JPH053558B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] This invention relates to a control rod withdrawal monitoring device for a boiling water reactor, and in particular, it guides the amount of control rod withdrawal during reactor power operation, and monitors the health of the fuel even when control rods are withdrawn. This invention relates to a control rod withdrawal monitoring device that prevents damage to the control rod. [Technical background of the invention and its problems] Boiling water reactors are equipped with a system to monitor the increase in reactor power caused by control rod withdrawal during power operation.
A control rod withdrawal monitoring device is provided. This extraction monitoring device uses a plurality of fixed in-core neutron detectors (hereinafter referred to as this) placed around the selected extraction control rod when a extraction control rod is selected to increase the reactor output. LPRM) is selected and the number of neutron fluxes around the pull-out rod is detected. The neutron flux detection signals from the selected LPRMs are averaged and
When the average value exceeds the average value of the neutron flux signal before withdrawal by a predetermined value, a control rod withdrawal prevention signal that prevents withdrawal of the control rod is output. This prevents the control rods from being pulled out any further, suppressing the increase in reactor power below a predetermined level, and maintaining the safety of the reactor. FIG. 5 is a diagram showing an example of the arrangement of in-core neutron detectors in a conventional control rod withdrawal monitoring device. As is clear from this figure, there are 16 LPRMs, namely in-reactor neutron detectors 2,
3 is placed. These LPRMs 2 and 3 are divided into two systems, for example, A system and B system, and eight LPRMs are allocated to each system. and,
The A-system and B-system LPRMs 2 and 3 have different neutron flux signal strength-control rod withdrawal position characteristics, as shown in FIGS. 6A and 6B, respectively. The neutron flux signal intensity-control rod withdrawal position characteristic curve when all LPRMs 2 and 3 of A and B systems are operating normally is the solid line l,
The response characteristics of both systems are fast. The intersection point P _a of the characteristic curve shown by the solid lines l and m and the preset control rod withdrawal prevention level straight line n,
Draw a perpendicular line from P _b to obtain the control rod withdrawal prevention positions a ₁ and b ₁ . Therefore, in the control rod withdrawal monitoring device, a control rod withdrawal prevention signal is output from the A system and the B system LPRM 2 and 3, whichever system reaches the above-mentioned intersection P _a or P _b earlier. By the way, with the withdrawal of the control rod,
There are variations in the reactor output detected by LPRM. This variation includes some that are undesirable from the viewpoint of preventing control rod withdrawal.
For example, due to a failure of a certain LPRM, each LPRM
If the detected neutron value from the control rod becomes an extremely large output or a small output compared to the average value, it may cause the control rod to be pulled out by mistake. Therefore, from the viewpoint of preventing control rod withdrawal, undesirable LPRMs are bypassed to stop their LPRM detection function, and a control rod withdrawal prevention signal is output based only on the output signal from the preferred LPRM. Therefore, in some special cases, A system,
One LPRM of the B system may be completely bypassed. A system bypass, B system listed in the table below
System bypass means this complete bypass state.

[Purpose of the invention]

This invention has been made in consideration of the above-mentioned circumstances, and includes a method of predicting and calculating in advance changes in the thermal margin of nuclear fuel due to control rod withdrawal based on the current state of the reactor core, and setting a control rod withdrawal prevention position. It is an object of the present invention to provide a control rod withdrawal monitoring device that improves the operational efficiency of a nuclear reactor and improves the ability to monitor control rod operations. [Summary of the Invention] In order to achieve the above-mentioned object, a control rod withdrawal monitoring device according to the present invention uses the withdrawal distance of a selected control rod as a parameter to determine an index of the thermal margin of nuclear fuel with respect to the amount of control rod withdrawal. A prediction calculation device that predicts and calculates the amount of change in the neutron flux signal of the in-reactor neutron detector surrounding the withdrawn control rod, and a prediction calculation device that predicts and calculates the amount of change in the neutron flux signal of the in-reactor neutron detector surrounding the withdrawn control rod, and uses these predicted calculation values as a preset limit value of the nuclear fuel thermal margin index. and a comparison circuit that performs comparison calculations, and the prediction calculation device estimates the removable distance of the retractable control rod within the limit value of the thermal margin index, and calculates the retractable distance of the retractable control rod. The neutron flux signal at the control rod withdrawal limit value is set as the withdrawal limit value, and the neutron flux signal at the control rod withdrawal limit value is set as the limit value of the neutron flux signal for the plurality of in-reactor neutron detectors with large changes in the neutron flux signal, and the comparison circuit During the extraction operation of the selected extraction control rod,
The position signal of the withdrawn control rod and the neutron flux signal around the withdrawn control rod are compared with the control rod withdrawal limit value and the neutron flux signal limit value, respectively, and one of the control rod position signal and the neutron flux signal exceeds the above limit value. When the limit is exceeded, the control rod control device outputs a control rod withdrawal signal. [Embodiments of the Invention] Hereinafter, an embodiment of a control rod withdrawal monitoring device according to the present invention will be described with reference to the accompanying drawings. In FIG. 1, reference numeral 10 indicates a reactor core housed in a reactor pressure vessel (not shown) of a boiling water reactor, and this reactor core, together with a large number of fuel assemblies (not shown), is used to adjust and control the reactor output. A control rod 11 and a fixed type in-reactor neutron detector (LPRM) 12 are stored. The control rods 11 are controlled by a control rod drive mechanism 13 provided below the reactor pressure vessel, and are inserted into and withdrawn from the reactor core 10.
The control rod drive mechanism 13 is operated and controlled by a drive control signal _S1 from the control rod controller 14, and the control rod drive mechanism 13
The withdrawal position of the control rod is detected positionally by the control rod position detection device 15. The control rod position detection signal S ₂ from the detection device detection device 15 is the control rod position detection signal S 2 from the control rod control device 1
4 and the control rod withdrawal monitoring device 16. On the other hand, the LPRM 12 installed in the reactor core 10 is connected to the control rod withdrawal monitoring device 16, and each system
A neutron flux detection signal _S3 from the LPRM 12 is output. By the way, when the extraction control rod 11 to be extracted is selected from among the control rods installed in the reactor core 10, the control rod selection signal _S4 is input to the control rod control device 14. The control rod control device 14 also receives a control rod position detection signal S ₂ from the control rod position detection device 15 to monitor the control rod position, and receives the control rod selection signal S ₄ to control the control rod position. Predictive calculation information S ₅ necessary for predictive calculation of all control rod positions in the current core 10 and selected withdrawn control rod positions is input to the rod withdrawal monitoring device 16 . The control rod withdrawal monitoring device 16 also receives current data from each detector 17 of the reactor plant and core, such as reactor power, core flow rate, core temperature, recirculation flow rate, feed water flow rate, etc. (hereinafter referred to as core data). The neutron flux detection signal (LPRM signal) S ₆ and the neutron flux detection signal (LPRM signal) S ₃ from the LPRM 12 are input. The control rod withdrawal monitoring device 16 predicts and calculates the thermal margin of the nuclear fuel and the change in the LPRM signal _S3 using the withdrawal distance of the selected withdrawal control rod 11 as a parameter from the input information _S3 , _S5 , _S6 . , the distance at which the selected withdrawal control rod 11 can be withdrawn is determined. This removable distance information S ₇ is transmitted to the control rod controller 14 to provide distance information S ₈
will be shown to the operator as On the other hand, when the control rod operation request signal _S9 is input to the control rod controller ₁₄ after the control rod selection signal S4 is input, the control rod controller 14 sends the drive signal _S1 to the control rod drive mechanism 13. The control rod drive mechanism 13 is operated and the selected extraction control rod 11 is extracted. During this withdrawal operation, the withdrawal position of the withdrawal control rod 11 is detected positionally by the control rod position detection device 15, and its detection signal _S2 is input to the control rod withdrawal monitoring device 16. In addition, the control rod monitoring device 16 compares the extracted control rod position detection signal S ₂ and the LPRM signal S ₃ with the nuclear fuel thermal margin limit value set in advance by predictive calculation, and either one of them is the limit value. When the value is exceeded, a control rod withdrawal prevention signal S ₁₀ is output to the control rod control device 14, thereby stopping the drive signal S ₁ and stopping the control rod drive mechanism 13. The control rod withdrawal monitoring and control device 16 is comprised of a prediction calculation device 18 and a comparison circuit 19, as shown in FIG. When selecting a control rod, the prediction calculation device 18
The distance at which the selected withdrawal control rod 11 can be withdrawn;
The limit value of the LPRM signal S ₃ is predicted, and the prediction result information S ₁₁ is output to the comparison circuit 19. The control rod position detection signal S ₂ and the LPRM signal S ₃ during the actual control rod withdrawal operation are input to the comparison circuit 19, and these input signals S ₂ and S ₃ are based on the preset thermal margin of the nuclear fuel. It is compared with the limit value of the index, that is, the control rod withdrawal prevention set value, and when the set value is exceeded, a control rod withdrawal prevention signal _S10 is output. Next, a method of calculating the control rod withdrawal prevention set value by the prediction calculation device 18 will be explained. The prediction calculation device 18 calculates the thermal margin of the nuclear fuel when the selected withdrawal control rod is withdrawn.
Changes in the LPRM signal S ₃ are determined from the core data signal S ₆ by calculating the reactor power distribution, thermal neutron flux distribution, etc. in the reactor core 10 using a nuclear thermal hydraulic calculation code based on the neutron diffusion equation. .
The thermal margin of nuclear fuel is an index for determining the soundness of nuclear fuel, and this nuclear fuel thermal margin index includes critical power ratio (hereinafter referred to as CPR) and linear power density (hereinafter referred to as LHGR). ). In any case, the thermal margin of the nuclear fuel is calculated using the withdrawal distance of the selected withdrawal control rod 11 as a parameter. 3A and 3B show the amount of change in the critical power ratio (ΔCPR) and the amount of change in the linear power density (ΔLHGR) with respect to the withdrawal distance of the control rod 11. The distance at which the selected extraction control rod 11 can be extracted is:
These changes (△CPR and △LHGR) are determined as the point at which the nuclear fuel is considered to exceed a predetermined limit pressure for its health, and the control rod withdrawal distance is determined from the critical power ratio (CPR). The control rod withdrawal distance △L _L determined from the control rod withdrawal distance △L C (see Figure 3 A) and _the linear power density (LCGR)
(See Figure 3B), whichever is smaller (Figure 3C)
). In addition, the control rod withdrawal prevention set point L _B at the control rod withdrawal position is calculated from the current control rod insertion position L _O and the possible displacement (withdrawal) amount △ _L , as shown in Figure 3D _. -△L However, △L is expressed as min(△L _C , △L _L )...(1) and is stored. On the other hand, the LPRM signal S ₃ for the control rod withdrawal distance
The amount of change △R in the LPRM signal S ₃ when the control rod is withdrawn to the possible withdrawal distance △L _C (△L _L ) determined in Figure 3 A and B and the current LPRM signal are calculated. The control rod withdrawal prevention set point R _B of the LPRM signal is determined from the value R _O as R _B =R _O +ΔR (2) and is stored. The control rod withdrawal prevention set point R _B of the LPRM signal is the
Calculated for LPRM. For this reason, LPRM
When the number of 12 is N, there are N pull-out prevention set points R _B (n) where n=1 to N. This number is 16 if the number of selected extraction control rods 11 is one.
However, if the number of extraction control rods 11 is four, for example, the number increases to 64, and all LPRMs are removed during the extraction operation.
Monitoring the signal complicates the configuration of the comparator circuit 19. By the way, in general, in control rod operation, not all the LPRM signals around the operated pull-out control rod 11 change significantly, and the amount of change in the LPRM signal is small. LPRM of this small amount of change
Since the signal does not need to be monitored and the amount of change in the LPRM signal with respect to the control rod withdrawal distance is small, it is not preferable as information for issuing a control rod withdrawal prevention signal. Therefore, the amount of change △R of each LPRM signal
(n) However, the control rod withdrawal prevention set point R _B must be determined only for the set number of control rods where n = 1 to N is larger than a certain set value or the LPRM signal change amount △R (n) is large. Bye. Naturally, LPRMs that have been bypassed due to failure etc. are excluded from this monitoring target. When withdrawing the control rod 11, the control rod position determined in FIG. 4 is the control rod withdrawal prevention set point L _B
When either one of the values and the withdrawal prevention set point R _B of the LPRM signal is exceeded, the withdrawal of the withdrawal control rod 11 is prevented. Figure 4A shows control rod withdrawal prevention due to the LPRM signal from the in-reactor neutron detector, in which the curve P represents the predicted change in the LPRM signal, and the curve q represents the actual change in the LPRM signal. . Control rod withdrawal prevention is performed at point B, where the actual LPRM signal reaches the control rod withdrawal prevention set point R _B. Figure 4B shows the control rod 11 in the control rod withdrawn position.
curves r and s show the withdrawal inhibition of the fourth
Similar to the one shown in Figure A, the predicted and actual
It shows changes in the LPRM signal. Figure 4B shows a case where the actual LPRM signal is smaller than the predicted increase in the LPRM signal;
The control rod 11 is prevented from being withdrawn by the LPRM signal at the withdrawal prevention set point L _B . Thus, having two control rod withdrawal prevention set points greatly increases reliability. In conventional control rod withdrawal monitoring devices, control rod withdrawal is prevented when the actual control rod withdrawal causes the LPRM reading to exceed a predetermined amount compared to before the control rod was withdrawn. In the rod withdrawal monitoring system, the distance that the control rod can be withdrawn is calculated in advance, so the validity of the control rod withdrawal operation, which would result in an excessive increase in output due to the health of the fuel, is checked in advance, making it extremely safe. improve.
In addition, in conventional control rod withdrawal monitoring systems, the control rod withdrawal prevention position is set to ensure that the integrity of the fuel is sufficiently maintained. In the past, control rod withdrawal was prevented even though there was sufficient thermal margin, impeding operability and potentially damaging the economic efficiency of the reactor, but the control rod withdrawal monitoring device according to the present invention prevents the withdrawal of control rods. ,
Considering the current state of the reactor core, the control rod withdrawal prevention position is set individually for each control rod.
Improves drivability. Furthermore, control rod withdrawal prevention is
Reliability and safety are improved because two types of control rod withdrawal prevention setting points are used: one based on the LPRM signal and the other based on the control rod position. Even when multiple control rods are operated and withdrawn at the same time, the number of LPRM signals to be detected can be reduced by monitoring only those with a large amount of change in the LPRM signal predicted in advance. , the circuit configuration is simplified. [Effects of the Invention] As described above, in the control rod withdrawal monitoring device according to the present invention, the predictive calculation device and the comparison circuit are used to determine whether a selected control rod is Predictively evaluate the thermal margin of the fuel when it is withdrawn, estimate the possible withdrawal distance within the limit value of the thermal margin, and set it as the withdrawal limit value. Changes in the neutron flux signal are also predicted, and the neutron flux signal at the control rod withdrawal limit value is set as the neutron flux signal limit value for multiple in-reactor neutron detectors with large changes in the neutron flux signal, and control is performed. When rod withdrawal starts, both the control rod position and the neutron flux signal are monitored, and if either one exceeds the previously set limit value, a control rod withdrawal prevention signal is output to the control rod controller. As a result, the amount of control rods to be withdrawn during reactor power operation is individually guided, improving monitoring performance and allowing selected control rods to be withdrawn within a range that maintains the integrity of the nuclear fuel. The positions of the control rods can be known individually in advance, and if an attempt is made to pull out the control rods, which could impair the integrity of the nuclear fuel, the control rods will be prevented from being pulled out, thereby reducing the operability of the reactor. This has the effect of improving nuclear reactor efficiency, ensuring the integrity of the reactor core and nuclear fuel, and improving the economic efficiency and safety of the reactor.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a control rod drive system of a boiling water reactor equipped with a control rod withdrawal monitoring device according to the present invention;
FIG. 2 is a detailed diagram showing an embodiment of the control rod withdrawal monitoring device according to the present invention, and FIGS. 3A, B, C, and D
4A and 4B are diagrams showing a method for determining the control rod withdrawal prevention set point in the control rod withdrawal monitoring device according to the present invention, and FIGS. 4A and 4B show the control rod withdrawal prevention operation in the control rod withdrawal monitoring device according to the present invention. FIG. 5 is a diagram showing the arrangement relationship between a control rod to be extracted by a conventional control rod extraction monitoring device and a fixed in-core neutron detector.
FIGS. 6A and 6B are characteristic diagrams showing the relationship between signal strength and control rod withdrawal position in a conventional control rod withdrawal monitoring device. DESCRIPTION OF SYMBOLS 10... Reactor core, 11... Extraction control rod, 12... In-reactor neutron detector, 13... Control rod drive mechanism, 14... Control rod control device, 15... Control rod position detection device, 16...
Control rod withdrawal monitoring device, 18... Prediction calculation device, 19
...comparison circuit.

Claims (1)

[Claims] 1 Using the extraction distance of the selected extraction control rod as a parameter, predict and calculate the change in index of the thermal margin of nuclear fuel and the amount of change in the neutron flux signal of the in-reactor neutron detector surrounding the extraction control rod with respect to the amount of control rod extraction. and a comparison circuit that compares and calculates these predicted calculation values with a preset limit value of a nuclear fuel thermal margin index, and the prediction calculation device is configured to calculate the limit value of the thermal margin index of nuclear fuel. Estimate the pullable distance of the pullout control rod within the range, and set the pullout distance as the pullout limit value of the pullout control rod,
Further, for the plurality of in-reactor neutron detectors whose neutron flux signals have a large variation, the neutron flux signal at the control rod withdrawal limit value is set as the neutron signal limit value, and the comparison circuit During the control rod withdrawal operation, the position signal of the withdrawn control rod and the neutron flux signal around the withdrawn control rod are compared with the control rod withdrawal limit value and the neutron flux signal limit value, respectively, and the control rod position signal and neutron flux signal are A control rod withdrawal monitoring device characterized in that when one of the control rod withdrawal signals exceeds the above-mentioned limit value, a control rod withdrawal signal is output to a control rod control device.