JPH0152717B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a control rod withdrawal monitoring device that monitors the local output around a control rod to be withdrawn and prevents the control rod from being withdrawn if this output exceeds a predetermined set value. Regarding.

Generally, in a boiling water reactor, four fuel assemblies 2 are arranged around a control rod 1 having a cross-shaped cross section to form a unit cell 3, as shown in FIG. As shown in the figure, reactor cores 4 are arranged in a lattice pattern and have a planar shape close to a circle, and are housed in a reactor pressure vessel 9. In addition, one square in FIG. 2 represents one fuel assembly 2, and each large white circle represents a control rod 1. In addition, inside the core 4, there are many power range neutron detectors 5a..., 5b... (hereinafter referred to as LPRM).
(referred to as a detector) is provided. Furthermore, this
The LPRM detector is shown as a small white or black circle in FIGS. 2 and 3. Planarly, these LPRM detectors 5a..., 5b... are arranged in a lattice pattern every two unit grids 3, that is, every two control rods, as shown in FIG. 3, and as shown in FIG. 4 in the vertical direction, that is, in the insertion and withdrawal direction of control rod 1...
It is located on steps. The outputs from these LPRM detectors 5a..., 5b... are processed by the average power range monitor system (APRM) to detect the average power of the entire core 4, and the local power range monitor system (LPRM) detects the local output power. is detected. When the control rod 1... is withdrawn, the local output around it is monitored, and if the local output exceeds a predetermined control rod withdrawal prevention setting value due to inappropriate withdrawal of the control rod 1... In such a case, a control rod withdrawal monitoring device 6 is provided which prevents the control rods 1 from being withdrawn to ensure the integrity of the reactor. For example, when the control rod 1' at the shaded position in FIG. 16
A group of LPRM detectors 5a..., 5b... is selected, and this group of LPRM detectors 5a..., 5b...
A local power range monitoring system for monitoring the local power around this control rod 1' is constructed. In addition, this designated control rod 1' and a group of LPRM detectors 5a
..., 5b... is shown in FIG. 4. In order to increase reliability, this group of LPRM detectors is further divided into a plurality of systems, for example, half each into two systems. In addition, the LPRM detector 5a belonging to one system (hereinafter referred to as A system) in FIGS. 2 to 4...
is indicated by a white circle, and the LPRM detector 5b belonging to the other system (hereinafter referred to as B system) is indicated by a black circle. And the output of these LPRM detectors 5a..., 5b... is A
The average value is calculated for each system and B system, and when the average value of either system A or B exceeds a preset control rod withdrawal prevention setting value, the control rod withdrawal monitoring device 6 sends an alarm to the control rod control device. A withdrawal prevention signal is sent to the control rod controller 7, and the control rod 1 is removed from the control rod controller 7.
A signal is sent to the control rod drive mechanism 8 of... to prevent the control rod 1 from being withdrawn.
In addition, when an abnormality occurs in the LPRM detectors 5a..., 5b..., the control rod withdrawal monitoring device 6 excludes, that is, bypasses, the LPRM detectors 5a..., 5b... in which the abnormality has occurred, and the remaining LPRM detectors 5a... ...,
The average value of the output of 5b... is calculated, and if an abnormality occurs in the entire A system or B system, the entire system in which the abnormality has occurred is excluded or bypassed, and the remaining systems are used for monitoring. The aim is to improve reliability. However, the distribution of neutron flux or power within the reactor core 4 is not uniform; for example, the control rod 1'
When the control rod 1' is pulled out halfway, the output increases locally in the portion above the upper end of the control rod 1', and the output distribution also changes in the horizontal direction. Therefore, each of the above LPRMs for the withdrawal amount of control rod 1'
The changes in the signal output of the detectors 5a..., 5b... are different, and therefore the characteristics of the signal output with respect to the amount of withdrawal of the control rod 1' are different between the A system and the B system. Curve a ₁ and system B have the characteristics as shown in curve b ₁ in Figure 5b, and when the control rod 1' is gradually withdrawn, for example, the signal output of system B reaches the control rod withdrawal prevention setting value L first. Reach _B. Normally, signals from this system with good responsiveness, in this example, the B system, are prioritized and monitored. Therefore, if the A system with poor responsiveness is bypassed, there will be no effect on the overall characteristics of the control rod withdrawal monitoring device 6, but if the B system with good responsiveness is bypassed, the A system with poor responsiveness will be affected. Monitoring is based on signals, so
The overall responsiveness of the control rod withdrawal monitoring device 6 was reduced, which was undesirable in terms of maintaining the integrity of the reactor core 4. Also, although there is no bypass for the entire system, the LPRM detectors 5a..., 5 have the highest responsiveness.
b... is bypassed, the remaining
Since the outputs of the LPRM detectors 5a..., 5b... are averaged, the response of system A or system B decreases as shown by curves a ₂ and b ₂ in Figure 5 a and b, and the control rod withdrawal monitoring device 6. Overall responsiveness decreases. On the other hand, the LPRM detectors 5a..., 5b... which have the worst response
When the system is bypassed, the responsiveness of the A system or the B system will be improved, and the responsiveness of the control rod withdrawal monitoring device 6 as a whole will be improved. Therefore, the system or
When the LPRM detectors 5a..., 5b... are bypassed, the responsiveness of the control rod withdrawal monitoring device 6 changes depending on which system is bypassed or which LPRM detector 5a..., 5b... is bypassed. However, the position at which the control rods 1' are prevented from being withdrawn changes, resulting in a problem that the stability of operation is impaired. For this reason, in the past, for example, as shown in FIG.
Two LPRM detectors 5b... belonging to the system are arranged diagonally in each stage, and the arrangement of these LPRM detectors 5a..., 5b... is shifted by 90 degrees for each adjacent stage, and the output It is configured so that the unevenness of distribution affects both systems as equally as possible, and
Efforts have been made to improve stability by reducing the difference in response of the system, but no fully satisfactory effect has been obtained.

The present invention has been made based on the above circumstances, and its purpose is to
An object of the present invention is to provide a control rod withdrawal monitoring device capable of reducing changes in responsiveness when a detector is bypassed and improving stability.

The present invention will be explained below with reference to an embodiment shown in FIGS. 6 to 10. Reference numeral 101 in FIG. 6 is a control rod, and around the control rod 101, four fuel assemblies 102 are arranged as shown in FIG. 6, forming a unit grid 103. Then, these unit cells 103... are arranged in a lattice shape, and the seventh
As shown in the figure, a reactor core 104 having a planar shape close to a circle is constructed, and this reactor core 104 is housed in a reactor pressure vessel 105. Note that a square in FIG. 7 indicates one fuel assembly 102, and a large white circle indicates a control rod 101. Inside this core 104 is a power range neutron detector 106.
a..., 106b... (hereinafter referred to as LPRM detector)
is provided. These LPRM detectors 106a..., 106b... are arranged at the corners of the unit grid 103... every second unit grid 103..., that is, every two control rods 101..., as shown in FIG. At each of these two-dimensional positions, as shown in FIG. 8, the control rod 101...
There are four stages each in the insertion/extraction direction. In addition, these LPRM detectors 106a..., 106b... are shown by small white circles or black circles in FIGS. 7 and 8. In addition, a control rod drive mechanism 107 is provided for each control rod 101, and each control rod 101 is provided with a control rod drive mechanism 107.
01... are configured to be inserted and withdrawn by these control rod drive mechanisms 107....
These control rod drive mechanisms 107 are configured to be controlled by a control rod control device 108. And the LPRM detector 106a...,
106b... are configured to detect local neutron flux, that is, local output within the core 104, and output a signal corresponding to this. The signals from these LPRM detectors 106a..., 106b... are sent to an average power range monitor system (APRM) (not shown) and processed, and the average power of the entire reactor core 104 is detected and used for normal operation of the reactor. The output is converted into a percentage of the output at a time. In addition, these LPRM detectors 106
The signals from a..., 106b... are configured to be sent to the control rod withdrawal monitoring device 109. This control rod withdrawal monitoring device 109 detects the local output around the control rods 101... that are operated when the control rods 101... are pulled out, and detects the local output around the control rods 101... due to inappropriate withdrawal of the control rods 101... This system prevents the control rods 101 from being withdrawn when the control rods locally exceed the set value for preventing withdrawal of the control rods, thereby maintaining the integrity of the core 104. explain. In the figure, reference numeral 110 denotes a selection circuit, which is configured so that signals S _{1 .} . . from the respective LPRM detectors 106a . . . , 106b . The selection circuit 110 also includes a control rod 101 to be extracted from the control rod control device 108.
It is configured such that a control rod designation signal _S2 specifying... is input. In this selection circuit 110, a group of control rods surrounding the specified control rod 101...
LPRM detectors 106a..., 106b... are configured to be selected. Therefore, for example, if control rod 101' shown with diagonal lines in FIG.
A group of LPRM detectors 106a with a total of 16 stages...,
106b... is configured to be selected. In this case, a group of LPRM detectors 106a...,
106b... and the control rod 101' is shown in a perspective view in FIG. In addition, this selection circuit 110 divides the group of LPRM detectors 106a, 106b, . In addition, the LPRM detector 1 belonging to one of the systems (hereinafter referred to as A system) in FIGS. 7 to 9
06a... is a white circle, the other system (hereinafter referred to as B system)
The LPRM detectors 106b belonging to... are indicated by black circles.
Then, the LPRM detector 106a belonging to the A system...
Two LPRM detectors 106b... belonging to the and B systems are arranged in each stage, and the LPRM detectors 106a..., 106b... belonging to the same system are arranged diagonally to each other, and this arrangement is adjacent to each other. Each row is offset by 90 degrees. In addition, when a control rod 101... arranged in the central part of the reactor core 104 like the above-mentioned control rod 101' is specified, a total of 16 control rods (4 rods, 4 stages, 16 rods) surrounding it as described above are specified.
The LPRM detectors 106a..., 106b... are selected, but if the control rods 101... arranged in the peripheral part of the reactor core 104 are specified, the LPRM detectors 106a..., 106b... are selected, and if the control rods 101... arranged around the core 104 are specified, the LPRM detectors 106a..., 106b... are selected. 4 stages total 8
A group of LPRM detectors 106a..., 106b
is selected. Then, this selection circuit 110
It is configured to send signals from the LPRM detectors 106a... belonging to the system to the A-system averaging circuit 111a, and to send signals from the LPRM detectors 106b... belonging to the B-system to the B-system averaging circuit 111b. The A-system averaging circuit 111a and the B-system averaging circuit 111b average the signal outputs of the A-system and B-system LPRM detectors 106a, . . . , 106b, respectively, and calculate the average value thereof. For example, the output of the A-system averaging circuit 111a is sent to the A-system gain adjustment circuit 112a.
12a is the average output monitor system (APRM) mentioned above.
The signal from the A system of the reactor core 104, that is, the average output signal Sa of the reactor core 104 is input. Then, the A-system gain adjustment circuit 112a compares the average output signal Sa with the signal output from the A-system average circuit 111a, and if the level of the average output signal Sa is higher, the normal operating state of the furnace is The gain is constantly corrected so that the levels of both signals match, that is, when the average output signal Sa is 100%, the signal level from the A-system averaging circuit 111a is also 100%, and the A-system averaging circuit 111a
When the signal output from the A-system average circuit 111a is larger than the average output signal Sa, the signal output from the A-system average circuit 111a is directly used as the output of the A-system gain adjustment circuit. The output signal from the A-system gain adjustment circuit 112a is sent to an A-system correction circuit 113a, which will be described later, to be corrected, and further sent to an A-system determination circuit 114a. Similarly to the A system, the signal output of the B system averaging circuit 111b is also gain adjusted by the B system gain adjustment circuit 112b, corrected by the B system correction circuit 113b, and the B system
The signal is configured to be sent to the system determination circuit 114b. These A-system determination circuits 114a and B-system determination circuits 114a and B
In the system determination circuit 114b, the above-mentioned A system correction circuit 113
Signals from the a and B system correction circuits 113b and the preset control rod withdrawal prevention setting value L _B
(usually set at about 105% to 107%), and when the signal level from the A system correction circuit 113a or the B system correction circuit 113b exceeds this control rod withdrawal prevention set value L _B , withdrawal is prevented. A signal S ₄ is sent to the control rod controller 108 to control the control rod 10.
1... is configured to prevent it from being pulled out. In addition, the LPRM detectors 106a..., 106b...
If an abnormality occurs in the
Stopping the signals of the LPRM detectors 106a..., 106b... from being sent from the selection circuit 110 to the A-system averaging circuit 111a or the B-system averaging circuit 111b,
The LPRM detectors 106a..., 106b... are excluded or bypassed. Also, A type or B type
If an abnormality occurs in a system circuit or the like, the operator removes or bypasses the entire system. Note that the A-system average circuit 111a and the B-system average circuit 111
b is the bypassed LPRM detector 106a
..., 106b..., exclude the signals from the LPRM detectors 106a..., 106b...,
It is configured to calculate the average value of the signals of the remaining LPRM detectors 106a..., 106b.... Reference numeral 115 denotes a bypass number determining circuit, which receives signals from the respective LPRM detectors 106a..., 106b... output from the selection circuit 110, and receives signals from the LPRM detectors 106 that are bypassed for each system.
a..., 106b... or a bypassed system. The signal from the bypass number determining circuit 115 is configured to be sent to the A-system correction circuit 113a and the B-system correction circuit 113b. In these A-system correction circuit 113a and B-system correction circuit 113b, based on the signal from the bypass number determination circuit 115, the A-system gain adjustment circuit 112a
And the signal from the B-system gain adjustment circuit 112b is configured to be modified as follows.

That is, first, a group of LPRM detectors 106a selected when a certain control rod 101...
Let the number of ..., 106b... be N, and the number of self-system
The LPRM detectors 106a..., 106b...that is, the A-system correction circuit 113a uses the A-system LPRM.
Regarding the detectors 106a..., B-system correction circuit 113b, the number of bypassed LPRM detectors 106b... of the B-system is N' _s , and the number of bypassed LPRM detectors 106a..., 106b..., that is, the A-system correction circuit Regarding circuit 113b, it is LPRM of B system.
As for the detectors 106b, . . . and the B-system correction circuit 113b, the number of bypassed LPRM detectors 106a, . . . of the A-system is assumed to be N' _p . Also,
Set the control rod withdrawal prevention setting value to L _B (usually 105 to 107%).
), the operating limit value is L _R , and the limit increase rate R is R = L _B - L _R /L _R ... (1). If none of the bypassed LPRM detectors 106a, 106b, etc. in the A system or the B system is bypassed, that is, in a normal state, the signal is not modified. If any of the LPRM detectors 106a..., 106b... of the own system is bypassed, the A system correction circuit 113a or the B system correction circuit 11 of that system
3b, the A system gain adjustment circuit 112a or B
It is configured to perform correction by multiplying the signal output from the system gain adjustment circuit 112b by 1+R·N's+C/N (2). Note that in this example, the above L _R is 100%, and
L _B is 105%, and the marginal increase rate R in this case is
It is 0.05. Further, in this embodiment, the above constant C
are the selected LPRM detectors 106a..., 106
It is assumed that it is changed according to the number of b..., and is set as C=N/2...(3). Therefore, in the case of this embodiment, equation (2) becomes +0.05N' _s /N+0.5 (4).

In one embodiment of the present invention configured as described above, the control rod 101 to be pulled out...for example, the control rod 10
1' is specified, the selection circuit 110 selects a group of 16 in total in 4 stages surrounding this control rod 101'.
LPRM detectors 106a..., 106b... are selected, and a group of these LPRM detectors 106a
..., 106b... are divided into A system and B system.
And these LPRM detectors 106a..., 106
The signals from b... are averaged by the A-system averaging circuit 111a and the B-system averaging circuit 111b for each A-system and B-system, respectively, and these average values are averaged by the A-system gain adjustment circuit 1.
12a and the B-system gain adjustment circuit 112b, respectively, and are compared with the average output signals Sa and Sb, and when they are smaller than the average output signals Sa and Sb, the average output signal is
The gain is adjusted to match Sa and Sb,
Further, the signal is sent via the A-system correction circuit 113a and the B-system correction circuit 113b to the A-system judgment circuit 114a and the B-system judgment circuit 114b, where the local output around the control rod 101' being pulled out is monitored. When the local output around the control rod 101' increases excessively due to improper withdrawal of the control rod 101', the input signals of the A-system determination circuit 114a and the B-system determination circuit 114b increase, and these input signals are When the prevention setting value L _B is exceeded, a withdrawal prevention signal is output, preventing withdrawal of the control rod 101' and ensuring the integrity of the fuel. The signals sent to the A-system determination circuit 114a and the B-system determination circuit 114b are sent to the bypassed self-system LPRM detectors 106a..., 1.
A system correction circuit 113a corresponding to the number of 06b...
Since the signal output is corrected as described above in the B system correction circuit 113b, the LPRM detector 1
When a bypass of 06a..., 106b... occurs, the change in the responsiveness of the control rod withdrawal monitoring device 109 as a whole is reduced, and the operation becomes stable. The reason will be explained below. First, as a result of analyzing the characteristics of the conventional control rod withdrawal monitoring device described above, the present inventors found that, for example, in the control rod pattern shown in FIG. 11, the control rod 1' shown with diagonal lines in FIG. It was found that the characteristics when pulled out were as shown in FIG. In addition, FIG. 11 is a diagram similar to FIG. 2 and FIG.
It shows the number of withdrawal notches when the period from full insertion to full withdrawal is divided into 48 notches, with total insertion as 0 and full withdrawal as 48.If no number is attached, the number of withdrawal notches for control rod 1 is 48. 48, indicating that it is fully pulled out. In addition, the solid line X ₁ in FIG. 12 indicates the withdrawal prevention position of the control rod 1' when the responsiveness is the lowest, and the broken line Y ₁ is the withdrawal prevention position of the control rod 1' when the responsiveness is the highest. This shows that. That is, when the control rod 1' is withdrawn, the surrounding output distribution is not uniform as described above, so the signal outputs of the group of LPRM detectors 5a..., 5b... are not equal, and therefore, when the control rod 1' is withdrawn, The change in the signal output of each LPRM detector 5a..., 5b..., that is, the response to the signal is different from each other. and,
The local power distribution around the control rods that occurs when withdrawing them is analyzed in advance during core design, so
This output distribution and each LPRM detector 5a..., 5b...
The order of responsiveness of each LPRM detector 5a..., 5b... is determined from the arrangement of , and _the above solid line The dashed line Y ₁ is the one with the worst responsiveness.
This is an analysis of the case where it is assumed that the LPRM detectors 5a..., 5b... are bypassed in order. Therefore, in the former case, the responsiveness is good.
Since the LPRM detectors 5a..., 5b... are bypassed, the average value of the signal output of the remaining LPRM detectors 5a..., 5b... decreases, the overall response deteriorates, and the number of notches required to prevent control rod withdrawal increases. In other words, the control rod is prevented from being pulled out more slowly, and in the latter case, the response becomes better and the control rod is prevented from being pulled out faster.In reality, the solid line X ₁ and the broken line
The control rod 1' is prevented from being withdrawn in the range surrounded by _Y1 . By the way, in FIG. 12, the broken line _Y1 , that is, the LPRM detector 5a with poor response...
When the control rods are bypassed in order from 5b..., the control rod withdrawal prevention position becomes smaller, and the control rod withdrawal device operates on the safe side, and the change is 8 notches compared to 10 notches in the normal case, and 2 Since the change is about the size of a notch, the degree of the problem is small. On the other hand, if the solid line _X1 , that is, the LPRM detectors 5a..., 5b..., which have good response, are bypassed in order, the control rod 1' is delayed in being prevented from being withdrawn, and its change also changes by 6 notches, so the control rod is withdrawn. The impact on the reliability and safety of operation as a monitoring device is extremely large. Moreover, since the LPRM detectors 5a..., 5b... with good responsiveness always receive a larger neutron flux than the others, the probability of their failure is high, and therefore, the LPRM detectors 5a..., 5b with good responsiveness also have a high probability of failure. The probability of being bypassed is higher in the order of... Therefore, in order to improve the reliability and stability of the control rod withdrawal monitoring device, the most important requirement is to modify this solid line _X1 so that it is as close as possible to the control rod withdrawal prevention position in the normal case.
In this embodiment of the present application, the signal outputs from the A-system gain adjustment circuit 112a and the B-system gain adjustment circuit 112b are corrected by the A-system correction circuit 113a and the B-system correction circuit 113b.
When the bypass of the LPRM detectors 106a..., 106b... occurs, the A-system gain adjustment circuit 11
The signal level sent from the 2a and B system gain adjustment circuit 112b to the A system determination circuit 114a and the B system determination circuit 114b is increased, and modifications are made to speed up the prevention of control rod withdrawal to improve responsiveness.
This is to compensate for the delay in preventing control rod withdrawal that occurs when the LPRM detectors 106a, . . . , 106b . . . are bypassed in order. However, in the actual core 104, the bypassed LPRM detector 106
Even if it is possible to identify a..., 106b..., the bypassed LPRM detector 106a..., 106
b... is selected as a group of LPRM detectors 106a.
..., 106b..., it is extremely difficult to determine where it is in the responsiveness ranking, and the parameters used for correction are bypassed.
Only the number of LPRM detectors 106a..., 106b... can be used. By the way, the conventional LPRM detectors 5a..., 5b... have good responsiveness.
Characteristics when bypassed in order from 12th
Further consideration of the solid line _X1 in the figure shows that when the number of LPRM detector bypasses is one, the control rod withdrawal prevention position reaches 14 notches, and when there are two, it reaches the maximum of 16 notches, and from then on, even if the number of LPRM detector bypasses increases. Maintaining approximately 16 notches, with 6 to 6 bypasses on the way
The number of notches decreases only slightly around 9 notches.
Therefore, if we pay attention to this characteristic and make a predetermined correction when the self-system LPRM detectors 106a..., 106b... are bypassed, the _entire solid line You can approach the 10th notch, which is the control rod withdrawal position. Therefore, in this case, the correction can be made by multiplying the signal by 1+R・A...(5). Here, R is the above-mentioned critical increase rate, and A is a positive constant of 1 or less.
However, the group of LPRM detectors 106a...,
The number N of 106b... is the number of control rods 101 to be pulled out.
For example, when operating the control rods 101 in the center of the reactor core 104, 4
There are a total of 16 LPRMs in four stages, but when operating the control rods 101 in the periphery of the core 104, a group of LPRMs is required.
The number N of the detectors 106a..., 106b... is 3 in 4 stages for a total of 12 or 2 in 4 stages for a total of 8. Therefore, this group of LPRM detectors 106a...,1
As the number N of 06b... decreases, the weight of one LPRM detector 106a..., 106b... increases, so the characteristics as shown in FIG. 12 are also emphasized. Therefore, this group of LPRM detectors 10
To cancel the influence of the number N of 6a..., 106b..., the signal correction coefficient is
It is necessary to change it corresponding to the weight of 06a..., 106b.... Therefore, if the above A is replaced with a positive constant C' equal to or less than N divided by N, the following formula is obtained: 1+R.C'/N...(6). By the way, considering the case where the LPRM detectors 106a..., 106b... are actually bypassed, in many cases, a small number of LPRM detectors 106a..., 106b... are bypassed, and too many LPRM detectors 106a..., 106b... are bypassed. Detector 106
The probability that a state in which a..., 106b... will occur is small. Therefore, considering the stability of the operation of the control rod withdrawal monitoring system as a whole, it is preferable to emphasize the improvement of characteristics when a small number of rods are bypassed. Examining the characteristics of the solid line _X1 shown in Fig. 12 when there are a small number of bypasses, it is found that in the range of 1 to 2 bypasses, the number of notches that prevent control rod withdrawal increases in proportion to the number of bypasses, that is, the prevention of withdrawal tends to be delayed. . Therefore, if emphasis is placed on improving the characteristics when a small number of bypasses are used, it is desirable to change the amount of signal modification in accordance with the number of bypasses. In addition, for the control rod withdrawal monitoring system as a whole, it is only necessary to consider the number of bypassed LPRM detectors 106a, 106b, regardless of system A or B, and the characteristics of the control rod withdrawal prevention position. If we consider only the system or B system, it is the LPRM of the own system that directly affects that system.
Number of bypasses of detectors 106a..., 106b...
There is only N′ _s . Of course, the outputs of system A and system B are compared with each other, and the higher level is given priority.
Although the systems are related to each other to some extent, each A
A system correction circuit 113 provided for each system and B system
It is appropriate to use the number N _'s of bypasses in the own system as a parameter for signal modification performed separately in the a and B system modification circuits 113b. Therefore, in the above equation (6), the self-system LPRM detector 106a...,
By introducing the number of bypasses N' _s of 106b..., the above-mentioned equation 1+R·N' _s +C/N (2) is obtained. Here, C is a positive constant equal to or less than N/2. The feature of this equation (2) is that it can greatly modify the response of a system with a bypass of the LPRM detector. Furthermore, since C is added to N _'s , it is possible to correct more than a certain value even with a small number of bypasses. Note that in the above embodiment, the A system correction circuit 1
In order to simplify the circuit configurations of the B-system correction circuit 13a and the B-system correction circuit 113b, as described above, C=N/2 (3), and with respect to the constant C, N is excluded from the correction parameters. Therefore, when withdrawing the control rods 101 from the periphery of the core 104, one LPRM detector 10 is required due to the decrease in N as described above.
As the weight of 6a..., 106b... increases, the delay in preventing control rod withdrawal due to the bypass appears significantly, but in the periphery of the reactor core 104, there are many neutrons leaking out of the core 104, so the local power is originally low, and control Even if there is some delay in preventing rod withdrawal, the integrity of the fuel will not be impaired, and even if configured as in this embodiment, the reliability and stability of the control rod withdrawal monitoring system as a whole will not be impaired. There isn't. FIG. 13 shows the results of analyzing the characteristics of the control rod and pattern shown in FIG. 11 for this embodiment. As is clear from this result, the example of the present application has the highest responsiveness.
The solid line in Fig. 13 is the characteristic when the LPRM detectors 106a..., 106b... are bypassed in order.
_X2 has a maximum of 12 notches, which is close to the control rod withdrawal prevention position under normal conditions. Therefore, in this embodiment, the LPRM detectors 106a..., 1
In the worst case, the delay in preventing control rod withdrawal in the event of a bypass of 06b... is 12 notches, and the reliability and stability of the control rod withdrawal monitoring device have been significantly improved. In addition, the broken line _Y2 , which is the characteristic when the LPRM detectors 106a..., 106b... with the lowest response are bypassed in order, shows that the pull-out prevention is applied 2 to 6 notches earlier than the conventional one. However, as mentioned above, these characteristics make it safer to use as a control rod withdrawal monitoring device.
The degree of the problem is small unless the withdrawal is significantly accelerated.

Note that FIGS. 14 to 17 show characteristics of analysis examples in which various analyzes were performed in addition to the one embodiment described above.

Figure 14 is the first analysis example, which takes into account the influence of the presence or absence of bypass in other systems, and when the entire other system is not bypassed, the signal output is 1 + R・N' _s + C ₁ /N... ( 7), and if there is a bypass for the other system, 1+R・N′ _s +C ₂ /N ...(8), C ₁ = N/4 ...(9) C ₂ = N/2 ...( 10) and when R=0.05. In this case, the responsive LPRM detector 106a...,1
Characteristics when bypassing in order from 06b... (solid line
X ₃ ) has decreased to only 14 notches, but the LPRM detectors 106a..., 106b have poor response.
Characteristics when bypassed in order from ... (dashed line Y ₃ )
The decrease was 6 notches, and in this respect it is more preferable than the previous example.

Moreover, FIG. 15 shows the characteristics of the second analysis example.
This one is a different system's LPRM detector 106a...,1
The number of bypasses N' _p of 06b... is also used as a correction parameter, and the weight of the number of bypasses N' _s of the own system is
It is larger than _N'p , and the signal output is 1+R・2N _'s + _N'p /N...(11). Note that R in this case is R=0.05. In this LPRM detector 1, the characteristics when the number of bypasses is small are not much different from the first analysis example, but when the number of bypasses is large, the response is good.
The characteristics when bypassing in order from 06a..., 106b... (solid line X ₄ ) and the characteristics (broken line Y ₄ ) when bypassing in order from LPRM detectors 106a..., 106b... with poor responsiveness are both significantly reduced,
Control rod withdrawal tends to be blocked very quickly.

Further, FIG. 16 shows the characteristics of the third analysis example, and FIG. 17 shows the characteristics of the fourth analysis example. These methods introduce the number N' _p of bypasses in the other system as a parameter, as in the second analysis example, but they do not give weight to the number N' _s of bypasses in the own system, and the signal output is 1+R・N' _s +N' _p /N...(12) It is multiplied. The third analysis example shown in FIG. 16 is set to R=0.05, and the fourth analysis example shown in FIG. 17 is set to R=0.07. The third analysis example is a highly responsive LPRM detector 106a..., 106.
Characteristics when bypassing in order from b... (solid line X ₅ )
increases to 16 notches when the number of bypasses is 2, and the LPRM detector 106a has poor response...
106b, etc. (broken line _Y5 ) shows the same characteristics as in the first analysis example of FIG. 14, except that the number of bypasses increases to 8 notches when the number of bypasses is 1 to 3. In addition, in the fourth analysis example, LPRM detectors 106a..., 106b... with good responsiveness are used.
Characteristics (solid line X ₆ ) when bypassed in order from LPRM detector 106a...,1 with poor response
Characteristics when bypassing in order from 06b... (dashed line)
Y ₆ ) both have characteristics similar to those in the second analysis example shown in FIG.

It should be noted that the present invention is not limited to the above-mentioned embodiment; for example, the number and arrangement of LPRM detectors, the number of systems,
The specific circuit configuration etc. are not necessarily limited to those described above.

As described above, the present invention selects a group of LPRM detectors surrounding a control rod designated for withdrawal operation, divides this group of LPRM detectors into a plurality of systems, and adjusts the signal output of the LPRM detector for each system. In a control rod withdrawal monitoring device that calculates the average value of and prevents the control rod from being withdrawn if this average value exceeds a control rod withdrawal prevention setting value, a correction circuit is provided for each system to modify the average value. A bypass number judgment circuit is provided to determine the number of LPRM detectors that are bypassed, and if one or more LPRM detectors belonging to the own system are bypassed, the critical increase rate is R, and the number of LPRM detectors in a group is determined. N, own lineage
The number of bypasses of the LPRM detector is N′ _s , and C is N/2.
The average value for each system is modified to be multiplied by 1+R·N' _s +C/N as the following positive constant. Therefore, the delay in preventing control rod withdrawal that occurs when the number of bypasses is small, especially when the LPRM detectors with good response are bypassed in order, is compensated for by modifying the above average value, and the LPRM detector's This has great effects, such as reducing the delay in preventing the control rod from being pulled out when a bypass occurs, and greatly improving operational reliability and stability. Furthermore, the above average value of the present invention is corrected when one or more LPRM detectors belonging to the own system are bypassed, and the magnitude of the correction depends on the number of bypasses of the LPRM detectors belonging to the own system. do.
Therefore, the response characteristics are modified according to the number of bypasses of the LPRM detector in the own system, which directly affects the response characteristics of the own system, so that effective modification can be achieved. In addition, since a constant C is added to the number of bypasses N _'s of the LPRM detector in its own system, it is possible to perform corrections greater than a certain value even with a small number of bypasses, and effective correction can be performed when a small number of bypasses are present. Can be done.

[Brief explanation of drawings]

Figures 1 to 5 a and b show conventional examples, where Figure 1 is a plan view of the unit cell, Figure 2 is a plan view of the core, Figure 3 is a schematic configuration diagram, and Figure 4 is a diagram showing the control rods.
A schematic perspective view showing the arrangement of the LPRM detector, and FIGS. 5a and 5b are diagrams showing the characteristics of the signal output with respect to the control rod withdrawal amount of the A system and the B system, respectively.
6 to 10 show an embodiment of the present invention, FIG. 6 is a plan view of a unit cell, FIG. 7 is a plan view of a reactor core, FIG. 8 is a schematic configuration diagram, and FIG. 9 is a control rod. and
FIG. 10 is a schematic perspective view showing the arrangement of the LPRM detector and a block diagram of the control rod withdrawal monitoring device. Furthermore, FIG. 11 is a plan view of the core showing the control rod patterns used to analyze the characteristics of the conventional example and an embodiment of the present invention, FIG. 12 is a diagram showing the characteristics of the conventional example, and FIG. 13 is a diagram showing the characteristics of the present invention. FIG. 3 is a diagram showing the characteristics of an example. FIG. 14 is a diagram showing the characteristics of the first analysis example,
Figure 15 is a diagram showing the characteristics of the second analysis example;
The figure is a diagram showing the characteristics of the third analysis example, and FIG. 17 is a diagram showing the characteristics of the fourth analysis example. 101, 101'...control rod, 102...fuel assembly, 104...core, 106a, 106b...
...LPRM detector, 108...control rod control device,
109... Control rod withdrawal monitoring device, 110... Selection circuit, 111a, 111b... Average circuit, 113
a, 113b...correction circuit, 114a, 114b
...Judgment circuit.

Claims (1)

[Claims] 1. Select a group of output area neutron detectors surrounding a control rod designated for withdrawal operation, divide this selected group of output area neutron detectors into a plurality of systems, and separate each system for each system. Calculate the average value of the outputs of the remaining neutron detectors in the output area that bypassed the neutron detector in the output area that has an abnormality, and control if any of the average values of these systems exceeds the control rod withdrawal prevention setting value. The control rod withdrawal monitoring device for preventing rod withdrawal includes a bypass number determining means for determining the number of bypassed power range neutron detectors in the group of power range neutron detectors, and a bypass number determining means provided for each of the above systems. If one or more of the output range neutron detectors belonging to the own system receive a signal from the bypass number determining means and one or more of the output range neutron detectors belonging to the own system are bypassed, the control rod withdrawal prevention setting value is set as L _B and the operation limit value is set as L _R. =L _B −L _R /L _R , and the number of output region neutron detectors in the above group is N, the number of bypassed output region neutron detectors belonging to the own system is Ns′, and C is N/
1. A control rod withdrawal monitoring device comprising: a correction means for multiplying the average value of the output of the output range neutron detector of each system by 1+RN _s '+C/N when the positive constant is 2 or less. 2. The control rod withdrawal monitoring device according to claim 1, wherein the correction means sets the constant to C=N/2.