JPH0812269B2 - Control rod withdrawal monitoring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention monitors the local output around the control rod to be pulled out, and when this output exceeds a predetermined set value. The present invention relates to a control rod withdrawal monitoring device that prevents the withdrawal of control rods.

(Prior Art) Generally, in a boiling water reactor (hereinafter referred to as BWR), as shown in FIG. 6, four fuel assemblies 2 are arranged around a control rod 1 having a cross-shaped cross section, which is arranged in a unit cell 3 The unit lattices 3 are arranged in a lattice form as shown in FIG. 7 to form a core 4 having a substantially circular planar shape. The reactor core 4 having such a configuration is installed in the reactor pressure vessel 9 shown in FIG. In addition, in FIG. 7, one square represents the fuel assembly 2 and the white circle represents the control rod 1.

A number of power range neutron detectors (hereinafter referred to as
5) is installed. Many of these LPRM5
In the plan view, every two unit lattices 3 are arranged as shown in FIG. 7, in other words, every two control rods 1 are arranged, and in the vertical direction there are four stages as shown in FIG. It is installed all over.

The output from a part of the LPRM 5 is processed by an average power range monitor system (APRM) (not shown) to detect the average output of the entire core 4. Also, a local output is detected by a local output area monitor system (LPRM) (not shown). When pulling out the control rod 1, the local output around it is monitored, and if this local output exceeds a predetermined control rod pull-out prevention set value due to improper pulling of the control rod 1. , The withdrawal of the control rod 1 is prevented to ensure the soundness of the nuclear reactor.
The control rod pull-out monitoring device 6 performs such control. For example, it is assumed that the control rod 1a at the position shown by hatching in FIG. 7 is the target of the pulling operation. In this case, 16 (4 × 4 stages) LPRM detectors 5 that surround the control rod 1a are selected, and these 16 LPRM detectors 5 constitute the local output range monitor system. The local power around the control rod 1a is monitored.

The LPRM5 is roughly divided into two systems to increase its reliability. Let us assume that one system is the A system and the other system is the B system. Then, the average value of the output of LPRM5 is calculated separately for each of the A system and the B system, and either the average value of the A system or the average value of the B system exceeds the preset control rod withdrawal prevention set value. In this case, the withdrawal prevention signal S6 is output from the control rod withdrawal monitoring device 6 to the control rod control device 7. The control rod control device 7 outputs a signal S7 to the control rod drive mechanism 8,
This prevents the control rod 1 from being withdrawn. When the LPRM detector 5 has an abnormality, the control rod pull-out monitoring device 6 excludes the LPRM detector 5 in which the abnormality has occurred and leaves the remaining LPs.
The average value of the output of the RM detector 5 is calculated. Further, when an abnormality occurs in the entire A system or B system, the system having the abnormality is excluded and the remaining systems are monitored. This improves reliability.

However, the control rod pull-out monitoring device 6 having the above-mentioned configuration is based on the premise that the number of pull-out control rods is one, and there is a problem that good control cannot be performed when there are a plurality of pull-out control rods. . That is, in recent years, in the control rod operation of BWR, a method of simultaneously controlling a plurality of control rods is adopted instead of the conventional control of one rod at a time. This is to reduce the number of times of pulling out the control rod to shorten the startup time and improve the control rod operability, and such an operation is called a control rod gang operation. Therefore, when the control rod gang operation for simultaneously pulling out the pull-out control rods 1a, 1b, 1c and 1d shown by hatching in FIG. 2 is performed, if the above-mentioned conventional control rod pull-out monitoring device 6 attempts to monitor it. Then, one of the pull-out control rods 1a to 1d is selected and the local power distribution in the vicinity of this one control rod is monitored.

Here, a comparison will be made between the case where a plurality of control rods are simultaneously pulled out and the case where a single control rod is pulled out. When a plurality of control rods are pulled out at the same time, even if the same amount is pulled out, the reactivity to be injected into the reactor is larger than when a single control rod is pulled out. The output also rises sharply. Along with this, the output rise near the withdrawal of the control rod also becomes sharp. In this way, when a plurality of control rods are pulled out at the same time, compared with the case where one control rod is pulled out, the output in the vicinity of each control rod rises sharply even if the amount of pulling out is the same. And, there are variations depending on the control rod. Therefore, the desired purpose cannot be achieved by arbitrarily selecting and monitoring the vicinity of the control rod as described above.

By the way, there is a minimum limit power ratio (hereinafter referred to as MCPR) as an index showing the degree of heat removal from the fuel to the coolant. This is the ratio (CP / BP) of the limit CP, which is the limit of the heat removal of the fuel due to the change of the boiling mechanism and the material temperature rises, which causes fuel loss, and the output BP of the fuel assembly 2 in operation. It shows the minimum value of. And it is decided that this MCPR is operated at a preset control value or more. This change in MCRP is considerably worse in the case of the control rod gearing operation than in the case of the single withdrawal.

Therefore, in the case of pulling out the control rod alone, even if an abnormality occurs in the LPRM detector, or even if an abnormality occurs in the entire system, the LPRM in which the abnormality occurred or the remaining LPRM that bypassed the entire system By monitoring with a detector or system, it was possible to avoid the reactor pressure rise due to control rod withdrawal by blocking the control rod withdrawal before the fuel damage occurred, but in the case of control rod gang operation Also,
Even if the control rod withdrawal prevention setting value is the same, the MCPR is severely deteriorated.Therefore, depending on how the LPRM detector or the system is bypassed, fuel damage has already occurred when the control rod withdrawal was blocked. Is also expected. This will be described in more detail with reference to FIG. In FIG. 9, the horizontal axis represents the amount of control rod withdrawal, and the vertical axis represents changes in the output signal and MCPR. In the figure, La is the MCPR limit value, and Lb is the control rod withdrawal prevention set value. Curve a ₁ is the average output signal from the LPRM detector when the individual control rod is withdrawn, and the entire B system is abnormal and 4 of the 8 LPRM detectors in the A system are abnormal, and these are bypassed. If you do. The curve a ₂ shows the average output when the four control rod gangs are operated under the same conditions. The curves b ₁ and b ₂ are MCs corresponding to a ₁ and a ₂ above.
It shows changes in PR. As is clear from FIG. 9, when the control rod is operated independently, MCPR is P ₁ and lower than La when the curve a ₁ reaches Lb (when the control rod pull-out is blocked). There is no. On the other hand, in the case of the four control rod gang operation, MCPR is P ₂ and is less than La. As described above, in the conventional control rod pull-out monitoring device, the control rod gang operation cannot be monitored, and its improvement is required to maintain the soundness of the fuel.

(Problems to be Solved by the Invention) As described above, in the conventional control rod pull-out monitoring device, the control rod gang operation for pulling out a plurality of control rods at the same time cannot be monitored. The purpose of the present invention is to provide a control rod pull-out monitoring device capable of monitoring the control rod gang operation even when the control rod gang operation is performed.

[Structure of the Invention] (Means for Solving the Problems) That is, the control rod pull-out monitoring device according to the present invention includes a plurality of channels which collectively form a group of output region neutron detectors surrounding one control rod, Select the channel corresponding to the plurality of control rods that are the target of the internal extraction operation of these multiple channels, and divide the detection signals from the group of output area neutron detectors that constitute the selected channel in advance for each channel. A selection circuit for selecting corresponding to a plurality of systems, and a plurality of signals which are installed corresponding to the plurality of systems for each of the channels and which are selected by the selection circuit, are input for each system and averaged. An averaging circuit and a plurality of correction circuits that individually input the signals from the plurality of averaging circuits and correct the core average output, and the signals from the plurality of correction circuits are input. And a control circuit for outputting a control rod withdrawal prevention signal to the control rod control device for preventing the withdrawal of the plurality of control rods when the preset control rod prevention set value is exceeded. Is.

(Operation) In other words, for all control rods to be extracted, select a channel consisting of a group of output region neutron detectors that surround the control rods, divide it into multiple systems, and divide each system. Then, the average value is calculated by the averaging circuit and corrected by the correction circuit, and each judgment circuit determines whether or not the value exceeds the preset control rod pull-out prevention set value, and the control rod pull-out prevention signal Is output.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. In FIG. 1, reference numeral 101 is a control rod, and four fuel assemblies 102 are arranged around the control rod 101 to form a unit lattice 103. The unit lattices 103 are arranged in a lattice pattern to form a core 104 having a substantially circular plan shape as shown in FIG. The reactor core 104 is housed in a reactor pressure vessel 105 as shown in FIG. In FIG. 2, one square represents one fuel assembly 102, and a large white circle represents the control rod 101.

An LPRM 106 is installed in the core 104. This LP
The RM 106 has two unit cells 103 as shown in FIG.
In other words, every two control rods 102 are arranged at the corners of the unit lattice 103, and as shown in FIG. 3, they are arranged in four stages in the vertical direction. In addition, as shown in FIG.
For LPRM, the code (106a ₁ , 106b ₁ , 106a ₂ , 106
b ₂ ), (106a ₃ , 106b ₃ , 106a ₄ , 106b ₄ ), (106a ₅ , 106
b ₅ , 106a ₆ , 106b ₆ ), (106a ₇ , 106b ₇ , 106a ₈ , 106b ₈ )
It is shown with a suffix.

A control rod drive mechanism 107 is installed in each of the nuclear control rods 101. The control rod 101 is inserted into / pulled out from the core 104 by these control rod drive mechanisms 107. Control rod drive mechanism 1
07 is controlled by the control rod controller 108. Also, the LPR
The M detector 106 detects a local output in the core 104 and outputs a signal corresponding to this. Signals from a part of these LPRM detectors 106 are average output area monitor system (APR) not shown.
M) and processed. As a result, the average output of the entire core 104 is detected, and based on this, the percentage of the output during normal operation of the reactor is converted and output. Also the above LP
Signals from RM detector 106 is output to the control rod withdrawal monitoring device 109, the control rod withdrawal monitoring device 109, the periphery of the control rod 101 for the total number of control rods 101 to operate the control rod 101 at the time of drawing operation The local output of the control rod 101 is detected, and when the surrounding output locally exceeds the control rod pull-out prevention set value due to improper pull-out of the control rod 101, the pull-out of the control rod 101 is blocked, and the core 104 It maintains soundness.

The configuration of the control rod pull-out monitoring device 109 will be described in more detail below with reference to FIG. Reference numeral 110 in FIG. 4 is a selection circuit. For example, the control rod 101a shaded in FIG.
When the four control rods 101b, 101c and 101d are pulled out by the control rod gang operation, the LPRM detector 106 corresponding to each control rod 101a, 101b, 101c and 101d is selected. That is, the control rod 101a includes LPRM detectors 106a ₁ and 106a ₁ .
b ₁ , 106a ₂ , 106b ₂ selected, LP for control rod 101b
The RM detectors 106a ₃ , 106b ₃ , 106a ₄ , 106b ₄ are selected. Similarly, for control rod 101c, LPRM detectors 106a ₅ , 105
b ₅ , 106a ₆ and 106b ₆ are selected, and LPRM detectors 106a ₇ , 106b ₇ , 106a ₈ and 106b ₈ are selected for the control rod 101d. The channels 111a, 111b, 111c and 111d are formed by the respective channels. The signals Sa, Sb, Sc and Sd from these channels 111a, 111b, 111c and 111d are transferred to the selection circuit 110.
Is output to On the other hand, in the selection circuit 110, the control rod designation signals Sa ₁ , Sb ₁ , for designating the four control rods 101a to 101d,
Sc ₁ is entered. Further, the selecting circuit 110 divides each of the group of LPRM detector signals Sa to Sd corresponding to each of the pull-out control rods 101a, 101b, 101c and 101d into two systems (A system and B system) by eight. . That is, the selection circuit 110 outputs the signal Sa to the A system averaging circuit 112a and the B system averaging circuit 112b. Similarly, Sb is output to A system averaging circuits 112a ₂ and 112b ₂ , Sc is output to A system averaging circuits 112a ₃ and 112b ₃ , and Sb is A
It outputs to the system averaging circuits 112a ₄ and 112b ₄ . These A system averaging circuits 112a _{1 to} 112a ₄ and B system averaging circuits 112b _{1 to} 1
12b ₄ averages the signal outputs to calculate an average value. Then, the output is converted into% of the normal operation output of these reactors. The output signal of the A-system averaging circuit 112a ₁ is input to the correction circuit 113a _1, and the output signal of the B-system averaging circuit 112b ₁ is B
It is input to the system correction circuit 113b ₁ . Similarly, from the A system averaging circuit 112a ₂ to the A system correction circuit 113a ₂ , the B system averaging circuit 112b ₂
To B system correction circuit 113b ₂ , from A system averaging circuit 112a ₃ to A system correction circuit 113a ₃ , and from B system averaging circuit 112b ₃ to B system correction circuit
113b ₃ includes A system averaging circuit 112a ₄ to A system correction circuit 113a.
₄ , the B system averaging circuit 112b ₄ outputs the B system averaging circuit 112b ₄ to the B system correcting circuit 113b ₄ , respectively. On the other hand, in the correction circuits 113a _{1 to} 113a ₄ and 113b _{1 to} 113b ₄ , the average signal S114 of the core 104 is transmitted to the APR
It is output from M. The correction circuits 113a _{1 to} 113a ₄ , and 113b _{1 to} 113b ₄ are based on the average signals S114,
The signals from each averaging circuit 112a _{1 to} 112a ₄ and 112b _{1 to} 112b ₄ are corrected. Then, the correction signal is sent to the A system determination circuit 115a ₁
To be output to 115a ₄ and the B-system determination circuit 115b ₁ to 115b _4. The A system determination circuits 115a _{1 to} 115a ₄ are provided when the signals from the A system correction circuits 113a _{1 to} 113a ₄ exceed a preset control rod pull-out prevention set value (usually about 105% to 107%). A pull-out prevention signal S116 is output to the control rod control device 108. Similarly, the B system determination circuits 115b _{1 to} 115b ₄ output the pull-out prevention signal S116 when the signals from the B system correction circuits 113b _{1 to} 113b ₄ exceed the control rod pull-out prevention set value.
Output to 8. When the withdrawal prevention signal S116 is output from any of the determination circuits, a signal is output from the control rod control device 108 to the drive mechanism of the four control rods to be withdrawn.
S108 is output, and as a result, pulling out of the four control rods is simultaneously blocked.

Further, the control rod withdrawal monitoring device 109 has a circuit diagnostic function. For example, when an abnormality occurs in a plurality of LPRM detectors, the occurrence of the abnormality is detected, and the signal from the LPRM detector having the abnormality is prohibited from being output to each averaging circuit via the selection circuit 110. Then, it functions so as to bypass the LPRM detector in which the above abnormality has occurred. At this time, the A-system averaging circuit and the B-system averaging circuit calculate average values based on signals from LPRM detectors other than the LPRM detector in which the abnormality has occurred.
Further, even when an abnormality occurs in the A-system averaging circuit, the B-system averaging circuit, etc., it functions to bypass the entire system.

An example of monitoring by the control rod pull-out monitoring device 109 having the above configuration will be described with reference to FIG. This is a case where the four control rods 101a to 101d are pulled out by the control rod gang operation as described above. In FIG. 5, the horizontal axis represents the control rod withdrawal amount, and the vertical axis represents changes in the output signal and MCPR. Also, Laa in the figure is the limit value of MCPR,
Lbb is the control rod pull-out prevention set value. The line a in the figure is an average output signal in the case where one of the A system and the B system is abnormal and the other is also bypassed due to the abnormality of four of the eight LPRM detectors. As shown in the figure, in the case of this embodiment, the control rod withdrawal is blocked when the withdrawal amount is Ia, and the MCPR at that time is Pa. This Pa does not fall below Laa, and therefore the control rod is prevented from being pulled out before it falls below the limit value Laa. On the other hand, in the conventional case (shown by the broken line b in the figure), when the control rod withdrawal is blocked (withdrawal amount is Ib), the MCPR is Pb, and the limit value is
Well below Laa. This encloses one control rod in the case of the conventional four-gang control rod gang operation.
While 16 LPRM detectors (4 × 4 stages) are used for monitoring, in the case of this embodiment, the four control rods 101a to 101d which are the targets of the gang operation are surrounded respectively. This is because the configuration is such that 64 (4 × 4 stages × 4 channels) LPRM detectors are used for monitoring, and there are many LPRs that are operating normally even under the same bypass conditions.

According to this embodiment, the following effects can be obtained. In other words, even in the case of four control rod gang operation, since the LPRM106 surrounding each of the four control rods is used for monitoring, the control rod withdrawal is blocked before the MCPR limit value is exceeded. It is possible to prevent the pulling out of the control rod, and thereby prevent a situation such as fuel damage due to the pulling out of the control rod.

The present invention is not limited to the above-mentioned embodiment,
For example, the number of control rods to be pulled out is not limited to four.

[Effects of the Invention] As described in detail above, according to the control rod pull-out monitoring device of the present invention, even in the case of a so-called control rod gang operation in which a plurality of control rods are pulled out at the same time, a situation of fuel damage occurs. Can prevent the pulling out of the control rod before,
Not only the integrity of the fuel, but also the integrity of the core and thus the integrity and safety of the entire reactor can be maintained, and such effects are great.

[Brief description of drawings]

1 to 5 show one embodiment of the present invention.
The figure is a plan view of the unit lattice, FIG. 2 is a plan view of the reactor core, FIG. 3 is a diagram showing the structure of the core and the lower part of the reactor pressure vessel and its vicinity, and FIG. Fig. 5 shows the signal output and MCPR for the control rod withdrawal amount.
FIG. 6 to FIG. 9 are views used for explaining the conventional example, FIG. 6 is a plan view of a unit cell, FIG. 7 is a plan view of a core, and FIG. 8 is a core and an atom. FIG. 9 is a diagram showing the configuration of the furnace pressure vessel and its vicinity, and FIG. 9 is a diagram showing changes in signal output and MCPR with respect to the control rod withdrawal amount. 101 ... Control rod, 102 ... Fuel assembly, 103 ... Unit cell, 104 ... Core, 106 ... Power region Neutron detection, 108
...... Control rod control device, 109・ ・ ・ Control rod withdrawal monitoring device, 110
...... Selection circuit, 111a to 111d …… Channel, 112a _{1 to} 112
a ₄ , 112b _{1 to} 112b ₄ ...... Averaging circuit, 113a _{1 to} 113a ₄ , 113b ₁ to 1
13b ₄ ... correction circuit, 115a _{1 to} 115a ₄ , 115b _{1 to} 115b ₄ ... judgment circuit.

Claims (1)

[Claims] Claims: 1. Corresponding to a plurality of channels formed by grouping a group of output region neutron detectors surrounding a single control rod, and a plurality of control rods to be extracted from these channels. A selection circuit for selecting a channel and selecting a detection signal from a group of output region neutron detectors constituting the selected channel corresponding to a plurality of systems predivided for each channel, and a plurality of the above for each channel The core averaging is performed by inputting each signal selected by the above-mentioned selection circuit for each system and averaging by inputting each signal for each system, and by individually inputting the signals from the plurality of averaging circuits. A plurality of correction circuits that make corrections based on the output, and the signals from these correction circuits are input to prevent the pulling out of these control rods when the preset control rod prevention set value is exceeded. That the control rod withdrawal inhibit outputs signals to the control rod control system, characterized by comprising a determination circuit control rod withdrawal monitoring device.