JP5099700B2 - Nondestructive measuring device detector bank and nondestructive measuring device - Google Patents

Description

  The present invention relates to a non-destructive measurement apparatus that measures the content of fission material contained in waste by a non-destructive method when disposing of waste containing fissile material such as uranium and plutonium. More specifically, the present invention is not limited to a specific waste, and a non-destructive measuring apparatus capable of accurately measuring fissile materials even for waste including high-density metal compacts and neutron poisons, and The present invention relates to a detector bank used in a nondestructive measuring apparatus.

  An active neutron method has been proposed and used as a nondestructive inspection method for radioactive waste solid fissionable materials (see, for example, Patent Document 1). In the method described in Patent Document 1, fast neutrons emitted from a neutron generator tube are scattered in a solid waste that is a measurement object, decelerated to thermal neutrons, and the thermal neutrons are fissile materials in the solid waste. The time-count component of fission neutrons released when fissioning is performed is selectively extracted by fitting by the least square method, the sum of the count values of the components is obtained, and the sum of the count values is discarded as the measurement object This is a method of setting a value indicating the total amount of fissile material incorporated in a solid.

  As a nondestructive measuring apparatus that implements such a nondestructive inspection method, for example, an apparatus including a detector bank incorporating a neutron detector shown in FIG. 11 has been proposed (Patent Document 2). In this detector bank, a plurality of detectors 303 are surrounded by a neutron moderator 302 made of polyethylene, and the entire neutron moderator 302 is covered with a thermal neutron absorber 301 made of cadmium.

  FIG. 12 shows a detection process in the conventional detector bank having such a structure. In the detection system, fast neutrons, epithermal neutrons, and thermal neutrons are mixed depending on the degree of deceleration of fast neutrons. Among them, the thermal neutron 601 is absorbed and separated by the thermal neutron absorber 301 made of cadmium constituting the outer wall of the detector bank, so that epithermal neutrons and fast neutrons enter the detector bank. Epithermal neutrons and fast neutrons are decelerated by a neutron moderator 302 made of polyethylene constituting the detector bank. The neutron detector 303 detects the thermal neutrons 602 caused by epithermal neutrons in the measurement system, the thermal neutrons 603 caused by fast neutrons, the epithermal neutrons 604, and the epithermal which arrived without deceleration. They are neutron 605 and fast neutron 606.

In such a conventional nondestructive measurement method, the target is to maximize the detection efficiency, and the slower the neutron velocity, the higher the detection efficiency. A method of detecting by decelerating to thermal neutrons as efficiently as possible with a neutron moderator is adopted. In this method, fission neutrons and irradiated neutrons, which are fast neutrons mixed in the measurement system, and epithermal neutrons and thermal neutrons in which the fast neutrons are slowed down by the measurement system and the solid waste are detected in the detector bank. It absorbs and separates with a thermal neutron absorber made of cadmium constituting the outer wall, and decelerates fast neutrons (fission neutrons and irradiated neutrons) and epithermal neutrons that have passed therethrough with a neutron moderator made of polyethylene surrounding the neutron detector. Thus, thermal neutrons are used to detect fast neutrons efficiently. In the irradiated neutron spectrum component, neutrons caused by epithermal neutrons in the neutron detector are dominant, and as a result, the object to be measured is high-density metal compression waste or neutron poison (thermal neutron absorbing material; Gd). If included, the fission neutron time spectrum is buried in the irradiation neutron time spectrum, and even if fission neutrons are generated, it becomes impossible to separate and extract fission neutron components by spectral analysis, so only specific waste is accurately measured. There was a problem that it was not possible (see FIG. 9).
JP-A-11-64528 Japanese Patent No. 3845685

  Therefore, the object of the present invention is not limited to specific waste, and it is possible to accurately measure fissile materials in various wastes and objects such as high-density metal compacts and wastes containing neutron poisons. The object is to provide a destructive measuring device and a detector bank for use in the device.

As a result of intensive studies to solve the above problems, the present inventors have found that the above object can be achieved by improving the structure of the neutron detector bank, and have completed the present invention.
That is, the present invention is a detector bank used in a non-destructive measurement device for radioactive waste solid fissile material, a neutron detector, and a thermal neutron absorber made of cadmium surrounding the neutron detector, A neutron moderator made of polyethylene surrounding the thermal neutron absorber, a thermal neutron absorber and an epithermal neutron absorber made of a boron-containing material surrounding the neutron moderator, and the thermal neutron and epithermal neutron absorber. A detector bank comprising a surrounding structural member.

The boron-containing material is preferably boron or boron carbide.
The structural material is preferably made of iron or an iron alloy, particularly stainless steel.

  Further, according to the present invention, the detector bank of the present invention and the neutron generator tube are arranged at opposing positions around the radioactive waste solid to be inspected inside the neutron reflector, A non-destructive measuring device for radioactive waste solid fissile material is provided.

  According to the nondestructive measuring apparatus for fissile material comprising the detector bank of the present invention, various kinds of waste such as high density metal compacts and waste containing neutron poisons are not limited to specific wastes. Can accurately measure fissile material in objects.

  In more detail, the nondestructive measuring apparatus of the present invention can selectively detect epithermal neutrons caused by fast neutrons in the neutron detector and fast neutrons reached without decelerating. It is possible to measure wastes containing thermal neutron absorbing substances (neutron poisons) such as Gd and the like, which have been impossible to measure and detect.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
Here, FIG. 1 is a schematic view showing an embodiment of the nondestructive measuring apparatus of the present invention. FIG. 2 is an enlarged perspective plan view showing the internal structure of the neutron detector bank of the present invention shown in FIG. 1, and FIG. 3 schematically shows a neutron detection process in the neutron detector bank of the present invention shown in FIG. It is a schematic diagram shown in FIG.

The nondestructive measuring apparatus 1 shown in FIG. 1 is a nondestructive measuring apparatus for the amount of fissile material formed by surrounding fast neutron generating tubes 102a and 102b and a detector bank 103 with a built-in neutron detector with a neutron reflector 101. The radioactive waste solid 104 is positioned at the center of the nondestructive measuring apparatus 1. The fast neutron generator tubes 102a and 102b and the detector bank 103 with a built-in neutron detector are arranged at positions facing each other around the radioactive waste solid.

Specifically, SUS304 is used as the neutron reflector 101 surrounding the measurement space 100 having a square cross section. Neutron generating tubes 102a and 102b are arranged at two corners of the measurement space 100 (two corners on the upper side in FIG. 1), respectively. Further, 30 neutron detectors are distributed and arranged in three neutron detector banks 103 on the lower side of FIG. In addition, a metallic compression-solidified waste body 104 to be inspected is disposed in the central portion of the measurement space 100 in a state where 100 mg of ²³⁹ Pu105 is disposed in the central portion. The entire structure of the nondestructive measuring apparatus (a structure excluding the neutron detector bank 103 ) is the same as the apparatus described in Japanese Patent No. 3845685, and detailed description thereof is omitted.

As shown in FIG. 2, the neutron detector bank 103, which is a characteristic part of the apparatus of the present invention, includes a neutron emitter 205 having a plurality of neutron emitters arranged at a predetermined interval, and a cadmium surrounding the neutron detector 205. a thermal neutron absorber 204 made of a neutron moderator 203 made of polyethylene surrounding the thermal neutron absorber 204, the thermal neutrons and epithermal neutron absorbers 202 consisting of boron-containing material surrounding the neutron moderator 203 And a structural member 201 surrounding the thermal neutron and epithermal neutron absorber 202.

In the present embodiment, the SUS304 as a structural material 201, a boron carbide _(B 4 C) as thermal neutrons and epithermal neutron absorbers 202, polyethylene neutron moderator 203, using cadmium as thermal neutron absorber 204 Yes. Can be preferably used those containing ¹⁰ B 95% as boron carbide, polyethylene as can be preferably used a general high-density polyethylene, its density is 0.98 g / cm ^3, the thickness of 12mm Is optimal. In addition, a clearance space for inserting the detector is provided between the thermal neutron absorber 204 and the detector 205.

In the neutron detector of the present embodiment, the thickness of the structural member 201 is 4 mm, thermal neutrons and epithermal neutron absorbers 202 3 mm in _{B 4} C thickness containing ¹⁰ B 95% ^{of, 10} B natural (19 .6%) B ₄ C is 10 mm, and the thickness of the neutron moderator 203 (thickness in the narrowest part between the neutron absorber 202 and the thermal neutron absorber 204) is 12 mm, and thermal neutron absorption The thickness of the body 204 is 1 mm. The distance between the thermal neutron absorber 204 and the detector 205 is 1 mm, and the diameter of the detector 205 is 25.4 mm. Although these thicknesses are not limited, for example, when boron carbide containing 95% of ¹⁰ B is extremely thick as 1 cm or 2 cm as the thermal neutron and epithermal neutron absorber 202, the detection sensitivity is very poor. It may be impossible to detect.

The detection process by the neutron detector bank 103 of this embodiment is shown in FIG. As shown in FIG. 3, thermal neutrons 701 and epithermal neutrons 702 from outside the neutron detector bank 103 are absorbed by the neutron absorber 202 made of boron carbide (B ₄ C). Fast neutrons that have entered the detector bank are decelerated by a neutron moderator 203 made of polyethylene, and differentiated into thermal neutrons, epithermal neutrons, and fast neutrons that have not been decelerated. The thermal neutrons 703 whose fast neutrons are decelerated are absorbed by the thermal neutron absorber 204 made of cadmium disposed around the neutron detector 205. Therefore, what is finally detected by the neutron detector 205 is epithermal neutrons 704 caused by fast neutrons in the measurement system and fast neutrons 705 that arrive without deceleration.

  The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

EXAMPLES Hereinafter, although this invention is demonstrated further more concretely based on an Example and a comparative example, this invention is not restrict | limited to these.
[Example 1 and Comparative Example 1]
The Monte Carlo simulation calculation for confirming the effectiveness was performed using the nondestructive measuring apparatus 1 including the neutron detector bank 103 shown in FIGS. 1 and 2. The calculation is performed by generating approximately 10 billion particles of 14 MeV fast neutrons from the neutron generator tubes 102a and 102b, and performing a calculation with a time-dependent function for the neutrons detected by all the neutron detectors 205. Exactly the same time-dependent calculation data was obtained.

  The result is shown in FIG. FIG. 4 shows a result obtained by fitting a result obtained by calculation into an irradiation neutron component 401 and a fission neutron counting component 402 by the least square method and separating them. FIG. 5 shows the results obtained by using the conventional neutron detector bank shown in FIGS. 11 and 12 for comparison.

From the results shown in FIG. 5, it can be seen that in the apparatus having the conventional neutron detector bank shown in FIG. 11 , the fission neutron counting component 502 is buried in the irradiated neutron component 501 and cannot be confirmed. On the other hand, since the apparatus of the present invention has the neutron detector bank shown in FIG. 2, as shown in FIG. 4, the time for the irradiation neutron component to completely disappear is changed from 0.82 msec to 0.12 msec. As a result, the counting component of the fission neutron 502 to be measured, which was hidden behind the irradiated neutron component and could not be confirmed by the conventional apparatus, is indicated by reference numeral 402 in FIG. It can be seen that the detection sensitivity and the detection limit value can be dramatically improved.

  FIG. 6 shows time-dependent data according to neutron energy in the neutron moderator region in the conventional neutron detector bank shown in FIG. 12 as a comparison object. Similarly, FIG. 7 shows the time-dependent data for each neutron energy in the neutron moderator region in the neutron detector bank in the apparatus of the present invention divided by neutron energy.

  Comparing both, in FIG. 6, the annihilation time of the irradiated neutrons is the epithermal neutron 801 caused by epithermal neutrons or fast neutrons in the measurement system and epithermal neutrons entered in the measurement system, or epithesis caused by fast neutrons. Thermal neutron 802 is dominant.

On the other hand, in FIG. 7, the annihilation time of irradiated neutrons is determined by absorbing and separating the thermal neutrons and epithermal neutrons in the measurement system with boron carbide (B ₄ C), and the thermal neutrons 901 caused by the fast neutrons in the measurement system. Epithermal neutrons become dominant, and the time until the irradiated neutrons disappear is 0.72 ms to 0.3 ms, which is about 1/2 of that of a conventional neutron detector bank.

Further, FIG. 8 shows the time-dependent data for each neutron energy in the detector region of the neutron detector bank shown in FIG.
Comparing FIG. 7 with FIG. 8, thermal neutrons caused by fast neutrons in the measurement system that were dominant in the annihilation time of irradiated neutrons in the neutron moderator region of FIG. In the extinction time of the irradiated neutrons, the epithermal neutrons 1001 that have been absorbed and separated by the action of the thermal neutron absorber 204 made of cadmium and have entered the detector region without decelerating due to the fast neutrons 1001 in the measurement system. As a result, the extinction time is reduced from 0.82 msec to 0.12 msec to about 1/7.

From these results, it can be seen that the apparatus of the present invention can be measured more accurately.
[Example 3 and Comparative Example 3]
FIG. 9 shows the result of an experiment conducted in the same manner as in Example 1 on a cement solidified waste mixed with 200 g of Gd using a conventional neutron detector bank. In this case, the fission neutron counting component 2002 is completely hidden behind the interrogation neutron component 2001 from the neutron generating tube, and measurement is impossible, as in the case of measurement of a highly compressed metal waste.

  In contrast, the apparatus of the present invention uses the neutron detector bank having the configuration shown in FIG. 2, so even when applied to a waste containing Gd, as shown in FIG. Like the body, the fission neutron counting component 3002 was separable without being hidden by the interrogation neutron 3001.

  From this result, it can be seen that according to the apparatus of the present invention, it is possible to detect the fission material present in the waste containing the neutron poison, which is impossible with the conventional apparatus.

FIG. 1 is a schematic view showing an embodiment of the nondestructive measuring apparatus of the present invention. FIG. 2 is an enlarged perspective plan view showing the internal structure of the neutron detector bank of the present invention used in the nondestructive measuring apparatus shown in FIG. FIG. 3 is a schematic diagram schematically showing a neutron detection process in the neutron detector bank of the present invention shown in FIG. FIG. 4 is a diagram showing time-dependent measurement data obtained when a metal waste drum having a density of 4.0 g / cm ³ is measured by Monte Carlo simulation calculation using the new detector bank of the present invention. FIG. 5 is a diagram showing time-dependent measurement data obtained when a metal waste drum having a density of 4.0 g / cm ³ is measured by Monte Carlo simulation calculation using a conventional detector bank. FIG. 6 is a diagram showing time-dependent data according to neutron energy in the neutron moderator region in the conventional detector bank. FIG. 7 is a diagram showing time-dependent data according to neutron energy in the neutron moderator region in the new detector bank of the present invention. FIG. 8 is a diagram showing time-dependent data according to neutron energy in the He-3 detector region in the new detector bank of the present invention. FIG. 9 is a diagram showing time-dependent measurement data obtained when a cement-solidified waste drum in which 200 g of Gd is uniformly mixed is measured by Monte Carlo simulation calculation using a conventional detector bank. FIG. 10 is a diagram showing time-dependent measurement data obtained when a cement solidified waste drum in which 200 g of Gd is uniformly mixed is measured by Monte Carlo simulation calculation using the new detector bank of the present invention. FIG. 11 is a schematic diagram of a conventional detector bank. FIG. 12 is a schematic diagram showing the neutron separation and detection process in the conventional detector shown in FIG.

Explanation of symbols

101: Neutron reflector 102a: Neutron generator tube 102b: Neutron generator tube 103: Neutron detector bank 104: Radioactive waste 105: Nuclear material 201: Structural material (iron or SUS)
202: Absorber of thermal and epithermal neutrons (boron carbide)
203: Neutron moderator (polyethylene)
204: Thermal neutron absorber (cadmium)
205: Neutron detector
301: Thermal neutron absorber made of cadmium
302: Neutron moderator made of polyethylene
303: Neutron detector
501: Irradiation neutron component
502: Fission neutron counting component
701: Thermal Neutron
702: Epithermal neutron
703: Thermal neutrons decelerated by fast neutrons
704: Epithermal neutrons caused by fast neutrons
705: Fast neutrons reached without deceleration
801: Thermal neutrons caused by fast neutrons
802: Epithermal neutrons caused by fast neutrons
901: Thermal neutrons caused by fast neutrons
1001: Epithermal neutrons caused by fast neutrons
1002: Fast neutrons that have entered the detector region without slowing down
2001: Questioning neutron components from neutron generator tubes
2002: Fission neutron counting component
3001: Interrogation Neutron
3002: Fission neutron counting component

Claims (2)

Non-destructive measurement of fissile material in radioactive waste solids, with the detector bank and the neutron generator tube positioned opposite each other, centered on the radioactive waste solid to be inspected, inside the neutron reflector A device, wherein the detector bank is
A neutron detector;
A thermal neutron absorber made of cadmium completely surrounding the neutron detector;
A neutron moderator made of polyethylene that completely surrounds the thermal neutron absorber;
Thermal and epithermal neutron absorbers composed of boron-containing materials that completely surround the neutron moderator,
And a structural material surrounding the thermal neutron and epithermal neutron absorber ,
A non-destructive measuring device that selectively detects epithermal neutrons caused by fast neutrons in neutron detectors and fast neutrons that arrive without deceleration . The non-destructive measuring apparatus according to claim 1, wherein the boron-containing substance is boron or boron carbide.