Patent Number: 051981858
Section: summary

Discussion of Background In a nuclear reactor, coolant, usually water, is used to remove heat from the fissioning fuel. Depending upon the reactor design, water may also serve to increase the rate of fission by moderating the speed of neutrons so that they are more likely to cause fission in the nuclei of fuel material. In the event of an abnormal occurrence during reactor operations or a reactor accident, the control system of the reactor can be activated to shut down the fission process. Although the rate of fission can be reduced very quickly, the fuel continues to generate considerable heat as a result of the radioactive decay of the fission products that resulted from nuclear fission of fuel material prior to shutdown. A particularly severe type of accident is a loss-of-coolant accident, wherein the flow of coolant to the reactor core is abruptly reduced. Although the core is shut down by activating the control system, the decay heat can be sufficient to cause melting of the fuel material. Good reactor design anticipates this accident scenario and seeks to minimize the effects of decay heat following a LOCA. In normal power operation the distribution of heat produced by the reactor core, that is, the collection and arrangement of nuclear fuel elements, is not uniform. Fuel elements toward the center of the core tend to produce more power than those toward the periphery of the core. If the distribution of power varies too much, several reactor performance-related problems can occur. Therefore, in many reactors, design features are incorporated to reduce power variation across the core and, indeed, to "shape" the power distribution. To permit an increase in total reactor power, the coolant distribution should be shaped to correspond to the power distribution. In certain reactors, coolant flows into a plenum above the core and then down into the core through various orifices. Briefly, and referring to FIG. 1 which illustrates an example of this type of reactor, the coolant flows from the plenum through slots in a first sleeve surrounding each fuel element position, then through an array of holes in a universal sleeve housing into the region directly above the fuel element. An orifice plate is positioned in the sleeve housing, below the holes, to reduce the flow to the element. Each position in the core may have a different orifice plate. The different plates have different numbers and arrangements of holes so that the flow in each position may vary. Reducing the flow in the outer positions and increasing the flow to the more centrally-located elements improves flow distribution generally to the higher powered fuel elements. Unfortunately, this design, although working well during normal operation, does not provide optimum flow distribution during the very low flow conditions that occur in the event of a LOCA and result in an unnecessarily restrictive operating power limit. Various other designs exist to improve reactor flow. In U.S. Pat. No. 4,947,485, Oosterkamp discloses a design for better flow during "load follow" (the adjusting or reactor power level to accommodate changes in electrical demand during the day). His improved flow results from better mixing and by establishing flow between the downcomer region and the chimney. Veronesi provides holes of different sizes and patterns in an upper-core, plenum shroud, as described in U.S. Pat. No. 4,793,966. The use of vanes is described by Dotson, et al. in U.S. Pat. No. 3,623,948 to improve flow distribution at the entrance of the fuel region. Zmola, et al., use a variety of structural elements to force more flow to the hotter, higher power density regions of the core from the cooler, lower-power density regions, as described in U.S. Pat. No. 3,623,999. In particular, Zmola, et al. use entrance reduction elements and holes in the sides of tubular members to achieve the improved flow. There remains, however, a need for improved flow of coolant in reactors under both nominal and accident conditions. SUMMARY OF THE INVENTION According to its major aspects and broadly stated, the present invention is a coolant flow distribution that results in improved flow during accident conditions without degrading flow during nominal conditions. The modification comprises imposing a variation in the number and size of holes in the sleeve housings from one sleeve to another to increase amount of coolant flowing to the fuel in the center of the core and decrease, relatively, flow to the peripheral fuel. Preferably, the holes are arranged in rows and columns from the bottom of the upper portion of the sleeves to the top, where the plenum ends, with all holes having the same diameter and some sleeves having more rows than others to create the different flows. Those sleeves with the greatest number of rows are placed in the center of the core; those with the least are placed in the core periphery. An important feature of the present invention is the use of the variations in the number and possibly the size of holes in the sides of the sleeve to change the flow distribution in the core to a more favorable one. This feature eliminates the orifice plate and improves distribution of coolant flow during both accident and nominal conditions. Moreover, considerable flexibility is available to a designer in varying the number of holes and size of holes to meet a particular power shape across the core. Another feature of the present invention is the variation of the number of rows of holes by eliminating rows of holes from the top of the upper portion of the sleeve. Preferably the core is divided into zones that correspond roughly to rings. Beginning with the next-to-central ring, one row of holes is eliminated with each ring until the peripheral ring is reached. Eliminating a few rows of holes in the outer rings substantially increases flow during LOCA without impacting the flow during nominal conditions. Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of a Preferred Embodiment presented below and accompanied by the drawings.