JP2016024154A - Estimation method of subcritical state and subcritical state estimation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an estimation method of a subcritical state and a subcritical state estimation system capable of grasping a subcritical degree of fuel debris without introducing large-scaled facilities.SOLUTION: A subcritical state estimation system 1 includes: an image acquisition part 3 for acquiring an image of fuel debris and generating three-dimensional shape data based on image data; a neutron detector 4 for measuring neutrons emitted from fuel debris 11; and a subcritical state estimation device 2 for simulating a state of the fuel debris 11 based on the image data and the three-dimensional shape data, extracting a neutron multiplication factor of the fuel debris 11 based on simulation results and measured neutron fluxes, and estimating a subcritical degree of the fuel debris 11. The subcritical state estimation device 2 is constituted of an input condition preparation part 22, a subcritical state estimation part 24, and a simulation part 23, and includes a structure DB 25 for storing a composition of the structure inside a nuclear reactor building and a composition of nuclear fuel materials.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a subcritical state estimation method and estimation system when handling nuclear fuel materials, and more particularly, to a subcritical state estimation method and subcriticality when handling molten nuclear fuel materials in boiling water nuclear plants and pressurized water nuclear plants. It relates to a state estimation system.

In nuclear power plants such as the current boiling water nuclear power plant (hereinafter BWR) and pressurized water nuclear power plant (hereinafter PWR), the weight ratio of fissile uranium isotope ²³⁵ U to the total uranium is about 5% at the maximum. Low enriched uranium is mainly used as nuclear fuel material. In such low-enriched uranium, uranium and water, which is a neutron moderator, are mixed at a critical volume ratio or atomic ratio (hereinafter collectively referred to as a mixing ratio), and the uranium and water mixture is mixed. It becomes critical only when it exceeds a certain size. In addition, the mixture of uranium and water includes a case where water exists between fuel rods storing uranium, such as a fuel assembly. The size of the mixture of uranium and water indicates the volume of each mixture when there is substantially no neutron exchange between one mixture and another mixture. The fuel may contain fissionable isotopes other than ²³⁵ U. Hereinafter, these fissionable isotopes are collectively referred to as uranium.

  When handling nuclear fuel materials, care must be taken not to be critical, not only in the reactor, but also during manufacturing and transportation. If the enrichment of uranium and the mixing ratio of uranium and water are known, the shape and volume of the mixture of uranium and water that becomes critical can be known (for example, Table 4.4 of Critical Management Handbook No. 2, P111). For example, the volume of the container for storing the fission material is set so that the volume does not exceed that.

  On the other hand, even if the enrichment of uranium and the mixing ratio of uranium and water are unknown, Patent Document 1 describes a method of measuring the so-called subcriticality, how far the system is from the critical state. . For fluid nuclear fuel materials containing fissionable isotopes and isotopes that emit spontaneous neutrons, a container containing nuclear fuel materials, measuring neutrons leaking from the container at multiple sites with different shapes, and subcritical Calculate the degree. Even if the enrichment of uranium and the mixing ratio of uranium and water are the same, the amount of neutrons leaking to the outside of the nuclear fuel material will change if the shape of the nuclear fuel material is different, It uses what is determined depending on the subcriticality of nuclear fuel material.

  Further, Patent Document 2 is known as another method for measuring the subcriticality. A plurality of neutron detectors are installed on one side of an elongated or flat nuclear fuel material, and one or more of a neutron source, a neutron reflector, and a neutron absorber are installed on the other side. A neutron source, a neutron reflector, and a neutron absorber are moved closer to or away from nuclear fuel material to locally change the neutron flux level, and neutron measurements between multiple neutron detectors are used to determine the neutrons between neutron detectors. Evaluate the propagation of Neutron propagation utilizes the fact that it depends on the subcriticality of the system, and evaluates the subcriticality between neutron detectors.

Japanese Patent Laid-Open No. 62-294998 Japanese Patent Laid-Open No. 2-222887 Japanese Patent Laid-Open No. 9-81737

  However, since the method for measuring the subcriticality described in Patent Document 1 is to store a uniform liquid nuclear fuel material in a known container in advance, it is possible to apply to a molten nuclear fuel material having an unknown shape. Application is difficult. That is, it is difficult to apply to estimation of the subcriticality of a molten nuclear fuel material having various unknown shapes or a mixture of a molten nuclear fuel material and a structure after being melted (hereinafter collectively referred to as fuel debris). It becomes.

  In addition, the subcriticality measuring method described in Patent Document 2 requires that the neutron source, the neutron reflector, and the neutron absorber be operated so as to be close to or away from the nuclear fuel material, and such equipment is installed. Space is required. However, it is assumed that the fuel debris is in the reactor pressure vessel. In this case, it is conceivable that the fuel debris having an unknown shape exists in the narrow portion. Therefore, the above-mentioned equipment cannot be installed in a narrow part, and it is difficult to apply it to estimation of the subcriticality of fuel debris.

  Accordingly, the present invention is to provide a subcritical state estimation method and a subcritical state estimation system capable of grasping the subcriticality of fuel debris without introducing a large-scale facility.

  In order to solve the above problems, a subcritical state estimation system according to the present invention acquires an image of fuel debris, generates an image acquisition unit that generates three-dimensional shape data based on the acquired image, and neutrons emitted from the fuel debris. A neutron detector to be measured, and a state of the fuel debris is simulated based on the acquired image data and three-dimensional shape data, and a neutron multiplication factor of the fuel debris is extracted based on the simulation result and the measured neutrons, And a subcritical state estimating device for estimating the subcriticality of the fuel debris.

  Further, the subcritical state estimation method for estimating the subcriticality of fuel debris existing in the reactor building of the present invention includes: (1) a first support member in which an image acquisition unit is installed at a tip portion; A step of lowering in the vertical direction in the reactor building and positioning the image acquisition unit in the vicinity of the fuel debris; and (2) a second support member in which a neutron detector is installed at the tip of the reactor building. And (3) positioning the neutron detector in the vicinity of the fuel debris, and (3) determining the fuel debris based on the fuel debris image data and the three-dimensional shape data acquired by the image acquisition unit. (4) extracting the neutron multiplication factor of the fuel debris based on the simulation result and the measurement result by the neutron detector, and determining the subcriticality of the fuel debris. Characterized by comprising the subcriticality estimation step of constant.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to implement | achieve the subcritical state estimation method and subcritical state estimation system which can grasp | ascertain the subcriticality of fuel debris, without introducing a large-scale installation.

  Also, for example, when the fuel debris existing in the reactor pressure vessel is cut out and taken out, the subcriticality of the fuel debris can be grasped in advance, so that appropriate criticality prevention measures can be taken as necessary. The risk of becoming critical during the fuel debris retrieval operation can be reduced.

  Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

1 is an overall configuration diagram of a subcritical state estimation system according to an embodiment of the present invention. It is a figure which shows the state which installed the subcritical state estimation system shown in FIG. 1 in the reactor building. It is a figure which shows the positional relationship with a fuel debris, an image acquisition part, and a neutron detector. It is a figure which shows the simulation system in the simulation part which comprises a subcritical state estimation apparatus. It is a figure which shows the evaluation result of the relationship between the neutron flux and neutron multiplication factor by the simulation system shown in FIG. It is a figure which shows the non-erosion state to the structure by the fuel debris which has an unknown shape. It is a figure which shows the erosion state to the structure by the fuel debris which has an unknown shape. It is a figure which shows the relationship between a neutron flux and a neutron multiplication factor by the arrangement position of the neutron detector on a simulation model. It is a figure which shows the relationship between the neutron flux and neutron multiplication factor by the other arrangement position of the neutron detector on a simulation model.

    Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 shows an overall configuration diagram of a subcritical state estimation system according to a first embodiment of the present invention. As shown in FIG. 1, the subcritical state estimation system 1 includes an image acquisition unit 3, a neutron detector 4, a subcritical state estimation device 2, an input unit 5, and a display unit 6. The image acquisition unit 3 irradiates an image data captured by an imaging device such as a camera and a device that generates three-dimensional shape data from the captured image data, or a fuel debris that is a target object with a laser or an ultrasonic wave, The apparatus includes a device that generates three-dimensional shape data and image data of an object from the reflected wave or reflected light. Hereinafter, a case where a camera is used as the image acquisition unit 3 will be described as an example.

The subcritical state estimation device 2 includes an I / OIF 21, an input condition creation unit 22, a simulation unit 23, a subcriticality estimation unit 24, a structure database (hereinafter, structure DB) 25, and an internal bus 26. Here, the input condition creation unit 22, the simulation unit 23, and the subcriticality estimation unit 24 include, as a hardware configuration, a ROM that stores various programs, a calculation target data, a calculation process data, a RAM that stores calculation results, and the like. A storage device including a memory, a program such as a CPU that reads a program stored in a ROM, executes the read program on data to be calculated stored in the RAM, and stores the calculation result in the RAM . The I / OIF 21 includes data input by an operator via the input unit 5, image data and three-dimensional shape data transmitted from the image acquisition unit 3 such as a camera via the first signal line 20a, and a neutron detector. 4, the measured neutron data transmitted via the second signal line 20b, for example, the neutron flux is captured, and the captured data is input via the internal bus 26 to the input condition creating unit 22 and the simulation unit. 23 and a function of transmitting to the subcriticality estimation unit 24. In addition, the I / OIF 21 transmits image data transmitted from the image acquisition unit 3 such as a camera via the first signal line 20 a to the display unit 6, which is used in an operation process described later, to the display unit 6. It has a function to be displayed on the screen.
Here, the state which installed the subcritical state estimation system 1 in the reactor building in FIG. 2 is shown. First, the structure inside the reactor building 12 will be described. In a normal state, the BWR includes a reactor (not shown) in the reactor containment vessel 13. The reactor containment vessel 13 is installed in the reactor building 12, and a reactor containment vessel upper lid 17 is attached to an upper end portion of the reactor containment vessel 13 so as to be sealed. The reactor containment vessel 13 has a pressure suppression chamber (wet well) 33 in which a pressure suppression pool is formed. One end of the vent passage 34 connected to the reactor containment vessel 13 is immersed in the cooling water of the pressure suppression pool in the pressure suppression chamber 33. A shield plug, which is a radiation shield divided into a plurality of parts, is disposed directly above the reactor containment vessel upper lid 17, and these shield plugs are installed on the operation floor 32 of the reactor building 12.

  A reactor containment vessel 13 is installed inside the reactor building 12, and a reactor pressure vessel upper lid 18 is opened at the top of the reactor containment vessel 13 when the reactor is shut down to show a fuel assembly (not shown). 2) is taken out and moved to the adjacent spent fuel storage pool 31. The fuel assembly is taken out and transferred to the spent fuel storage pool 31 by a fuel exchanger (not shown). Further, a spent fuel storage pool for temporarily storing a dryer / separator pool 30 and spent fuel on both sides of a shield plug (not shown) directly above the reactor containment vessel top cover 17 is sandwiched. 31 is provided. The dryer / separator pool 30 is used as a place for temporarily placing in-furnace components such as the steam dryer 15 and the steam / water separator 16 during periodic inspection.

  A reactor pressure vessel 14 configured by attaching a reactor pressure vessel top lid 18, a core loaded with a plurality of fuel assemblies containing nuclear fuel materials, a steam dryer 15, and a steam / water separator 16 are provided. The core, the steam dryer 15 and the steam separator 16 are disposed in the reactor pressure vessel 14. The steam / water separator 16 is disposed above the upper lattice plate located at the upper end of the core, and the steam dryer 15 is disposed above the steam / water separator 16. Here, the fuel assembly has a fuel rod in which a plurality of pellets of, for example, MOX fuel as a nuclear fuel material (not shown) are filled in a stainless steel cladding tube in the axial direction. A fuel assembly is formed by arranging a plurality of fuel rods in a square lattice shape in a channel box having a quadrangular cross section. The reactor pressure vessel 14 is installed on a cylindrical pedestal 19 provided on a concrete mat 35 provided at the bottom of the reactor containment vessel 13.

  In the state shown in FIG. 2, the shield plug is removed, and the reactor containment vessel upper lid 17, the reactor pressure vessel upper lid 18, the steam dryer 15, and the steam / water separator 16 are taken out, and the dryer is filled with water. A state where the separator pool 30 has already been temporarily placed is shown. On the operation floor 32, the subcritical state estimation device 2, the input unit 5, and the display unit 6 are installed, and above the removed shield plug, that is, the upper opening of the reactor containment vessel 13 and the upper portion of the reactor pressure vessel 14. A work table 8 is installed at a position covering the opening. For example, a camera is attached to the tip, for example, a first support member 9 that moves vertically in the reactor containment vessel 13 or the reactor pressure vessel 14 and a neutron detector 4 is attached to the tip. The second support member 10 that moves up and down in the reactor containment vessel 13 or the reactor pressure vessel 14 in the vertical direction, and the first support member 9 and the second support member 10 are suspended, and the work table 8 is placed horizontally. A moving device 7 that can move in the direction is attached to the work table 8. These steps such as the installation of the work table 8 will be described later. In addition, the image data from the image acquisition unit 3 attached to the tip of the first support member 9 and the three-dimensional shape data generated based on the image data are the first support member 9 and the first signal line 20a. It is possible to input to the subcritical state estimation device 2 via The neutron flux measured by the neutron detector 4 attached to the tip of the second support member 10 is input to the subcritical state estimation device 2 via the second support member 10 and the second signal line 20b. It is configured to be possible. In FIG. 2, the fuel debris 11, which is a molten nuclear fuel material or a mixture of molten nuclear fuel material and a structure, is present on the concrete mat 35 at the bottom of the reactor pressure vessel 14 and inside the pedestal 19. Shows the case.

  The moving device 7 shown in FIG. 2 moves on the work table 8 in the horizontal direction and is positioned at the upper opening of the reactor pressure vessel 14 as described above, and thereafter, the first support member 9 and the second support member 9 are moved. The support member 10 is lowered or raised toward the bottom in the reactor pressure vessel 14 at a desired feed amount. Here, as the moving device 7, for example, a mechanism for moving the first support member 9 and the second support member 10 up and down using a fuel debris boring device with a drill attached to the tip for cutting the fuel debris 11 is used. It is configured by providing. The moving device 7 is not limited to the fuel debris boring device, but can move in the horizontal direction on the work table 8 and can hold the first support member 9 and the second support member 10 so as to be movable up and down. Any of them may be used.

  Here, returning to FIG. 1, the structure DB 25 will be described. The structure DB 25 is not limited to the reactor pressure vessel 14, but is a pressure suppression chamber (wet) in which a reactor containment vessel 13, a steam / water separator 16, a steam dryer 15, and other devices and a pressure suppression pool are formed. The shape data and material data of the structure such as the well) 33, the composition data of the nuclear fuel material, the content rate of the material having a different composition, and the like are stored.

  In FIG. 3, the positional relationship with the fuel debris 11, the image acquisition part 3, and the neutron detector 4 is shown. In FIG. 3, the fuel debris 11 exists in the bottom part of the reactor pressure vessel 14 shown in FIG. 2, and the positional relationship with the camera as the image acquisition part 3 and the neutron detector 4 is shown. As shown in FIG. 3, the fuel debris 11 has an irregular shape, and the fuel debris 11 and its surrounding image data are acquired by a camera as the image acquisition unit 3, and the fuel is based on the acquired image data. Three-dimensional shape data of the debris 11 is generated and input to the I / OIF 21 of the subcritical state estimation device 2 outside the reactor pressure vessel 14 via the signal line 20a. Similarly, the neutron flux measured by the neutron detector 4 is input to the I / OIF 21 of the subcritical state estimation apparatus 2 outside the reactor pressure vessel 14 through the signal line 20b.

  The image data including the fuel debris 11 and its surroundings input to the I / OIF 21 is transmitted to the display unit 6, and the image data, the three-dimensional shape data generated based on the image data, and the measured neutron data are And transmitted to the input condition creating unit 22 via the internal bus 26. The input condition creation unit 22 has a function of converting into an input format that can be calculated by the simulation unit 23. For example, the boundary of the fuel debris 11 that is the target object is set from the image data obtained from the image acquisition unit 3 and the three-dimensional shape data obtained from the image data, and is input to the input format from the input unit 5. Further, the composition contained in the region of the fuel debris 11 that is the target object divided by the boundary, for example, the content rate of carbon, iron, etc. is read from the structure DB and input to the input format. Here, in setting the boundary of the fuel debris 11, for example, image data and three-dimensional shape data transmitted from the I / OIF 21 are displayed on the display unit 6, and the boundary by the worker on the display screen is displayed with a mouse cursor or What is necessary is just to comprise so that the operation which traces with a touch pen may be received and the received coordinate data on the display screen may be input into the input condition preparation part 22. FIG. Note that the input is not limited to the input by the worker as described above, and if necessary, for example, the boundary is set based on the feature amount in the image data and the three-dimensional shape data, or the boundary is automatically set by the edge enhancement processing of the image data. It may be configured to set.

  As described above, the input condition creating unit 22 creates an input condition for enabling the simulation unit 23 to execute the simulation. Here, the purpose of the simulation is to predict neutron data (neutron flux and neutron multiplication factor) measured by the neutron detector 4 placed in the reactor pressure vessel 14 or the reactor containment vessel 13. As a suitable method therefor, there is a neutron transport calculation method using the Monte Carlo method (hereinafter referred to as Monte Carlo calculation). An example of such a Monte Carlo calculation is an address on the web, https: // mcnp. lanl. There is a calculation code shown in gov /. Information necessary for performing the Monte Carlo calculation is a boundary between regions mainly composed of different materials and a material composition in each region. Further, as a method of generating three-dimensional information of an object from image data acquired by a camera as the image acquisition unit 3, that is, three-dimensional shape data, for example, a method described in Patent Document 3 is known, Boundaries between regions composed of different materials can be created automatically.

  In addition, after creating automatically to some extent, as described above, when the worker makes corrections manually while looking at the image displayed on the display unit 6, or the input unit 5 and the I / OIF 21 only by manual operation. It is good also as a structure input into the input condition preparation part 22 via.

  The material composition contained in each region divided by the boundary setting is held in the structure DB 25 in advance, and the composition and density of the structure existing in the reactor pressure vessel 14 or the reactor containment vessel 13 are known. Yes, it can be stored in the structure DB 25 as described above. However, in the case of the fuel debris 11, unlike the case where the fuel debris 11 exists in a healthy state in the reactor pressure vessel 14, it is assumed that structures other than nuclear fuel materials are mixed in or the density is changed. The It is also conceivable that the composition of the nuclear fuel material changes as a result of combustion in the reactor pressure vessel 14, for example, the uranium concentration changes. Considering the above, it is desirable to prepare a plurality of patterns assumed as the material composition of the fuel debris 11 and store them in the structure DB 25. Moreover, based on the image data acquired by the camera as the image acquisition part 3, it is also possible to create a new substance composition as information used for the Monte Carlo calculation. The input condition creation unit 22 sets the material composition and density data prepared as described above for each region divided by the boundary setting. In this case, since the position and material composition of the structure in the reactor pressure vessel 14 are stored in the structure DB in advance, the material composition can be automatically set for each region by comparing with the boundary data. .

  Next, simulation by the simulation unit 23 shown in FIG. 1 will be described. Using the input conditions created by the input condition creation unit 22, the simulation unit 23 executes a simulation. A plurality of patterns are stored in advance in the structure DB 25 as the material composition of the fuel debris 11, and a plurality of simulations are executed corresponding to the plurality of material composition patterns. In order to efficiently perform necessary calculations such as speeding up a single simulation and parallel processing of a plurality of simulations, it is desirable that the simulation unit 23 has a hardware configuration in which a plurality of processors are connected in parallel.

  In FIG. 4, the simulation system (simulation model) in the simulation part 23 which comprises the subcritical state estimation apparatus 2 is shown. The present inventors paid attention to the uranium enrichment and the mixing ratio of nuclear fuel material and water as parameters that have a large influence on the subcriticality in the material composition of the fuel debris 11 and are likely to be unknown. Therefore, in this embodiment, the fuel debris 11 and water are simulated as a sphere having a diameter of 40 cm, and the neutron detector 4 is simulated as a sphere having a diameter of 10 cm. Moreover, as shown in FIG. 4, the sphere of the neutron detector 4 was arranged adjacent to the sphere composed of the fuel debris 11 and water, and the periphery of these was defined as water. In the simulation of changing the mixing ratio of the nuclear fuel material and water, the uranium enrichment was constant at about 2%, and the volume ratio of the nuclear fuel material and water was changed within the range of the following formula (1).

Nuclear fuel material: water = 0.1: 0.9 to 1.0: 0.0 (1)
Further, in the simulation for changing the uranium enrichment, the condition that maximizes the neutron multiplication factor obtained when the simulation is performed under the condition of the expression (1), that is, the volume ratio of the nuclear fuel material and water is expressed by the following expression ( 2) constant,
Nuclear fuel material: Water = 0.4: 0.6 (2)
The conditions were such that the uranium enrichment was varied in the range of about 1% to about 5%.

  FIG. 5 shows the relationship between the neutron flux and the neutron multiplication factor obtained when simulating under the above conditions. In the simulation for changing the uranium concentration, as shown in FIG. 5, when the uranium enrichment is increased, both the neutron flux and the neutron multiplication factor are increased. The reason why the neutron flux is not zero even when the neutron multiplication factor is less than 1.0, that is, in the subcritical state, is that the fuel debris 11 contains spontaneous fissionable isotopes. In addition to neutrons generated by spontaneous fission leaking out of fuel debris 11 (molten fuel), neutrons generated by spontaneous fission cause fission of fissionable isotopes contained in fuel debris 11, and the generated neutrons generate fuel debris. 11 leaks out. When uranium enrichment is high, that is, the ratio of fissionable isotopes is high, the probability of fission is increased by neutrons generated by spontaneous fission or fission of other fissionable isotopes, and more neutrons are generated by fission. Therefore, more neutrons leak out of the fuel debris 11. For this reason, when the uranium enrichment increases, the neutron flux at the position where the neutron detector is arranged on the simulation model also increases.

  On the other hand, in the simulation of changing the mixing ratio of nuclear fuel material and water, both the neutron flux and the neutron multiplication factor increase to some extent when the ratio of the nuclear fuel material is increased within the range shown in the above equation (1). This is because the amount of spontaneous fissionable isotopes increases as the proportion of nuclear fuel material increases, and the amount of neutrons generated by spontaneous fission increases. This is because it becomes energy and neutrons generated by fission increase. However, as the proportion of nuclear fuel material increases, the neutron multiplication factor starts to decrease along the way. This is because the ratio of water in the fuel debris 11 is reduced, so that neutrons are not decelerated, fissionable isotopes are less likely to undergo fission, and neutrons generated by fission are reduced.

  As described above, the simulation unit 23 outputs the relationship between the neutron flux obtained at the position of the neutron detector on the simulation model and the neutron multiplication factor as a simulation result. That is, the neutron data (neutron flux) by the neutron detector simulated by the simulation, which can be compared with the neutron flux measured by the actual neutron detector 4, is output as a simulation result in the subcritical estimation unit 24 described later. The simulation result is transmitted to the subcriticality estimation unit 24 via the internal bus 26.

The subcriticality estimation unit 24 takes in the neutron flux measured by the neutron detector 4 through the second signal line 20b, the I / OIF 21 and the internal bus 26. Then, the measurement result of the captured neutron flux is compared with the simulation result indicating the relationship between the neutron flux obtained at the neutron detector position on the simulation model and the neutron multiplication factor, and the subcriticality is estimated. That is, the subcriticality is determined by estimating the neutron multiplication factor of the fuel debris 11. Specifically, from the material composition pattern of the fuel debris 11 that was unknown at the time of the simulation, a material composition that can be considered to match the measured value of the neutron flux by the neutron detector 4 with the simulation result is selected. The criticality is regarded as the subcriticality of the fuel debris 11. In the simulation results shown in FIG. 5, for example, when the neutron flux measured by the neutron detector 4 is 5 × 10 ³ / cm ² / s ² , the neutron multiplication factor is changed in the case where the uranium enrichment is changed. About 0.7 is obtained. In the case where the mixing ratio of nuclear fuel material and water is changed, a neutron multiplication factor of about 0.6 is obtained. Therefore, it is difficult to distinguish which material composition of the fuel debris 11 is close by only comparing with the measured value of the neutron detector 4, but from the viewpoint of grasping the risk of becoming a critical state, the obtained neutron Even if the multiplication factor is high, it is sufficient to know that it is about 0.7 (when the uranium enrichment is high).

  In the present embodiment, even if the material composition of the fuel debris 11 cannot be specified as described above, the maximum value of the neutron multiplication factor, that is, the range including the minimum subcriticality can be estimated. That is, by comparing the simulation result with the measurement result of the neutron flux by the neutron detector 4, the neutron multiplication factor can be estimated more strictly and the subcriticality of the fuel debris 11 can be estimated.

  In this example, in order to make the explanation easy to understand, the uranium enrichment and the mixing ratio of the nuclear fuel material and water were evaluated by simulation as independent parameters, but a combination in which both were changed was evaluated, You may survey the neutron multiplication factor which becomes the maximum on the conditions which reproduce the measured value of the neutron flux by the neutron detector 4. Moreover, although the neutron flux was shown as data compared with the data measured by the neutron detector 4 in the present Example, it is not restricted to this. For example, any quantity that can be measured by the neutron detector 4 such as a count rate (count / second) and can be evaluated by the simulation can be used as a parameter in the simulation.

  As the influence on the measurement data and subcriticality of the neutron detector 4, the ratio of the spontaneous fissionable isotope contained in the material composition of the fuel debris 11, the type and ratio of the substance of the structure, and the like are also conceivable. The ratio of spontaneous fissionable isotopes can be estimated from operation management data or combustion composition calculation based on operation management data. In addition, the types and ratios of the structural materials can be estimated from accident analysis. In the initial stage, the subcriticality of the fuel debris 11 is estimated based on the ratio of these spontaneous fissile isotopes or the ratio of the substance of the structure, and then the substance composition is clarified by sampling analysis of the fuel debris 11 Thus, the subcriticality can be determined using data with higher accuracy. At this time, it is possible to estimate the maximum neutron multiplication factor under the conditions that give the same neutron data (neutron flux or neutron count rate) at the position of the neutron detector on the simulation model. Since it is the purpose of the simulation, the ratio of the spontaneous fissionable isotope or the kind and ratio of the substance of the structure may be simulated as parameters within the estimated range.

  Returning to FIG. 2, each step of the subcritical state estimation method according to this embodiment will be described below. First, as a preparatory step, a shielding step of pouring shielding water (water) downward from the operation floor 32 is performed. Next, the work table 8 is installed on the operation floor 32 with the radiation dose reduced by the shielding process. At this time, the moving device 7 to be installed on the work table 8 later, the reactor opening with the reactor containment vessel upper lid 17 removed, or the reactor pressure with the reactor pressure vessel upper lid 18 removed. The work table 8 is installed so as to be positioned at the upper opening of the container 14 (work table installation process).

  Subsequently, the moving device 7 is attached to the installed work table 8. At this time, the first support member 9 with the camera as the image acquisition unit 3 attached to the tip and the second support member 10 with the neutron detector 4 attached to the tip are used as the drive unit of the moving device 7. (Not shown) is engaged (moving mechanism installation step). Further, the subcritical state estimation device 2, the input unit 5, and the display unit 6 are installed at desired positions on the operation floor 32, and the moving device 7 and the subcritical state estimation device 2 are connected to the first signal line 20a and the second signal line 20a. The signal line 20b is connected. Thereby, the installation of the subcritical state estimation system 1 including the image acquisition unit 3, the neutron detector 4, the subcritical state estimation device 2, the input unit 5, and the display unit 6 is completed (subcritical state estimation system installation step) ).

  Here, for example, a case where the fuel debris 11 is present at the bottom of the reactor pressure vessel 14 as shown in FIG. 2 is assumed. The moving device 7 moves on the work table 8 in the horizontal direction in accordance with the operation by the worker input via the input unit 5. When the moving device 7 moves to a position almost directly above the upper opening of the reactor pressure vessel 14, a stop command is sent from the input unit 5 to the moving device 7 via the I / OIF 21 in the subcritical state estimating device 2. Is output, and the moving device 7 stops. Thereafter, the moving device 7 is driven, and the first support member 9 and the second support member 10 are lowered in the vertical direction in the reactor pressure vessel 14. During the descent, the image data obtained from the image acquisition unit 3 attached to the tip of the first support member 9 and the three-dimensional shape data generated based on the image data are the first support member 9, It is displayed on the display unit 6 via the first signal line 20a and the I / OIF 21 of the subcritical state estimation apparatus 2. Similarly, the neutron flux and / or neutron count rate (count / second) measured by the neutron detector 4 attached to the tip of the second support member 10 is also displayed on the display unit 6. The operator visually confirms the image data, the three-dimensional shape data, the neutron flux and / or the neutron count rate (count / second) displayed on the screen of the display unit 6, and the input unit 5 and The feed amounts (operation amounts) of the first support member 9 and the second support member 10 are sent to the moving device 7 via the I / OIF 21. The moving device 7 lowers the first support member 9 and the second support member 10 toward the bottom side of the reactor pressure vessel 14 according to the received feed amount. In the present embodiment, the image data, the three-dimensional shape data, the neutron flux and / or the neutron count rate are displayed on the display unit 6, but the present invention is not necessarily limited thereto. That is, only the image data may be displayed on the display unit 6 so as to support the operation by the worker, or the image data and the measured value of the neutron flux may be displayed on the display unit 6. .

  As the first support member 9 and the second support member 10 are lowered, the image acquisition unit 3 and the neutron detector 4 attached to the respective tip portions are arranged in the vicinity of the fuel debris 11 that is the target object. This state is the state shown in FIGS. Thereby, the positioning process of the image acquisition part 3 and the neutron detector 4 is complete | finished.

  After the positioning process of the image acquisition unit 3 and the neutron detector 4, the boundary of the fuel debris 11 that is the target object is set from the image data displayed on the display unit 6 or the three-dimensional shape data. Here, as for the boundary setting, as described above, the set boundary is set in a predetermined input format by setting through the input unit 5 by an operator or by automatic setting of a boundary between regions composed of different substances. And is input to the input condition creation unit 22 via the I / OIF 21.

  As described above, the input condition creation unit 22 refers to the input boundary and structure DB 25, generates a plurality of patterns as the material composition of the fuel debris 11, and transmits the pattern to the simulation unit 23 via the internal bus 26. . Thereby, the input condition creation process is completed.

  The simulation unit 23 executes the above-described processing, and outputs the relationship between the neutron flux obtained at the position of the neutron detector on the simulation model and the neutron multiplication factor as a simulation result. Further, as described above, the subcriticality estimation unit 24 takes in the neutron flux measured by the neutron detector 4 via the second signal line 20b, the I / OIF 21 and the internal bus 26. Then, the measurement result of the captured neutron flux is compared with the simulation result indicating the relationship between the neutron flux obtained at the neutron detector position on the simulation model and the neutron multiplication factor, and the subcriticality is estimated. The estimated value of the subcriticality, that is, the neutron multiplication factor of the fuel debris 11 is displayed on the display unit 6 via the internal bus 26 and the I / OIF 21, and the process is terminated (subcriticality estimation step).

  In this embodiment, as shown in FIG. 2, the subcritical state estimation device 2 is installed on the operation floor 32 in the reactor building 12 and wired by the first signal line 20a and the second signal line 20b. However, although the image acquisition unit 3 and the neutron detector 4 are configured to be electrically connected, the present invention is not necessarily limited thereto. For example, a wireless communication terminal is installed on the wall surface of the operation floor 32 or the reactor building 12, and the first signal line 20a and the second signal line 20b are connected to the wireless communication terminal. The subcritical state estimation device 2, the input unit 5, and the display unit 6 are installed at a desired location outside the reactor building 12, and a wireless communication IF is newly incorporated in the subcritical state estimation device 2, thereby enabling remote operation. It may be configured. Note that. In this case, the processing after the positioning step of the image acquisition unit 3 and the neutron detector 4 described above may be executed remotely, and an articulated manipulator is installed on or above the operation floor 32 in the reactor building 12. By newly attaching and enabling remote operation in the same manner, all processes including the shielding process as a preparation process may be executed by remote operation.

  Further, the image acquisition unit 3 attached to the distal end portion of the first support member 9 and the neutron detector 4 attached to the distal end portion of the second support member 10 may be configured to be rotatable by remote operation.

  According to the present embodiment, it is possible to realize a subcritical state estimation method and a subcritical state estimation system capable of grasping the subcriticality of fuel debris without introducing a large-scale facility.

  Further, according to the present embodiment, for example, when the fuel debris existing in the reactor pressure vessel is cut out and taken out, the subcriticality of the fuel debris can be grasped in advance, so that appropriate criticality prevention can be performed as necessary. It is possible to take measures and reduce the risk of becoming critical during fuel debris retrieval work.

  FIG. 6 shows a non-erosion state of the structure by the fuel debris having an unknown shape, and FIG. 7 shows an erosion state of the structure by the fuel debris having an unknown shape. In the first embodiment, assuming that fuel debris exists on the bottom of the reactor pressure vessel 14 or on the concrete mat 35 of the reactor containment vessel 13, the fuel debris obtained by the image acquisition unit 3 and the neutron detector 4 is assumed. The subcriticality of the fuel debris 11 having an unknown shape is estimated by detecting image data, three-dimensional shape data generated based on the image data, and neutron flux emitted from the fuel debris 11. In this embodiment, the fuel debris 11 having an unknown shape is a structure, for example, when it erodes the bottom of the reactor pressure vessel 14 or erodes the concrete mat 35 in the reactor containment vessel 13. In this case, the subcriticality of these fuel debris 11 is different from that of the first embodiment. Also in the present embodiment, the configuration shown in FIGS. 1, 2, 4 and 5 is the same as that of the first embodiment, and hence the description thereof is omitted below.

  As shown in FIG. 6, when the fuel debris 11 is present on the upper surface without eroding the structure, the image acquisition unit 3 is based on, for example, image data acquired by a camera and the image data. The shape of the fuel debris 11 can be grasped from the generated three-dimensional shape data. This is because the shape of the structure can be grasped from the drawing (design data) of the atomic pressure vessel 14 in addition to the image data obtained by the image acquisition unit 3, so that the three-dimensional shape data of the fuel debris 11 is generated based on the image data. Depending on what you can do. In this case, the subcriticality of the fuel debris 11 can be estimated in the same manner as in the first embodiment.

  However, when a part of the fuel debris 11 is eroded by the structure as shown in FIG. 7, the shape of the eroded portion (molten fuel) to the structure cannot be grasped by the image acquisition unit 3. Therefore, in the subcritical state estimation system 1 according to the present embodiment, even if the fuel debris 11 is eroded into the structure, it is assumed that there is no erosion. 23, the simulation is executed in the same manner as in the first embodiment. As described above, assuming that the fuel debris 11 does not erode into the structure, the amount (volume) of the fuel debris 11 (molten fuel) is estimated to be smaller than that actually eroded. When the number of neutrons at the position of the neutron detector on the simulation model is the same, the smaller the amount of fuel debris 11, that is, the smaller the amount of spontaneous fissionable isotopes, the more fission by fissionable isotopes is required. There is. That is, assuming that the erosion is not eroded, the neutron multiplication factor of the fuel debris 11 obtained as a result of the simulation by the simulation unit 23 is evaluated to be larger than actual. As described in the first embodiment, the maximum neutron multiplication factor is obtained under the condition of providing the same neutron data (neutron flux or neutron counting rate) at the position of the neutron detector on the simulation model. Therefore, by ignoring the erosion part of the structure due to the fuel debris 11, the range enclosing the maximum value of the neutron multiplication factor can be estimated.

  Note that, as an aspect in which the shape of the fuel debris 11 cannot be grasped from the image data obtained by the image acquisition unit 3, the case where the fuel debris 11 is eroding the structure has been described as an example. However, the present invention is not limited to this, and as a mode in which the shape of the fuel debris 11 changes, the fuel debris 11 may flow into a gap between structures. Even in this case, the subcritical state estimation system 11 of the present embodiment can be applied.

  According to the present embodiment, in addition to the effects of the first embodiment, in the state where the fuel debris 11 is eroding the structure, the neutron multiplication factor obtained by the simulation is the fuel existing on the structure ignoring the eroded portion. By calculating based on the neutron flux emitted from the debris 11, it becomes possible to determine the subcritical state under more severe conditions. That is, the safety in the fuel debris retrieval operation can be more reliably evaluated.

  FIG. 8 shows the relationship between the neutron flux and the neutron multiplication factor depending on the position of the neutron detector on the simulation model, and FIG. 9 shows the neutron flux and neutron increase when the neutron detector is placed at a position different from FIG. The relationship with magnification is shown. In the second embodiment, it is assumed that there is no erosion of the structure by the fuel debris 11 with respect to the contact portion between the fuel debris 11 and the structure that is difficult to acquire by the image acquisition unit 3. Also evaluated the neutron multiplication factor by simulation higher, and as a result, showed the method to estimate the range enveloping the maximum value of neutron multiplication factor. However, in some cases, it may be considered that the neutron multiplication factor is evaluated too high by assuming that there is no erosion. Therefore, in the present embodiment, when the fuel debris 11 erodes into the structure, the neutron detector is arranged in consideration of the eroded portion, so that the subcriticality of the fuel debris 11 is estimated with higher accuracy. Is different from the second embodiment. Also in the present embodiment, the configuration shown in FIGS. 1, 2, 4 and 5 is the same as that of the first embodiment, and hence the description thereof is omitted below.

  As shown in FIG. 8, when the fuel debris 11 (molten fuel) is hemispherically on the structure and there is no erosion of the structure, the hemispherical erosion, or the cylindrical erosion, the fuel The graph on the left side of FIG. 8 shows the result of evaluating the neutron flux at the position of the neutron detector A shown on the right side of FIG. 8 by simulation using the uranium enrichment of the debris 11 as a parameter.

  In the arrangement position of the neutron detector A on the simulation model shown in FIG. 8 (arranged immediately above the hemispherical fuel debris 11), when there is no erosion, when eroded into a hemisphere, and when eroded into a cylindrical shape, As shown in the left graph, the relationship between the neutron flux and the neutron multiplication factor is almost the same. That is, even if evaluation is performed by simulation assuming that there is no erosion, the neutron multiplication factor is not evaluated too high as compared with the case where erosion actually occurs.

  On the other hand, when it is assumed that there is no erosion at the arrangement position of the neutron detector B on the simulation model shown in FIG. 9 (placed in contact with the structure of the hemispherical fuel debris 11, that is, on the side), Compared with the case of erosion, as shown in the graph on the left, the neutron multiplication factor when the neutron flux obtained by simulation is the same is about 0.2 larger, and when there is erosion, it is evaluated higher. I understand that it is too much.

  The difference shown in FIG. 8 and FIG. 9 is that neutrons emitted from the fuel debris 11 in the state where the fuel debris 11 is eroded by the structure are both spontaneous fission neutrons and neutrons that are multiplied inside the fuel debris 11. Is included. In the arrangement position of the neutron detector A on the simulation model shown in FIG. 8, the spontaneous fission neutrons in the erosion part, that is, in the hemispherical erosion part or the cylindrical erosion part cannot reach the neutron detector A, The neutron flux detected by the detector A is only the neutron flux multiplied within the fuel debris 11. On the other hand, in the arrangement position of the neutron detector B, all the spontaneous fission neutrons emitted from both the erosion part and the fuel debris 11 on the structure are detected. Originally, the neutrons to be detected in order to determine the subcritical state are neutrons emitted from the fuel debris 11 according to the neutron multiplication factor, and are not spontaneous fission neutrons. Therefore, as described above, the number of spontaneous fission neutrons is detected more in the arrangement position of the neutron detector B than in the arrangement position of the neutron detector A, and noise in estimation of the neutron multiplication factor is detected. Will be included. That is, although the spontaneous fission neutron can be detected more accurately at the position of the neutron detector B, the neutron detector itself cannot distinguish (identify) the spontaneous fission neutron from the neutron by the neutron multiplication factor. This is noise in the magnification estimation.

  In the present embodiment, based on the above examination, the neutron detector on the simulation model and the actual position in the position where the relationship between the neutron flux and the neutron multiplication factor is almost the same depending on whether the structure is eroded by the fuel debris 11 or not. What is necessary is just to arrange | position the neutron detector 4 which measures the neutron flux emitted from the fuel debris 11. Specifically, at the simulation stage, a plurality of neutron detector positions are set on the simulation model, and the presence / absence of erosion of the structure by the fuel debris 11 (molten fuel) is also input as a parameter in the simulation unit 23. Evaluate in advance. The actual neutron detector 4 may be disposed at a place where the change in the relationship between the neutron flux and the neutron multiplication factor is the smallest due to the evaluation result by simulation and the presence or absence of erosion. Thereby, the range which envelops the maximum value of a neutron multiplication factor can be estimated, without having excessive maintainability.

  In the present embodiment, the data measured by the neutron detector 4 was evaluated as a neutron flux. However, as described in the first embodiment, the count rate (count / second) and the like can be measured by the neutron detector 4. Any one can be used as long as it can be evaluated by simulation.

  Further, as described in the second embodiment, as an example in which the shape of the fuel debris 11 cannot be grasped, the present invention can be similarly applied to the case of flowing into the gap of the structure.

  According to the present embodiment, in addition to the effects of the second embodiment, the arrangement position of the neutron detector on the simulation model and the arrangement position of the actual neutron detector 4 can be optimized, and without having excessive maintainability. It is possible to estimate the range that envelops the maximum value of the neutron multiplication factor.

  As shown in the first embodiment, the uranium enrichment of the fuel debris 11 and the ratio of the structural substances are subcritical by sampling the fuel debris 11 and appropriately reflecting the analysis result and performing simulation by the simulation unit 23. The degree of accuracy can be improved.

  In the present embodiment, the mixing ratio of the nuclear fuel material and water, which is one of the material composition parameters of the fuel debris 11, is more detailed by bringing a neutron absorber such as boron or gadolinium close to the fuel debris 11, for example. It becomes possible to obtain simple data. Specifically, when the mixing ratio of water is large with respect to the nuclear fuel material, the neutron spectrum is soft. Here, when the neutron spectrum is soft, when the mixing ratio of water as a moderator is large, neutrons are decelerated, and a large number of neutrons with low energy levels occupy. In this specification, this state is said to have a soft neutron spectrum.

  Therefore, when the mixing ratio of water is high, it is considered that the decrease in the number of neutrons at the position of the neutron detector when the neutron absorber is brought close to the fuel debris 11 is larger than when the mixing ratio of water is small. . In this way, by using the difference that appears remarkably depending on the mixing ratio of water to the nuclear fuel material when using the neutron absorber, the simulation evaluation result by the simulation 23 and the measurement result by the actual neutron detector 4 are compared. Thus, the mixing ratio of nuclear fuel material and water in the fuel debris 11 is estimated.

  According to the present embodiment, as described above, the facility for bringing the neutron absorber close to the fuel debris 11 is required, but information on the material composition of the fuel debris 11 can be obtained and It becomes possible to improve the estimation accuracy of the criticality.

  In the first to fourth embodiments, the configuration including one image acquisition unit 3 and one neutron detector 4 constituting the subcritical state estimation system 1 of the present invention has been described as an example. However, the present invention is not limited thereto. Alternatively, a plurality of image acquisition units 3 and / or neutron detectors 4 may be provided.

  In the first to fourth embodiments, as illustrated in FIG. 1, the case where the input condition creating unit 22, the simulation unit 23, and the subcritical estimation unit 24 are independently configured as functional blocks has been described as an example. The present invention is not limited to this, and these three functional blocks, the input condition creation unit 22, the simulation unit 23, and the subcriticality estimation unit 24 may be realized by software processing by one or a plurality of processors connected in parallel.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add, delete, and replace the configurations of other embodiments with respect to a part of the configurations of the embodiments.

DESCRIPTION OF SYMBOLS 1 ... Subcritical state estimation system 2 ... Subcritical state estimation apparatus 3 ... Image acquisition part 4 ... Neutron detector 5 ... Input part 6 ... Display part 7 ... Moving apparatus 8 ... Workbench 9 ... First support member 10 ... Second support member 11 ... Fuel debris 12 ... Reactor building 13 ... Reactor containment vessel 14 ... Atom Reactor pressure vessel 15 ... steam dryer 16 ... steam separator 17 ... reactor containment vessel top lid 18 ... reactor pressure vessel top lid 19 ... pedestal 20a ... first signal line 20b ... second signal line 21 ... I / OIF
22 ... Input condition creation unit 23 ... Simulation unit 24 ... Subcriticality estimation unit 25 ... Structure database (structure DB)
26 ... Internal bath 30 ... Trier / Separator pool 31 ... Spent fuel storage pool 32 ... Operation floor 33 ... Pressure suppression chamber (wet well)
34 ... Vent passage 35 ... Concrete mat

Claims (10)

An image acquisition unit that acquires an image of fuel debris and generates three-dimensional shape data based on the acquired image;
A neutron detector for measuring neutrons emitted from the fuel debris;
The state of the fuel debris is simulated based on the acquired image data and three-dimensional shape data, the neutron multiplication factor of the fuel debris is extracted based on the simulation result and the measured neutrons, and the subcriticality of the fuel debris And a subcritical state estimation device for estimating a subcritical state. The subcritical state estimation system according to claim 1,
The subcritical state estimation device is:
An input condition creation unit for setting a boundary of fuel debris in the image data and the three-dimensional shape data;
A structure database storing at least the composition of nuclear fuel material and the composition of the structure in the reactor building related to the fuel debris;
A simulation unit for extracting a neutron multiplication factor of the fuel debris based on the set boundary of the fuel debris and the composition stored in the structure database;
A subcritical state estimation system, comprising: a subcriticality estimation unit that estimates the subcriticality of the fuel debris based on the simulation result and the neutrons measured by the neutron detector. The subcritical state estimation system according to claim 2,
The simulation unit outputs, as the simulation result, a relationship between a neutron flux and a neutron multiplication factor of fuel debris obtained by a neutron detector on a simulation model,
The subcritical estimation unit extracts the neutron multiplication factor of the fuel debris from the relationship between the neutron flux measured by the neutron detector and the neutron flux and the neutron multiplication factor, and outputs the neutron multiplication factor as the subcriticality. Subcritical state estimation system characterized by In the subcritical state estimation system according to any one of claims 1 to 3,
A first support member having the image acquisition unit attached to the tip;
A second support member having the neutron detector attached to the tip;
A subcritical state estimation system comprising a moving device that allows the first and second support members to move vertically in a reactor pressure vessel and is electrically connected to the subcritical state estimation device. . In the subcritical state estimation system according to any one of claims 2 to 4,
In a state where the fuel debris erodes the structure in the reactor building, the simulation unit is configured to obtain the fuel on the structure obtained from the three-dimensional shape data of the fuel debris on the structure from the image acquisition unit. A subcritical state estimation system, wherein the neutron multiplication factor is calculated using a volume of debris. In the subcritical state estimation system according to any one of claims 2 to 4,
In a state where the fuel debris erodes structures in the reactor building,
The simulation unit sets an arrangement position of a plurality of neutron detectors for fuel debris that erodes the structure on the simulation model, and the neutron flux and neutron increase of the fuel debris are set for each of the set arrangement positions. A subcritical state estimation system characterized by obtaining a relationship with magnification and obtaining an arrangement position of a neutron detector that minimizes a change in the relationship between the obtained neutron flux and neutron multiplication factor. The subcritical state estimation system according to claim 2,
The simulation unit outputs, as the simulation result, a relationship between a neutron count rate and a neutron multiplication factor of fuel debris obtained by a neutron detector on a simulation model,
The subcritical estimator extracts the neutron multiplication factor of the fuel debris from the neutron count rate measured by the neutron detector and the relationship between the neutron count rate and the neutron multiplication factor, and outputs it as the subcriticality A subcritical state estimation system characterized by: A subcritical state estimation method for estimating the subcriticality of fuel debris existing in a reactor building,
A step of lowering a first support member having an image acquisition unit installed at a distal end thereof in a vertical direction in the reactor building, and positioning the image acquisition unit in the vicinity of the fuel debris;
A step of vertically moving a second support member having a neutron detector installed at a tip thereof in the reactor building, and positioning the neutron detector in the vicinity of the fuel debris;
Simulating the state of the fuel debris based on the image data and three-dimensional shape data of the fuel debris acquired by the image acquisition unit;
Subcriticality estimation characterized by comprising a subcriticality estimation step of extracting a neutron multiplication factor of the fuel debris based on the simulation result and the measurement result by the neutron detector and estimating the subcriticality of the fuel debris Method. In the subcritical state estimation method according to claim 8,
The simulation step includes a step of setting a boundary of fuel debris in the image data and the three-dimensional shape data,
The subcriticality estimation step refers to a structure database storing at least the composition of nuclear fuel material and the composition of the structure in the reactor building related to the fuel debris, and based on the simulation result and the measurement result by the neutron detector A subcritical state estimation method comprising a step of extracting a neutron multiplication factor of the fuel debris. In the subcritical state estimation method according to claim 9,
Before the step of positioning the image acquisition unit and the neutron detector in the vicinity of the fuel debris, a work table is installed above a reactor pressure vessel in the reactor building, and the first and second support members A subcritical state estimation method comprising: preparing a moving device that can move up and down and move horizontally on the workbench on the workbench.

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