Patent Number: 055286399
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a typical BWR power/flow operating map showing a conventional protection system having the alarm setpoint a distance A above the operating point 1 and the scram setpoint a distance B above the operating point 1, both setpoints being above the maximum operating line. After startup, the permissible operating range for a BWR is above the cavitation region, below the maximum operating line and bounded by the minimum normal flow line and the maximum normal flow line. In conventional protection systems, when the BWR is operating within the operating zone, an unplanned transient that does not increase the power level above the maximum operating line will not be detected by the setpoints and reactor trip will not occur. The invention overcomes this problem by providing safety system setpoints (at which transient mitigation action is initiated) which are adjusted so that they are much closer to the operating power level (for example, point 1) than conventional protection designs. This principle can be applied as needed throughout the entire normal power/flow operating range shown in FIG. 1. FIG. 2 is an example of a BWR power/flow operating map showing the enhanced protection provided by the invention. If the reactor is operating at 100% power or along the maximum operating line, the setpoints provided by the invention will be automatically adjusted to essentially the same position as in conventional protection systems (a distance of either A or B above operating point 1 in FIG. 1). However, if the reactor is operating at a partial power condition (such as point 1 in FIG. 2), the invention provides alarm and scram setpoints that are closer to that point. In FIG. 2, A1 and A2 represent the adjusted margins between the operating point and two alarm setpoints and B represents the adjusted margin between the operating point and a scram setpoint. The tracking logic in accordance with the invention controls the adjustment of the alarm and scram setpoints so that they are set the desired amount (A1, A2 and B) above any operating condition within the BWR's range of operation. Typical signals and functions included in the invention are shown in FIG. 4. The new portions of logic added by the invention are demarcated from conventional design elements by a dashed line 10. This simplified diagram is-intended to illustrate the essential principles of the invention. It does not show the redundancy necessary for reactor protection functions, nor is it to be construed as the only manner in which the functional logic of the invention can be implemented. Referring to FIG. 4, the invention's tracking scram setpoint logic 12 and tracking alarm setpoint logic 14 (only one alarm function is shown for simplicity) maintain the desired trip margin during planned power increases by automatically increasing the scram and alarm setpoints. These setpoints are respectively used by the scram trip unit 16 and alarm trip unit 18 to monitor the STP signal output from filter 20. Reactor scram can be initiated by the output of a scram signal from either scram trip unit 16 or high power trip unit 22. Planned power changes are identified by a permissive input signal which may be generated manually by the reactor operator or in association with normal methods of increasing power (e.g., control rod withdrawals or recirculation flow setpoint increases). However, when an unplanned power increase occurs, the tracking logic will not increase the setpoints except as controlled, thereby providing enhanced protection. The setpoint adjustment will also automatically track any reactor maneuver which significantly reduces the power level. In this way the protection setpoints are re-established near the new, final operating point. The upper and lower values of the setpoints may also be limited to bound the function of the invention to a desired operating range. The tracking logic of the invention may use one or more alarms (A1, A2) in conjunction with the STP scram signal (B) (shown in FIG. 2). The use of alarm signals to perform active functions to avoid full reactor trip (or scram) is another important attribute of the invention. In addition to alerting the operator, various actions may be initiated at the alarm setpoint(s) (A1, A2) to stop the power increase without imposing the operational penalties associated with total shutdown of the reactor caused by scram. Such actions include, but are not limited to, blocking of control rod withdrawal, reduction of reactor recirculation flow and insertion of selected control rods. An additional feature of the invention is the option to include supplementary adjustment of the high-power trip setpoints based on signals from other reactor parameters. For example, the setpoints may be adjusted in dependence on reactor pressure, reactor recirculation flow or feedwater temperature. The reactor protection system in accordance with the invention also includes the ability to use either the filtered STP signal and/or the direct neutron flux signal (i.e., the "power range monitor signal" in FIG. 4) as the input to the tracking scram setpoint logic 12. The setpoints used with a direct neutron flux signal must be set higher than those used with the STP filter 20 method to avoid inadvertent actuation. FIG. 3 shows an example of an application of the invention with tracking setpoints supplemented by a recirculation flow signal. If the reactor is operating at full power or along the maximum operating line, it is similar in many respects to the example shown in FIG. 2, with the maximum expected setpoints adjusted to be essentially equal to the setpoints of conventional protection systems. The setpoints are also adjusted to be the desired margin above any partial power operating point (for example, operating point 1 in FIG. 3). The unique aspect of this application is that the scram and alarm setpoints are also varied automatically with changes in reactor recirculation flow (the setpoints have a flow-referenced slope above point 1 in FIG. 3). Also shown in FIG. 3 is an example of the alternative to use a direct neutron flux signal in conjunction with the tracking setpoint logic (set above the STP setpoints at C). The amount of the variation with flow (the slope of the setpoint lines above point 1 in FIG. 3) can be chosen to optimize performance of the invention during reactor flow and power maneuvers. The variation of the setpoints with supplemental reactor parameters (e.g., recirculation flow in this example) may also be limited in magnitude and/or direction of change to optimize the effectiveness of the application of the invention. If the flow-referenced option is used, as shown in FIG. 3, the setpoints will also be automatically increased if the power increase is caused by an unplanned reactor recirculation flow increase. However, the setpoint increase will be a controlled amount according to the slope of the flow-dependent setpoint variation. The invention therefore provides enhanced reactor protection by adjusting the trip setpoints so that they remain close to the operating point anywhere in the power/flow operating range of the reactor. In conjunction with this closer safety trip (scram) protection, the invention provides alarms that are simultaneously adjusted so that automatic actions can also be initiated to avoid full shutdown of the unit during transient events. The setpoints automatically track power decreases, but increases of the setpoints are restricted so that they provide enhanced protection for all unplanned transients that increase reactor power. The tracking protection of the invention responds favorably to simulated reactor transients, including the postulated, slow events. A few transient examples are presented hereinbelow to demonstrate the performance of the present invention. EXAMPLE 1 Temperature Transient, Basic Invention One type of event that can occur in a BWR is a change in the temperature of the coolant flow being supplied to the reactor core. One way that this can happen is if a portion of the feedwater heaters fail to operate properly. FIGS. 5A and 5B show the calculated response of the reactor and the enhanced protection logic over time to this type of an event. The initial power is 70% and the reactor is assumed to be operating with maximum normal core flow. This operating condition is a significant amount below the conventional scram setpoint (shown in FIG. 5B). FIGS. 5A and 5B show that as the cooler water reaches the reactor, the power gradually increases. In this case, the STP signal increases almost up to the tracking scram setpoint provided by the invention. In FIG. 5B, the margins to the tracking alarm and scram setpoints are shown as the event progresses. In this example, only one alarm was simulated, and no scram avoidance actions were initiated when the alarm was reached (near 60 seconds, well ahead of when the scram setpoint is approached). The transient simulated in FIGS. 5A and 5B is equal to the maximum change in feedwater temperature currently allowed (100.degree. F.). Any larger change in temperature is unlikely. But should it occur, it would reach the scram setpoint provided by the invention. Therefore, acceptable reactor fuel protection is assured by the reactor protection system of the present invention. In contrast, conventional systems would not provide such protection if the same event were to occur because the conventional STP setpoint is well above the power transient. Therefore, manual operator actions would be required under conventional systems to provide protection. The performance shown in this example applies primarily to the basic invention. However, it also applies to the flow-referenced logic option if the reactor core flow remains constant during the event (manual flow control). Response in automatic flow control with the flow-referenced option is provided in the next example. EXAMPLE 2 Temperature Transient, Flow-Referenced Tracking Option In this example, an unplanned temperature transient similar to the one described in Example 1 is postulated to occur, but the reactor is assumed to be operating at full power in automatic flow control mode. The purpose of the automatic flow control is to hold reactor power at the initial power level setpoint. In this control mode, the reactor recirculation flow is automatically reduced during this event to counteract the power increasing effects of the transient. FIGS. 6A, 6B and 6C show a typical response to this type of event. These figures show that as the simulated temperature change tries to increase the reactor power, the automatic controls decrease the core recirculation flow so that power remains essentially constant. FIGS. 6A and 6B show the response of key reactor parameters versus time. As in Example 1, the currently limiting magnitude of the temperature change has been simulated. The transient settles to a final operating condition without the need for any protection. Since the controlled power level is supported, however, by less core coolant flow, it is approaching a condition in which insufficient cooling may be available to the reactor fuel. FIG. 6C shows how the reactor operating point moves along at constant power, but decreasing core flow characteristic during the simulated event. The invention with the flow-referenced option reduces the tracking setpoints as recirculation flow is reduced, so that by the end of this case, the scram setpoint is just above the final operating point. Tracking alarm actions are ignored in this case. Any larger temperature transient would initiate the new protection. The existing flow-referenced scram setpoint is also shown. It follows the characteristic shown in FIG. 2, and is further away from the operating condition. If the event had been simulated at lower initial power (e.g., 70% as in Example 1), the difference between the operating point and the conventional scram setpoint would be larger, while the setpoint provided by the invention will remain close to the operating point. EXAMPLE 3 Core Flow and Power Increase One common reactor maneuver that must be accommodated without reactor trip is the normal increase of power using the reactor core flow control system. FIGS. 7A, 7B and 7C demonstrate how the invention is able to accommodate this type of maneuver. In this situation, the operators will have planned and prepared for the power increase, and the permissive logic of the invention is activated at the start of the increase. The responses of the reactor and the tracking logic of the invention are shown in FIGS. 7A and 7B. Core flow and power are increased gradually in this ramp-like maneuver. The tracking setpoints of the invention increase with the reactor power. Margin is maintained, as required, between the STP signal and the alarm and scram setpoints. FIGS. 7A and 7B shows responses of key reactor parameters versus time. FIG. 7C shows the tracking action of the setpoint logic in accordance with the invention. The trip avoidance margins for the alarms (two in this example) and the scram are shown at the bottom of FIG. 7B to stay almost equal to the initial margin throughout the maneuver. The preferred embodiments have been disclosed for the purpose of illustration only. Variations and modifications of those embodiments will be readily apparent to engineers of ordinary skill in the art of boiling water reactor protection systems. All such variations and modifications are intended to be encompassed by the claims appended hereto.