Nuclear fuel measurement system, and nuclear fuel measurement method

Abstract

To enable a weight and degree of burning of a nuclear fuel contained in an object to be measured appropriately even when advance information on the object is a few.SOLUTION: A nuclear fuel measurement system has: a muon trajectory detector 11 that is provided in a facing position across an object 1, and measures a muon μ to detect a muon trajectory; a radiation detector 12 that is installed in the vicinity of the object, and measures a radioactive ray such as a gamma ray and the like to be radiated from the object; a trajectory determination unit 26 that reconstructs the muon trajectory detected by the muon trajectory detector, and determines whether the muon trajectory transmits the radiation detector; a nuclear fuel weight determination unit 27 that selects an event in which the muon trajectory does not transmit the radiation detector, and derives a weight of a nuclear fuel in the object on the basis of a measurement value of the muon trajectory detector; and a degree-of-burning determination unit 28 that selects an event in which the muon trajectory does not transmit the radiation detector, and derives a degree of burning of the nuclear fuel in the object on the basis of each measurement value of the muon trajectory detector and radiation detector.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] An embodiment of the present invention relates to a nuclear fuel measurement system and a nuclear fuel measurement method for measuring nuclear fuel contained in an object. [Background technology]

[0002] Nuclear fuels such as uranium are subject to strict international management from the perspective of nuclear security. In particular, spent fuel used in nuclear reactors is highly radioactive because it contains a large amount of fissile components, and generates heat due to decay heat. Therefore, it is necessary to take measures to prevent criticality, shield it, cool it, and manage it strictly. Therefore, in addition to the weight and initial composition of the nuclear fuel, information such as its usage history and burnup must be accurately managed.

[0003] Ordinary spent nuclear fuel is stored in a fuel pool for a certain period of time to allow short-lived nuclides to decay, and then transported to a reprocessing facility or long-term storage facility. Among the nuclides contained in the fuel, the composition of radioactive nuclides related to shielding management and the remaining amount of fissile nuclides such as uranium-235 related to criticality management vary mainly depending on the burnup, so evaluation of the burnup is important to ensure the safety of spent fuel.

[0004] The burnup assessment of spent fuel is carried out by measuring radiation from fission products accumulated in the fuel. In particular, gamma rays emitted from cesium (Cs-137) and neutron rays emitted from curium (Cm-244) are measured. A gamma ray spectrum detector such as a germanium semiconductor detector is used to measure gamma rays, and a detector with gamma ray isolation such as a fission ionization chamber is used to measure neutron rays. In addition, by measuring charged particle rays such as alpha rays with high accuracy, it is possible to directly assess the burnup and evaluate the contamination of the surface.

[0005] Existing burnup measurement devices are used when receiving spent fuel generated in nuclear reactors at reprocessing facilities. The most conservative criticality control method is the "fresh fuel assumption," which assumes that the fuel is not burning. This fresh fuel assumption requires longer fuel intervals during storage and the use of expensive neutron absorbing materials, which tends to be costly.

[0006] Considering that spent fuel is burned and its reactivity decreases, the rationalization of spent fuel transportation and storage facilities is called "burnup credit." In designs using burnup credit, technology to grasp burnup, such as the burnup measurement device mentioned above, is important.

[0007] Generally, the shape and weight of spent fuel generated in a nuclear reactor are managed, as well as its initial composition and operating history. Since a great deal of information related to the fuel is held prior to measurement, there is no need for overly conservative management such as that based on the new fuel assumption.

[0008] On the other hand, from the viewpoint of nuclear security, it is also necessary to inspect fuels that have no initial composition and no operating records. In addition, fuel debris generated by nuclear power plant accidents is a mixture of multiple fuels in the core, so the burnup of the fuel contained in the debris cannot be calculated from the operating records. In addition, when nuclear fuel is mixed with other substances, it is difficult to estimate the weight of the nuclear fuel and the weight of fissile nuclides contained. Under such conditions, it is necessary to manage the target under the most conservative assumptions, as in the case of new fuel assumptions in the management of spent fuel, which significantly increases the management costs. Therefore, a method is required to grasp the burnup by measurement even for nuclear fuels for which prior information is lacking.

[0009] Several methods have been proposed to measure the weight of nuclear material in nuclear fuel or fuel debris for which there is insufficient prior information. For example, a method has been proposed to measure the weight of nuclear fuel contained in material of unknown composition by measuring the scattering angle distribution of cosmic ray muons, which have strong penetrating power.

[0010] In addition, a method has been proposed to improve the accuracy of estimating the weight of nuclear fuel contained in a material by measuring gamma rays and neutron rays in addition to cosmic ray muons, and a method has been proposed to simultaneously determine the material composition and material shape by combining measurements of cosmic ray muons and X-rays. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2-222884 [Patent Document 2] JP 2017-161465 A [Patent Document 3] Patent Publication No. 2021-189050 [Patent Document 4] JP 2022-187538 A [Patent Document 5] JP 2016-4024 A Summary of the Invention [Problem to be solved by the invention]

[0012] Considering the above proposals, in order to measure the burnup and the ratio of fissile nuclides, it is possible to consider a device configuration that simultaneously performs measurements of muons and neutrons, gamma rays, and charged particle beams. For example, a method of monitoring radiation dose near a muon measurement device has been proposed (Patent Document 5). However, this proposal is intended to ensure reliability, not to measure the radiation dose of a sample, because a high radiation environment can adversely affect muon measurements. In order to improve the detection efficiency of neutrons and gamma rays, it is desirable to perform measurements near the sample, and in order to inspect the surface of radioactive materials for radioactive contamination, it is necessary to measure charged particle beams such as electron beams and alpha rays, which necessitates measurements near the sample.

[0013] However, if a radiation detector is placed near a sample, the event of a muon passing through the radiation detector will adversely affect the results of the muon measurement. In other words, in an apparatus that combines muon measurement and radiation measurement (measurement of at least one of neutron rays, gamma rays, electron beams, and alpha rays) simultaneously when inspecting the burnup and the ratio of fissile nuclides for waste containing nuclear fuel, if radiation measurement is performed near a sample, the event of a muon passing through the radiation detector may adversely affect the results of the muon measurement.

[0014] An embodiment of the present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a nuclear fuel measurement system and a nuclear fuel measurement method that can suitably measure the weight and burnup of nuclear fuel contained in an object even when there is little prior information about the object.

[0015] Another object of the present invention is to provide a nuclear fuel measurement system and a nuclear fuel measurement method that can suitably measure the weight and burnup of nuclear fuel contained in an object, as well as the proportion of fissile nuclides, even when there is little prior information about the object. [Means for solving the problem]

[0016] A nuclear fuel measurement system in an embodiment of the present invention is characterized in that it is configured to include: a muon trajectory detector that is provided at opposing positions sandwiching a measurement target object and measures muons to detect a muon trajectory; a radiation detector that is provided near the target object and measures at least one of gamma rays, neutron rays, and charged particle rays emitted from the target object; a trajectory determination unit that reconstructs the muon trajectory detected by the muon trajectory detector and determines whether or not the muon trajectory has passed through the radiation detector; a nuclear fuel weight determination unit that selects an event in which the muon trajectory has not passed through the radiation detector and derives a weight of the nuclear fuel contained in the target object based on the measurement values ​​of the muon trajectory detector; and a burnup determination unit that selects an event in which the muon trajectory has not passed through the radiation detector and derives a burnup of the nuclear fuel contained in the target object based on the measurement values ​​of the muon trajectory detector and the radiation detector.

[0017] A nuclear fuel measurement method in an embodiment of the present invention is characterized by comprising the steps of: preparing a muon trajectory detector, which is provided at opposing positions sandwiching an object to be measured and measures muons to detect a muon trajectory; and a radiation detector, which is provided near the object and measures at least one of gamma rays, neutron rays, and charged particle rays emitted from the object; a trajectory determination unit reconstructing the muon trajectory detected by the muon trajectory detector and determining whether or not the muon trajectory has passed through the radiation detector; a nuclear fuel weight determination unit selecting an event in which the muon trajectory has not passed through the radiation detector and deriving a weight of the nuclear fuel contained in the object based on a measurement value of the muon trajectory detector; and a burnup determination unit selecting an event in which the muon trajectory has not passed through the radiation detector and deriving a burnup of the nuclear fuel contained in the object based on each measurement value of the muon trajectory detector and the radiation detector. Effect of the Invention

[0018] According to an embodiment of the present invention, the weight and burnup of nuclear fuel contained in an object can be suitably measured even when there is little prior information about the object. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a configuration diagram showing a nuclear fuel measurement system according to an embodiment. [Diagram 2] 2 is a flowchart showing the procedure for measurement and evaluation of the nuclear fuel measurement system of FIG. 1. [Diagram 3] A graph showing the relationship between nuclear fuel weight and muon scattering. [Figure 4] A graph showing the relationship between the amount of gamma radiation emitted from Cs-137 and burnup. [Diagram 5] A graph showing the relationship between the neutron dose emitted from Cm-244 and burnup. [Figure 6] A graph showing the relationship between the U-235 content and burnup. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 1 is a block diagram showing a nuclear fuel measurement system 10 according to an embodiment. The nuclear fuel measurement system 10 includes a muon track detector 11 provided at a position facing an object 1 to be measured, a gamma ray detector 13 which is a radiation detector 12, a neutron detector 14, and a charged particle detector 15, and a measurement computer 16. The muon track detector 11, the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15 are connected to the measurement computer 16. The nuclear fuel measurement system 10 measures the weight and burnup of nuclear fuel contained in the object 1 to be measured, and further derives the content ratio of fissile nuclides.

[0021] There is little prior information about the substances contained in the object 1. For example, the object 1 may be waste (nuclear fuel debris) containing nuclear fuel generated at a nuclear power plant or the like. The object 1 may also be spent nuclear fuel.

[0022] Muons μ exist mainly as cosmic rays. Muons μ are a type of secondary cosmic rays that are generated when primary cosmic rays that enter the earth from space react with the earth's atmosphere. Muons μ have a positive or negative charge and a high energy of 3 to 4 GeV on average, and therefore have high penetrating power. Muons μ can also be artificially generated using an accelerator. In this embodiment, a form in which muons μ of cosmic rays are used is exemplified, but artificially generated muons μ may also be used.

[0023] At least one pair of muon trajectory detectors 11 are provided at positions facing each other across the target 1, and measure muons μ, which are charged particles, to detect muon trajectories. When cosmic ray muons are used, these muon trajectory detectors 11 are provided at positions above and below the target 1. As the muon trajectory detector 11, a device in which drift tube detectors (not shown) using ionized gas are stacked, or a device in which multi-channel scintillation detectors (not shown) are stacked, is used.

[0024] The gamma ray detector 13 is installed near the object 1 and measures gamma rays emitted from the object 1. In the example of Fig. 1, two gamma ray detectors 13 are provided to measure gamma rays emitted from one object 1. The number of gamma ray detectors 13 may be one, or three or more.

[0025] The gamma ray detector 13 is, for example, a detector such as a scintillator or a semiconductor detector (specifically, a germanium semiconductor detector). If the radiation dose is high and interferes with measurement, a collimator 17 may be formed to send only a portion of the gamma rays to the gamma ray detector 13. The collimator 17 is a material composed of heavy elements, and is, for example, an opening such as a slit or hole provided between the object 1 and the gamma ray detector 13.

[0026] The neutron detector 14 is installed near the object 1 and measures the neutron radiation emitted from the object 1. In the example of Fig. 1, two neutron detectors 14 are provided to measure the neutron radiation emitted from one object 1. The number of neutron detectors 14 may be one, or three or more.

[0027] The neutron detector 14 may be, for example, a liquid scintillator for measuring neutron rays classified as fast neutrons having an energy of 0.5 MeV or more, or a He-3 gas proportional counter or a BF3 counter for measuring neutron rays classified as thermal neutrons having an energy of 0.5 eV or less that are in thermal equilibrium with the medium. The neutron detector 14 may also be housed in a shielding body 18 for eliminating the influence of gamma rays when measuring neutron rays.

[0028] The charged particle detector 15 is installed near the object 1 and measures a charged particle beam (electron beam, proton beam, α-ray, etc.) emitted from the object 1. In the example of Fig. 1, one charged particle detector 15 is provided to measure a charged particle beam emitted from one object 1. Note that the number of charged particle detectors 15 may be two or more. The charged particle detector 15 is, for example, a radiation detector such as a scintillation detector or a semiconductor detector.

[0029] In this embodiment, the nuclear fuel measurement system 10 is illustrated as having a gamma ray detector 13, a neutron detector 14, and a charged particle detector 15. However, the nuclear fuel measurement system 10 may have one or two of the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15.

[0030] The measurement computer 16 has a processing circuit 20, an input unit 21, an output unit 22, a storage unit 23, and a communication unit 24. The processing circuit 20 includes a data accumulation unit 25, a trajectory determination unit 26, a nuclear fuel weight determination unit 27, a burnup determination unit 28, and a fissile nuclide ratio determination unit 29. These are realized by a CPU (Central Processing Unit) executing a program stored in a memory or an HDD (Hard Disk Drive). The nuclear fuel measurement method of this embodiment is realized by having a computer execute various programs.

[0031] The processing circuit 20 is a circuit including, for example, a CPU, a GPU (Graphics Processing Unit), or a dedicated or general-purpose processor. This processor executes various programs stored in the storage unit 23 to realize various functions. The processing circuit 20 may also be configured with hardware such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit). Various functions can also be realized by such hardware. The processing circuit 20 can also realize various functions by combining software processing by a processor and a program with hardware processing.

[0032] The input unit 21 allows a user who uses the measurement computer 16 to input predetermined information in response to an operation of the user. The input unit 21 includes input devices such as a mouse, a keyboard, and a touch panel. In other words, predetermined information is input to the measurement computer 16 in response to an operation of the input unit 21.

[0033] The output unit 22 outputs predetermined information. The measurement computer 16 includes a device for displaying images, such as a display that outputs analysis results. In other words, the output unit 22 controls the images displayed on the display.

[0034] The memory unit 23 stores various information required when performing measurements and evaluations of nuclear fuel. For example, the memory unit 23 stores a predetermined program executed by the processing circuit 20. The memory unit 23 also stores the measurement values ​​acquired by the muon trajectory detector 11, the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15.

[0035] The communication unit 24 communicates with other computers and predetermined devices via a predetermined communication line. The predetermined devices include the muon trajectory detector 11, the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15.

[0036] The data accumulation unit 25 in the processing circuit 20 accumulates the respective measurement values ​​acquired by the muon trajectory detector 11, the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15. The accumulated calculation values ​​are stored in the memory unit 23.

[0037] The trajectory determination unit 26 reconstructs the muon trajectory based on the measurement values ​​acquired by the muon trajectory detector 11, and calculates the scattering position and scattering angle. Furthermore, the trajectory determination unit 26 determines whether or not the muon trajectory detected by the muon trajectory detector 11 has passed through the radiation detector 12 (gamma ray detector 13, neutron detector 14, charged particle detector 15), i.e., whether or not the muon μ has entered the radiation detector 12, from the above calculation results. Using this determination result, the nuclear fuel weight determination unit 27 determines the weight of the nuclear fuel by excluding events in which the muon trajectory has passed through the radiation detector 12, as described later. Furthermore, using the determination result of the trajectory determination unit 26, the burnup determination unit 28 derives the burnup by excluding events originating from muon μ from the time information and trajectory information for each measurement result of the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15, as described later.

[0038] Here, the reconstruction of the muon trajectory will be explained. The muon trajectory detector 11 measures muons μ, which are cosmic rays incident on the ground from the sky, to detect the muon trajectory. Muons μ have high penetrating power, and many of the muons μ that are incident on the object 1 to be measured pass through the material. Furthermore, muons μ cause multiple Coulomb scattering depending on the composition and shape of the material. The average scattering angle θ0 of this muon scattering is shown in the following formula 1, and changes depending on the radiation length X0 specific to the atomic number of the material.

[0039]

number

[0040] Here, p is the momentum of the muon μ. βc is the speed of the muon μ. z is the charge of the muon μ. x is the distance that the muon μ travels through the material. Since the radiation length X0 varies depending on the material, it is possible to estimate the material present at the coordinates where muon scattering occurred from the analyzed scattering angle θ0.

[0041] The nuclear fuel weight determination unit 27 shown in FIG. 1 selects events in which the muon trajectory has not passed through the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15, and derives the weight of the nuclear fuel contained in the target 1 based on the measurement value of the muon trajectory detector 11 from the measurement values ​​accumulated by the data accumulation unit 25.

[0042] That is, first, the trajectory determination unit 26 measures the change in the muon trajectory when the muon μ passes through the target 1 based on the measurement value of the muon trajectory detector 11, and derives muon scattering from the magnitude of the change in the muon trajectory. Furthermore, the trajectory determination unit 26 determines whether or not the muon trajectory has passed through the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15. Next, based on the determination result of the trajectory determination unit 26, the nuclear fuel weight determination unit 27 selects an event in which the muon trajectory has not passed through the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15, and derives the weight of the nuclear fuel contained in the target 1 from the relationship between muon scattering and the nuclear fuel weight.

[0043] Here, muon scattering has the property that it is largely scattered by heavy elements with high atomic numbers. Therefore, the weight of the nuclear fuel contained in the material is the dominant factor that determines the magnitude of muon scattering in a material that contains a large amount of nuclear fuel material. Figure 3 shows an example of the relationship between the nuclear fuel weight (uranium weight) and muon scattering (average muon scattering angle). Since the value of muon scattering increases with an increase in the nuclear fuel weight, the analysis of muon scattering by the trajectory determination unit 26 enables the nuclear fuel weight determination unit 27 to measure the weight of the nuclear material contained in the target 1.

[0044] However, muon scattering is not sensitive to differences in scattering due to differences in isotopes of the same element, so it cannot distinguish between U-238 and U-235, for example. Therefore, the analysis result by muon scattering is the total amount of heavy elements such as uranium and plutonium, regardless of whether they are fissile or not. In nuclear fuel criticality control, the proportion of fissile nuclides such as uranium 235 or plutonium 239 contained in the fuel is important, so the analysis result by muon scattering alone cannot provide the information necessary for fuel criticality control.

[0045] The burnup determination unit 28 shown in FIG. 1 selects events in which the muon trajectory does not pass through the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15, and derives the burnup of the nuclear fuel contained in the target 1 based on the measurement values ​​accumulated in the data accumulation unit 25 (the measurement values ​​of the muon trajectory detector 11, the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15).

[0046] That is, the burnup determination unit 28 selects an event in which the muon trajectory has not passed through the gamma ray detector 13 from the determination results of the trajectory determination unit 26, and derives the nuclide amount (weight of cesium) of the nuclide to be evaluated, such as cesium, contained in the object 1, from the measurement value of gamma rays as radiation by the gamma ray detector 13. Furthermore, the burnup determination unit 28 derives the nuclide amount per unit weight of the object 1 from the weight of the nuclear fuel derived from the measurement value of the muon trajectory detector 11 and the above-mentioned nuclide amount derived from the measurement value of the gamma ray detector 13. Then, the burnup determination unit 28 derives the burnup of the object 1 from the relationship between the nuclide amount per unit weight and the burnup.

[0047] Here, the measurement of gamma rays will be described. The gamma ray detector 13 measures gamma rays emitted from the target 1. It is recommended that the gamma ray detector 13 be a detector capable of energy spectrum resolution in order to discriminate the energy of the incident gamma rays. An example of a detector with good energy discrimination is a germanium semiconductor detector. The gamma ray detector 13 may perform measurement through a collimator 17 in order to remove gamma rays from the surrounding environment and adjust the incident direction and position of the gamma rays to be measured.

[0048] In burnup determination by the burnup determination unit 28, 662 keV gamma rays originating from Cs-137 are mainly measured. Cs-137 undergoes beta decay with a half-life of about 30 years to produce Ba-137m, which emits 662 keV gamma rays to become stable Ba-137. Figure 4 shows the radioactivity value (gamma radiation amount) per unit weight of Cs-137 versus burnup. Figure 4 shows cooling periods of 10, 15, and 20 years.

[0049] The weight of Cs-137 increases almost linearly with burnup. In addition, the weight of Cs-137 attenuates during the cooling period from the end of irradiation. Therefore, by measuring the amount of gamma rays originating from Cs-137 with the gamma ray detector 13, the burnup can be estimated by the burnup determination unit 28. In addition, if the cooling period from the end of irradiation is known, the accuracy of the above estimation can be improved. The derivation of the burnup by the above measurement utilizes the relationship that the amount of gamma rays increases with burnup under the condition that the weight of the burnup is assumed to be the same, so if the weight of the target object 1 is unknown, it is difficult to estimate the burnup.

[0050] 1 selects an event in which the muon trajectory has not passed through the neutron detector 14 from the determination results of the trajectory determination unit 26, and derives the nuclide amount (weight of curium) of the nuclide to be evaluated, such as curium, contained in the object 1 from the measurement value of the neutron ray as radiation by the neutron detector 14. Furthermore, the burnup determination unit 28 derives the nuclide amount per unit weight of the object 1 from the weight of the nuclear fuel derived from the measurement value of the muon trajectory detector 11 and the above-mentioned nuclide amount derived from the measurement value of the neutron detector 14. Then, the burnup determination unit 28 derives the burnup of the object 1 from the relationship between the nuclide amount per unit weight and the burnup.

[0051] Here, the measurement of neutron rays will be described. The neutron detector 14 measures neutron rays emitted from the target 1. Since there is a large amount of gamma rays in the measurement environment, the neutron detector 14 used is required to be able to distinguish between neutron rays and gamma rays. Examples of neutron detectors 14 with good gamma ray discrimination performance include a fission chamber (nuclear fission ionization chamber) and the above-mentioned liquid scintillator.

[0052] FIG. 5 shows an example of the radioactivity value (neutron dose) per unit weight of Cm-244 versus burnup. In FIG. 5, graphs are shown for cooling periods of 10, 15, and 20 years. Cm-244 has a half-life of 18.1 years and undergoes alpha decay, and also emits neutrons through spontaneous fission. The amount of Cm-244 produced increases approximately squared according to the burnup. Therefore, the burnup determination unit 28 can determine the burnup from the neutron dose measured by the neutron detector 14. As with the derivation of burnup using gamma rays, the relationship between burnup and neutron dose is based on the assumption that the weight of the fuel is constant, and therefore it is difficult to determine the burnup when the weight is unknown.

[0053] 1 selects an event in which the muon trajectory does not pass through the charged particle detector 15 from the determination result of the trajectory determination unit 26, and derives the nuclide amount (weight of U-235, U-238) of the nuclide to be evaluated, such as U-235 or U-238, contained in the object 1 from the measurement value of the charged particle beam as radiation by the charged particle detector 15. Furthermore, the burnup determination unit 28 derives the nuclide amount per unit weight of the object 1 from the weight of the nuclear fuel derived from the measurement value of the muon trajectory detector 11 and the nuclide amount derived from the measurement value of the charged particle detector 15. Then, the burnup determination unit 28 derives the burnup of the object 1 from the relationship between the nuclide amount per unit weight and the burnup.

[0054] Here, the measurement of the charged particle beam will be described. The charged particle detector 15 measures alpha rays, electron beams, proton beams, etc. as the charged particle beams emitted from the target 1. For example, when detecting alpha rays emitted from U-235 or U-238, a thin scintillation detector can be used as the charged particle detector 15. In the derivation of burnup based on the charged particle beam by the burnup determination unit 28, the charged particle beam emitted from the nuclear fuel can be directly detected, so that it becomes possible to obtain the burnup from the measurement value of the charged particle beam.

[0055] The burnup determination unit 28 determines whether or not the burnup values ​​derived from multiple types of measurement values ​​(measurement values ​​of gamma rays, neutron rays, and charged particle rays) by multiple types of radiation detectors 12 (gamma ray detector 13, neutron detector 14, and charged particle detector 15) are within a predetermined allowable range, and derives the burnup using only the values ​​that are within the allowable range. In this manner, it becomes possible to derive one burnup from the burnups derived from different types of measurement values ​​(measurement values ​​of gamma rays, neutron rays, and charged particle rays) by the radiation detectors 12. For example, it becomes possible to derive the burnup by excluding values ​​derived from measurement values ​​by the radiation detectors 12 that have low reliability, and thus improve the accuracy of burnup determination.

[0056] Here, the above-mentioned allowable range may be set by the average value of burnup derived from multiple types of measurement values ​​(measurement values ​​of gamma rays, neutron rays, and charged particle beams).Also, the allowable range may be set by the average value of values ​​obtained by multiplying the burnup value derived from multiple types of measurement values ​​by a weight that takes into account a certain outflow amount of the nuclide associated with each measurement value of gamma rays, neutron rays, and charged particle beams.Also, when the burnup value derived from one or two types of measurement values ​​is not reliable, another burnup value may be used.

[0057] The fissile nuclide proportion determination unit 29 shown in Fig. 1 derives the content ratio of fissile nuclides based on the measured values ​​of the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15. In other words, the fissile nuclide proportion determination unit 29 evaluates the content ratio (weight ratio) of the fissile nuclides contained in the target 1 based on the weight of the nuclear fuel derived by the nuclear fuel weight determination unit 27, the burnup derived by the burnup determination unit 28, and the relationship of the nuclide amount of the fissile nuclides to the burnup evaluated in advance. In this way, it becomes possible to evaluate the content ratio of the fissile nuclides together with the burnup.

[0058] Here, the evaluation of the content ratio of fissile nuclides will be explained. Figure 6 shows the content ratio of U-235 (fissile nuclide) with respect to burnup. For example, Figure 6 shows the change in the weight ratio of U-235 accompanying the burning of UO2 (uranium dioxide) fuel with an enrichment of 3.5 wt%. As the fuel burns, the weight ratio of U-235, which is a fissile nuclide, gradually decreases. Therefore, when the burnup is determined, it becomes possible to analyze the weight ratio of fissile nuclides such as U-235 contained in the fuel. In practical operation, the content ratio of fissile nuclides other than U-235 can also be estimated by assuming the initial composition of the fuel and performing a burnup calculation up to the derived burnup. For example, a burnup calculation code such as ORIGEN2 is used for the burnup calculation.

[0059] Next, a nuclear fuel measurement method using the nuclear fuel measurement system 10 will be described with reference to FIG. This nuclear fuel measurement system 10 mainly performs four types of measurements: muon trajectory measurement (S1), gamma ray spectrum measurement (S4), neutron beam measurement (S6), and charged particle beam measurement (S8), as well as procedures for analyzing each measurement result (S5, S7, S9, S10 to S13).

[0060] First, in step S1, the muon trajectory detector 11 measures muons and detects the muon trajectory. In the next step S2, the trajectory determination unit 26 calculates muon scattering and determines whether the muon trajectory has passed through the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15. Then, in the next step S3, the nuclear fuel weight determination unit 27 uses the determination result of the trajectory determination unit 26 to select an event in which the muon trajectory has not passed through the gamma ray detector 13, the neutron detector 14, and the charged particle detector 15, and derives the weight of the nuclear fuel from the calculation result of the muon scattering.

[0061] In the next step S4, the gamma ray detector 13 measures the gamma ray spectrum. Then, in step S5, the burnup determination unit 28 uses the determination result of the trajectory determination unit 26 to select events in which the muon trajectory has not penetrated the gamma ray detector 13, and derives the amount of nuclides per unit weight emitting gamma rays from the gamma ray measurement value and the weight of nuclear fuel obtained by the nuclear fuel weight determination unit 27. Then, the burnup determination unit 28 derives a provisional value of burnup from the amount of nuclides per unit weight.

[0062] Simultaneously with step S4, in step S6, the neutron detector 14 measures the neutron beam. Then, in step S7, the burnup determination unit 28 uses the determination result of the trajectory determination unit 26 to select events in which the muon trajectory has not penetrated the neutron detector 14, and derives the amount of nuclides per unit weight emitting the neutron beam from the measurement value of the neutron beam and the weight of the nuclear fuel obtained by the nuclear fuel weight determination unit 27. Then, the burnup determination unit 28 derives a provisional value of burnup from the amount of nuclides per unit weight.

[0063] Furthermore, simultaneously with step S4, in step S8, the charged particle detector 15 measures the charged particle beam. Then, in step S9, the burnup determination unit 28 uses the determination result of the trajectory determination unit 26 to select events in which the muon trajectory has not penetrated the charged particle detector 15, and derives the amount of nuclides per unit weight that emit the charged particle beam from the measurement value of the charged particle beam and the weight of nuclear fuel obtained by the nuclear fuel weight determination unit 27. Then, the burnup determination unit 28 derives a provisional value of burnup from the amount of nuclides per unit weight.

[0064] In the next step S10, the burnup determination unit 28 performs a comprehensive evaluation of each burnup (provisional value). For example, independent provisional values ​​of burnup are obtained from the measurement results of gamma rays and neutron rays, and a comprehensive evaluation is performed on these. In this case, the burnup determination unit 28 evaluates the burnup by taking the average of the two burnups (provisional values) as a simple method (step S11). On the other hand, the content of the fuel debris is lower than the original value due to the leakage of nuclides generated during operation due to the storage state, and it is considered that the burnup is underestimated. Under such conditions, the burnup determination unit 28 derives the burnup by adopting the higher burnup (provisional value) from the evaluation results of the burnup (provisional value) by gamma rays, neutron rays, and charged particle beams (step S11).

[0065] In the next step S12, the fissile nuclide fraction determination unit 29 evaluates the content ratio of fissile nuclides. In this evaluation, the fissile nuclide fraction determination unit 29 derives the content ratio of fissile nuclides using the burnup derived in step S11 and the weight of the nuclear fuel derived in step S3 (step S13). As described above, the nuclear fuel measurement system 10 analyzes and evaluates the weight, burnup, and content ratio of fissile materials of the nuclear fuel by executing each procedure (steps S1 to S13).

[0066] As configured as above, this embodiment provides the following advantages. Even when the radiation detector 12 (gamma ray detector 13, neutron detector 14, charged particle detector 15) is installed near the object 1 to be measured to improve the measurement accuracy of radiation (gamma rays, neutron rays, charged particle rays) emitted from the object 1, the nuclear fuel weight determination unit 27 uses the determination result of the trajectory determination unit 26 to select an event in which the muon trajectory does not pass through the radiation detector 12 and derives the weight of the nuclear fuel in the object 1, and the burnup determination unit 28 uses the determination result of the trajectory determination unit 26 to select an event in which the muon trajectory does not pass through the radiation detector 12 and derives the burnup of the nuclear fuel in the object 1. Therefore, it is possible to eliminate adverse effects on the measurement values ​​of the muon trajectory detector 11 and the radiation detector 12 (gamma ray detector 13, neutron detector 14, charged particle detector 15) that occur when the muon trajectory passes through the radiation detector 12. As a result, the weight and burnup of the nuclear fuel contained in the object 1, as well as the content ratio of fissile nuclides, can be suitably measured even when there is little prior information on the object 1. This makes it possible to store the object 1 containing nuclear fuel at an appropriate management cost.

[0067] In this embodiment, the measurements of gamma rays, neutron rays, and charged particle beams may be performed simultaneously or individually. However, since each measurement is a statistical event, the statistical accuracy can be improved by ensuring sufficient measurement time and increasing the number of counts. Therefore, it is more efficient to perform each measurement simultaneously over a period of time. In addition, the nuclear fuel measurement system 10 itself can be made smaller by adopting a structure that allows all measurements to be performed simultaneously.

[0068] Although the embodiment of the present invention has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This embodiment can be implemented in various other forms, and various omissions, substitutions, changes, and combinations can be made without departing from the gist of the invention, and these substitutions, changes, and combinations are included in the scope and gist of the invention, as well as in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0069] 1... object, 10... nuclear fuel measurement system, 11... muon trajectory detector, 12... radiation detector, 13... gamma ray detector, 14... neutron detector, 15... charged particle detector, 26... trajectory determination unit, 27... nuclear fuel weight determination unit, 28... burnup determination unit, 29... fissile nuclide ratio determination unit, μ... muon

Claims

1. a muon trajectory detector that is provided at opposing positions across an object to be measured and that measures muons and detects muon trajectories; a radiation detector disposed near the object and configured to measure at least one of gamma rays, neutron rays, and charged particle rays emitted from the object; a trajectory determination unit that reconstructs the muon trajectory detected by the muon trajectory detector and determines whether or not the muon trajectory has passed through the radiation detector; a nuclear fuel weight determination unit that selects an event in which the muon trajectory does not pass through the radiation detector and derives a weight of the nuclear fuel contained in the object based on a measurement value of the muon trajectory detector; a burnup determination unit that selects an event in which the muon trajectory does not pass through the radiation detector, and derives the burnup of the nuclear fuel contained in the object based on each measurement value of the muon trajectory detector and the radiation detector.

2. the trajectory determination unit measures a change in a muon trajectory when the muon passes through an object based on a measurement value of the muon trajectory detector, and derives muon scattering from the magnitude of the change in the muon trajectory; 2. The nuclear fuel measurement system according to claim 1, wherein the nuclear fuel weight determination unit is configured to select an event in which the muon trajectory has not passed through a radiation detector from the determination result of the trajectory determination unit, and to derive the weight of the nuclear fuel contained in the object from the relationship between the muon scattering and the weight of the nuclear fuel.

3. the burnup determination unit selects an event in which the muon trajectory has not passed through a gamma ray detector serving as a radiation detector from the determination result of the trajectory determination unit, and derives the nuclide amount of the nuclide to be evaluated contained in the object from the measurement value of gamma rays serving as radiation by the gamma ray detector; 2. The nuclear fuel measurement system according to claim 1, wherein the nuclear fuel amount per unit weight of the object is derived from the weight of the nuclear fuel derived from the measurement value of the muon track detector and the nuclide amount, and the burnup of the object is derived from the relationship between the nuclide amount per unit weight of the object and the burnup.

4. the burnup determination unit selects an event in which the muon trajectory has not passed through a neutron detector serving as a radiation detector from the determination result of the trajectory determination unit, and derives the nuclide amount of the nuclide to be evaluated contained in the object from the measurement value of the neutron ray serving as radiation by the neutron detector; 2. The nuclear fuel measurement system according to claim 1, wherein the nuclear fuel amount per unit weight of the object is derived from the weight of the nuclear fuel derived from the measurement value of the muon track detector and the nuclide amount, and the burnup of the object is derived from the relationship between the nuclide amount per unit weight of the object and the burnup.

5. the burnup determination unit selects an event in which a muon trajectory does not pass through a charged particle detector serving as a radiation detector from the determination result of the trajectory determination unit, and derives the nuclide amount of the nuclide to be evaluated contained in the object from the measurement value of the charged particle beam serving as radiation by the charged particle detector; 2. The nuclear fuel measurement system according to claim 1, wherein the nuclear fuel amount per unit weight of the object is derived from the weight of the nuclear fuel derived from the measurement value of the muon track detector and the nuclide amount, and the burnup of the object is derived from the relationship between the nuclide amount per unit weight of the object and the burnup.

6. 2. The nuclear fuel measurement system according to claim 1, wherein the burnup determination unit is configured to determine whether or not a burnup value derived from a plurality of types of measurement values ​​obtained by a plurality of types of radiation detectors is within a predetermined tolerance range, and to derive the burnup using a value included in the tolerance range.

7. 2. The nuclear fuel measurement system according to claim 1, further comprising a fissile nuclide ratio determination unit that derives a content ratio of fissile nuclides based on each measurement value by the muon track detector and the radiation detector.

8. 8. The nuclear fuel measurement system according to claim 7, wherein the fissile nuclide ratio determination unit is configured to evaluate the content ratio of the fissile nuclides contained in the object based on the weight of the nuclear fuel derived by the nuclear fuel weight determination unit, the burnup derived by the burnup determination unit, and a relationship between the nuclide amount of the fissile nuclides and the burnup evaluated in advance.

9. a muon trajectory detector that is provided at opposing positions across an object to be measured and that measures muons and detects muon trajectories; a radiation detector that is installed near the target and measures at least one of gamma rays, neutron rays, and charged particle rays emitted from the target; a trajectory determination unit reconstructing the muon trajectory detected by the muon trajectory detector and determining whether or not the muon trajectory has passed through the radiation detector; A nuclear fuel weight determination unit selects an event in which the muon trajectory does not pass through the radiation detector, and derives the weight of the nuclear fuel contained in the object based on the measurement value of the muon trajectory detector; a burnup determination unit selecting an event in which the muon trajectory does not pass through the radiation detector, and deriving the burnup of the nuclear fuel contained in the object based on the measurement values ​​of the muon trajectory detector and the radiation detector.

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