JPH0198995A - Method and device for monitoring drawing-out of control rod - Google Patents

Abstract

PURPOSE:To enlarge the flexibility related to drawing-out of a control rod at the time of high output, and also, to execute an economical operation by using a heat flux corresponding value instead of a neutron flux as a signal utilized by a monitoring device (RBM). CONSTITUTION:First of all, when a control rod to the operated is selected, maximum four bodies of detector assemblies 1 being adjacent to the control rod are utilized. Subsequently, an average value of a signal of a detector LPRM assigned to A and B channels of the RBM is calibrated by using an average output area instrumentation (APRM) by an initialized data generating part 5. In this case, an APRM signal shows a neutron flux, therefore, it is converted to a heat flux corresponding value by multiplying a time constant of 6sec by a time constant circuit 3. When drawing-out of a control rod is started, the RBM decides by a comparing circuit 4 whether a signal of averaging circuits A, B exceeds a set value or not, and when an output of one of A and B has exceeded the set value, a drawing-out obstructing signal is outputted so that the control rod cannot be drawn out further than that. In such a way, the flexibility related to drawing-out of the control rod at the time of high output can be enlarged.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention provides a method and apparatus suitable for monitoring control rod withdrawal at high power, particularly in the core of a nuclear reactor loaded with silicone liner fuel. It is related to.

[Prior art]

Regarding this type of monitoring technology, please refer to the application form for permission to change reactor equipment for the expansion of No. 2 reactor at Shimane Nuclear Power Station, 8.8.5.
3 (4) Control rod monitoring devices are known.

Hereinafter, among the above-mentioned known techniques, matters related to the present invention will be summarized and briefly described.

The control rod withdrawal monitor (hereinafter abbreviated as RBM) utilizes a local core power level signal around the control rod selected for withdrawal, and controls when this signal power reaches a preset level. This is to issue a signal to prevent the rod from being pulled out. The level of RBM is determined in relation to the average core power.

In current boiling water light water reactors (hereinafter abbreviated as BWR), control rods are inserted from below the reactor core.

Also, when withdrawing control rods, one control rod can be selected at a time.

The RBM has two channels, A and B, and each channel averages the signals of up to eight local power range instrumentation detectors (hereinafter abbreviated as LPRM) near the operation control rod. There is a function to generate an instruction value, and a function to compare this instruction value with a set value and prevent the control rod from being pulled out.

Once a control rod has been selected for operation, the RBM detects up to eight detectors in the detector assembly, up to four closest to the control rod, as LPRMs suitable for monitoring the withdrawal of this control rod. The LPRM signal is assigned to one channel according to the RBM input selection matrix. FIG. 2 shows the positional relationship between the selected control rod and the detector assembly selected at that time. In the figure, four detector assemblies 9 are selected for the control rods 12 in the center of the core, but three or two detector assemblies 9 are selected for the control rods 13 and 14 in the periphery of the core. A detector assembly 10.11 is selected.

There are four LPRMs in each detector assembly, and they are A, B, and 'C from the bottom depending on the height position.

Let D represent LPRM. FIG. 3 shows A, 82 of the 16 LPRMs in the four selected detector assemblies.
Indicates distribution to channels. A channel is at height position A
The eight B channels belonging to and C take in and average the signals from the eight LPRMs belonging to B and D.

The signal averaged from 8 outputs each for A and B channels is:
Before the control rod is withdrawn, it is automatically compared with the indicated value of the average power range instrumentation (hereinafter abbreviated as APRM),
Calibrated.

After control rod withdrawal is started, if the output of either channel A or B of this monitoring system exceeds a preset value, a withdrawal prevention signal is issued to prevent any further control rod withdrawal.

The setpoint is set depending on the recirculation flow rate.

There are two methods for adjusting the core power in BWR: a method by adjusting the recirculation pump flow rate and a method by operating the control rods. When core power is adjusted by the recirculation pump flow rate, the power distribution changes almost like the whole core, but when the control rods are manipulated, the local power distribution near the operating side control rod changes significantly.

In the current BWR, restrictions on PCIOMR rules (fuel running-in operation method) restrict locally severe changes in power distribution at high outputs, that is, control rod operations.

[Problem that the invention seeks to solve]

Zirconium liner fuel, which has been put into practical use in recent years, is not subject to the restrictions of the PCIOMR rules. Therefore, it is fully expected that control rod operations will be carried out frequently at high output in order to improve operational economy.

Now, the LPRM measures neutron flux, but since there is "fluctuation" in the neutron flux, "fluctuation" of about 2% also occurs in the RBM itself, which averages and uses the LPRM signal. . Therefore, due to fluctuations, the RBM may issue a withdrawal prevention signal even though it is actually possible to withdraw.

At low power, the RBM has sufficient margin for the control rod withdrawal prevention set value, so current BWRs do not have any problems. However, at high power, the margin becomes smaller, so in a core loaded with zirconium-based fuel, it is necessary to take measures for monitoring control rod operation at high power, that is, for RBM.

It is an object of the present invention to provide a control rod withdrawal monitoring method and a control rod withdrawal monitoring device that have great flexibility regarding control rod withdrawal at high power and allow economical operation for a reactor core loaded with zirconium liner fuel. The purpose is to

[Means for solving problems]

The basic principle of the present invention created to achieve the above object is summarized as follows.

That is, the above object is achieved by using a value equivalent to heat flux instead of neutron flux as a signal used in RBM. The heat flux equivalent value is obtained by multiplying the neutron flux by a time constant.

Based on the above principle, as a concrete configuration for applying the same to a practical aspect, the method of the present invention includes (a) a plurality of control rods and a plurality of detectors for local power area instrumentation; (b) When one or more control rods to be extracted are selected, a plurality of neutron flux detectors near the control rods are selected; c.
) When the average output of the plurality of detectors selected above exceeds a preset value after control rod withdrawal has started, the control rod withdrawal operation is prevented from continuing any further. In a method of monitoring control rod withdrawal using a control rod withdrawal monitoring device having a structure in which a signal is emitted, (d) converting the average output of the plurality of neutron flux detectors into a heat flux equivalent value using a time constant circuit; (e) When the thermal fluid equivalent value reaches a preset value, the blocking signal is generated to stop the withdrawal operation of the control rod.

Furthermore, the monitoring device of the present invention created to carry out the above method is applicable to the same monitoring device as that to which the above method of the invention is applied, and (f) averages the outputs of a plurality of neutron flux detectors. A time constant circuit is provided between the averaging circuit and a comparison circuit that compares the detected signal output with a preset value.

[Effect]

According to the monitoring method having the configuration described above, the LPR
When the neutron flux signal measured by M is multiplied by a time constant of generally several seconds, the neutron flux signal is converted into a heat flux equivalent value. It is desirable to use heat flux equivalent values for control rod withdrawal monitoring in RBM from the following two viewpoints.

(■) Reducing "fluctuation" The "fluctuation" of neutrons is about 2%. In contrast,
With a heat flux equivalent value multiplied by a time constant of about 6 seconds, "fluctuation" is reduced to about 0.3%.

Physically, this can be understood as follows. The heat flux is proportional to the temporal average value of the heat output. Also, thermal output is proportional to neutron flux. As a result, the heat flux is proportional to the temporal average value of the neutron flux, and "fluctuation" is greatly reduced.

In the present invention, multiplying j by a time constant of rt seconds corresponds to dividing a definite integral value of t by t in terms of a calculation method.

(n) Physical meaning: Preventing control rod withdrawal is performed to prevent damage to the cladding due to boiling transition of the fuel rods. Therefore, the boiling transition of the fuel flow is
It is related to surface heat flux, not neutron bodies. Therefore, R.B.
In M, it is logically more accurate to monitor control rod withdrawal with a signal equivalent to heat flux rather than a neutron flux signal.

Now, FIG. 6 shows the relationship between the value of the time constant for converting neutron flux into heat flux and linear power density. It is understood from the figure that the value of the time constant changes depending on the linear power density.

Normally, it is sufficient to consider a range of 2 to 15 KW/ft as the linear power density, so when implementing the present invention, it is sufficient to consider a time constant of about 3 to 10 seconds.

〔Example〕

Next, an example of implementing the monitoring method according to the present invention using the monitoring device according to the present invention will be described.

FIG. 1 shows a case where the RBM of the present invention is applied to a boiling water reactor that satisfies the following two conditions.

(A) It is not allowed to pull out two or more control rods at the same time.

That is, only one control rod is allowed to be withdrawn at a time.

(B) In RBM, the recirculation flow rate is used as a signal representing the core flow rate.

The configuration and mechanism of the RBM will be described below with reference to FIG. 1.

The RBM uses two signals for monitoring: the local power range instrumentation detector (LPRM) and the recirculation flow rate. LPR
Four M are arranged in the detector assembly 1 shown in FIG. The four LPRMs 16 to 19 are A, B, and C from the bottom depending on the height position.

It can be called D.

Figure 2 shows the arrangement of the detector assembly within the core. In the figure, detector assemblies 1 are regularly arranged in the reactor core at a ratio of one for every 16 fuel assemblies 15.

Note that one control rod 7 is arranged in the reactor core for every four fuel assemblies.

Now, when a control rod to be operated is selected, up to four detector aggregates close to the control rod are used. That is,
In FIG. 2, four detector assemblies 9 are selected for the control rods 12 in the center of the core, but the control rods 12 in the periphery of the core
3 or 14, three detector assemblies 10 or two detector assemblies 11 are selected.

The RBM has two channels, A and B, and the LPRMs in a maximum of four selected detector aggregates are distributed as follows. That is, as shown in FIG. 3, the LPRM at height positions A and C is placed on the A channel of the RBM and the LPRM is placed on the A channel of the RBM at height position B.

D's LPRMt! :Assign each to the B channel. Therefore, a maximum of 8 LPRMs are allocated.

The distributed LPRM is divided into two averaging circuits A shown in FIG.
, B and averaged. Since LPRM measures neutron flux, there is "fluctuation" of about 2% even in the averaged signal. Therefore, in the present invention, a time constant of 6 seconds is applied by the time constant circuit 3 so that the averaged signal indicates a value equivalent to heat flux. FIG. 4 shows the magnitude of fluctuation when the average value of the LPRM signal is multiplied by a time constant. From the figure, it can be seen that by multiplying by a time constant of 6 seconds, the "fluctuation" from 1.0% can be reduced to 0.3%.

When a control rod is selected, A of RBM is also selected.

It is also necessary to calibrate the average value of the LPRM signal assigned to the B channel using average power range instrumentation (APRM). This calibration is performed by the initialization data creation unit 5 shown in FIG.
The ratio to the M average value (L PRM' ave), that is, the gain (G°=APRM°/L'PRM' ave
) as initialization data. At this time, since the APRM signal represents neutron flux,
The time constant circuit 3 multiplies the value by a time constant of 6 seconds to convert it into a heat flux equivalent value.

When control rod withdrawal begins, the RBM switches between average circuits A and B.
It is determined in comparison round 4 whether the signal of A or B exceeds the set value, and if the output of either A or B exceeds the set value, a withdrawal prevention signal is output to prevent the control rod from being pulled out any further. In addition, in the comparison circuit, a value is calibrated by multiplying the signal (L PRM 'ave) obtained by multiplying the outputs of averaging circuits A and B by a time constant of 6 seconds by the gain (Go) created before control rod operation. (Go · LPRM″ave). Also, the set value is determined as follows using the recirculation flow rate.

If the recirculation flow rate is W, the set value is set to 0 by the calculation circuit 7.
．． It is calculated as 55W+50, and this setting value is used in the comparison circuit.

The present invention is based on LPR, which represents a large neutron flux with "fluctuation".
By using a heat flux equivalent value with a small "fluctuation" instead of the M average value, the control rod withdrawal margin can be expanded.
It can also handle cores loaded with zirconium liner fuel, where control rod operation at high power is considered.

Another embodiment of the invention is shown in FIG. FIG. 5 shows a case where the present invention is applied to a new type of boiling water reactor (ABWR). ABWR has the following characteristics.

(a) It is possible to pull out two or more control rods at once, which is a gigantic pull-out.

(b) Since the ABWR does not have a recirculation pump and uses an internal pump, the core flow rate is calculated from the differential pressure across the lower support plate.

Due to the differences in (a) and (b) above, the following two points are different compared to the embodiment shown in FIG. 1, but the others are the same.

(a) For control rod withdrawal, each control rod is monitored using a two-channel RBM. In other words, in Guyang extraction where four control rods are pulled out at the same time, 2X4=8
Monitor with channel RBM.

The selection and allocation of LPRMs at this time are the same as in the embodiment described in FIG.

(b) The core flow rate used in calculating the set value is derived from the differential pressure across the lower support plate. Use core flow rate.

〔Effect of the invention〕

According to the method of the present invention, since there is great flexibility in control rod withdrawal, there is also great flexibility in control rod operation at high power, and it can also be applied to a core loaded with zirconium liner fuel.

Moreover, according to the apparatus of the present invention, the above-described method of the invention can be easily and reliably carried out, and its effects can be fully exhibited.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention. FIG. 2 is a diagram showing the arrangement of the LPRM and control rods inside the reactor. FIG. 3 is an explanatory diagram of assignment of LPRM to RBM. FIG. 4 is a chart showing the relationship between time constant and fluctuation. FIG. 5 is a block diagram of another embodiment of the present invention. FIG. 6 is a chart showing the relationship between linear power density and time constant. DESCRIPTION OF SYMBOLS 1... Detector assembly, 2... Average circuit, 3... Time constant circuit, 4... Comparison circuit, 5... Initialization data creation section, 6... Recirculation flow rate, 7 ...Arithmetic circuit, 8...
・Control rod, 9-11...Detector assembly, 12-14・
...Control rod, 15...Fuel assembly, 16-1.9...
・LPRM height position, 20...core flow rate.

Claims (1)

[Claims] 1. (a) A control rod withdrawal monitoring device installed in a nuclear reactor equipped with a plurality of control rods and a plurality of local power range instrumentation detectors, which (b) When one or more control rods to be removed are selected, a plurality of neutron flux detectors close to the control rod are selected, and (c) after control rod withdrawal is started, the selected plurality of neutron flux detectors are selected. When the average output of the detector exceeds a preset value, a control rod withdrawal monitoring device is configured to issue a signal that prevents the control rod withdrawal operation from continuing any further. In the monitoring method, (d) the average output of the plurality of neutron flux detectors is converted into a heat flux equivalent value using a time constant circuit, and (e
) When the above heat flux equivalent value reaches a preset value,
A control rod withdrawal monitoring method, characterized in that the control rod withdrawal operation is stopped by generating the aforementioned blocking signal. 2. (a) A control rod withdrawal monitoring device installed in a nuclear reactor equipped with a plurality of control rods and a plurality of local power range instrumentation detectors, and (b) a control rod withdrawal monitoring device that monitors the control rod to be withdrawn. When one or more neutron flux detectors are selected, a plurality of neutron flux detectors close to the control rod are selected, and (c) after control rod withdrawal is started, the average output of the selected plurality of detectors is In a control rod withdrawal monitoring device that is structured so that when a preset value is exceeded, a signal is issued to prevent the withdrawal operation of the control rod from being continued any further, (f) the output of a plurality of neutron flux detectors; A control rod withdrawal monitoring device, characterized in that a time constant circuit is provided between an averaging circuit that averages the signal output and a comparison circuit that compares the detected signal output with a preset value.