Patent Number: 053533208
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Illustrated schematically in FIG. 1 is an exemplary pressure vessel 10 containing a conventional boiling water reactor core 12 submerged in reactor water 14. Other conventional details inside the pressure vessel 10 are not illustrated for clarity of presentation. The pressure vessel 10 is conventionally disposed in a containment building 16 which includes among other things a conventional annular wetwell or suppression pool 18 which surrounds the pressure vessel 10 and is disposed at a suitable elevation above the reactor core 12. The suppression pool 18 contains water 20 which is used for various functions in the normal operation of the power plant. For example, a first supply pipe or line 22 is joined at one end to the suppression pool 18 and at an opposite end to a first inlet nozzle 24 on the pressure vessel 10 and includes a first valve 26 for controlling the flow of water 20 from the suppression pool 18 by gravity into the pressure vessel 10 in the event of loss of coolant accident (LOCA). In the event of a LOCA, the pressure vessel 10 is conventionally depressurized and the first valve 26 is opened for allowing gravity flow of the water 20 into the pressure vessel 10. During normal operation, the first valve 26 is closed and prevents flow through the first supply line 22. The exemplary embodiment illustrated in FIG. 1 is representative of a simplified boiling water reactor (SBWR) wherein the containment building 16 further includes a gravity-driven cooling system (GDCS) which has a GDCS pool 28 containing water 30 disposed at an elevation above both the suppression pool 18 and the reactor core 12. A second supply pipe or line 32 is joined at one end in flow communication with the GDCS pool 28 and at an opposite end to a second inlet nozzle 34 of the pressure vessel 10 disposed at an elevation above the first nozzle 24. Disposed in the second supply pipe 32 is a conventional second valve 36 which is selectively openable, before opening the first valve 26, for allowing the water 30 in the GDCS pool 28 to flow by gravity through the second supply pipe 32 and into the pressure vessel 10 through the second nozzle 34 following a LOCA condition. During normal operation, the second valve 36 is closed. Both the suppression pool 18 and the GDCS pool 28 are conventionally used to provide makeup water into the pressure vessel 10 in the event of a LOCA wherein the break occurs in any of the various pipes (not shown) leading to the pressure vessel 10 except, however, for a break in either of the first and second supply lines 22, 32 themselves. Since the normal level of water within the pressure vessel 10 is higher than the elevation of the first and second nozzles 24, 34, a leak in the supply lines 22, 32 between the nozzles 24, 34 and the respective valves 26, 36 will allow the reactor water 14 to escape from the vessel 10. However, in accordance with the present invention, each of the nozzles 24, 34 has a preferred configuration to provide a relatively low flow resistance and pressure drop in the normal, forward flow direction from the respective pools 18, 28 to the vessel 10, and a relatively large resistance to flow in the backflow direction from the pressure vessel 10 through the nozzles 24, 34. In this way, the nozzles 24, 34 allow gravity draining of the pools 18, 28 into the pressure vessel 10 following a LOCA without substantial flow resistance to provide makeup water into the vessel 10, but in the event of a LOCA created in the supply lines 22, 32 themselves, a substantial flow resistance is created for reducing leakage of the reactor water 14 from the pressure vessel 10 through the respective nozzles 24 or 34. Illustrated in cross-section in FIG. 2 is an exemplary embodiment of the second inlet nozzle 34 joined to the pressure vessel 10 and the second supply pipe 32 in accordance with one embodiment of the present invention. The first nozzle 24 is identical to the second nozzle 34 except for specific dimensions, with the description of the second nozzle 34 applying equally as well to the first nozzle 24. The nozzle 34 includes a tubular body 38 having a longitudinal or axial centerline axis 40 and a proximal end 42 adapted for being fixedly joined to the pressure vessel 10. In the exemplary embodiment illustrated in FIG. 2, the proximal end 42 has a larger diameter than that of the main body 38 and is conventionally welded into the wall of the pressure vessel 10. The body 38 also includes a distal end 44 adapted for being joined to the supply pipe 32, and in the exemplary embodiment illustrated in FIG. 2, the distal end 44 is cylindrical and conventionally welded to the cylindrical supply pipe 32. Extending completely axially through the body 38 is an annular or preferably circular flow channel or passage 46 which includes several portions in serial flow communication from the distal end 44 to the proximal end 42, all disposed coaxially about the centerline axis 40. The portions include an annular first port or inlet 48 at the distal end 44 which is disposed in flow communication with the supply pipe 32 when the distal end 44 is welded thereto. The first port 48 has a first inner diameter D.sub.1 which is constant for a suitable axial distance to provide a substantially constant flow area preferably equal to that of the supply pipe 32. The supply pipe 32 has an inner diameter D.sub.p which, therefore, is preferably substantially equal to the first inner diameter D.sub.1 of the first port 48. The flow passage 46 further includes a throat 50 spaced axially from the first port 48 which has a second inner diameter D.sub.2 which is less than the first inner diameter D.sub.1 to provide backflow resistance through the nozzle 34. A conical channel 52 extends axially from one end thereof at the throat 50 to an opposite end thereof adjacent the nozzle proximal end 42 and has an inner diameter D which increases axially from one end at the throat 50, i.e., D.sub.2, to a maximum or third inner diameter D.sub.3 at its opposite end. A second port or outlet 54 is disposed at the nozzle proximal end 42 and is joined in flow communication with the pressure vessel 10. Accordingly, in the event of a LOCA condition, forward flow of the water 30 from the GDCS pool 28 (see FIG. 1) occurs through the nozzle 34 illustrated in FIG. 2 from the supply pipe 32 at the right to the pressure vessel 10 at the left as represented by the flow arrow labeled 30. The water, therefore, flows through the throat 50 of decreased flow area, which causes flow resistance and a corresponding pressure drop, but then continues to flow through the conical channel 52 which in the forward flow direction to the left as illustrated in FIG. 2 is a diverging channel or diffuser having a preferred half-angle H selected for obtaining diffusion of the water 30 to maximize recovery of the pressure drop resulting from the throat 50 for improving the flow rate of the water 30 in the forward direction into the pressure vessel 10 without flow separation from the walls of the channel 52. However, in the event of a LOCA caused by a break in the supply pipe 32 upstream from the nozzle 34, the reactor water 14 will enter the second port 54 for flow outwardly from the vessel 10 through the nozzle 34 to the site of the break in the supply pipe 32. Since in this backflow direction to the right in FIG. 2, as represented by the phantom arrow labeled 14, the conical channel 52 is a converging channel, it provides increasing resistance to the discharge of the reactor water 14 therethrough, with the throat 50 and first port 48 providing additional pressure losses in the backflow. Although the throat 50 provides a smaller flow area than that of the supply pipe 32, the preferred configuration of the nozzle 34 provides more resistance to flow in the backflow direction from the nozzle 34 than in the forward flow direction into the nozzle 34. In this way, the GDCS pool 28 may be designed to provide a predetermined flow rate of water 30 through the nozzle 34 and into the pressure vessel 10 by gravity, but in the event of a backflow condition through the nozzle 34, resistance to leakage of the reactor water 14 therethrough is provided. In the preferred embodiment, the throat 50 has a substantially constant flow area for a predetermined axial throat length L.sub.t, with the second inner diameter D.sub.2 being the same for the entire length of the throat 50. The preferred length L.sub.t of the throat 50 is at least 10 cm so that the conventionally known homogeneous flow model may be used to calculate blowdown flow rates from the pressure vessel 10. Since the pressure vessel 10 is normally under relatively high pressure and contains steam therein in addition to the reactor water 14, discharge of the reactor water 14 from the vessel 10 through the nozzle 34 will cause the water 14 to flash boil and form steam bubbles. The throat 50 is so configured for ensuring the generation of an equilibrium saturated mixture of steam and water (homogeneous mixture) being discharged from the nozzle 34 into the supply pipe 32. Furthermore, the conical channel 52 is preferably straight with its sidewalls having linearly varying diameters between its two ends over its axial length L.sub.C. In this way, a substantially uniform rate of diffusion is provided in the forward flow direction of the water 30 from the first port 48 and out through the second port 54 for maximizing pressure recovery in the water 30. And, in the backflow direction from the second port 54 and out the first port 48, the decreasing flow area correspondingly increases the pressure drop and, therefore, resistance to flow of the reactor water 14 therethrough. Also in the preferred embodiment, the axial length L.sub.C of the conical channel 52 and the third inner diameter D.sub.3 at its largest end are selected to ensure that flow in the forward direction into the vessel 10 does not separate from the walls of the channel 52 to maximize pressure recovery. In order to further reduce pressure losses in the water 30 in the forward direction through the nozzle 34 into the vessel 10, the flow passage 46 further includes a first bellmouth 56 integrally joining the first port 48 to the throat 50 for providing a relatively smooth transition of decreasing flow area in the forward flow direction from the first port 48 to the throat 50. And, the flow passage 46 further includes a second bellmouth 58 integrally joining the conical channel 52 to the second port 54 to provide a relatively smooth discharge from the channel 52 into the vessel 10 with the second bellmouth 58 increasing in diameter in the forward flow direction from the channel 52 to the second port 54. In a preferred and exemplary embodiment of the present invention, the conical channel 52 has a half-angle H of about 3.8.degree. for the first nozzle 24 and 4.05.degree. for the second nozzle 34 for obtaining maximum pressure recovery from the forward flow of the water 20, 30, while still providing an effective.sup.amount of backflow resistance to the reactor water 14 through the nozzles 24, 34 in the event of a leak downstream therefrom. Accordingly, the nozzle 34 provides relatively low resistance to flow in the normal or inward flow direction with recovery of pressure through the diverging channel 52, while, in the reverse or outward direction from the nozzle 34, provides the necessary restriction in flow by a simple reduction in flow area and ensures a homogeneous bubbly mixture of reactor water 14 and flash steam to further increase backflow resistance. Referring again to FIG. 1, the first nozzle 24 is disposed at an elevation below the second nozzle 34 and is therefore subject to higher head pressure in the reactor water 14 in the vessel 10. The first nozzle 24 may be configured substantially identically to the second nozzle 34 illustrated in FIG. 2 except, however, the respective diameters of the flow passage 46 thereof are preferably made smaller than in the second nozzle 34 to further increase the backflow resistance against the higher driving pressure force of the reactor water 14 in the vessel 10. Since the nozzle 34 includes the throat 50, the nozzle 34 may be used if desired to measure the normal forward water flow through the nozzle 34 from the GDCS pool 28. A first conventional pressure sensor 60 may be preferably operatively mounted in the throat 50 for measuring the pressure, P.sub.1 therein. And, a second conventional pressure sensor 62 may be operatively joined to the pressure vessel 10 for measuring the pressure P.sub.2 therein, adjacent to the nozzle second port 54. The difference in pressure P.sub.1 -P.sub.2 may be determined in a conventional comparator 64 which is effective for providing a flow value proportional to the pressure differential. While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims: