JP2022191692A - Nuclear material measurement system and nuclear material measurement method - Google Patents

Abstract

To prevent a detector from being inoperative when measuring a nuclear material contained in wastes, stored materials and damaged/molten fuel-containing materials from an atomic power plant.SOLUTION: A nuclear material measurement system includes an electron beam accelerator 1, a target 20 that receives a pulse high-energy electron beam emitted from the electron beam accelerator to generate a pulse bremsstrahlung radiation and a pulse high-speed neutron, and a neutron detector 50 that applies a voltage to measure a neutron. It has a configuration to irradiate an object to be measured 30 with the pulse bremsstrahlung radiation and the pulse high-speed neutron emitted from the target by irradiation of the pulse high-energy electron beam. The neutron detector measures the pulse high-speed neutron emitted from the object to be measured by irradiation of the pulse high-speed neutron, and in a time zone when being irradiated with the pulse high-energy electron beam, the voltage to be applied to the neutron detector is set to less than a predetermined value, and after the elapse of the time zone, the voltage to be applied to the neutron detector is set to the predetermined value or more, so that the neutron detector can measure the neutron.SELECTED DRAWING: Figure 1

Description

The present invention relates to a nuclear material measurement system and a nuclear material measurement method.

As a conventional method, the object to be measured is irradiated with pulsed fast neutrons, and based on the irradiation time of the pulsed fast neutrons, the neutrons whose energy is moderated by the fast neutrons and the object to be measured react with nuclear matter (nuclear fission reaction). There is a method for detecting and measuring nuclear material by measuring the time-dependent change in the production rate of fission neutrons.

According to Patent Document 1, fission neutrons generated by a nuclear fission reaction decay exponentially with a certain constant time constant (annihilation time). This annihilation time depends on the neutron absorption and moderation properties of the material contained in the measurement object, the total count number of fission neutrons measured with a neutron detector is proportional to the amount of It varies with the neutron absorption and moderation properties of the materials contained within. Therefore, by performing a calibration test in advance and changing the physical property values that indicate the neutron absorption and moderation performance of the substance contained in the measurement object, fissile properties that are detected in a state containing a known amount of known fissile material Obtain the relationship between the neutron annihilation time and the total number of counts. Based on this relationship, it is a method of deriving the amount of unknown fissile material in the object to be measured.

Patent Document 2 proposes a method for measuring fissile material in fuel debris using an X-ray CT apparatus. This is a method to measure the amount of nuclear material by correcting the measurement results of gamma rays of nuclear materials in damaged or melted nuclear fuel or nuclides that behave similarly with the results of density measurement by X-ray CT.

Patent Document 3 proposes a system that combines X-ray CT, photonuclear reaction, photoneutron mixing beam utilization, and neutron activation analysis using an electron linear accelerator and an X-ray generation converter.

Patent Document 4 discloses an X-ray CT apparatus that irradiates an object to be measured with two or more types of X-rays having different energy ranges and detects transmitted X-rays, and an X-ray CT apparatus that irradiates the object to be measured with a neutron beam to perform measurement. and a neutron measuring device for counting the number of neutrons emitted from an object.

Japanese Patent No. 6179885 Japanese Patent No. 6468931 Japanese Patent No. 4649580 Japanese Unexamined Patent Application Publication No. 2020-94906

In the method of Patent Document 1, the amount of fissile material cannot be derived without a database of physical property values indicating the neutron absorption and moderation performance of the material contained in the measurement object. In particular, at the Fukushima Daiichi Nuclear Power Station, it is assumed that the fissile material in the fuel assembly is in a molten state, and the structural material, neutron absorbing material, and concrete material are mixed. It is difficult to build a database of physical properties that indicate neutron absorption and moderation performance in advance.

In the technique of Patent Document 2, even if density correction is performed by X-ray CT, it is difficult to directly measure fissile material because the energy of gamma rays from fissile material is low.

In the method of Patent Document 3, in X-ray CT, it is difficult to measure fissile substances only with density distribution information, and in photonuclear reactions, for example, measurement of difficult-to-measure nuclides such as iodine-129 is difficult. Possible, but difficult to measure fissile material. In the use of photoneutron mixing rays, it is difficult to distinguish and measure neutrons produced by photonuclear reactions and neutrons produced by fission reactions by neutrons, so measurement of fissile materials is difficult. In the neutron activation analysis method, it is possible to measure the content of chlorine, lead, etc., but it is difficult to measure the fissile material.

In the nuclear material amount measurement device described in Patent Document 4, a transport mechanism is provided between the X-ray CT device and the neutron measurement device, and the measurement object is moved to separately measure X-rays and neutrons. It has become. Therefore, it is difficult to unify the conditions under which the object to be measured is irradiated with X-rays and neutrons, and the system becomes large.

Although it is possible to apply multiple measurement methods such as X-ray CT and active neutron method, it is difficult to apply multiple methods without changing electron beam conditions and targets. In addition, when using neutrons generated by irradiating a neutron generation target with bremsstrahlung radiation as the neutron source used in the active method, there is a problem that the neutron detector becomes inoperable due to the bremsstrahlung radiation irradiation. there were.

As described above, no effective systems and measurement methods have been proposed for detecting and measuring fissile material (nuclear material) in wastes and stored materials from nuclear power plants and materials containing damaged/melted fuel. It is in.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent detectors from becoming inoperable when measuring nuclear material contained in waste and deposits from nuclear plants and materials containing damaged or melted fuel.

The nuclear material measurement system of the present invention includes an electron beam accelerator, a target that receives a pulsed high-energy electron beam emitted from the electron beam accelerator and generates pulsed bremsstrahlung and pulsed fast neutrons, and voltage is applied to generate neutrons. and a neutron detector for measurement, and has a configuration for irradiating the measurement object with pulsed bremsstrahlung and pulsed fast neutrons emitted from the target by irradiation with the pulsed high-energy electron beam, wherein the neutron detector includes pulsed fast neutrons to measure the fast neutrons emitted from the object to be measured due to the irradiation of the pulsed high-energy electron beam. , the voltage applied to the neutron detector is set to a predetermined value or higher, and the neutron detector can be used for measurement.

Advantageous Effects of Invention According to the present invention, it is possible to prevent the detector from becoming inoperable when measuring nuclear material contained in waste and stored material from a nuclear power plant and damaged/melted fuel-containing material.

1 is a configuration diagram showing a nuclear material measurement system of Example 1. FIG. 4 is a graph showing an example of the amount of bremsstrahlung generated by electron beam irradiation. 1 is a graph showing a reaction cross section of a reaction in which one neutron is generated by irradiating beryllium with bremsstrahlung. 1 is a graph showing a reaction cross section of a reaction in which one neutron is generated by irradiating deuterium with bremsstrahlung radiation. 5 is a graph showing an example of peak value spectrum when a boron-10 (B-10) detector is used as a neutron detector according to Example 1. FIG. 5 is a graph showing an example of peak value spectrum when a helium-3 (He-3) detector is used as the neutron detector according to Example 1. FIG. 4 is a graph showing an example of the change over time of the fast neutron count rate in Example 1. FIG. 4 is a graph showing an example of temporal changes in the voltage applied to the neutron detector, the trigger signal, the intensity of the pulsed high-energy electron beam, the intensity of the pulsed bremsstrahlung, and the intensity of the pulsed fast neutrons in Example 1. FIG. FIG. 11 is a configuration diagram showing a nuclear material measurement system of Example 2; 7 is a graph showing an example of temporal changes in the voltage applied to the neutron detector, the trigger signal, the intensity of the pulsed high-energy electron beam, the intensity of the pulsed bremsstrahlung, and the intensity of the pulsed fast neutrons in Example 2. FIG. FIG. 11 is a configuration diagram showing a nuclear material measurement system of Example 3; 7 is a graph showing an example of temporal changes in the voltage applied to the neutron detector, the trigger signal, the intensity of the pulsed high-energy electron beam, the intensity of the pulsed bremsstrahlung, and the intensity of the pulsed fast neutrons in Example 3. FIG. FIG. 11 is a configuration diagram showing a nuclear material measurement system of Example 4;

Hereinafter, embodiments will be described with reference to the drawings.

Embodiment 1 will be described with reference to FIGS. 1 to 7. FIG.

FIG. 1 shows an example of the configuration of the nuclear material measurement system of this embodiment.

The nuclear material measurement system shown in this figure includes an electron beam accelerator 1, an electron beam accelerator controller 2, a target 20, a neutron detector 50, a switching circuit 55, and a data processing/analyzing device 70. there is The object 30 to be measured is placed downstream of the target 20 in the traveling direction of the pulsed high-energy electron beam 10 that irradiates the target 20 from the electron beam accelerator 1 .

A neutron detector preamplifier 51 a is connected to the neutron detector 50 . The neutron detector preamplifier 51a is connected to a data processing/analyzing device 70 via a neutron detector amplifier 52a. The neutron detector preamplifier power supply 53a is a power supply for the neutron detector preamplifier 51a. The neutron detector high voltage power supply 54a is connected to the neutron detector preamplifier 51a through a neutron detector switching circuit 55a.

A pulsed high-energy electron beam 10 emitted from an electron beam accelerator 1 irradiates a target 20 for generating pulsed bremsstrahlung and pulsed fast neutrons. As a result, pulsed bremsstrahlung 21 and pulsed fast neutrons 22 are generated, and the object 30 to be measured is irradiated with these. The pulsed fast neutrons 22 irradiated to the measurement object 30 are decelerated within the measurement object 30 . The slowed fast neutrons undergo a nuclear reaction with the nuclear material in the measurement object 30 to generate fast neutrons. The fast neutrons are measured by a neutron detector 50 installed outside the object 30 to be measured.

The target 20 is a combination of a target made of heavy metal such as tungsten, platinum, gold, lead (heavy metal target) and a target made of beryllium (beryllium target). The heavy metal target is placed before (that is, upstream of the pulsed high-energy electron beam 10), and the beryllium target is placed after the heavy metal target (that is, downstream of the pulsed high-energy electron beam 10). Note that deuterium can be used in place of beryllium in the subsequent stage of the target 20 . In this case, deuterium may be combined with other metal elements and used in the solid state as a deuteride. Alternatively, it may be used in a liquid state such as heavy water. Subsequent targets containing beryllium or deuterium are referred to herein as "light element targets."

The electron beam accelerator controller 2 issues a control signal 3 to the electron beam accelerator 1 and a trigger signal 4 to the switching circuit 55 . The electron beam accelerator 1 that receives the control signal 3 is configured to emit a pulsed high-energy electron beam 10 . The switching circuit 55 that receives the trigger signal 4 is configured to send the gate signal 5 to the neutron detector switching circuit 55 a and the data processing/analyzing device 70 . The data measured by the neutron detector 50 is sent to the data processing/analyzing device 70 via the neutron detector preamplifier 51a and the neutron detector amplifier 52a.

FIG. 2 is a graph showing an example of the amount of bremsstrahlung generated by electron beam irradiation.

As shown in this figure, bremsstrahlung is generated when electron beams are irradiated to heavy metals such as tungsten and platinum. The maximum energy of bremsstrahlung produced is approximately equal to the energy of the electron beam. Bremsstrahlung radiation with lower energy is generated in greater quantity.

FIG. 3A is a graph showing a reaction cross-section of a reaction in which one neutron is generated by bremsstrahlung irradiation of beryllium. The horizontal axis is the energy of bremsstrahlung, and the vertical axis is the reaction cross section σ _tot .

As shown in this figure, neutrons are generated when beryllium is irradiated with bremsstrahlung with a certain energy or higher.

Beryllium has a reaction cross section that generates neutrons even when subjected to relatively low energy bremsstrahlung radiation. Therefore, beryllium receives bremsstrahlung radiation generated by electron beam irradiation and generates neutrons even when the electron beam has a relatively low acceleration energy of several MeV or less.

Therefore, when an electron beam is irradiated to a member such as the target 20 (FIG. 1) in which a heavy metal is placed in the front stage and beryllium is placed in the rear stage of the heavy metal, both bremsstrahlung and neutrons can be generated.

In this embodiment, both pulsed bremsstrahlung 21 and pulsed fast neutrons 22 can be generated by pulsing the electron beam and irradiating the target 20 with the pulsed high-energy electron beam 10 .

The measured object 30 is irradiated with the generated pulsed bremsstrahlung radiation 21 and pulsed fast neutrons 22 . If a neutron reflector is installed inside or outside the measurement object 30, the pulsed fast neutrons 22 irradiated to the measurement object 30 are also decelerated. Here, graphite (graphite), lead, iron, tungsten carbide, etc. are used as the neutron reflector.

The pulsed fast neutrons 22 that have been slowed down cause a nuclear reaction with the nuclear material in the measurement object 30 to generate fast neutrons. These fast neutrons are measured by a neutron detector 50 installed outside the object to be measured.

The neutron detector 50 may be surrounded by a neutron absorbing material that is not mixed in the measurement object 30, and neutrons that are not absorbed by the neutron absorbing material may be measured. Here, boron, cadmium, xenon, hafnium, etc. are used as the neutron absorber.

The pulsed fast neutrons 22 are generated corresponding to the irradiation time and irradiation time width of the pulsed high-energy electron beam 10 . Therefore, by using the trigger signal 4 of the pulsed high-energy electron beam 10, the peak value and measurement time of the measurement signal from the neutron detector 50 can be measured based on the generation time of the pulsed fast neutrons 22. By collecting the peak value data at each measurement time, the change over time of the peak value spectrum in the neutron detector 50 is obtained. If the number of peak value data at each measurement time is small, irradiation with pulsed fast neutrons 22 may be repeated multiple times until a sufficient amount of peak value data is collected.

FIG. 3B is a graph showing the reaction cross section of the reaction in which one neutron is generated by irradiating deuterium with bremsstrahlung. The horizontal axis is the energy of bremsstrahlung, and the vertical axis is the reaction cross section σ _tot .

As shown in this figure, deuterium also generates neutrons when exposed to bremsstrahlung with a certain energy or more, as with beryllium. In addition, even when exposed to relatively low-energy bremsstrahlung, there is a reaction cross-section that generates neutrons.

Therefore, when deuterium is arranged behind the heavy metal as the target 20 (FIG. 1) and irradiated with an electron beam, both bremsstrahlung and neutrons can be generated.

FIG. 4 is a graph showing an example of peak value spectrum when a boron 10 (B-10) detector is used as a neutron detector. The horizontal axis is the peak value, and the vertical axis is the count. This is a detector in which B-10 is applied to the inner wall of the detector, and shows the results of measuring alpha particles and lithium 7 (Li-7), which are charged particles generated by a nuclear reaction between B-10 and neutrons.

FIG. 5 is a graph showing an example of peak value spectrum when a helium-3 (He-3) detector is used as a neutron detector. This is a detector in which He-3 gas is sealed inside, and shows the results of measuring protons and tritium, which are charged particles generated by a nuclear reaction between He-3 and neutrons.

The peak value spectrum measured by the neutron detector 50 includes a noise signal that is not a signal due to neutrons in a range in which the peak value is equal to or less than a certain value. The noise signal includes, in addition to the electrical noise signal, a noise signal due to gamma rays when the measurement object 30 contains a nuclide that emits gamma rays. As with the electrical noise signal, the gamma ray noise signal also has a lower crest value than the neutron signal.

FIG. 6 is a graph showing an example of the change over time of the fast neutron count rate in this example.

Of the curves in this figure, the solid line is the fast neutron count rate. The dashed line is pulsed fast neutrons from a target that produces pulsed bremsstrahlung radiation and pulsed fast neutrons. The dashed-dotted line is the neutrons generated by the reaction between the nuclear material and the neutrons in the measurement object. A two-dot chain line is neutrons generated from spontaneous fission nuclides contained in the measurement object. Based on the fast neutron count rate indicated by the solid line, the neutron count rates indicated by the dashed line, dashed-dotted line, and dashed-two dotted line are calculated.

The amount of nuclear material is calculated from the relationship between the amount of neutrons generated by the reaction between the nuclear material and neutrons, which has been measured in advance, and the amount of nuclear material.

A He-3 neutron detector, which is an example of a neutron detector, has a thin wire placed in the center of a hollow cylindrical electrode (cathode) as an electrode for charge collection (anode), and He-3 gas is placed inside. filled. A nuclear reaction between neutrons and He-3 produces tritium and protons, and the produced tritium and protons cause ionization in the gas to produce ion-electron pairs. Ions are attracted to the cathode and electrons to the anode. Due to the gas amplification of electrons occurring in the vicinity of the wire to which a high voltage is applied, they are amplified in proportion to the number of electrons produced. This results in a large output pulse.

Since the neutron flux incident on the He-3 neutron detector is usually not high, neutrons can be measured one by one by measuring the crest value of this output pulse.

Pulsed bremsstrahlung irradiation also causes ionization of the gas enclosed inside the neutron detector, generating ion-electron pairs. Because the intensity of pulsed bremsstrahlung is very high, a large number of ion-electron pairs are generated during the short duration of the pulse. A discharge state occurs due to the gas amplification of electrons occurring in the vicinity of the wire to which a high voltage is applied. This renders the neutron detector inoperable.

When the pulsed bremsstrahlung is no longer irradiated, a large amount of ion-electron pairs are no longer generated and the discharge state ceases, but it takes several milliseconds to return to the original state.

Pulsed bremsstrahlung produces very high intensity bremsstrahlung within the duration of the pulse and not outside the duration of the pulse.

The applied voltage is lowered within the range of time width during which very high intensity bremsstrahlung is irradiated. In this case, the applied voltage may be zero. As a result, the gas amplification effect can be suppressed, so that the discharge state can be prevented, and the neutron detector can be prevented from becoming inoperable.

FIG. 7 is a graph showing an example of temporal changes in the voltage applied to the neutron detector, the trigger signal, the intensity of the pulsed high-energy electron beam, the intensity of the pulsed bremsstrahlung, and the intensity of the pulsed fast neutrons.

As shown in the figure, a target 20 is irradiated with a pulsed high-energy electron beam 10 in response to a trigger signal 4 , and pulsed bremsstrahlung 21 and pulsed fast neutrons 22 are generated from the target 20 . Neither the pulsed bremsstrahlung 21 nor the pulsed fast neutrons 22 are generated during the time when the pulsed high-energy electron beam 10 is not irradiated. The irradiation time of the pulsed high-energy electron beam 10 is about several microseconds to 10 microseconds. For example, when the pulse repetition frequency is 50 Hz, the irradiation time interval of the pulsed high-energy electron beam 10 is 2 milliseconds. be.

As shown in FIG. 1, a high voltage from a neutron detector high voltage power supply 54a is input to a neutron detector switching circuit 55a and then to the neutron detector 50 via a neutron detector preamplifier 51a. Also, the gate signal 5 generated by the switching circuit 55 based on the trigger signal 4 from the electron beam accelerator controller 2 is input to the neutron detector switching circuit 55a. The neutron detector switching circuit 55a outputs a high voltage when the gate signal 5 is input, and does not output a high voltage when the gate signal 5 is not input.

By setting the output time of the gate signal 5 to the time during which the pulsed high-energy electron beam 10 from the electron beam accelerator 1 is not output, that is, the time during which the pulsed bremsstrahlung 21 is not generated, a very high intensity During the time period when the bremsstrahlung is irradiated, the application of high voltage to the neutron detector 50 can be made zero, and the neutron detector 50 can be prevented from becoming inoperable.

The application of the high voltage to the neutron detector 50 may be started immediately after the irradiation of the pulsed bremsstrahlung radiation 21, or may be delayed by about 150 microseconds as shown in FIG. The application stop time may be before the next pulsed bremsstrahlung 21 is irradiated. In FIG. 7, fast neutrons are measured in less than 1.8 milliseconds.

In this way, fast neutrons produced by nuclear reactions between the slowed fast neutrons and the nuclear material in the measurement object 30 can be detected without being affected by pulsed neutrons.

A second embodiment will be described with reference to FIGS. 8 and 9. FIG.

In this embodiment, only points different from the first embodiment will be described.

FIG. 8 is a block diagram showing the nuclear material measurement system of this embodiment.

In the nuclear material measurement system shown in the figure, in the traveling direction of the pulsed high-energy electron beam 10 irradiated from the electron beam accelerator 1 to the target 20, the measurement object 30 and the transmitted bremsstrahlung detector array are located downstream of the target 20. 40 are installed in this order. That is, a transmitted bremsstrahlung detector array 40 is added. In other words, the target 20, the measurement object 30, and the transmitted bremsstrahlung detector array 40 are arranged in this order in the direction in which the pulsed high-energy electron beam 10 travels.

The transmitted bremsstrahlung detector array 40 measures bremsstrahlung that has passed through the measurement object 30 and has been attenuated. The transmitted bremsstrahlung detector array 40 is connected to a data processing and analysis device 70 .

FIG. 9 is a graph showing an example of temporal changes in the voltage applied to the neutron detector, the trigger signal, the intensity of the pulsed high-energy electron beam, the intensity of the pulsed bremsstrahlung, and the intensity of the pulsed fast neutrons.

This figure differs from FIG. 7 of Example 1 in that the intensity distribution of the bremsstrahlung transmitted through the measurement object 30 is measured by the transmitted bremsstrahlung detector array 40 within the generation time of the pulsed bremsstrahlung 21. .

It is conceivable that the measurement object 30 may contain nuclides that emit gamma rays. This gamma ray is emitted at a constant emission rate regardless of time. On the other hand, the pulsed bremsstrahlung 21 is generated with a very high intensity during the pulse period and is not generated outside the pulse period.

Therefore, the transmitted bremsstrahlung is measured only during the period when the pulsed bremsstrahlung 21 is generated. Then, the value of the pulsed bremsstrahlung 21 can be measured by comparing with the gamma rays from the nuclide that emits gamma rays. This makes it possible to eliminate the influence of gamma rays from nuclides that emit gamma rays.

In addition, the intensity distribution of transmitted bremsstrahlung from the measurement object 30 can be measured from various directions by setting the measurement object 30 on the measurement object rotation table and rotating it. In other words, the measuring object 30 is rotatably installed. The intensity distribution data of transmitted bremsstrahlung from all directions of 360 degrees can be used to calculate the density distribution of the measurement object 30 .

In summary, the density distribution of the measurement object 30 is calculated using the intensity distribution data of the transmission bremsstrahlung radiation measured by the transmission bremsstrahlung detector array 40, and the amount of nuclear material contained in the measurement object 30 is calculated from the density distribution. Calculate

The amount of nuclear material calculated from the density distribution in the measurement object 30 may be compared with the amount of nuclear material calculated from the neutron measurement results. If the difference between these measurement results is within a certain range, the amount of nuclear material calculated from the density distribution in the measurement object 30 or the amount of nuclear material calculated from the neutron measurement results is It can be the amount of nuclear material contained within. If the difference between the two measurement results exceeds a certain range, the one with the greater amount of nuclear material may be used as the amount of nuclear material contained in the object 30 to be measured.

Example 3 will be described with reference to FIGS. 10 and 11. FIG.

In this embodiment, only points different from the first embodiment will be described.

FIG. 10 is a configuration diagram showing the nuclear material measurement system of this embodiment.

In the nuclear material measurement system shown in this figure, a gamma ray detector consisting of a gamma ray detection element 60 and a photomultiplier tube 61 is installed near the object 30 to be measured.

The signal obtained from the photomultiplier tube 61 is sent to the data processing/analyzing device 70 via the gamma ray detector preamplifier 51b and the gamma ray detector amplifier 52b. A gamma ray detector preamplifier power source 53b is connected to the gamma ray detector preamplifier 51b. Also, the photomultiplier tube 61 is connected to a gamma ray detector switching circuit 55b. A photomultiplier tube high voltage power supply 54b is connected to the gamma ray detector switching circuit 55b. The gamma ray detector switching circuit 55 b is configured to receive the signal from the switching circuit 55 .

The photomultiplier tube high voltage power supply 54b, the gamma ray detector preamplifier 51b, the gamma ray detector preamplifier power supply 53b, and the gamma ray detector switching circuit 55b are desirably installed at positions where they are not directly irradiated with bremsstrahlung. Alternatively, the photomultiplier tube high voltage power supply 54b, the gamma ray detector preamplifier 51b, the gamma ray detector preamplifier power supply 53b, and the gamma ray detector switching circuit 55b may be covered with a bremsstrahlung radiation shield such as lead. good. When radiation impinges on the gamma ray detection element 60, photons are generated. The generated photons enter the photomultiplier tube 61 and are converted into electrons, which are then amplified and converted into voltage signals. When pulsed bremsstrahlung is incident on the gamma ray detector element 60, a very large number of photons are produced in a short time. A large amount of generated photons enter the photomultiplier tube 61 in a short period of time, are converted into a large amount of electrons, and are amplified, thereby rendering the photomultiplier tube 61 inoperable. When no high voltage is applied to the photomultiplier tube 61, the photomultiplier tube 61 does not become inoperable because there is no amplification.

As shown in the figure, a high voltage from a photomultiplier tube high voltage power supply 54b is input to a gamma ray detector switching circuit 55b, and its output is input to a photomultiplier tube 61. FIG. Also, the gate signal 5 generated by the switching circuit 55 based on the trigger signal 4 from the electron beam accelerator controller 2 is input to the gamma ray detector switching circuit 55b. The gamma ray detector switching circuit 55b outputs a high voltage when the gate signal 5 is input, and does not output a high voltage when the gate signal 5 is not input.

By setting the output time of the gate signal 5 to the time during which the pulsed high-energy electron beam 10 from the electron beam accelerator 1 is not output, that is, the time during which the pulsed bremsstrahlung 21 is not generated, a very high intensity During the time period when the bremsstrahlung is irradiated, the application of high voltage to the photomultiplier tube 61 can be made zero, and the photomultiplier tube 61 can be prevented from becoming inoperable. Due to the neutron irradiation, the element contained in the object to be measured reacts with the neutron to emit a gamma ray having energy peculiar to the element, that is, a crest value. The count of gamma rays (prompt gamma rays) immediately generated by reaction with neutrons reflects the amount of reaction between neutrons and elements at that time. In the peak value spectrum of gamma rays at each time, the count rate at the peak peculiar to the element is calculated, and the change over time is obtained. The data of the temporal change of the neutron count rate obtained by the neutron detector 50 may be corrected using the data of the temporal change of the peak count rate of gamma rays.

In summary, the gamma ray detection element 60 receives gamma rays from the measurement object 30 and generates photons, and the photomultiplier tube 61 applies a predetermined voltage (interelectrode voltage) between built-in electrodes. As a result, photons generated by the gamma ray detection element 60 are converted into electrons, and the electrons are amplified. The inter-electrode voltage is set to be less than a predetermined value, and after the above time period has passed, the inter-electrode voltage applied to the neutron detector 50 is set to a predetermined value or more to enable measurement.

Alternatively, the amount of nuclear material contained in the measurement object 30 may be calculated using measurement data of fast neutrons from the measurement object 30 and measurement data of the peak value of gamma rays.

FIG. 11 is a graph showing an example of temporal changes in the voltage applied to the neutron detector, the trigger signal, the intensity of the pulsed high-energy electron beam, the intensity of the pulsed bremsstrahlung, and the intensity of the pulsed fast neutrons.

7 of Example 1 is different from FIG. 7 of Example 1 in that gamma rays are measured instead of measuring fast neutrons in a time zone in which pulses are not generated.

Example 4 will be described based on FIG.

FIG. 12 is a configuration diagram showing the nuclear material measurement system of this embodiment.

In this figure, a shield 25 is installed around the target 20 . The solid angle of the opening of the shield 25 viewed from the point where the pulsed bremsstrahlung 21 and the pulsed fast neutrons 22 are thought to be generated when the target 20 is irradiated with the pulsed high-energy electron beam 10 (two single points ), and the neutron detector 50 is installed outside the solid angle range. The shield 25 prevents the neutron detector 50 from being directly irradiated with the pulsed bremsstrahlung radiation 21, thereby preventing the neutron detector 50 from becoming inoperable.

It should be noted that the above points that are considered to be inside the target 20 may be assumed to be located at the center of gravity of the target 20 .

Methods for measuring nuclear material are collectively described below.

A method for measuring nuclear material includes a step of irradiating a target with a pulsed high-energy electron beam from an electron beam accelerator to generate pulsed bremsstrahlung and pulsed fast neutrons, and irradiating the object to be measured;
a step of measuring fast neutrons emitted from an object to be measured by irradiation with pulsed fast neutrons with a neutron detector, wherein the voltage applied to the neutron detector during the period of irradiation with the pulsed high-energy electron beam is set to be less than a predetermined value, and after the above time period has elapsed, the voltage applied to the neutron detector is set to a predetermined value or more to enable measurement by the neutron detector.

It is desirable that the method for measuring nuclear material further includes the step of measuring bremsstrahlung that passes through the measurement object with a transmitted bremsstrahlung detector array.

It is desirable that the nuclear material measurement method further include the step of rotating the measurement object.

The method for measuring nuclear material further includes the step of measuring gamma rays from the object to be measured with a gamma ray detector, and during the time period during which the pulsed high energy electron beam is irradiated, the photomultiplier tube of the gamma ray detector is applied It is desirable that the inter-electrode voltage applied to the photomultiplier tube is set to be less than a predetermined value, and after the above-mentioned period of time has elapsed, the inter-electrode voltage to be applied to the photomultiplier tube is set to a predetermined value or more so that the gamma ray detector can measure.

The nuclear material measurement method calculates the density distribution of the object to be measured using the intensity distribution data of the transmitted bremsstrahlung radiation measured by the transmitted bremsstrahlung detector array, and from the density distribution, the amount of nuclear material contained in the object to be measured. It is desirable to further include the step of calculating

According to the nuclear material measurement system and the nuclear material measurement method according to the present disclosure, the following effects are also obtained.

In an active neutron method application system using an electron beam accelerator, the time variation of the fast neutron count rate can be measured based on the generation time of pulse fast neutrons without the neutron detector becoming inoperable. In addition, by using a transmission bremsstrahlung detector, it is possible to simultaneously measure the intensity distribution of the bremsstrahlung that penetrates the measurement object. It will be possible to measure nuclear material in waste and archives from nuclear plants and material containing damaged or melted fuel by multiple measurement techniques such as line-of-sight CT.

1: electron beam accelerator, 2: electron beam accelerator controller, 3: control signal, 4: trigger signal, 5: gate signal, 10: pulsed high energy electron beam, 20: target, 21: pulsed bremsstrahlung, 22: pulse Fast neutrons, 25: shield, 30: object to be measured, 40: transmitted bremsstrahlung detector array, 50: neutron detector, 51a: neutron detector preamplifier, 51b: gamma ray detector preamplifier, 52a: neutron Detector amplifier 52b: Gamma ray detector amplifier 53a: Neutron detector preamplifier power supply 53b: Gamma ray detector preamplifier power supply 54a: Neutron detector high voltage power supply 54b: Photomultiplier tube high voltage Power supply 55: switching circuit 55a: neutron detector switching circuit 55b: gamma ray detector switching circuit 60: gamma ray detection element 61: photomultiplier tube 70: data processing/analysis device.

Claims (15)

an electron beam accelerator;
a target that receives a pulsed high-energy electron beam emitted from the electron beam accelerator and generates pulsed bremsstrahlung and pulsed fast neutrons;
A neutron detector that measures neutrons by applying a voltage,
A configuration for irradiating a measurement object with the pulsed bremsstrahlung and the pulsed fast neutrons emitted from the target by irradiation with the pulsed high-energy electron beam,
The neutron detector measures fast neutrons emitted from the measurement object by irradiation with the pulse fast neutrons,
setting the voltage to be applied to the neutron detector to less than a predetermined value during the time period during which the pulsed high-energy electron beam is irradiated;
A nuclear material measurement system having a configuration in which the voltage applied to the neutron detector is set to a predetermined value or higher after the elapse of the time period so that the neutron detector can perform measurement. 2. The target according to claim 1, wherein said target comprises a heavy metal target receiving said pulsed high energy electron beam and generating said pulsed bremsstrahlung radiation, and a light element target receiving said pulsed bremsstrahlung radiation and generating said pulsed fast neutrons. nuclear material measurement system. the heavy metal target comprises tungsten, platinum, gold or lead;
3. The nuclear material measurement system of claim 2, wherein said light element target comprises beryllium or deuterium. 2. The nuclear material measurement system according to claim 1, wherein a neutron absorbing material is installed around said neutron detector. further comprising a transmitted bremsstrahlung detector array;
2. A nuclear material measurement system according to claim 1, wherein said target, said object to be measured, and said transmission bremsstrahlung detector array are arranged in this order in the traveling direction of said pulsed high-energy electron beam. further comprising a gamma ray detector including a gamma ray detection element and a photomultiplier tube;
The gamma ray detection element receives gamma rays from the measurement object and generates photons,
The photomultiplier tube converts the photons generated by the gamma ray detection element into electrons by applying a voltage between the electrodes, amplifies the electrons,
during the time period during which the pulsed high-energy electron beam is irradiated, the inter-electrode voltage applied to the photomultiplier tube is less than a predetermined value;
2. The system for measuring nuclear material according to claim 1, wherein said inter-electrode voltage applied to said neutron detector is set to a predetermined value or higher after said period of time elapses so that said measurement can be performed. further comprising a shield placed around the target;
The measurement object is placed within a solid angle range of the opening of the shield viewed from a predetermined point located inside the target,
2. The nuclear material measurement system according to claim 1, wherein said neutron detector is installed outside said solid angle range. 2. The nuclear material measurement system according to claim 1, wherein said measurement object is rotatably installed. The measurement object is rotatably installed,
calculating the density distribution of the object to be measured using the intensity distribution data of the transmitted bremsstrahlung radiation measured by the transmitted bremsstrahlung detector array, and calculating the amount of nuclear material contained in the object to be measured from the density distribution; 6. The nuclear material measurement system according to claim 5. The measurement object is rotatably installed,
7. The nuclear material measurement system according to claim 6, wherein the measurement data of said fast neutrons from said measurement object and the measurement data of gamma ray crest values are used to calculate the amount of nuclear material contained in said measurement object. A step of irradiating a target with a pulsed high-energy electron beam from an electron beam accelerator to generate pulsed bremsstrahlung and pulsed fast neutrons, and irradiating the object to be measured;
measuring the fast neutrons emitted from the measurement object by the irradiation of the pulsed fast neutrons with a neutron detector;
During the time period during which the pulsed high-energy electron beam is irradiated, the voltage applied to the neutron detector is set to be less than a predetermined value,
A method of measuring nuclear material, wherein after the time period has elapsed, the voltage applied to the neutron detector is set to a predetermined value or higher, and the neutron detector is enabled to measure. 12. The method of measuring nuclear material according to claim 11, further comprising the step of measuring the bremsstrahlung that passes through said measurement object with a transmitted bremsstrahlung detector array. 12. The method of measuring nuclear material according to claim 11, further comprising the step of rotating said object to be measured. Further comprising the step of measuring gamma rays from the measurement object with a gamma ray detector,
during the time period during which the pulsed high-energy electron beam is irradiated, the inter-electrode voltage applied to the photomultiplier tube of the gamma-ray detector is less than a predetermined value;
12. The method of measuring nuclear material according to claim 11, wherein after the time period has elapsed, the inter-electrode voltage applied to the photomultiplier tube is set to a predetermined value or more to allow the measurement by the gamma ray detector. calculating the density distribution of the object to be measured using the intensity distribution data of the transmitted bremsstrahlung radiation measured by the transmitted bremsstrahlung detector array, and calculating the amount of nuclear material contained in the object to be measured from the density distribution; 13. The method of measuring nuclear material according to claim 12, further comprising:

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