Patent Number: 051924935
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 illustrates a simplified schematic representation of a typical pressurized water reactor-steam generator system in which the method and apparatus of the present invention to provide a median signal selector for feedwater control systems may be employed. Like reference numerals are employed among the various figures to designate like elements. The reactor vessel 50 has coolant flow inlet means 51 and coolant flow outlet means 52. The vessel 50 contains a nuclear core (not shown) consisting mainly of a plurality of clad nuclear fuel elements which generate substantial amounts of heat, depending primarily upon the position of control rods 53. The heat generated by the reactor core is conveyed from the core by coolant flow entering through inlet means 51 and exiting through outlet means 52. The flow exiting through outlet means 52 is conveyed through an outlet conduit 54 to a heat exchange steam generator system 55. The heated coolant is conveyed through heat exchange tubes 56 which are in a heat exchange relationship with water 57 which is used to produce steam. The steam produced by the steam generator 55 is utilized to drive a turbine 58 for the production of electricity as described more fully below. The flow of the coolant is then conveyed from the steam generator 55 through an inlet conduit 59 to inlet means 51. Thus, a closed recycling primary loop couples the reactor vessel 50 and the steam generator 55. The system shown in FIG. 2 is illustrated with one closed fluid flow loop although the number of loops and hence the number of steam generators 55 varies from plant to plant and commonly two, three, or four are employed. The secondary side of the steam generator 55 is isolated from the primary loop by the heat exchange tubes 56. The water 57 in the steam generator 55 is placed into a heat exchange relationship with the primary coolant, whereby the water 57 is heated and converted to a vapor or steam. The vapor flows through a steam conduit 60 to the turbine 58. The steam, after passing through the turbine 58, is condensed in a condenser 61. The condensate or water is returned to the secondary side of the steam generator 55 through conduit 62. Thus, a recycling, secondary loop couples the steam generator 55 to the turbine 58. Completing the description of the system shown in FIG. 2, three water level channels 10, 11 and 12 measure the level of the water 57 in the steam generator 55 and generate water level signals 13, 14 and 15, respectively, representative of the water level 57 in steam generator 55. A steam generator low water level reactor protection system and feedwater control system constructed according to the teachings of the present invention is shown in FIG. 3. The reactor protection system is constructed as follows. Water level signals 13, 14 and 15 generated by water level channels 10, 11 and 12, respectively, are input to water level comparators 16, 17 and 18, respectively. The water level signals 13, 14 and 15 are compared by water level comparators 16, 17 and 18, respectively, to a predefined steam generator water level set point. Low-low water level signals 19, 20 and 21 from water level comparators 16, 17 and 18, respectively, are input to coincidence gate 22. A low-low water level indication from any two of signals 19, 20 and 21 will cause a signal 23 to be generated which is available at an output of coincidence gate 22 to thereby initiate a reactor trip. A reactor trip is accomplished by inserting control rods 5 (shown in FIG. 2) into the nuclear core (not shown) to take the reactor to a subcritical state. Water level signals 13, 14 and 15 also serve as inputs to the feedwater control system. Water level signals 13, 14 and 15 are input to the feedwater control system through electrical isolation devices 70, 71 and 72, respectively. Electrically isolated water level signals 73, 74 and 75 from isolation devices 70, 71 and 72, respectively, serve as inputs to microprocessor 81 which is programmed to serve as the median signal selector 80. Signal 82, representative of the median water level signal, alarm signal 84 and feedwater control system operating mode signal 85 are output through known output interface 83 to the feedwater control system. The operation of the median signal selector 80 may be implemented as illustrated in the flow chart of FIG. 4. The flow chart begins at step 100 where the microprocessor 81 of FIG. 3, through known data acquisition techniques, samples the electrically isolated water level signals 73, 74 and 75. In step 101, electrically isolated water level signal 73 is stored in microprocessor 81 memory as Signal A; electrically isolated water level signal 74 is stored in microprocessor 81 memory as Signal B; electrically isolated water level signal 75 is stored in microprocessor 81 memory as Signal C. The microprocessor 81 then selects the high value between Signal A and Signal B in step 102 and stores the high value in microprocessor 81 memory as Signal D. Program control continues at step 103 where the high value between Signal B and Signal C is selected and stored in microprocessor 81 memory as Signal E. The microprocessor 81, in step 104, then selects the high value between Signal C and Signal A and stores the selected value in microprocessor 81 memory as Signal F. Program execution continues at step 105 where the low value between Signal D and Signal E is selected and stored in microprocessor 81 memory as Signal G. The microprocessor 81 determines the median signal as between Signal A, Signal B, and Signal C in step 106 where the low value between Signal G and Signal F is selected. The median signal 82 is then output by microprocessor 81 in step 107 to the feedwater control system through output interface 83. An example of the operation of the median signal selector 80 follows. Suppose that Signal A, Signal B and Signal C are signals representing 30%, 40% and 50% of maximum steam generator water level. After the high values are selected in steps 102, 103 and 104, Signal D, Signal E, and Signal F are each equal to 40%, 50% and 50% of maximum steam generator water level, respectively. Selection of the low value between Signal D and Signal E in step 105 yields a Signal G of 40% of maximum steam generator water level. Finally, the low value as between Signal G and Signal F, the median signal 82, is equal to 40% of maximum steam generator water level. Thus, the median signal selector 80 will always select the median of Signal A, Signal B and Signal C. A failure high or low of any water level channel 10, 11 or 12 (FIG. 3) will result in the corresponding water level signal 13, 14 or 15, respectively, being rejected by the median signal selector 80 thereby preventing the failure from causing a control system disturbance and initiating a transient which may require protective action. Several failure detection features may also be implemented in the median signal selector 80. These failure detection routines are functionally represented in step 108 of the flow chart of FIG. 4. If the value of any of the electrically isolated water level signals 73, 74 or 75 differs from the value of either of the remaining two signals by more than an allowable predetermined difference value, an alarm signal 84 is generated by microprocessor 81 and is output to the feedwater control system through output interface 83. Additionally, if the value of any of the electrically isolated water level signals 73, 74 or 75 is greater than a predetermined high limit signal value or is less than a predetermined low limit signal value, an alarm signal 84 is generated by microprocessor 81 and is output to the feedwater control system through output interface 83. In either case, the median signal 82 as calculated in step 106 is output to the feedwater control system through output interface 83 in step 107. The detection of a failure of any two electrically isolated water level signals 73, 74 or 75 (difference value, out-of-range) will cause the microprocessor 81 to generate a signal 85 output to the feedwater control system through output interface 83 to effect a transfer of the feedwater control system from automatic to manual. The last median signal 82 calculated by microprocessor 81 in step 106 of the flow chart of FIG. 4 prior to the failure detection will be output through output interface 83 to the feedwater control system in step 107. The median signal selector 80 eliminates the need to postulate the second random water level channel failure as required by standard IEEE-279 because the initiating water level channel failure does not result in a nuclear power plant condition requiring protective action. The median signal selector 80 prevents the failure of a single water level channel 10, 11 or 12 from initiating a feedwater control system transient. It is not necessary, therefore, to postulate the second random failure and, thus, two out of three water level channels 10, 11 and 12 remain in service. These two remaining water level channels are sufficient to satisfy the two out of three reactor trip logic implemented in the low-low water level reactor trip. The low feedwater flow reactor trip logic is, therefore, no longer required. The median signal selector 80 has eliminated the need for the low feedwater flow reactor trip thereby eliminating the need for the feedwater flow channels 27 and 28 and the steam flow channels 25 and 26 in the reactor protection system. While the present invention has been described in connection with an exemplary embodiment thereof, it will be understood that many modifications and variations will be readily apparent to those of ordinary skill in the art. This disclosure and the following claims are intended to cover all such modifications and variations.