Patent Number: 047175320
Section: description

DETAILED DESCRIPTION The present invention provides a pressure control system for a pressurized water nuclear reactor plant containing an improved two stage sparger for use in the pressurizer relief tank that provides efficient steam distribution and condensation in the pressurizer relief tank while limiting the pressure drop for both normal and anticipated transient without trip events. In FIG. 1, there is schematically illustrated a pressure control system for a pressurized water nuclear reactor which can incorporate the two stage sparger apparatus of the present invention. The pressure control system 1 contains a pressurizer 3, normally formed as a vertical, cylindrical vessel, composed of carbon steel with austenitic stainless steel cladding on all surfaces exposed to primary reactor coolant. Electrical heaters 5 are provided in the bottom portion of the pressurizer 3 and spray nozzles 7 are provided in the upper portion thereof. The pressurizer is designed to accommodate positive and negative surges caused by load transients on the system. A surge line 9, attached to the bottom of the pressurizer 3 connects the pressurizer with the hot leg of a reactor coolant loop. During an insurge, the spray nozzles 7 which are fed from the cold leg of the reactor coolant loop through line 11, spray water into the upper portion of the pressurizer 3 to condense steam in the pressurizer 3 to prevent the pressure in the pressurizer from reaching the setpoint of power operated relief valves 13 in lines 15. During an outsurge, flashing of water to steam and generating of steam by actuation of heaters 5 keep the pressure above the low pressure reactor trip setpoint. The pressurizer 3 is also provided with safety relief valves 17, three being shown in FIG. 1. The safety relief valves 17, in lines 19, are spring loaded or self-activated with back pressure compensation, and loop seals 21 are provided in lines 19 for valve protection. Such loop seals (not shown) are also normally provided in lines 15 for the protection of power operated relief valves 13. The combined capacity of the safety relief valves 17 is equal to, or greater than, the maximum surge rate resulting from the complete loss of load without reactor trip or any other control. Power operated relief valves 13 discharge to line 23 to a pressurizer relief tank 25 which contains a sparger, while safety relief valves 17 discharge into branch lines 27 which communicate with line 23 and to the pressurizer relief tank 25. In such conventional systems, the line 23 to the pressurizer relief tank was normally 12 inches in diameter and the sparger, attached to the end of line 23, within the pressurizer relief tank, was also a 12" diameter pipe with 1/2 inch orifices therein, and having an end cap. In the improved pressure control system of the present invention, the sparger within the pressurizer relief tank is a two stage sparger which is comprised of a primary conduit having orifices, a secondary conduit having orifices, and an intermediate valve means actuated by a pressure differential between the two conduits. Referring now to FIGS. 2 and 3, the two stage sparger 27 in a closed pressurizer relief tank 25 has a primary conduit 31 which has orifices 33 formed in walls 35 thereof, and a secondary conduit 37 attached to the terminus 39 of the primary conduit, the secondary conduit 37 having orifices 41 formed in the walls 43 thereof. The orifices 33 and 41 are formed in long rows, along the horizontal axis of the conduit and are located, in the side walls of the conduit, normally within an area no more than about 30.degree. above and below the horizontal axis. A coupling 45 is attached to the inlet end of the primary conduit 31 for connection thereof to the line 23 of the pressure control system. The two stage sparger is located in the pressurizer relief tank within a supply of liquid coolant 47 such as water, and rupture disks (not shown) are provided on the closed pressurizer relief tank. A valve means 49 (FIGS. 4 and 5) connects the primary conduit 31 and the secondaary conduit 37 together, such that communication between the two conduits is effected, when desired. The valve means 49 preferably comprises a spring loaded check valve having a valve seat 51, closure element 53 and a biasing spring 55 that biases the closure element against the valve seat to close the valve. As illustrated, the terminus 39 of the primary conduit may have a step-down section 57 thereon which enables the use of a secondary conduit 37 of a diameter d' less than the diameter d of the primary conduit 31. As an example of such dimensions, the line 23 from the power operated relief valves and safety relief valves can be about 16 inches in diameter, the primary conduit 31 would be about 16 inches in diameter, and the secondary conduit 37 of about 12 inches in diameter. At least a portion of the orifices 41 in the secondary conduit 37 should be below the level, or horizontal plane. of the orifices 33 in the primary conduit 31 so as to insure that water will pass through the primary conduit 31, and valve 49 to the secondary conduit 37 for discharge therefrom. Also, the total area of flow through the orifices 41 of the secondary conduit 37 should be greater than the total area of flow through the orifices 33 of the primary conduit 31, and preferably about 125 percent of the flow area of the orifices of the primary conduit. This increase in flow area through the orifices in the secondary conduit can be effected by the use of a larger number of such orifices or by providing orifices through the walls of the secondary conduit of a diameter greater than the diameter of the orifices through the walls of the primary conduit. As illustrated, a preferred construction of the secondary conduit 37 has a connecting section 59 communicating with the check valve 49, and is bifurcated (FIG. 2) at cross section 61 to form two leg portions 63 which extend back towards the primary conduit 31. The leg sections have end caps 65 at the ends thereof. The orifice 39 can be formed in the connecting section 59, cross section 61, the leg sections 63, and the end caps 65, the number of orifices and placement thereof along the secondary conduit dependent upon the flow are desired. In the operation of the pressure control system of the present invention, in the event of an anticipated transient without trip, the following would occur. With a loss of load on the reactor, without a reactor trip, or shutdown, the reactor coolant system pressure in the pressurizer 3 increases rapidly. The power operated relief valves 13 and safety valves 17 would open to discharge cold water loop water seals and steam into the line 23 to the pressurizer relief tank 25. The loop seal "plugs", or water seals, compress nitrogen in the line 23 which forces water in the primary conduit 31 out through the orifices 33. As the pressure in the primary conduit 31 exceeds a predetermined pressure, preferably of about 50 psig (such pressure could rise to a value of about 400 psig in a conventional sparger), the check valve 49 opens and the fluid passes to the secondary conduit 37, which, with orifices 41 more than doubles the total flow area for fluid from the sparger. Water in the primary conduit 31 is discharged therefrom to the secondary conduit 37, and out of the orifices 41, reducing the pressure in the primary conduit to below the predetermined value and the check valve 49 closes as steam reaches the pressurizer relief tank through line 23. Steam discharged through the orifices 33 of the primary conduit 31 is condensed in the water 47 in the pressure relief tank 25 since the design of the primary conduit is effective to maintain a pressure differential below the predetermined value that would actuate the check valve. The steam content of the pressurizer 3 is discharged with the pressurizer relief tank at about 50 psig pressure and 200.degree. F. The reactor coolant system pressure increases (2500 psig to 2750 psig) when the pressurizer becomes water filled and water discharge from the pressurizer 3 through power operated relief valves 13 and safety valves 17 to line 23 begins. With the pressurizer filled with, and discharging water, the volumetric discharge through the power operated relief valves and safety valves is unchanged but mass flowrate is increased by roughly a factor of 5. As the water flow increases through the line 23 to the primary conduit 31, valve backpressure increases and the pressure differential between the primary conduit 31 and secondary conduit 37 increases to above the predetermined value and the check valve 49 again opens to provide for flow through the orifices of both sparger conduits, until the reactor is tripped either automatically or manually. The flow through the sparger is illustrated in FIGS. 4 and 5 which illustrate fluid flow through the orifices 33 of the primary conduit 31 when the check valve 49 is closed, and fluid flow through both the orifices 33 of primary conduit 31 and orifices 41 of secondary conduit 37 when the check valve 49 is in open position.