impeller (23); a purge water inlet (41) for injecting purge water into the motor case (32) and for supplying purge water to the pressure vessel (1) through the water passage (B); the purge water serving for cooling the inside of the motor case (32) and for preventing clad from the pressure vessel (1) from entering the motor case (32); characterized by swirling flow generating means (112, 114, 116) for generating a swirling flow of the purge water in the passage (B) caused by the rotation of the pump shaft (26). The nuclear reactor internal pump as claimed in claim 16, wherein said swirling flow generating means comprises spiral projections (112) formed on the pump shaft (26, 110). The nuclear reactor internal pump as claimed in claim 16, wherein said swirling flow generating means comprises vanes (114) formed on the pump shaft (26, 110). The nuclear reactor internal pump as claimed in claim 16, wherein said swirling flow generating means comprises spiral grooves (116) formed on the pump shaft (26, 110).

Inventors: FUJII TAKAO, MINAKAMI KO, OHTOMI KOICHI, SHIOYAMA TSUTOMU, FUJII, TAKAO, MINAKAMI, KO, OHTOMI, KOICHI, SHIOYAMA, TSUTOMU
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: G21C15
IPC codes: G21C15