JP5907836B2 - Measurement management method - Google Patents

Description

  The present invention relates to a method for measuring and controlling a mixture of nuclear fuel material and non-nuclear fuel material. More specifically, the present invention relates to a mixture of nuclear fuel material and non-nuclear fuel material (hereinafter referred to as a management target) suitable for metering management of a material such as fuel debris in which an unknown amount of nuclear fuel material and an unknown amount of material are mixed. Is related to the management method.

  In Japan, nuclear fuel materials used in nuclear power plants are required to accurately grasp the weight and location of uranium, that is, to control the weight, from the viewpoint of security. In normal operation of a nuclear power plant, all nuclear fuel is contained in the fuel assembly. Therefore, the measurement control is performed in units of the fuel assembly, and all substances originating from uranium in the fuel assembly remain in the fuel assembly. It is done as a thing. On the other hand, at the time of the three-mile island nuclear power plant (hereinafter referred to as TMI) accident in the United States, fuel debris was formed by melting part of the fuel rods, control rods, and core substructure. The fuel debris is cut and collected and stored in a container called a canister. Such fuel debris measurement management needs to be performed in units of canisters. However, since measurement control is not obligatory in the United States, no technology has been developed to grasp the weight of uranium enclosed in each canister. On the other hand, in the Fukushima Daiichi power plant accident, it is thought that fuel debris was generated as in the case of TMI, and it is expected that these will be collected in canisters.

  In addition, although it is not about the thing whose component ratio is unknown like fuel debris, there exists patent document 1 as a technique regarding the measurement management of the used nuclear fuel whose component ratio is known or can be traced.

JP 2000-162371 A

  By the way, as for fuel debris, the mixing amount of substances other than nuclear fuel is unknown, and the weight of nuclear fuel contained in fuel debris cannot be measured accurately.

  An object of the present invention is to provide a measurement management method for an object to be managed that can perform measurement management even when the amount of the non-nuclear fuel material contained therein is unknown.

In order to achieve this object, the method for measuring and managing a management object according to claim 1 is caused by a neutron capture reaction of a measurement target nuclide released from a control object including a nuclear fuel nuclide and a measurement target nuclide that causes a neutron capture reaction. Γ-rays and γ-rays generated by nuclear fuel nuclide fission reactions were measured, and the neutron capture reaction rate of the target nuclide was calculated based on the measured values of γ-rays generated by the neutron capture reaction, and produced by the fission reaction. The fission rate of the nuclear fuel nuclide is calculated based on the measured value of the gamma ray, and the ratio between the neutron capture reaction rate of the target nuclide and the fission rate of the nuclear fuel nuclide is divided by the ratio of the microneutron capture cross section and the micro fission cross section. to after having determined the ratio N _{i /} N _u the number density N _i and number density N _u nuclear fuel nuclide to be measured nuclides, the ratio N _{i /} N _u, of the measurement target nuclide atomic weight [rho _i and nuclear fuel nuclide atomic weight ρ _u of As well as the managed object of weight W using Wu = W / (1+ (ρ i N i / ρ u N u)), if the managed object is assumed to be composed of a nuclear fuel nuclide a measurement target nuclide and performs metering management seeking weight Wu of the nuclear fuel nuclides managed object in the. Therefore, even if the mixing amount of the measurement target nuclide is unknown, the weight of the nuclear fuel nuclide can be obtained, and the measurement management of the nuclear fuel material can be appropriately performed .

  According to a second aspect of the present invention, there is provided a method for measuring and managing gamma rays by irradiating a management object with neutrons from the outside. Therefore, in addition to γ-rays of nuclear reactions caused by internal neutrons, γ-rays of nuclear reactions caused by external neutrons can be generated. By significantly increasing the γ-ray generated by the reaction, it becomes easy to measure the γ-ray generated by the neutron capture reaction of the measurement target nuclide and the fission reaction of the nuclear fuel material.

  According to a third aspect of the present invention, there is provided a method for measuring and managing a management object, wherein the management object is fuel debris in which nuclear fuel materials and structural materials are broken or melted and mixed. Therefore, the fuel debris can be measured and managed.

  According to the method for measuring and managing a management object according to claim 1, it becomes possible to perform measurement management for a management object whose mixing ratio of nuclear fuel nuclides is unknown, and based on the obtained ratio of nuclear fuel nuclides and the weight of the management object. Since the weight of the nuclear fuel nuclide in the controlled object is obtained and the controlled object is measured and managed, it is possible to appropriately perform the controlled measurement of the material containing both the nuclear fuel material and the non-nuclear fuel material.

  In addition, according to the measurement management method of the management object according to claim 2, in addition to gamma rays of nuclear reaction caused by internal neutrons, gamma rays of nuclear reaction caused by external neutrons can be measured. γ-ray measurement is facilitated, and the management object can be measured and managed more accurately.

  In addition, according to the measurement management method of the management object according to the third aspect, since the fuel debris is the management object, the fuel debris measurement management can be performed.

It is a flowchart which shows an example of embodiment of the measurement management method of the management target object of this invention. It is a conceptual diagram which shows a mode that a control target object is irradiated with neutron and a gamma ray is measured. It is a schematic block diagram which shows the mode of experiment. It is a graph which shows an experimental result.

  Hereinafter, the configuration of the present invention will be described in detail based on the form shown in the drawings.

  1 and 2 show an example of an embodiment of the management object management method of the present invention.

When a controlled object in a subcritical state is irradiated with neutrons, gamma rays are generated by fission and capture reactions. These gamma rays have an energy spectrum that is unique to nuclides. When a γ-ray spectrum is measured, a measurement wave height spectrum in which various γ-ray spectra are mixed is obtained. When a mathematical operation called unfolding is performed on the measured wave height spectrum, it is possible to obtain the intensity ratio of various γ rays. From the intensity ratio, a specific neutron capture reaction rate and fission ratio can be estimated. This ratio is _expressed as <σ _{c, i} N _i φ> / <σ _fU N _U φ>. Here, i is a nuclide that causes the specific neutron capture reaction, and u is a nuclide that causes the fission. σ _c is the microscopic cross section of the neutron capture reaction, σ _fu is the microscopic cross section of fission, N is the number density, φ is the neutron flux, and <> is the integration in the spatial region irradiated with neutrons . Then, from the ratio <σ _{c, i} N _i φ> / <σ _fU N _U φ>, it is assumed that the neutron flux φ of the numerator is the same, and using the data of the microscopic cross section, <N _i > / <N _U > is obtained. From this, it is possible to determine the ratio of nuclear fuel nuclides contained in the controlled object when it is assumed that the controlled object consists of nuclear fuel nuclides and measurement target nuclides, and refer to the weight of the controlled object. The weight of the nuclear fuel nuclide in the control object can be obtained.

Examples of the measurement target (measurement target nuclide) of the capture reaction include ⁵⁶ Fe contained in stainless steel.

  That is, the method of measuring and managing the management target object according to the present embodiment includes the nuclear fuel nuclide and the γ-ray and nuclear fuel generated by the neutron capture reaction of the measurement target nuclide released from the control target object 1 including the measurement target nuclide that causes the neutron capture reaction. Measures γ-rays generated in the fission reaction of the nuclide, calculates the neutron capture reaction rate of the target nuclide based on the measured value of the γ-ray generated in the neutron capture reaction, and measures the value of γ-ray generated in the fission reaction The nuclear fission rate of the nuclear fuel nuclide is calculated, and the ratio of the nuclear fuel nuclide contained in the control target 1 is obtained by using the neutron capture reaction of the measurement target nuclide and the microscopic cross section of the nuclear fuel nuclide fission. The weight of the nuclear fuel nuclide in the management object 1 is obtained based on the ratio of the above and the weight of the management object 1 to perform measurement management.

In the present embodiment, fuel debris generated in a boiling water reactor (BWR) is assumed as the management object 1, and measurement management at the time of processing this fuel debris will be described as an example. Since the fuel debris generated in the BWR contains a large amount of nuclear fuel ²³⁵ U and ²³⁸ U, in this embodiment, ²³⁵ U and ²³⁸ U are selected as nuclear fuel nuclides that cause the fission reaction γ-rays to be measured. .

The target nuclide is a nuclide that causes a neutron capture reaction. In this embodiment, since the ratio of the nuclear fuel nuclide contained in the management target object 1 is calculated when it is assumed that the management target object 1 is composed of the nuclear fuel nuclide and the measurement target nuclide, the measurement target nuclide is the management target object. 1 is preferably contained in a large amount. In the BWR, a structure made of stainless steel containing a large amount of ⁵⁶ Fe is provided under the pressure vessel. Therefore, the fuel debris generated in the BWR contains a large amount of ⁵⁶ Fe. In the present embodiment, this ⁵⁶ Fe is used as a measurement target nuclide that is a measurement target of γ rays by a neutron capture reaction.

For example, ²⁵² Cf, Am-Be neutron source, Sb-Be neutron source, accelerator neutron source and the like can be used as the neutron beam source 2 for irradiating the management object 1 with neutrons, but is not limited thereto. .

  As the γ-ray measuring device 3 for measuring γ-rays, any gamma ray can be used as long as it can measure γ-rays generated by the neutron capture reaction of the target nuclide and γ-rays generated by the fission reaction of the nuclear fuel nuclide. For example, Nal, BGO, HP-Ge, LaBr3, a Cherenkov counter with a low signal resolution but a good signal-to-noise ratio, and the like can be used.

  Further, a shield and a scatterer are appropriately combined and arranged, and γ-ray measurement is performed in a region where the γ-ray count rate at the γ-ray measurement value 3 is appropriate and under a condition with a good signal-to-noise ratio.

  The management object 1 is accommodated in a container, for example. The management object 1 is irradiated with neutrons from an external neutron beam source 2 and γ rays are measured by the γ ray measuring instrument 3 (step S11 in FIG. 1, FIG. 2). Gamma rays of various energies are radiated from the management object 1, and a measured wave height spectrum in which various gamma ray spectra are mixed is obtained.

Next, the ratio (<σ _{c, i} N _i φ> / <σ _fU) of the neutron capture reaction rate <σ _{c, i} N _i φ> of the measurement target nuclide and the fission rate <σ _fU N _U φ> of the nuclear fuel nuclide. N _U φ>) is calculated (step S12). Therefore, first, based on the basic data of γ-rays generated by the neutron capture reaction of the target nuclides and the fission reaction of the nuclear fuel nuclides, simulation of γ-ray transport and energy application to the γ-ray measuring instrument 3 is performed. A response function used to derive <σ _{c, i} N _i φ> and <σ _fU N _U φ> is calculated. Here, as basic data, neutron capture reaction and γ-ray generation number data per fission, γ-ray generation spectrum data, and reaction cross-sectional data of γ-rays for fuel debris are used. In the simulation, for example, MCNP code, EGS-5 code or the like is used. However, it is not limited to these.

Then, when a mathematical operation called unfolding is performed using the response function on the measured wave height spectrum, the neutron capture reaction rate of the measurement nuclide <σ through the evaluation of the intensity of the γ-ray generated by the neutron capture reaction of the measurement nuclide _{c, i} N _i φ>, and the relative value of the fission rate <σ _fU N _U φ> of the nuclear fuel nuclide is obtained through the intensity of the γ-ray generated by the nuclear fission reaction of the nuclear fuel nuclide, so that the ratio between the two (<σ _{c , I} N _i φ> / <σ _fU N _U φ>).

After _{obtaining the} ratio (<σ _{c, i} N _i φ> / <σ _fU N _U φ>), the management target 1 is assumed to be composed of the nuclear fuel nuclide and the measurement target nuclide. The ratio of nuclear fuel nuclides contained in 1 is obtained. In the present embodiment, the ratio (<σ _{c, i} N _i φ> / <σ _fU N _U φ>) is the ratio of the micro neutron capture cross section σ _{c, i} and the micro fission cross section σ _fU σ _{c, i} / σ A ratio (N _i / N _u ) between the number density N _i of the measurement target nuclide and the number density N _u of the nuclear fuel material is _obtained by dividing by _fU (step S13). As the micro fission cross section σ _fU and the micro neutron capture cross section σ _{c, i} , known ones are used. Then, (ρ _i N _i / ρ _u N _u ) is obtained based on the ratio (N _i / N _u ) using the respective atomic weights ρ _i and ρ _u (step S14). The weight ratio of the nuclear fuel nuclide contained in the management object 1 when the management object 1 is assumed to be composed of the nuclear fuel nuclide and the measurement object nuclide is obtained by (ρ _i N _i / ρ _u N _u ).

Thereafter, the management object 1 is subjected to measurement management using the obtained weight ratio of the nuclear fuel nuclide contained in the management object 1. In the present embodiment, the weight Wu of the nuclear fuel nuclide in the fuel debris is obtained in order to perform the measurement management of the fuel debris (management object 1). Therefore, first, the weight W of the management object 1 is measured, and from the obtained weight information (weight W), (ρ _i N _i / ρ _u N _u ) is used to calculate the fuel debris and the nuclear fuel nuclide and the measurement target The weight Wu of the nuclear fuel nuclide in the fuel debris when it is assumed that it is composed of the nuclide is obtained (step S15). Measurement management is performed using the obtained weight Wu (step S16).

[Equation 1]
Wu = W / (1+ (ρ _i N _i / ρ _u N _u ))

  In the present invention, since the ratio of nuclear fuel nuclides contained in the management target 1 can be calculated in consideration of the neutron absorber (measurement target nuclide) contained in the management target 1, the management target is based on this ratio. The weight of the nuclear fuel nuclide in 1 can be determined. That is, even when the ratio between the nuclear fuel nuclide and other materials is unknown, such as fuel debris, the weight of the nuclear fuel nuclide in the management target 1 can be calculated, and the measurement management of the management target 1 It becomes possible to do.

  Moreover, since the weight of the nuclear fuel nuclide in the management target 1 is obtained based on the ratio of the nuclear fuel nuclide and the weight of the management target 1, the management of the management target 1 is performed, so that the measurement management can be performed appropriately. Become.

  In addition, since γ-rays are measured by irradiating the controlled object 1 with neutrons from the outside, the γ-rays generated by the neutron capture reaction of the measurement target nuclide and the fission reaction of the nuclear fuel material are greatly increased. By doing so, it becomes easy to measure the γ-rays generated by the neutron capture reaction of the measurement target nuclide and the fission reaction of the nuclear fuel material, and the ratio of the nuclear fuel nuclide contained in the control target 1 is more accurately determined. Can be sought.

  The above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the scope of the present invention.

 For example, in the above description, the fuel debris generated in the BWR is assumed as the management target 1, but the present invention is not limited to this. Fuel debris generated in PWR, spent nuclear fuel, unused nuclear fuel, dissolution tank for dissolving spent fuel for reprocessing, uranium mine, etc., including nuclear fuel nuclides and nuclides that cause neutron capture reaction It ’s fine.

In the above description, ⁵⁶ Fe is the measurement target nuclide, but the present invention is not limited to this. For example, ⁵⁸ Ni, nitrogen, materials used for control rods (boron, etc.), etc. may be used. Moreover, since fuel debris may contain concrete, Al of alumina contained in the concrete may be used as the measurement target nuclide. In addition, any nuclide that can generate γ-rays by neutron capture reaction can be used. Further, the measurement target nuclide may be one type or plural.

  In the above description, γ rays are measured while irradiating the management target 1 with neutrons from the outside. However, it is not necessary to perform neutron irradiation from the outside. In other words, it is more preferable to use external neutrons for gamma ray measurement, but gamma rays can be measured without using external neutrons. Although accurate quantitative evaluation of materials such as uranium and iron is difficult by this measurement, it is useful for more detailed evaluation of the mixing state of a plurality of structural materials by measuring a plurality of types of captured γ rays.

  Furthermore, in the above description, the γ-ray measurement is performed in a state where the management object 1 is accommodated in the container, but the γ-ray measurement may be performed without accommodating the management object 1 in the container. For example, the management object 1 may be lifted with a tool or the like in water to perform γ-ray measurement.

In the above description, the ratio (<σ _{c, i} N _i φ> / <σ _fU N _U φ>) of the neutron capture reaction rate of the target nuclide and the nuclear fuel nuclide is _{expressed as the microneutron} capture cross section σ _{c. , I} and the ratio of the nuclear fission cross section σ _{fU divided by} the ratio σ _{c, i} / σ _fU to obtain the ratio (N _i / N _u ) of the number density N _i of the measurement target nuclide and the number density N _u of the nuclear fuel material However, it is not necessarily limited to this. For example, it is also possible to obtain N _i and N _u and obtain the ratio (N _i / N _u ) as follows. That is, when the size of the management object 1 is relatively small, a reference sample with a known weight is used to irradiate neutrons with a known intensity and a known energy spectrum. Measure and compare the γ-ray generation rate from the management target 1 by measuring it. In this case it is possible to obtain the <σ _{c, i} φ> and N by <σ _fU φ> is assumed to same in the managed object 1 and the reference sample _i and N _U.

At the Kyoto University critical assembly test equipment, ²⁵² Cf (which emits neutrons and γ-rays) is loaded on a subcritical core consisting mainly of ²³⁵ U, Al, water, and SUS floor plate, and a BGO detector is placed on the outer periphery. A spectrum measurement of γ-rays was performed (FIG. 3). For the measured values in FIG. 4, ²³⁵ U fission, γ-ray generated by neutron capture reaction of Al and SUS, and γ-ray transport simulation spectrum, ²³⁵ U fission, γ of Al and SUS The line generation rate was determined. This roughly coincided with the calculated γ-ray generation rate calculated by neutron transport calculation. This means that (<σ _{c, i} N _i φ> / <σ _fU N _U φ>) can be measured, confirming the effectiveness of the present invention.

1 Controlled object 2 Neutron source 3 Gamma ray measuring instrument

Claims (3)

Measure the γ-rays generated in the neutron capture reaction of the measurement target nuclide released from the controlled object including the nuclear fuel nuclide and the measurement target nuclide causing the neutron capture reaction and the γ-ray generated in the fission reaction of the nuclear fuel nuclide, Calculate the neutron capture reaction rate of the target nuclide based on the measured value of γ-rays generated in the neutron capture reaction, and calculate the fission rate of the nuclear fuel nuclide based on the measured values of γ-rays generated in the fission reaction and, wherein a number density N _i of the measurement target nuclide the ratio of the neutron capture reaction rate and fission rate of the nuclear fuel nuclide of the measurement target species divided by the ratio of the microscopic neutron capture cross-section and microscopic fission cross-sectional area after having determined the ratio N _{i /} N _u the number density N _u the nuclear fuel nuclides, the ratio N _{i /} N _u, the measured atomic weight of the target nuclide [rho _i and the nuclear fuel nuclide atomic weight [rho _u, and the management target Equation 1 using the weight W follows
(Formula 1) Wu = W / (1+ (ρ _i N _i / ρ _u N _u ))
The control object is subjected to measurement management by obtaining the weight Wu of the nuclear fuel nuclide in the control object when it is assumed that the control object is composed of the nuclear fuel nuclide and the measurement target nuclide. A method of measuring and managing the management object   The method for measuring and managing a management object according to claim 1, wherein the gamma ray measurement is performed by irradiating the management object with neutrons from the outside.   The method for measuring and managing a management target according to claim 1 or 2, wherein the management target is fuel debris in which nuclear fuel materials and structural materials are broken or melted and mixed.

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