JP7344745B2 - Subcriticality measurement device and subcriticality measurement method - Google Patents

Description

The present invention relates to a subcriticality measuring device and a subcriticality measuring method.

Conventionally, for example, Patent Document 1 describes a method of measuring the neutron level in the reactor while the reactor is stopped in a subcritical state and stopping the reactor when there is an abnormal increase in the output of the reactor. Are listed.

Japanese Patent Application Publication No. 2000-19284

In light water reactor cores for power generation, for example, when the core is temporarily stopped due to an abnormal situation and the state is changed by cooling the core, indirect parameter monitoring such as neutron flux inside and outside the reactor is required to prevent recriticality. I do. However, for indirect parameter monitoring, it is necessary to allow a large margin. If we know the degree of subcriticality, which is the degree of reactivity during core shutdown, we can directly understand how much margin there is before recriticality occurs, and we can more rationally protect the core and prevent recriticality. It becomes possible.

The present disclosure solves the above-mentioned problems, and aims to provide a subcriticality measuring device and a subcriticality measuring method that can accurately measure the subcriticality.

In order to achieve the above object, a subcriticality measurement device according to one aspect of the present disclosure includes a neutron detector disposed near a nuclear fuel material, and a detection signal of the neutron detector that measures a count value per predetermined time. and a calculation section that calculates the degree of subcriticality from the count value obtained from the signal processing section.

In order to achieve the above object, a method for measuring subcriticality according to one aspect of the present disclosure applies a neutron detector placed near a nuclear fuel material, and during shutdown of a nuclear reactor, a subcriticality measurement method is provided. The method includes the steps of processing a detection signal obtained from the above as a count value per predetermined time, and calculating a degree of subcriticality from the count value.

According to the present disclosure, the neutron detector placed near the nuclear fuel material can detect neutrons generated by the nuclear fuel material with high sensitivity. Then, by processing the detection signal of the neutron detector as a count value per predetermined time and calculating the degree of subcriticality from the count value, the degree of subcriticality can be measured with high accuracy.

FIG. 1 is a schematic configuration diagram showing a subcriticality measuring device according to an embodiment of the present disclosure. FIG. 2 is a flowchart showing the operation of the subcriticality measuring device according to the embodiment of the present disclosure. FIG. 3 is a graph showing an example of the operation results of the subcriticality measurement device according to the embodiment of the present disclosure. FIG. 4 is a graph showing an example of an operation result of the subcriticality measuring device according to the embodiment of the present disclosure. FIG. 5 is a longitudinal sectional view of a nuclear reactor showing an example of the arrangement of the subcriticality measuring device according to the embodiment of the present disclosure. FIG. 6 is a schematic cross-sectional view of the core of the nuclear reactor shown in FIG.

Embodiments according to the present disclosure will be described in detail below based on the drawings. Note that this disclosure is not limited to this embodiment. Furthermore, the constituent elements in the embodiments described below include those that can be easily replaced by those skilled in the art, or those that are substantially the same.

The subcriticality measuring device, subcriticality measuring method, and neutron detector of this embodiment are applied to a nuclear power plant. A nuclear power plant includes a reactor core in which a fuel assembly as a nuclear fuel material is arranged inside a reactor vessel. Spent fuel assemblies and unused fuel assemblies are stored in the fuel pit of a nuclear power plant. Spent fuel assemblies are stored in casks and transported to storage facilities. The subcriticality measuring device, subcriticality measuring method, and neutron detector of this embodiment measure the subcriticality of a fuel assembly in such a nuclear power plant. Note that the nuclear fuel material includes nuclear fuels configured in various forms, and is not limited to a fuel assembly in which a plurality of fuel rods are bundled. In the following description, a fuel assembly will be used as an example. Plants that handle fuel include fuel reprocessing plants and fuel processing facilities, and are not limited to nuclear power plants.

FIG. 1 is a schematic configuration diagram showing a subcriticality measuring device according to this embodiment.

The subcriticality measurement device 1 according to this embodiment includes a detection section 2, a signal processing section 3, and a monitoring operation section 4.

The detection unit 2 includes a neutron detector 21 and a preamplifier 22. The neutron detector 21 detects neutrons. The preamplifier 22 amplifies the detection signal output from the neutron detector 21.

The signal processing unit 3 includes a power supply device 31, a signal processing device 32, and an MCA 33. The power supply device 31 supplies power to the signal processing device 32 and MCA 33 in the signal processing section 3 . The signal processing device 32 inputs the detection signal amplified by the preamplifier 22 in the detection unit 2 and removes unnecessary noise for measurement. The MCA 33 is a multi-channel analyzer, which inputs the detection signal from which noise has been removed by the signal processing device 32, and converts this detection signal into a count value per predetermined time, that is, a pulse event of high count rate neutrons. Process as . The MCA 33 then records this pulse event.

The monitoring operation unit 4 includes a power supply device 41, a calculation unit 42, and a monitoring control device 43. The power supply device 41 supplies power to the calculation section 42 and the supervisory control device 43 in the supervisory operation section 4 . The calculation unit 42 calculates the degree of subcriticality from the count value of pulse events recorded by the MCA 33 of the signal processing unit 3. The calculation unit 42 is, for example, a computer, and although not shown in the figure, is realized by a calculation processing device, a storage device, etc., and includes a display device, an input device, an audio output device, a drive device, and an input/output interface device. It's okay. The arithmetic processing device includes a microprocessor such as a CPU (Central Processing Unit). Storage devices include memory and storage such as ROM and RAM. The arithmetic processing device performs arithmetic processing according to a computer program stored in a storage device. The display device includes a flat panel display. The input device generates input data when operated, and includes at least one of a keyboard and a mouse. Note that the input device may include a touch sensor provided on the display screen of the display device. The audio output device includes a speaker. The drive device reads data from a recording medium in which data such as a program for executing processing is recorded. Recording media include recording media that record information optically, electrically, or magnetically, such as CD-ROMs, flexible disks, and magneto-optical disks, and semiconductors that record information electrically, such as ROMs and flash memories. Various types of recording media can be used, such as memory. The input/output interface device performs data communication between the arithmetic processing unit, the storage device, the display device, the input device, the audio output device, and the drive device.

In the calculation unit 42, the subcriticality is calculated based on, for example, the Feynman α method. Furthermore, since the arrangement of the target fuel assembly affects the degree of subcriticality, the measured result is multiplied by a coefficient to convert it to the degree of subcriticality. The coefficients are obtained by, for example, being calculated in advance based on the shape of the reactor core. In addition, the degree of subcriticality can be calculated repeatedly using similar cores, and past data (pulse event table) can be fed back and revised to further reduce uncertainty.

The monitoring control device 43 monitors the degree of subcriticality calculated by the calculation unit 42. The supervisory control device 43 is, for example, a computer, which is realized by an arithmetic processing device, a storage device, etc., and includes a display device, an input device, an audio output device, a drive device, and an input/output interface device, although not shown in the figure. You may. These configurations are similar to the calculation section 42, and the monitoring control device 43 can be configured together with the calculation section 42.

In the monitoring and control device 43, the degree of subcriticality can be monitored by displaying the variation in the degree of subcriticality calculated by the calculation unit 42 on a display device. Furthermore, monitoring of the degree of subcriticality is performed by determining an abnormality when the degree of subcriticality is lower than a predetermined threshold value in the variation of the degree of subcriticality calculated by the calculation unit 42, and notifying it by a display device or an audio output device. This will enable you to issue a warning.

In the monitoring and control device 43, the degree of subcriticality is monitored by evaluating the degree of subcriticality based on the degree of subcriticality calculated by the calculation unit 42 and the conditions under which the fuel assembly is arranged. The conditions under which the fuel assemblies are arranged are based on, for example, the arrangement of the fuel assemblies, the arrangement relationship between the new fuel assemblies and the fuel assemblies in use, and the like. Evaluation of subcriticality is performed based on inference rules. The inference rules are stored in a storage device and consist of one or more of, for example, neural networks, Bayesian networks, support vector machines, and case-based inference. Note that the inference rules are input to the supervisory control device 43 via a recording medium or a network and stored in a storage device. Furthermore, the supervisory control device 43 can update the inference rules stored in the storage device based on information that changes from time to time. Therefore, an update program for updating the inference rules is stored in the storage device.

FIG. 2 is a flowchart showing the operation of the subcriticality measuring device according to this embodiment. 3 and 4 are graphs showing an example of the operation results of the subcriticality measuring device according to this embodiment.

As shown in FIG. 2, the subcriticality measurement device 1 performs neutron detection in the detection unit 2 (step: S1). Next, the subcriticality measurement device 1 counts the detection signals of neutrons detected by the detection unit 2 in the signal processing unit 3 (step: S2). Next, the subcriticality measuring device 1 calculates the subcriticality in the calculation unit 42 of the monitoring operation unit 4 (step: S3). Next, the subcriticality measuring device 1 displays the subcriticality on the monitoring control device 43 of the monitoring operation section 4 (step: S4). Next, the subcriticality measuring device 1 determines whether there is an abnormality in the subcriticality in the monitoring control device 43 of the monitoring operation unit 4 (step: S5).

In step S2, the neutron detection signal detected by the detection unit 2 counts the wave height of the detection signal every time, as shown in FIG. Furthermore, in step S3, the degree of subcriticality is calculated based on the count of detection signals during the gate time ΔT, as shown in FIG. Further, in step S4, the degree of subcriticality is displayed as a change in the degree of subcriticality per time, as shown in FIG. In addition, in step S5, a subcriticality abnormality is determined by arbitrarily setting a subcriticality threshold using the display data in FIG. judge. In step S5, the supervisory control device 43 of the supervisory operation unit 4 issues a notification if a subcriticality abnormality is determined.

FIG. 5 is a longitudinal sectional view of a nuclear reactor showing an example of the arrangement of the subcriticality measuring device according to this embodiment. FIG. 6 is a schematic cross-sectional view of the core of the nuclear reactor shown in FIG.

Although not clearly shown in the figure, a nuclear power plant uses, for example, a pressurized water reactor as a nuclear reactor. In a pressurized water nuclear power plant, light water, which is a primary coolant, is heated in a nuclear reactor, and then the high-temperature light water is sent to a steam generator via a first coolant pipe. In a nuclear power plant, the secondary coolant is evaporated by exchanging heat between the high-temperature light water and the secondary coolant in the steam generator, and the vapor of the evaporated secondary coolant is passed through the steam pipe. Electricity is generated by sending the water to a turbine and driving a generator. Furthermore, the high-temperature light water that has flowed into the steam generator is cooled by exchanging heat with the secondary coolant, returned to the reactor via the second coolant pipe, and heated to a high temperature again. In addition, the steam of the secondary coolant used in the turbine for power generation is cooled in the condenser, returned to liquid, and returned to the steam generator via the condensate water pipe, where it is re-steamed by heat exchange with the primary coolant. becomes. The reactor, the first and second coolant pipes, the steam generator, and the like constitute a primary cooling system of the nuclear power plant, and these primary cooling systems are housed in the reactor containment vessel. Note that the nuclear power plant to which the subcriticality measuring device 1 and the neutron detector 21 of the present embodiment are applied is not limited to a pressurized water reactor, but may be another nuclear reactor such as a boiling water reactor. .

As shown in FIG. 5, in a pressurized water reactor, a reactor vessel 101 is a pressure vessel, and a reactor vessel main body 101a is constructed so that reactor internals including a fuel assembly 120 can be accommodated therein. On the other hand, the reactor vessel lid 101b is fixed with a plurality of stud bolts 121 and nuts 122. The reactor vessel main body 101a has a cylindrical shape whose upper part can be opened by removing the reactor vessel lid 101b, and whose lower part is closed by a hemispherical lower mirror 101e.

Regarding the reactor internals, an upper core support plate 123 is arranged above the inlet nozzle 101c and the outlet nozzle 101d of the reactor vessel body 101a, and a lower core support plate 124 is arranged near the lower mirror 101e. be done. The upper core support plate 123 and the lower core support plate 124 are disk-shaped and have a large number of communication holes (not shown) formed therein. The upper core support plate 123 is connected downwardly to an upper core plate 126 via a plurality of core support rods 125 . The upper core plate 126 has a large number of communication holes (not shown) formed therein. Note that the upper core support plate 123 and a structure in which the upper core plate 126 is connected to the upper core support plate 123 via the core support rod 125 are referred to as an upper core structure.

Regarding the reactor internals, a cylindrical core barrel 127 is arranged inside the reactor vessel main body 101a with a predetermined gap from the inner wall surface thereof. A lower core support plate 124 is fixed to the lower end of the core barrel 127 . Further, the core barrel 127 is provided with a lower core plate 128 inside and below. The lower core plate 128 is disk-shaped and has a large number of communication holes (not shown) formed therein, and is supported by the lower core support plate 124 via a plurality of lower core support columns 129 . Note that the core barrel 127, the lower core plate 128 provided for the core barrel 127, and the lower core support plate 124 are referred to as a lower core structure. The upper core structure is inserted into the core barrel 127 of the lower core structure from above, and the upper core plate 126 is connected to the upper end. The lower core support plate 124 of the lower core structure is fixed to the reactor vessel body 101a. That is, the lower core structure and the upper core structure are supported by the reactor vessel body 101a via the lower core support plate 124.

Regarding the reactor internals, a core 130 is formed by an upper core plate 126, a core tank 127, and a lower core plate 128. In the core 130, a large number of fuel assemblies 120 are arranged inside a core tank 127 and loaded onto a lower core plate 128. Furthermore, a large number of control rods 135 are arranged inside the core 130 . The upper ends of the large number of control rods 135 are grouped together to form a control rod cluster 136, which is inserted into the fuel assembly 120. A large number of control rod cluster guide tubes 137 pass through and are fixed to the upper core support plate 123 .

A control rod drive device 138 for a magnetic jack is provided on the reactor vessel lid 101b constituting the reactor vessel 101. The control rod drive device 138 is housed in a housing 139 that is integrated with the reactor vessel lid 101b. A control rod cluster drive shaft 140 extending downward from the control rod drive device 138 passes through the control rod cluster guide tube 137 and extends to the fuel assembly 120, and is provided so as to be able to grip the control rod cluster 136. It will be done.

Further, in the reactor vessel 101, with the above-described configuration, an upper plenum 142 is formed in the reactor core barrel 127 and above the upper core plate 126 to communicate with the outlet nozzle 101d. On the other hand, a lower plenum 143 is formed outside the core barrel 127 and below the lower core support plate 124 . A downcomer portion 144 communicating with the inlet nozzle 101c and the lower plenum 143 is formed between the inner wall of the reactor vessel 101 and the core barrel 127.

Note that the reactor vessel main body 101a is provided with a large number of instrumentation nozzles 145 that pass through the lower mirror 101e. Each instrumentation nozzle 145 has an in-furnace instrumentation guide tube 146 connected to its upper end inside the furnace, and a conduit tube 147 to its lower end outside the furnace. Each in-core instrumentation guide tube 146 has an upper end connected to the lower core support plate 124 . Then, a thimble tube 148 equipped with a neutron flux detector (not shown) capable of measuring neutron flux passes from the conduit tube 147 through the instrumentation nozzle 145 and the in-core instrumentation guide tube 146, and passes through the lower core plate 128. It becomes possible to penetrate and insert up to the fuel assembly 120. Furthermore, upper and lower connecting plates 150 and 151 are attached to each in-core instrumentation guide pipe 146 to suppress vibration. The connecting plates 150 and 151 are connected to the lower core support plate 124 via a support column 152. Furthermore, the connecting plates 150 and 151 are supported by a shock absorber 153. The shock absorber 153 is arranged between the bottom plate 154 fixed to the bottom of the lower mirror 101e and the lower connecting plate 151.

For the above-mentioned nuclear reactor, the subcriticality measurement device 1 of this embodiment is configured so that the neutron detector 21 in the detection unit 2 is located in the reactor core 130 near the fuel assembly 120, as shown in FIGS. 5 and 6. Placed. The preamplifier 22, signal processing section 3, and monitoring operation section 4 in the detection section 2 are arranged outside the reactor vessel 101. A signal cable connecting the neutron detector 21 and the preamplifier 22 is led out from the neutron detector 21 to the outside of the reactor vessel 101 and connected to the preamplifier 22 using, for example, the in-core instrumentation guide pipe 146. Therefore, the preamplifier 22, signal processing section 3, and monitoring operation section 4 in the detection section 2 can be placed in an environment that is less susceptible to the effects of neutrons and gamma rays.

As shown in FIGS. 5 and 6, the neutron detector 21 is arranged in a narrow place such as the side surface of the fuel assembly 120 (between the fuel assemblies 120) or the inner surface of the core barrel 127 surrounding the reactor core 130. Ru. As shown in FIG. 5, the neutron detector 21 can detect neutrons generated from the entire length range of the fuel assembly 120 by having its outer dimensions satisfy the above-mentioned range. , it is preferable to arrange them in a range that matches the length of the fuel assembly 120. However, the present invention is not limited thereto, and the neutron detectors 21 may be arranged at multiple locations within the length of the fuel assembly 120. Moreover, as shown in FIG. 6, the neutron detectors 21 are preferably arranged at a plurality of locations in a reactor core 130 in which a plurality of fuel assemblies 120 are arranged. FIG. 6 shows an example in which the neutron detectors 21 are arranged in the center 130C and outside 130O of the core 130, which are indicated by diagonal lines, and in the area 130M in between. Further, FIG. 6 shows an example in which neutron detectors 21 are arranged at multiple locations in the circumferential direction of the core 130. By arranging the neutron detectors 21 at a plurality of locations in the core 130 in this way, the neutron detectors 21 can be arranged in each of a plurality of areas of the reactor core 130, and the degree of subcriticality in each area can be measured.

It is preferable that the neutron detector 21 is formed into, for example, a plate shape or a rod shape. By configuring the neutron detector 21 in a plate shape or a rod shape, it can be placed in a narrow place as shown in FIGS. 5 and 6. Therefore, it is possible to arrange the fuel assemblies 120 between the fuel assemblies 120 without significantly changing the layout design of the fuel assemblies 120, and it is possible to arrange the fuel assemblies 120 on the inner surface of the core barrel 127 without changing the design of the core barrel 127. be able to. If the neutron detectors 21 can be placed without reducing the number of fuel assemblies 120 while maintaining the current structure of the reactor core 130, the number of neutrons generated within the reactor during the current reactor operation will not change, so the core operation will be improved. It does not interfere with Further, by configuring the neutron detector 21 in a plate shape or a rod shape, gamma rays can be transmitted through the neutron detector 21, thereby making it possible to lower the detection sensitivity of gamma rays. Moreover, it is preferable that the neutron detector 21 has a plurality of detection elements arranged, and can output many detection signals.

The above-mentioned subcriticality measuring device 1 of the present embodiment includes a neutron detector 21 disposed near a fuel assembly (nuclear fuel material) 120 and a detection signal of the neutron detector 21 as a count value per predetermined time. It includes a signal processing section 3 for processing, and a calculation section 42 for calculating the degree of subcriticality from the count value obtained from the signal processing section 3.

By arranging the neutron detector 21 near the fuel assembly 120, neutrons generated by the fuel assembly 120 can be detected with high sensitivity. Then, the signal processing section 3 processes the detection signal of the neutron detector 21 as a count value per predetermined time, and the calculation section 42 calculates the degree of subcriticality from the count value obtained from the signal processing section 3. Therefore, the degree of subcriticality can be measured with high accuracy.

By the way, in a nuclear reactor, the control rod cluster 136 is pulled out from the fuel assembly 120 while the temperature of the primary coolant is raised from 280° C. to 300° C. to make it critical. In the current operation, a start-up test is performed in which the control rod cluster 136 is allowed to stand by and the control rod cluster 136 is gradually withdrawn to gradually increase the output. In the subcriticality measuring device 1 of this embodiment, since the neutron detector 21 is placed near the fuel assembly 120, the subcriticality is measured before the temperature of the primary coolant is raised during the shutdown of the reactor. can be measured. The subcriticality before heating the primary coolant corresponds to the performance of the control rod cluster 136 inserted into the fuel assembly 120. That is, by measuring the subcriticality using the subcriticality measuring device 1 of this embodiment, the performance of the control rod cluster 136 can be known in advance. As a result, the safety and reliability of the nuclear reactor can be improved. In addition, knowing the performance of the control rod cluster 136 in advance can be used as an alternative to measuring the characteristics of the reactor core 130, making it possible to simplify or eliminate the startup test at startup, and shorten the startup time. It is possible to improve the operating rate.

Furthermore, the subcriticality measuring device 1 of this embodiment further includes a monitoring control device 43 that displays the subcriticality and issues a notification when the subcriticality falls below a predetermined threshold.

By displaying the degree of subcriticality, fluctuations in the degree of subcriticality can be visually confirmed. By providing a notification when the subcriticality falls below a predetermined threshold value in this variation of the subcriticality, it is possible to recognize an abnormality and take prompt action.

Furthermore, in the subcriticality measurement device 1 of this embodiment, at least one neutron detector 21 is arranged between the plurality of fuel assemblies 120 in the reactor core 130 within the reactor vessel 101.

That is, the vicinity of the fuel assembly 120 is between the plurality of fuel assemblies 120 in the reactor core 130 within the reactor vessel 101, and by arranging the neutron detector 21 here, the fuel assembly 120 is generated. Neutrons can be detected with high sensitivity.

Furthermore, in the subcriticality measuring device 1 of this embodiment, at least one neutron detector 21 is arranged around the reactor core 130 within the reactor vessel 101.

That is, the vicinity of the fuel assembly 120 is the area around the reactor core 130 in the reactor vessel 101, and by placing the neutron detector 21 here, the neutrons generated by the fuel assembly 120 can be detected with high sensitivity. be able to.

Furthermore, in the subcriticality measuring device 1 of this embodiment, the neutron detectors 21 are arranged in each of a plurality of areas of the reactor core 130 within the reactor vessel 101.

By arranging the neutron detectors 21 in each of a plurality of areas of the reactor core 130, the monitoring and control device 43 can calculate the subcriticality of each area calculated by the calculation unit 42 and the conditions under which the fuel assemblies 120 are arranged in each area. The degree of subcriticality can be evaluated based on Therefore, the characteristics of the core 130 can be measured more accurately based on the conditions under which the fuel assemblies 120 are arranged.

Furthermore, in the subcriticality measurement device 1 of this embodiment, the area includes at least one of the center 130C of the core 130, the outer side 130O, and the middle 130M.

Therefore, the subcriticality of the center 130C, the outer side 130O, and the middle 130M of the core 130 can be evaluated.

In addition, the subcriticality measurement method of this embodiment applies the neutron detector 21 placed near the fuel assembly 120, and uses the detection signal obtained from the neutron detector 21 at a predetermined level during the shutdown of the nuclear reactor. The method includes a step of processing the count value per time, and a step of calculating the degree of subcriticality from the count value.

By arranging the neutron detector 21 near the fuel assembly 120, neutrons generated by the fuel assembly 120 can be detected with high sensitivity. Then, by processing the detection signal of the neutron detector 21 as a count value per predetermined time and calculating the degree of subcriticality from the count value, the degree of subcriticality can be measured with high accuracy.

In addition, in the subcriticality measurement method of this embodiment, since the neutron detector 21 can be placed near the fuel assembly 120, the neutron detector 21 can be placed near the fuel assembly 120, so that the The degree of subcriticality can be measured. The subcriticality before heating the primary coolant corresponds to the performance of the control rod cluster 136 inserted into the fuel assembly 120. That is, by measuring the subcriticality using the subcriticality measuring device 1 of this embodiment, the performance of the control rod cluster 136 can be known in advance. As a result, the safety and reliability of the nuclear reactor can be improved. In addition, knowing the performance of the control rod cluster 136 in advance can be used as an alternative to measuring the characteristics of the reactor core 130, making it possible to simplify or eliminate the startup test at startup, and shorten the startup time. It is possible to improve the operating rate.

Furthermore, the subcriticality measuring method of this embodiment further includes the step of multiplying the count value by a pre-calculated coefficient to convert it into a subcriticality.

According to this method for measuring subcriticality, by multiplying the measured result by a coefficient and converting it into subcriticality, it is possible to correct the influence of subcriticality due to the arrangement of fuel assemblies and the like.

1 Subcriticality measurement device 2 Detection section 21 Neutron detector 22 Preamplifier 3 Signal processing section 31 Power supply device 32 Signal processing device 4 Monitoring operation section 41 Power supply device 42 Arithmetic section 43 Monitoring control device 101 Reactor vessel 120 Fuel assembly (nuclear fuel material)
127 Core tank 130 Core

Claims (6)

Near the nuclear fuel materials, at least one of which is located on the inner surface of a core tank surrounding the reactor core, within a reactor vessel between a plurality of said nuclear fuel materials in the core, within a reactor vessel at the center of said core. , the outside, and neutron detectors arranged in each of a plurality of areas of the reactor core including at least one in between ;
a signal processing unit that processes the detection signal of the neutron detector as a count value per predetermined time;
a calculation unit that calculates the degree of subcriticality from the count value obtained from the signal processing unit;
A monitoring and control device that evaluates the degree of subcriticality based on the degree of subcriticality calculated by the calculation unit and the conditions under which the nuclear fuel material is placed based on the placement relationship between the new nuclear fuel material and the nuclear fuel material in use. and,
A subcriticality measuring device equipped with. The subcriticality measuring device according to claim 1 , wherein the neutron detector is formed in a plate shape. 3. The subcriticality measuring device according to claim 1 , wherein the monitoring and control device displays the subcriticality and issues a notification when the subcriticality falls below a predetermined threshold. At least one of the nuclear fuel materials may be disposed on the inner surface of a core tank surrounding the reactor core, or may be disposed within the reactor vessel between a plurality of said nuclear fuel materials in the reactor core; Applying neutron detectors that are placed in each of multiple areas of the reactor core, including at least one in the center, outside, and in between,
processing the detection signal obtained from the neutron detector as a count value per predetermined time during the shutdown of the nuclear reactor;
calculating a degree of subcriticality from the count value;
Evaluating the degree of subcriticality based on the calculated degree of subcriticality and the conditions under which the nuclear fuel material is placed based on the placement relationship between the new nuclear fuel material and the nuclear fuel material in use;
Subcriticality measurement method including. 5. The subcriticality measurement method according to claim 4, wherein the neutron detector is formed in a plate shape. 6. The subcriticality measurement method according to claim 4 , further comprising the step of multiplying the count value by a pre-calculated coefficient to convert it into subcriticality.

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