Patent Number: 
Section: description

The present invention will be described in detail in conjunction with what are presently considered as preferred or typical embodiments thereof with reference to the drawings. In the following description, like reference characters designate like or corresponding parts throughout the drawings. Also in the following description, it is to be understood that such terms as xe2x80x9crightxe2x80x9d, xe2x80x9cleftxe2x80x9d, xe2x80x9cupperxe2x80x9d, xe2x80x9clowerxe2x80x9d and the like are words of convenience and are not to be construed as limiting terms. FIG. 1 is a sectional view showing generally and schematically a structure of a pressurized water reactor equipped with flow stabilizing members according to a first embodiment of the present invention. In this figure, some components are omitted from illustration. It should first be mentioned that in the pressurized water reactor now under consideration, the internal structure or internals of the reactor are, for the most part, essentially the same as the conventional nuclear reactor described hereinbefore. Accordingly, repetitive description thereof will be unnecessary. The pressurized water reactor according to the instant embodiment of the invention differs from conventional ones in that a short member 1 serving as a flow stabilizing member is mounted within the upper plenum 40 in an outer region extending outside of the fuel region along the inner wall of the core barrel 30 at a position lower than the outlet nozzle 12. In this conjunction, the phrase xe2x80x9couter region extending outside of the fuel regionxe2x80x9d means a region on the upper surface of the upper core plate 21 extending outside of the fuel assembly region 33 (indicated by a double-dotted line in FIG. 2) within the upper plenum 40 the outer periphery thereof corresponding to that of the core. Further, the phrase xe2x80x9cat a position lower than the outlet nozzle 12xe2x80x9d means that the top end of the flow stabilizing member 1 in the mounted state within upper plenum 40 is lower than a lowermost portion of the bore of the outlet nozzle 12. Next, referring to FIGS. 2 and 3, description will be made of a second embodiment of the present invention which is directed to a nuclear reactor for a four-loop plant in which the short members 1 according to the invention are employed as the flow stabilizing members. FIG. 2 is a schematic top plan view showing half of an essential portion of the nuclear reactor, and FIG. 3 is a perspective view showing a region where the short members are mounted in the vicinity of the outlet nozzles. As can be seen from the figures, the short members 1 serving as the flow stabilizing members, respectively, are disposed in the outer region extending outside of the fuel region in the proximity of the inner wall of the core barrel 30. More specifically, the short member 1 is mounted on the upper core plate 21 within the upper plenum 40 in a region extending or located outside of the fuel assembly region 33 (indicated by a double-dotted broken line) at a position close to the inner wall 30a of the core barrel 30. It should, however, be noted that in practical applications, the short member 1 is mounted at a certain distance from the inner wall 30a of the core barrel 30 in consideration of the thermal expansion of the short member 1. Furthermore, the short member 1 is mounted so that the top end thereof is positioned lower than the lowermost portion of a coolant outlet 12a of the outlet nozzle 12. This arrangement is effective for stabilizing the flow of the coolant within the upper plenum 40 in the regions located below the outlets 12a of outlet nozzles 12 as well as for reducing the hydrodynamic load acting on the short members 1. The length of the flow stabilizing member 1 is selected so that the top end thereof is positioned lower than the lowermost portion of the outlet 12a of the outlet nozzle 12 when the flow stabilizing member 1 is mounted on the upper core plate 21. More specifically, the flow stabilizing member 1 should preferably be dimensioned so that the length thereof falls within a range of from a position midway between the upper surface of the upper core plate 21 and the lowermost portion of the outlet 12a of the outlet nozzle 12 to a position lower than the lowermost portion of the outlet 12a. If the top end of the flow stabilizing member 1 is positioned higher than the lowermost portion of the outlet 12a of the outlet nozzle 12, the flow resistance of the coolant flowing within the upper plenum 40 toward the outlet nozzle 12 from a center portion of the reactor core will increase, and the hydrodynamic load acting on the short member 1 will increase as well to ultimately adversely effect the mechanical or structural strength of the internal structure of the reactor. On the other hand, if the length of the short member 1 is excessively short, it is difficult to stabilize the flow of the coolant along the inner wall 30a of the core barrel 30. For these reasons, the flow stabilizing member 1 should be so dimensioned that the length thereof falls within the range defined above. Furthermore, installation of the flow stabilizing member 1 can be realized by securing a bracket 2 which is fixed to the flow stabilizing member 1 onto the upper core plate 21 by means of bolts 3. The short length and ease of mounting of the flow stabilizing members on the upper core plate 21 can facilitate installation of the flow stabilizing members in existing equipment as well. Next, with reference to FIGS. 4 and 5, description will be made of the flow behavior of the coolant in the vicinity of the outlet nozzles 12 in the structure where the short members 1 are installed within the upper plenum in the manner described above. FIG. 4 is a schematic top plan view of half of a nuclear reactor illustrating disposition of the internal structural members within the upper plenum 40 together with the flow of the coolant in the vicinity of the outlet nozzles 12, and FIG. 5 is a schematic side view showing behavior of the coolant flowing in the vicinity of adjacent outlet nozzles 12. As can be seen from the figures, the coolant flows radially outward along the upper surface of the upper core plate 21 from a center region of the core to reach the inner wall 30a of the core barrel 30, whereupon the coolant flows toward the outlet nozzles 12 along the inner wall 30a of the core barrel 30 in the region outside of the outer periphery of the core. Here, a portion of the coolant or light water flowing along the inner wall 30a of the core barrel 30 flows below the outlet nozzle 12. Accordingly, when the short members 1 serving as the flow stabilizing members are not disposed, as in conventional reactors, streams of the coolant flowing in opposite directions collide with each other in a region between the adjacent outlet nozzles 12. In contrast, by disposing the short flow stabilizing members 1 according to the instant embodiment of the invention lower than the outlet nozzles 12, respectively, as mentioned previously, streams F of the coolant along the inner wall 30a can flow smoothly into the outlets 12a of the outlet nozzles 12 under the guiding action of the flow stabilizing members 1, as can be seen in FIG. 5. Further, a stagnation region S occurring between the flow stabilizing members 1 is decreased and the coolant in the stagnation region S is forced to smoothly flow upward into the outlet 12a of the outlet nozzle 12 under the constraining action exerted by the two flow stabilizing members 1. Accordingly, the coolant flow entering the outlet nozzle 12 can stabilized, as a result of which temperature fluctuation of the coolant flowing through the outlet pipe 42 connected to the outlet nozzle 12 can be effectively suppressed. Furthermore, measurement of variation or fluctuation of the temperature within the outlet pipe 42 connected to the outlet nozzle 12 in a demonstration test simulating flow behavior of the outlet nozzle 12 and upper plenum 40 show that temperature fluctuation in the outlet pipe 42 of the reactor equipped with the flow stabilizing members 1 can be suppressed or reduced by approximately half when compared with the structure in which no flow stabilizing members 1 are employed. Furthermore, the flow stabilizing member 1 according to the instant embodiment of the invention is implemented in a hollow cylindrical shape in view of the fact that the hollow cylindrical flow stabilizing member 1 can be manufactured with light weight and relatively low cost. However, the present invention is not restricted to a flow stabilizing member with such a shape. As long as the stream F of the coolant flowing along the inner wall 30a of the core barrel 30 can be stabilized, as described above, flow stabilizing members 1 with different structures such as a solid cylindrical or columnar structure, a plate-like structure, a prism structure or the like can be employed as well. A third embodiment of the present invention is directed to application of the flow stabilizing member to a nuclear reactor for a two- or three-loop plant. First, it is to be noted that the disposition of the flow stabilizing member(s) according to the instant embodiment of the invention can be equally adopted in the four-loop reactor plant described above. FIG. 6 is a schematic top plan view showing an essential portion of the nuclear reactor for a three-loop plant, wherein some components are omitted from illustration, and FIG. 7 is a perspective view showing a region of the upper plenum in the vicinity of the outlet nozzles 12. Here, it should be mentioned that the shape and the length of the flow stabilizing member 1 are essentially the same as those of the flow stabilizing member described hereinbefore. Accordingly, repeated description thereof will be unnecessary. The third embodiment of the invention differs from the preceding embodiments in that the outlet nozzles 12 are not disposed adjacent to each other. The flow stabilizing member 1 according to the third embodiment of the invention is disposed directly underneath a central portion of the outlet 12a of the outlet nozzle 12 in close proximity to the inner wall 30a of the core barrel 30. As mentioned hereinbefore, disposition of the flow stabilizing member in close proximity to the inner wall 30a of the core barrel 30 is very effective for reducing the hydrodynamic load applied to the flow stabilizing member 1 and also stabilizes flow of the coolant F in a space within the upper plenum 40 below the outlet nozzle 12. Now, description will turn to the flow behavior of the coolant in the vicinity of the outlet nozzle 12 in the reactor equipped with the flow stabilizing members 1 according to the third embodiment of the invention. Portions of the coolant flowing opposite to one another along the inner wall 30a of the core barrel 30 tend to collide with each other beneath the outlet nozzle 12. However, because the flow stabilizing member 1 is disposed underneath the center portion of the bore of the outlet nozzle 12, the streams of the coolant flowing along the inner wall 30a of the core barrel 30 are forced to flow upward uniformly at both sides of the flow stabilizing member owing to the flow guiding action thereof. Thus, the coolant can flow smoothly into the outlet 12a defined by the outlet nozzle 12. In other words, collision of the coolant streams flowing opposite to one another along the inner wall 30a beneath the outlet nozzle 12 can be effectively suppressed, whereby the stream of the coolant flowing into the outlet nozzle 12 can be stabilized. Consequently, temperature fluctuation within the outlet pipe 42 connected to the outlet nozzle 12 can be suppressed. Although it has been described that the flow stabilizing member 1 employed in the instant embodiment of the invention is installed underneath a center portion of the outlet of the outlet nozzle 12, it goes without saying that the flow stabilizing member 1 can be mounted at practically any position as long as the mounting position of the flow stabilizing member is covered by a region extending below and across the outlet nozzle 12. What is important is to avoid collision of the coolant streams in the vicinity of the region beneath the outlet nozzle 12, because then the flow of the coolant flowing into the outlet nozzle 12 can be stabilized. Accordingly, when it is impossible to mount the flow stabilizing member underneath the central portion of the outlet nozzle in view of structural limitations, then the flow stabilizing member 1 can be mounted at a position more or less deviated from a position underneath the center of the bore of the outlet nozzle 12 as long as the deviated position lies within the region extending beneath and across the outlet nozzle 12. Thus, the present invention can be applied to existing nuclear reactors without any appreciable difficulty. In the foregoing, exemplary embodiments of the present invention which are considered preferable at present and other alternative embodiments have been described in detail with reference to the drawings. It should, however, be noted that the present invention is not restricted to these embodiments and other variations and modifications can be easily conceived and realized by those skilled in the art without departing from the spirit and scope of the present invention.