Patent Number: 061730289
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings. In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as "right", "left", "top", "bottom" and the like are words of convenience and are not to be construed as limiting terms. FIG. 1 is a partially enlarged vertical sectional view showing in detail a structure of a upper plenum and associated components in a pressurized water reactor in which short-length tubes 5 are employed as the heated fluid guide members of short length according to an embodiment of the present invention. In the figure, the structural or component members of the pressurized water reactor are, for the most part, essentially the same as those of the conventional reactor described hereinbefore. Accordingly, repetitive description thereof will be unnecessary. In this conjunction, it should first be mentioned that the short-length tubes 5 serving as the heated fluid guide member according to the present invention can be installed additionally as fresh members or components at all available locations in an outer peripheral region within the upper plenum 40 which are not occupied by any existing internal component members. Alternatively, the short-length tubes 5 may be disposed restrictively only at locations in an outlet or exit region adjacent to the outlet nozzle 12. The short-length tubes 5 according to an illustrated embodiment of the present invention are formed in a sleeve-like or tubular shape and have a top end and a bottom end both of which are opened. Thus, when the short-length tubes 5 are mounted on the upper core plate 21, the tubes 5 are brought into fluid communication with the reactor core by way of through-holes formed in the upper core plate 21 so that the coolant leaving the reactor core can flow into the short-length tubes 5 to flow therethrough, being ultimately ejected from the top open ends of the short-length tubes 5 into the upper plenum 40. At this juncture, it is to be mentioned that the overall length of the short-length tubes 5 should preferably be so selected that when they are moved on the upper core plate 21, the open top end of the short-length tubes 5 assume a height level or vertical position which is lower than the bottom of the bore of the outlet nozzle 12. More preferably, the length of the short-length tubes 5 should be so determined that the open top ends thereof are positioned substantially midway between the upper surface of the upper core plate 21 and the lower end of the outlet nozzle 12 in the mounted state. When the short-length tubes 5 have an excessively long length, the flow resistance to the coolant stream flowing through the upper plenum 40 toward the outlet nozzle 12 from the center portion of the core will increase to thereby exert undesirable influence on the strength of the structural members such as the upper core support columns 23 and the control rod cluster guide tubes 22. The short-length tubes 5 of the length determined as mentioned above can thus be mounted at all available locations on the upper core plate 21 substantially along the outer periphery of the core which are not occupied by existing structural members such as the upper core support columns, the control rod cluster guide tubes and others. Furthermore, with the length of the short-length tubes 5 mentioned above, the low temperature peripheral coolant stream can be positively discharged or introduced into the high temperature center coolant stream without fail under the effect of inertia of the coolant ejected from the short-length tubes 5. Furthermore, it should also be mentioned that the short-length tubes 5 can be installed within the upper plenum 40 with ease because of the short length thereof. Of course, the short-length tubes 5 can be installed in existing equipment without any appreciable accompanying difficulty. Referring to FIG. 1, it can be seen that a slot-formed tube 1 is mounted adjacent to a short-length tube 5. The structure of the slot-formed tube 1 is disclosed in detail in Japanese Patent Application No. 10-284532 entitled "APPARATUS FOR PROMOTING INTERMIXING OF HEATED FLUID STREAMS IN A NUCLEAR REACTOR" filed by the inventor(s) of the present application. Parenthetically, the disclosure of this preceding application is incorporated herein by reference. Described briefly, the slot-formed tube 1 has an open bottom end so that when it is mounted on the upper core plate 21, the slot-formed tube 1 is brought into fluid communication with the reactor core by way of through-holes formed in the upper core plate 21. Further, a plurality of slots 2 are formed in the slot-formed tube 1 in a window-like fashion at locations corresponding to the height level (vertical position) of the outlet nozzle 12 installed in the nuclear reactor vessel 10. Parenthetically, it should also be mentioned that the slot-formed tube 1 has a reduced diameter portion reduced at a ratio of about 20% of the diameter substantially over two thirds of the whole length thereof in order to prevent excessive hydrodynamic load from being applied to the structural members disposed within the upper plenum 40. Now, description will turn to disposition of the short-length tubes 5 within the upper plenum 40 by reference to FIG. 2. FIG. 2 is a plan view of a quarter cross section of the nuclear reactor vessel 10 and shows only major structural members disposed within the upper plenum 40. In this case, the slot-formed tube 1 mentioned above is also employed in combination with the short-length tube 5 according to the invention. In FIG. 2, each of the control rod cluster guide tubes 22 is shown in the form of a plain rectangle, the upper core support columns 23 are represented by X-like patterns, respectively, and the slot-formed tubes 1 are indicated by hatched circles, respectively. Further, the positions at which the short-length tubes 5 according to the present invention are mounted are indicated by plain circles, respectively. Next, description will be made of the flow behaviors of coolant or light water within the core in the nuclear reactor vessel 10 having the upper plenum 40 within which the short-length tubes 5 with the structure described above are installed. In this conjunction, it is noted that the flow behaviors of the light water is, for the most part, similar to that in the conventional reactor described hereinbefore. Accordingly, the following description will be directed to the flows or streams of light water within the upper plenum 40 and the outlet nozzle 12 by reference to FIG. 1. As described hereinbefore, the stream (indicated by an arrow d) of the coolant or light water flowing through the center portion of the reactor core where the nuclear fission reaction is vigorous is heated at a relatively high rate up to a relatively high temperature. Then, the heated coolant leaves the reactor core into the upper plenum 40 and flows along and through the control rod cluster guide tubes 22. Thus, the coolant reaches the lower surface of the upper core support plate 20 to be thereby deflected in a transverse direction toward the outlet nozzle 12, as indicated by the arrows e and f in FIG. 2. On the other hand, the peripheral stream a of the coolant flowing through the peripheral portion of the reactor core where the neutron flux density is relatively low is heated to a temperature which is relatively low when compared with the center stream of the coolant. Thus, the coolant passed through the peripheral portion of the reactor core enters the upper plenum 40 from the reactor core at a relatively low temperature and flows into the short-length tubes 5 disposed appropriately to be finally ejected therefrom upwardly into the upper plenum 40 through the open top ends of tubes 5. Although the open top end is positioned at a height lower than the bottom end of the outlet nozzle 12, the coolant can be ejected upwardly from the top end of the short-length tubes 5 under the effect of inertia of the coolant flowing therethrough and can flow toward the outlet nozzle 12 within the upper plenum 40, as indicated by arrow b'. As will now be appreciated, owing to the arrangement that at least some part of the coolant of relatively low temperature is ejected upwardly through the short-length tubes 5 in the manner described above, the peripheral coolant stream of relatively low temperature and the center coolant stream of relatively high temperature are intermingled or intermixed to a sufficient extent, whereon the intermixed coolant flows into the outlet nozzle 12, as indicated generally by an arrow A. As a result of this, temperature distribution of the coolant (i.e., light water) flowing through the outlet nozzle 12 can be made uniform. Thus, the mean temperature of the coolant can be measured with high accuracy and reliability. Furthermore, the short-length tubes 5 allow the center coolant stream of high temperature to flow smoothly above the short-length tubes 5 toward the outlet nozzle 12 because of the short length of the tubes 5. Thus, the hydrodynamic load applied to the structural member disposed in the vicinity of the outlet nozzle 12 can be reduced. Furthermore, intermixing of the peripheral coolant stream of low temperature and the center coolant stream of high temperature can be promoted further. Additionally, when the short-length tube 5 according to the present invention is employed in combination with the slot-formed tube 1, there can be obtained advantageous effects provided by the slot-formed tube 1. More specifically, some part of the coolant of relatively low temperature flows into the upper plenum 40 from the slot 2 formed in the slot-formed tube 1 at a location corresponding to the height of the outlet nozzle 12. Thus, the peripheral coolant stream of relatively low temperature and the center coolant steam of relatively high temperature can be intermixed appropriately, whereon the intermixed coolant flows into the outlet nozzle 12, as indicated generally by the arrow A. As a result of this, the temperature distribution of the coolant (i.e., light water) flowing through the outlet nozzle 12 is made more uniform. Thus, the coolant intermixed more sufficiently can flow through the outlet pipe 42 to ensure measurement of the mean temperature of the coolant with enhanced accuracy. In the foregoing, exemplary embodiments of the present invention which are considered preferable at present and other alternative embodiments have been described in detail by reference to the drawings. It should, however, be noted that the present invention is never restricted to these embodiments but other numerous variations and modifications of the structure for promoting intermixing of the heated fluid streams can be easily conceived and realized by those skilled in the art without departing from the spirit and scope of the present invention.