Patent Number: 
Section: description

A preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows a control system of a nuclear power plant according to a preferred embodiment of the present invention. A core 2 inside a reactor pressure vessel I comprises hundreds of fuel bundles. The output of a reactor is controlled by a core flow rate that depends on an insertion ratio of control rods 3 into the core and on the number of revolutions of an internal pump B. The control rods 3 are driven by a control rod drive 5, a motor 6 for activating the control rod drive 5 and a motor control circuit 7 for controlling the motor 6. The number of revolution of the internal pump 8 is controlled by a motor 9 and a motor speed control circuit 10. A local power range monitor 4 (hereinafter called the xe2x80x9cLPRMxe2x80x9d 4) for detecting the neutron bundles inside the reactor and a flow rate detector 17 for detecting the core flow rate are disposed inside the core 2 and their detection results are outputted to an automatic thermal limit monitor 11, respectively. The-control rod drive 5 is provided with a control rod position detector 18, and the detection result of this detector 18 is outputted to the automatic thermal limit monitor 11. The LPRMs 4 are disposed inside LPRM strings 40 as shown in FIG. 2. Generally, four LPRMs 4 are disposed inside one LPRM string 40. The portions of each LPRM 4 are called xe2x80x9cA, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDOxe2x80x9d levels from the bottom in an axial direction. The LPRM strings 40 are uniformly disposed inside the core and four LPRM strings 40 encompass sixteen fuel bundles. Referring to FIG. 1, the core monitoring system 12 calculates every ten minutes the maximum linear heat generation rate and the minimum critical power ratio and outputs them to the automatic thermal limit monitor 11. Though the maximum linear heat generation rate and the minimum critical power ratio are calculated every ten minutes in this embodiment, the calculation cycle is not particularly limited and may be an hour, for example. Needless to say, however, the shorter the calculation cycle, the higher becomes reliability of the thermal limit monitor value. The automatic thermal limit monitor 11 will be explained hereby in detail. FIG. 3 is a flowchart showing the processing content in this automatic thermal limit monitor 11. At the first step 301, the automatic thermal limit monitor 11 judges whether or not both maximum linear heat generation rate and minimum critical power ratio are calculated afresh by the core monitoring system 12. The flow then proceeds to the step 302 when the monitor 11 judges that the calculation is done afresh, and to the step 305 when it judges that the calculation is not done afresh. The maximum linear heat generation rate and the minimum critical power ratio as the calculation results of the core monitoring system 12 are fetched from the core monitoring system 12 at the step 302. At the next step 303, the maximum linear heat generation rate and the minimum critical power ratio are compared respectively with the thermal limit values that are set in advance for them, and whether or not the maximum linear heat generation rate and the minimum critical power ratio exceed the thermal limit values is judged. The flow proceeds to the step 304 when at least one of the maximum linear heat generation rate and the minimum critical power ratio exceeds the corresponding thermal limit value, and to the step 305 if neither of them is judged as exceeding the respective thermal limit value. An automatic operation exclusion instruction is outputted to the automatic power regulator system 13 at the step 304. The detection result of the LPRM 4 and plant data such as the control rod position information are fetched at the step 305. At the next step 306 is made a brief calculation of another maximum linear heat generation rate and another minimum critical power ratio on the basis of the plant data, the maximum linear heat generation rate and the minimum critical power ratio that are so fetched. The another maximum linear heat generation rate and the another minimum critical power ratio, that are determined by the brief calculation at the step 306, are then compared with the thermal limit values that are set beforehand for them, respectively, and whether or not these another maximum linear heat generation rate and minimum critical power ratio exceed the respective thermal limit values is judged at the next step 307. The flow proceeds to the step 308 if at least one of the another maximum linear heat generation rate and the minimum critical power ratio is judged as exceeding the thermal limit value, and to the step 309 if neither of them is judged as exceeding the thermal limit values. At the step 308, an operation hold instruction is outputted to the automatic power regulator system 13. Incidentally, the automatic power regulator system 13 enters the operation hold mode when the operation hold instruction is inputted thereto. Whether or not the automatic power regulator system 13 is in the operation hold mode is judged at the step 309. The flow proceeds to the step 310 if the automatic power regulator system 13 is judged as being in the operation hold mode. After the operation hold release instruction is outputted to the automatic power regulator system 13 at the step 310, the flow then returns to the step 301. If the automatic power regulator system 13 is not judged as being in the operation hold mode, the flow returns as such to the step 301. Next, the calculation method of the maximum linear heat generation rate and the minimum critical power ratio at the step 306 will be explained in detail. To calculate the another maximum linear heat generation rate, the area comprising the sixteen fuel bundles (with the index being defined as xe2x80x9cMxe2x80x9d) is divided into four segments (with the index being defined as xe2x80x9cKxe2x80x9d) in the A, B, C and D levels of the LPRM 4 in the axial direction, and the another maximum linear heat generation rate ALHGR (K, M) inside each divided area is calculated. The another maximum linear heat generation rate of the core is the greatest value among the another maximum linear heat generation rates ALHGR (K, M) determined for each area. The another maximum linear heat generation rate ALHGR (K, M) in the area (K, M) is obtained by correcting the maximum linear heat generation rate CLHGR (K, M) inside the area (K, M) calculated by the core monitoring system, on the basis of the mean value ALPRM (K, M) of the present LPRM of the area (K, M) and on the basis of the CLPRM (K, M) of the mean value of the LPRM, when the core monitoring system calculates the area (K, M), and also by executing the correction B (K, M) in accordance with the change of the control rod position from the start of the calculation by the core monitoring system. The calculation formula of the another maximum linear heat generation rate by the automatic thermal limit monitor 11 is given by the following expression (1): xe2x80x83ALHGR(K, M)=CLHGR(K, M)xc2x7ALPRM(K, M)/CLPRM(K, M)/B(K, M)xe2x80x83xe2x80x83(1) Here, the LPRM mean values ALPRM (K, M) and CLPRM (K, M) are the mean of the four LPRM indication values of each level of the height A, B, C and D in the LPRM axial direction as shown in FIG. 2. When the another minimum critical power ratio is calculated, the minimum critical power ratio inside each area comprising the sixteen fuel bundles is calculated, and the another minimum critical power ratio of the core is the smallest value among the another minimum critical power ratios ACPR(M) calculated for each area. The another minimum critical power ratio inside the area M is calculated by correcting the minimum critical output ratio CCPR(M) of the area M, that is calculated by the core monitoring system, by the mean value ALPRM(M) of the present LPRM of the area M and the LPRM mean value CLPRM(M) at the time of calculation of the area M by the core monitoring system, and by executing also the correction AC(M) in accordance with the change of the control rod position from the start of the calculation of the core monitoring system as well as the change in the core flow rate. The calculation formula of the another minimum critical power ratio by the automatic thermal limit monitor is represented by the following expression (2): ACPR(M)=CCPR(M)xc2x7CLPRM(M)/ALPRM(M)xc2x7AC(M)xc2x7AF(M)xe2x80x83xe2x80x83(2) Here, the LPRM mean values ALPRM(M) and CLPRM(M) are the mean values of twelve LPRMs other than the LPRM of the A level as shown in FIG. 4. FIG. 5 shows an example of the changes, with time, of the maximum linear heat generation rate calculated by the core monitoring system 12 and the another maximum linear heat generation rate calculated by the automatic thermal limit monitor 11. Though the maximum linear heat generation rate will be explained hereinafter, the explanation holds true of the minimum critical power ratio. In FIG. 5, the core monitoring system 12 calculates the maximum linear heat generation rate in the 10-minute cycle, and the automatic thermal limit monitor 11 executes the brief calculation of the another maximum linear heat generation rate between the calculations by the core monitoring system 12. As can be understood from FIG. 5, calculation accuracy of the another maximum linear heat generation rate, that is briefly calculated by the automatic thermal limit monitor 11, gets gradually deteriorated immediately after the calculation by the core monitoring system 12. The calculation error reaches the maximum immediately before the next calculation by the core monitoring system 12 and calculation accuracy is again improved immediately after this calculation. As shown in FIG. 5, the automatic thermal limit monitor 11 is designed in such a fashion that the calculation value of the another maximum linear heat generation rate becomes conservative. Therefore, a smaller value is set to the automatic thermal limit monitor 11 so as o secure a certain margin with respect to the practical thermal limit value. For this reason, the another maximum linear heat generation, rate calculated by the automatic thermal limit monitor 11 is likely to exceed the thermal limit value when the core condition exists in the proximity of the thermal limit value, or when the operation quantity of the fuel rods is great during the calculation by the core monitoring system 12. When the maximum linear heat generation rate reaches the thermal limit value as indicated by the point A in the drawing, the automatic thermal limit monitor 11 judges that the maximum linear heat generation rate exceeds the thermal limit value and outputs the operation hold instruction to the automatic power regulator system 13 (steps 307 and 308 in FIG. 3). Thereafter, when the calculation result by the core monitoring system 12 does not exceed the thermal limit value as indicated by the point B, the automatic thermal limit monitor 11 outputs the operation hold release instruction to the automatic power regulator system 13 (steps 303, 309 and 310). Incidentally, when the calculation result by the core monitoring system 12 exceeds the thermal limit value, the automatic thermal 1 imit monitor 11 outputs the automatic operation exclusion instruction to the automatic power regulator system 13 (steps 303 and 304). As described above, the operation hold instruction, the operation hold release instruction and the automatic operation exclusion instruction outputted from the automatic thermal limit monitor 11 are all outputted to the automatic power regulator system 13. Incidentally, a plant activation collective monitoring system 14 instructs at each break point a target electric output and an electric output change ratio to the automatic power regulator system 13 as shown in FIG. 1. FIG. 6 is a flowchart showing the processing content of the automatic power regulator system 13. At the first step 601, whether or not the automatic operation exclusion instruction is inputted from the automatic thermal limit monitor 11 is judged, and the flow proceeds to the step 602 when the automatic operation exclusion instruction is inputted, and to the step 603 when it is not inputted. The link of the automatic power regulator system 13 with the control rod control system 15 and with the re-circulation flow control system 16 is cut off at the step 602. when the link is cut off, the control rod control system 15 and the re-circulation flow control system 16 cannot be controlled any longer from the automatic power regulator system 13. To execute again the automatic power regulation by the automatic power regulator system 13, the link of the automatic power regulator system 13 with the control rod control system 15 and with the re-circulation flow control system 16 must be established once again, and the intervention of the operators becomes necessary. At the step 603, whether or not the operation hold instruction is inputted from the automatic thermal limit monitor 11 is judged, and the flow proceeds to the step 605 when the operation hold instruction is inputted and to the step 604 when it is not. When the flow proceeds from the step 603 to the step 604, the present electric output data are fetched from the plant. Next, a PI process is executed at the next step 606 by using the error between the present electric output so fetched and the target electric output inputted from the plant activation collective monitor system 14. The control rod operation or the re-circulation flow rate operation is carried out at the next step 607 on the basis of the result of the PI process of the step 606 so that the output can rise from the present electric output to the target electric output at a given electric output change ratio. The control rod operation is carried out by outputting the rod pull-out instruction/rod insertion instruction to the control rod control system 15, while the re-circulation flow rate operation is carried out by outputting a flow rate set value to the re-circulation flow control system 16. When the flow proceeds from the step 603 to the step 605, the instructions from the automatic power regulator system 13 to the control rod control system 15 and to the re-circulation flow control system 16 are held. In this holding state, the link of the automatic power regulator system 13 with the control rod control system 15 and with the re-circulation flow control system 16 remains established. At the next step 608, whether or not the operation hold release instruction is inputted from the automatic thermal limit monitor 11 is judged. The flow proceeds to the step 609 when the operation hold release instruction is inputted, and the hold state is released at this step 609. When the operation hold release instruction is not judged as being inputted at the step 608, on the other hand, the process of the step 608 is repeated until the operation hold release instruction is inputted. After the hold state is released at the step 609, the present electric output data is fetched from the plant at the step 610. A tie-back process (initialization process), is executed at the next step 611. Because the process returns to the normal PI control by the tie-back process, the control of the control rod and the recirculation flow rate is started once again. The control rod control system 15 controls the control rods by controlling the motor drive control circuit 7 in accordance with the rod pull-out instruction/insertion instruction that is inputted, and the re-circulation flow control system 16 controls the motor speed control circuit 10 on the basis of the inputted flow rate set value. The motor drive control circuit 7 and the motor speed control circuit 10 control the control rods 3 and the internal pump 8, respectively. In the embodiment described above, when at least one of the maximum linear heat generation rate and the minimum critical power ratio determined by the brief calculation exceeds the thermal limit value, the control signal is held while the link of the automatic power regulator system 13 with the control rod control system 15 and the re-circulation flow control system 16 remains established. Therefore, this embodiment can prevent the increase of the reactor output and can smoothly start once again the control of the control rods and the recirculation flow rate without the necessity for reestablishing the link when the calculation result of the maximum linear heat generation rate and the minimum critical power ratio obtained subsequently by the core monitoring system 12 is below the thermal limit values. In other words, this embodiment can easily execute the automatic output regulation by the automatic power regulator system 13 even when the maximum linear heat generation rate and the minimum critical power ratio are determined by the brief calculation in a short cycle. Particularly when the core condition is in the proximity of the limit values, the possibility of the occurrence of the phenomena described above is high, and this embodiment is effective for continuation of the automatic operation. If the maximum linear heat generation rate and the minimum critical power ratio that are determined accurately by the core monitoring system 12 are below the thermal limit values while the automatic power regulator system 13 is holding the control signal, automatic output regulation can be continued by stopping the holding operation of the control signal by the automatic power regulator system 13 if the actual maximum linear heat generation rate and the actual minimum critical power ratio are below the set values. Furthermore, if at least one of the maximum linear heat generation rate and the minimum critical power ratio that are accurately determined by the core monitoring system 12 exceeds the thermal limit value while the automatic power regulator system 13 is holding the control signal, automatic output regulation can be stopped by stopping the output of the control signal in the automatic power regulator system 13 when the actual maximum linear heat generation rate and the actual minimum critical power ratio exceed the thermal limit values. To quickly stop the control of the control rod and the core flow rate when exclusion of the automatic operation is required, the automatic thermal limit monitor 11 may directly output the operation stop signal to the control rod control system 15 and to the re-circulation flow control system 16. When the LPRM 4 as one of the means for detecting the plant data gets out of order such as short-circuit, the automatic thermal limit monitor 11 cannot calculate the maximum linear heat generation rate and the minimum critical power ratio. Therefore, the operation hold instruction may be given to the automatic power regulator system 13 in this case, too. If the calculation result by the core monitoring system 12 does not exceed the thermal limit value after bypassing the LPRM 4 which is out of order, the operation hold release instruction may be given to the automatic power regulator system 13. In this embodiment, the calculation of the maximum linear heat generation rate and the minimum critical power ratio in the core monitoring system 12 is made periodically (every 10 minutes). However, it is also possible to output a signal that requests the calculation of the maximum linear heat generation rate and the minimum critical power ratio to the core monitoring system 12 when the automatic thermal limit monitor 11 outputs the operation hold instruction to the automatic power regulator system 13, and to calculate the maximum linear heat generation rate and the minimum critical power ratio by the core monitoring system 12 upon receiving this request signal. Since it becomes possible by so doing to immediately judge whether or not the automatic output regulation is to be continued in the hold state of the automatic power regulator system 13, the automatic output regulation can be executed more smoothly.