Fast neutron detector and fast neutron detection device

Abstract

To provide a fast neutron detector capable of selectively detecting fast neutrons even under a high-temperature environment.SOLUTION: A fast neutron detector 50 according to an embodiment includes: a sealed container 11 having a tubular portion 11a, a first end portion 11b, and a second end portion 11c; a reaction portion 56 provided on an inner surface of the tubular portion 11a and containing a fast-fissionable nuclide; an internal gas 13 sealed in the sealed container 11 and ionized by nuclear fission fragments generated by a nuclear fission reaction of the fast-fissionable nuclide; a central conductor portion 15 accommodated in an internal space 11s of the sealed container 11, electrically insulated from the tubular portion 11a and extending in a longitudinal direction; and a high-temperature thermal neutron shielding portion 57 arranged to cover a radially outer side of the tubular portion 11a and containing a thermal neutron capturing nuclide. The high-temperature thermal neutron shielding portion 57 has a melting point higher than an environmental temperature at a position where the fast neutron detector 50 is installed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention relate to fast neutron detectors and fast neutron detection devices. [Background technology]

[0002] In core monitoring techniques such as estimating the power distribution in a reactor core, it is important to understand the neutron energy spectrum. The energy spectrum of neutrons produced by nuclear fission is an energy spectrum that is mainly in the fast neutron region. Due to elastic or inelastic scattering of fast neutrons in the reactor core, the energy of the fast neutrons decreases, dropping into the epithermal neutron region and even the thermal neutron region. In the thermal neutron region, the distribution approaches that corresponding to the thermal equilibrium state in the reactor. As a result, the neutron energy spectrum becomes a distribution that extends from the fast neutron region to the epithermal neutron region and thermal neutron region.

[0003] When monitoring a reactor core using signals from a neutron detector, it is desirable to selectively detect fast neutrons as soon as possible before information about the reactor core is lost due to scattering of the fast neutrons. In other words, there are cases where it is desirable to selectively detect fast neutrons having energies in the fast neutron region.

[0004] FIG. 12 is a conceptual structural diagram showing an example of the configuration of a fast neutron detector 10 as a first example of the prior art.

[0005] A conventional fast neutron detector (fast neutron detector) 10 has a nuclear fission counter 10a, a neutron moderator 18 arranged to cover the outside of the counter, and a thermal neutron shielding section 17 arranged to cover the outside of the neutron moderator 18.

[0006] The nuclear fission counter 10a has a central conductor portion 15, an internal gas 13, a reaction portion 16, and a housing 11 which is a sealed container that houses these components.

[0007] The housing 11 is a sealed container having a tube portion 11a extending in the longitudinal direction, and a first end portion 11b and a second end portion 11c that close both ends of the tube portion 11a.

[0008] The central conductor 15 functions as an anode. The tubular portion 11a of the housing 11 functions as a cathode. The reaction portion 16 contains fissionable nuclides and is provided on the inner surface of the tubular portion 11a. The reaction portion 16 is formed by coating, plating, or the like.

[0009] Here, as the thermal neutron capturing material of the thermal neutron shielding portion 17, cadmium, for example, has conventionally been used as a nuclide with a large capture cross section for thermal neutrons.

[0010] Conventionally, organic materials such as polyethylene have been used as the neutron moderator material of the neutron moderator section 18, as they contain nuclides with small mass numbers such as hydrogen.

[0011] The cathode material 16a is made of, for example, helium-3 (He-3) or boron-10 (B-10), which has a large cross section for reaction with thermal neutrons.

[0012] With this configuration, thermal neutrons incident on the fast neutron detector 10 are captured by the thermal neutron shielding section 17. Furthermore, the fast neutrons incident on the fast neutron detector 10 pass through the thermal neutron shielding section 17, are moderated by the neutron moderator section 18, and become neutrons in the thermal neutron range (thermal neutrons), which then flow into the housing 11. The thermal neutrons that have flowed into the housing 11 react with the cathode material 16a, which has a large cross section for reaction with thermal neutrons. In this way, the fast neutron detector 10 ensures high detection efficiency of fast neutrons.

[0013] As a second example of conventional technology, a method is used in which measurements are made using a fission counter that uses U-238, which has a large fast fission cross section, and the results are compared with the detection results of a separate detector for detecting thermal neutrons to measure the proportion of fast neutron flux. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Patent No. 5529144 [Non-patent literature]

[0015] [Non-Patent Document 1] Rei Kimura, Tadafumi Sano, Yuki Nakai, Atsushi Sakon, Satoshi Wada, Kunihiro Nakajima, Takashi Kanda, Masaki Goto, Yoshiyuki Takahashi and Cheol Ho Pyeon, “Power Distribution Estimation Method for SMR Using Ex-Core Detectors: Experimental Demonstration by Plural Control Rod Patterns at KUCA”, International Conference on Physics of Reactors 2022 (PHYSOR 2022), Pittsburgh, PA, May 2022 (2022) [Non-patent document 2] Rei Kimura, Yuki Nakai, Satoshi Wada, Atsushi Sakon and Tadafumi Sano, “Demonstration of power distribution estimation using ex-core detectors by reactor experiment at UTR-KINKI”, J. Nucl. Sci. Thecnol, publish online (2022). [Non-patent document 3] JENDL-4.0, Modified on 2023 / 04 / 10, Nuclear Data Center, Japan Atomic Energy Agency. Summary of the Invention [Problem to be solved by the invention]

[0016] As mentioned above, the first example of the prior art fast neutron detection method involves using cadmium or the like to shield thermal neutrons, and then slowing down the fast neutrons that pass through the cadmium using polyethylene or the like to detect them.

[0017] The melting point of cadmium, which is used to shield against thermal neutrons, is around 320°C. On the other hand, in nuclear reactors that operate at high temperatures, such as high-temperature gas reactors and high-temperature microreactors, the environmental temperature at the detector installation location is expected to be as high as around 800-900°C. This poses the problem that conventional methods cannot be used to measure fast neutrons in these high-temperature reactors.

[0018] Furthermore, the fast neutron detection method in the second example of the prior art requires both a fast neutron detector and a thermal neutron detector, and also requires processing to calculate the fast neutron flux and the thermal neutron flux based on the detection signals of the fast neutron detector and the thermal neutron detector, respectively, resulting in a problem of system complexity.

[0019] For these reasons, it is desirable to have a detector that can be used in high-temperature environments and that can selectively detect fast neutrons without using a complex system. In other words, it is desirable to be able to detect fast neutrons while hardly detecting epithermal or thermal neutrons using a single detector.

[0020] An object of the present invention is to provide a fast neutron detector that can selectively detect fast neutrons even in a high-temperature environment. [Means for solving the problem]

[0021] In order to achieve the above-mentioned object, the fast neutron detector of this embodiment is a fast neutron detector comprising: a housing having a tube section extending in the longitudinal direction and a first end section and a second end section closing both ends of the tube section; a reaction section provided on the inner surface of the tube section and containing fast fissionable nuclides; an internal gas sealed in the internal space of the housing and ionized by fission fragments produced by the fission reaction of the fast fissionable nuclides; a central conductor section housed in the internal space of the housing, electrically insulated from the tube section and extending in the longitudinal direction; and a high-temperature thermal neutron shielding section arranged to cover the radial outside of the tube section and containing thermal neutron capture nuclides, wherein the high-temperature thermal neutron shielding section has a melting point higher than the environmental temperature of a position where the fast neutron detector is installed. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a conceptual structural diagram showing the configuration of a fast neutron detector and a fast neutron detection device according to an embodiment. [Figure 2] 1 is a conceptual graph showing an example of a neutron energy spectrum in a nuclear reactor to explain the operation of a fast neutron detector according to an embodiment. [Figure 3] 10 is a conceptual graph for explaining the characteristics of the neutron capture cross section of a thermal neutron capture material used in a high-temperature thermal neutron shielding portion for explaining the operation of the fast neutron detector according to the embodiment. [Figure 4] 1 is a graph showing the neutron capture cross section of rhodium 103 as a thermal neutron capture material used in a high-temperature thermal neutron shielding portion of a fast neutron detector according to an embodiment. [Figure 5] 1 is a graph showing the neutron capture cross section of gadolinium 157 as a thermal neutron capture material used in a high-temperature thermal neutron shielding portion of a fast neutron detector according to an embodiment. [Figure 6] 4 is a conceptual graph showing an example of neutron energy spectra before and after passing through a thermal neutron capture material, for explaining the operation of the fast neutron detector according to the embodiment. [Figure 7]1 is a conceptual graph for explaining the characteristics of the fission cross section of a fast fissionable material for explaining the operation of the fast neutron detector according to the embodiment. [Figure 8] 4 is a graph showing the fission cross section of a first example of a fast fissionable material in a reaction section of a fast neutron detector according to an embodiment. [Figure 9] 10 is a graph showing the fission cross section of a second example of a fast fissionable material in a reaction section of a fast neutron detector according to an embodiment. [Figure 10] 10 is a conceptual graph showing an example of neutron energy spectrum before and after passing through a high-temperature thermal neutron shielding portion for explaining the operation of a conventional fast neutron detector. [Figure 11] 1 is a conceptual graph showing an example of neutron energy spectrum before and after passing through a neutron moderation section, for explaining the operation of a conventional fast neutron detector. [Figure 12] FIG. 1 is a conceptual structural diagram showing an example of the configuration of a fast neutron detector as a first example of the prior art. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a fast neutron detector and a fast neutron detection device according to an embodiment of the present invention will be described with reference to the drawings. Hereinafter, identical or similar parts will be denoted by common reference numerals, and overlapping descriptions will be omitted.

[0024] Here, the fast neutron detector is a neutron detector that can selectively detect fast neutrons inside a nuclear reactor. The nuclear reactor can be applied to any type of reactor, such as a light water reactor or a gas reactor. The fast neutron detector may be located either inside the reactor core (inside the reactor) or outside the reactor core. Here, inside the reactor refers to a location outside the location where the nuclear fuel that makes up the reactor core is located.

[0025] <Configuration explanation> FIG. 1 is a conceptual structural diagram showing the configuration of a fast neutron detector 50 and a fast neutron detection device 60 according to the embodiment.

[0026] The fast neutron detection device 60 includes a fast neutron detector 50, a coaxial cable 21, and a detector circuit 30. The detector circuit 30 includes a lead wire 31, a DC power supply 32, an ammeter 33, and a signal processing unit .

[0027] The fast neutron detector 50 has a nuclear fission counter 50a and a high-temperature thermal neutron shielding portion 57 arranged to cover the outside of the counter.

[0028] The fission counter 50a has a central conductor 15, an internal gas 13, a reaction section 56, and a housing 11 that is a sealed container that houses these components.

[0029] The housing 11 is a sealed container having a longitudinally extending tube portion 11a and conductive portions, a first end portion 11b and a second end portion 11c, which close both ends of the tube portion 11a. The housing 11 accommodates a central conductor portion 15 and an internal gas 13. The internal gas 13 is sealed within the internal space 11s of the housing 11 and is ionized by fission fragments produced by the nuclear fission reaction in the reaction portion 56. The internal gas 13 is a chemically stable gas, such as helium or argon. Hereinafter, the radial direction of the tube portion 11a will be referred to as the radial direction.

[0030] The central conductor portion 15 extends along the longitudinal direction of the tube portion 11a in the internal space 11s of the casing 11, which is a sealed container, and functions as a first electrode. The casing 11 also functions as a second electrode. For example, the first electrode is the anode side, and the second electrode is the cathode side. To electrically insulate the central conductor portion 15 from the casing 11, both ends of the central conductor portion 15 are fixed and supported by insulators 12a and 12b provided on both sides of the casing 11 in the longitudinal direction.

[0031] A coaxial cable 21 having an inner conductor 21a and an outer conductor 21b is attached to the second end 11c. An extension of the central conductor 15 penetrates the insulator 12b and is electrically connected to the inner conductor 21a. The second end 11c is also electrically connected to the outer conductor 21b.

[0032] The reaction section 56 is provided over the entire inner surface of the tubular section 11a. That is, it is provided over the entire circumference and along the longitudinal direction of the tubular section 11a. The reaction section 56 is formed by coating, plating, or the like. The reaction section 56 contains fast fissionable nuclides 56a.

[0033] Here, fast fission nuclides 56a refer to nuclides with a large fission cross section in the fast neutron region. The fast neutron region refers to the highest energy region above a certain energy when the neutron energy region is divided into the thermal neutron region, the epithermal neutron region, and the fast neutron region. Details of these will be explained later with reference to Figure 2.

[0034] A large fission cross section in the fast neutron region refers to either of the following two cases:

[0035] The first case is when, in the neutron energy spectrum of the fission cross section of the nuclide, the fission cross section in the fast neutron region is larger than the fission cross section in the thermal neutron region and the epithermal neutron region.

[0036] The second case is when the fission cross section of the nuclide in the fast neutron region is equal to or greater than the fission cross section of uranium-235 (U-235) in the fast neutron region.

[0037] The high-temperature thermal neutron shielding portion 57 is made of a metal or its compound or alloy that has a high melting point and a large neutron capture cross section in the thermal neutron region, as described below.

[0038] The high-temperature thermal neutron shielding portion 57 has a melting point higher than the environmental temperature at the location where the fast neutron detector 50 is installed. Here, the environmental temperature refers to the temperature of the gas, liquid, or solid when the fast neutron detector 50 is surrounded by a gas, liquid, or solid. For example, when the fast neutron detector 50 is installed in a nuclear reactor that operates at high temperatures, such as a high-temperature gas reactor or a high-temperature microreactor, the environmental temperature at the detector installation location is expected to be as high as about 800 to 900°C. Therefore, the high-temperature thermal neutron shielding portion 57 is made of a material that has a large neutron capture cross section in the thermal neutron region and a melting point higher than such environmental temperature.

[0039] The high-temperature thermal neutron shielding portion 57 can be made of a metal, compound, or alloy of a platinum group element or a lanthanoid element with a large thermal neutron capture cross section. Specifically, for example, rhodium (Rh) or a rhodium alloy can be used as the platinum group element, and gadolinium (Gd) or gadolinia (gadolinium oxide) can be used as the lanthanoid element. An example of a rhodium alloy is platinum-rhodium. Gadolinia is available in ceramic form, for example.

[0040] The melting point of rhodium is above 1900°C. Therefore, it can be used in an environment where it is installed, for example, in a high-temperature gas reactor at about 800°C. Also, for example, platinum-rhodium may be used as an alloy of rhodium. The melting point of platinum is above 1700°C, and so is the melting point of platinum-rhodium.

[0041] The only stable isotope of rhodium is Rh-103. There are also multiple radioactive isotopes, with the nuclide with the longest half-life being Rh-101, but its half-life is only 3.3 years. As will be explained later with reference to Figure 4, the stable isotope Rh-103 has a large neutron absorption cross section in the thermal neutron region, and its nuclear properties make it suitable as a material for the high-temperature thermal neutron shielding part 57.

[0042] The melting point of gadolinium is over 1300°C. Furthermore, the melting point of sintered gadolinium oxide (Gd2O3) is over 2400°C. Therefore, it can be used in an environment where it is installed, for example, in a high-temperature gas reactor at around 800°C.

[0043] The naturally occurring stable isotopes of gadolinium are Gd-154, Gd-155, Gd-156, Gd-157, Gd-158, and Gd-160. Furthermore, trace amounts of Gd-152 exist as a radioactive isotope. As will be explained later with reference to FIG. 5, the stable isotope Gd-157 has a large neutron absorption cross section in the thermal neutron region, and in terms of its nuclear properties, it is suitable as a material for the high-temperature thermal neutron shielding part 57. Here, the isotope abundance ratio of Gd-157 is approximately 15% or more, which can be said to be a sufficiently significant abundance ratio.

[0044] The coaxial cable 21 is connected to a detector circuit 30, and the signal detected by the fast neutron detector 50 is processed as follows.

[0045] That is, the coaxial cable 21 extends from the installation location of the fast neutron detector 50 to the outside as a lead wire 31. A DC voltage is applied between the inner conductor 21a and the outer conductor 21b of the coaxial cable 21 as the lead wire 31 by a DC power supply 32.

[0046] An ammeter 33 is attached to the lead wire 31, and a current signal is extracted. The extracted current signal is sent to a signal processing unit 35. The signal processing unit 35 has, for example, a preamplifier, a pulse height analyzer, etc., and processes the current signal obtained by the fast neutron detector 50.

[0047] <Explanation of the function of fast neutron detectors> 2 is a conceptual graph showing an example of a neutron energy spectrum in a nuclear reactor to explain the operation of the fast neutron detector 50 according to the embodiment. The horizontal axis represents neutron energy (eV), and the vertical axis represents neutron flux Φn (arbitrary scale).

[0048] FIG. 2 is an example of a neutron energy spectrum in the core of a high-temperature gas-cooled reactor. The curve shown by the dashed line indicates the neutron energy spectrum in the fuel. The curve shown by the solid line indicates the neutron energy spectrum outside the fuel in the core. Note that FIG. 2 shows the case of a high-temperature gas-cooled reactor as an example, but is not limited to this. That is, the same action and effect can be achieved even when the fast neutron detector 50 is used in other reactor types, i.e., light water reactors or fast reactors such as sodium-cooled reactors.

[0049] The neutron energy on the horizontal axis is divided into three regions. The three regions are the thermal neutron region, which is less than energy E1; the epithermal neutron region, which is greater than or equal to energy E1 and less than energy E2; and the fast neutron region, which is greater than or equal to energy E2. Here, the values ​​of energy E1 and energy E2 generally have a range, and are not fixed to a single value. In Figure 2, energy E1 is 0.1 eV and energy E2 is 1 x 10 5 eV, i.e., 100 KeV (0.1 MeV). In other words, in the case shown in Figure 2, the fast neutron region is the energy region of 100 KeV or more.

[0050] The neutron energy spectrum in the fuel, shown by the dashed line, has peaks in the fast neutron region and the epithermal neutron region. The epithermal neutron peak is located at an energy close to E1.

[0051] The neutrons in the fuel are reduced by capture by fission products, and the neutron flux outside the fuel is significantly reduced compared to the neutron flux inside the fuel. In addition, neutrons in the fast neutron region inside the fuel (fast neutrons) are slowed down by inelastic scattering with the components of the fuel assembly and fission products, etc., and move to the lower energy side.

[0052] As a result, the neutron energy spectrum outside the fuel, shown by the solid line, has a peak P in the fast neutron region. H In addition, there is a peak P in the thermal neutron region. TIn the example shown in FIG. 2, the peak P H The value of the peak P T This is about one-hundredth or less of the value of , or about two orders of magnitude lower.

[0053] In the following, an example will be described in which the fast neutron detector 50 is installed outside the fuel and inside the core, where the neutron energy spectrum is shown by the solid line. Note that the action in the following description is not limited to the case of the neutron energy spectrum shown by the solid line. That is, even if the neutron energy spectrum at the installation position of the fast neutron detector 50 is, for example, the neutron energy spectrum shown by the dashed line, the same action and effect can be achieved. Alternatively, the same action and effect can be achieved even in the case of an energy spectrum for another reactor type.

[0054] 3 is a conceptual graph illustrating the characteristics of the neutron capture cross section of the thermal neutron capture material used in the high-temperature thermal neutron shielding portion 57 to explain the function of the fast neutron detector 50 according to the embodiment. The horizontal axis represents neutron energy (eV), and the vertical axis represents the neutron capture cross section σc (barns). In the epithermal neutron region, as shown in the resonance region, there are multiple peaks in the cross section of the resonance absorption reaction.

[0055] The neutron energy spectrum of the neutron capture cross section of the thermal neutron capture material used in the high-temperature thermal neutron shielding portion 57 must be such that the neutron capture cross section values ​​in the thermal neutron region and resonance region are sufficiently larger than the neutron capture cross section value in the fast neutron region, as conceptually shown in Figure 3.

[0056] 4 is a graph showing the neutron capture cross section of rhodium-103 (Rh-103) as a thermal neutron capture material used in the high-temperature thermal neutron shielding portion 57 of the fast neutron detector 50 according to the embodiment. Fig. 4 is adapted from a diagram included in JENDL-4.0 (Non-Patent Document 3).

[0057] In Figure 4, the neutron capture cross section σc is shown by the dashed-dotted line, the elastic scattering cross section σes is shown by the dashed line, and the inelastic scattering cross section σies is shown by the dotted line. For ease of identification, the curve for the neutron capture cross section σc is indicated by the symbol C.

[0058] As shown in Figure 4, the neutron capture cross section σc of Rh-103 is 10 in the thermal neutron region, for example, about 0.05 eV. 2 On the other hand, in the fast neutron region, the neutron capture cross section σc of Rh-103 is, for example, 10 5 At 100 KeV (100 KeV), it is about 0.5 barn, and at 1 MeV it is about 0.1 barn. Therefore, the neutron capture cross section σc of Rh-103 in the thermal neutron region is two to three orders of magnitude larger than that in the fast neutron region.

[0059] 5 is a graph showing the neutron capture cross section of gadolinium-157 (Gd-157) as a thermal neutron capture material used in the high-temperature thermal neutron shielding portion 57 of the fast neutron detector 50 according to the embodiment. FIG. 5 also uses a diagram included in JENDL-4.0 (Non-Patent Document 3).

[0060] In Figure 5, the neutron capture cross section σc is shown by the dashed-dotted line, the elastic scattering cross section σes is shown by the dashed line, the inelastic scattering cross section σies is shown by the dotted line, etc. For ease of identification, the curve for the neutron capture cross section σc is indicated by the symbol C.

[0061] As shown in Figure 5, in the thermal neutron region, for example, about 0.07 eV, the neutron capture cross section σc is 10 5 On the other hand, in the fast neutron region, for example, 5 At 100 KeV (100 KeV), it is about 0.5 barn, and at 1 MeV it is about 0.1 barn. Therefore, the neutron capture cross section σc is about four orders of magnitude larger in the thermal neutron region than in the fast neutron region.

[0062] 6 is a conceptual graph illustrating an example of neutron energy spectra before and after passing through the thermal neutron capture material 57, for explaining the operation of the fast neutron detector 50 according to the embodiment. The solid curve shows the spectrum before the neutrons pass through the thermal neutron capture material 57, and the dashed curve in the thermal neutron region shows the spectrum after they pass through.

[0063] The capture of neutrons by the thermal neutron capture material shown in Figures 4 and 5 occurs particularly in the thermal neutron region. As a result, the neutron energy spectrum of neutrons that enter the fast neutron detector 50 after passing through the high-temperature thermal neutron shielding portion 57 hardly changes in the fast neutron region, but in the thermal neutron region, it decreases significantly from the curve shown by the solid line to the curve shown by the dashed line, as shown in Figure 6. Note that in Figure 6, the dashed line indicating the reduced thermal neutron flux is shown with a value larger than the reduced value, for convenience of explanation. As described above, the neutrons that enter the fast neutron detector 50 after passing through the high-temperature thermal neutron shielding portion 57 are almost exclusively fast neutrons.

[0064] FIG. 7 is a conceptual graph for explaining the characteristics of the fission cross section of fast fissionable material to explain the operation of the fast neutron detector 50 according to the embodiment.

[0065] As mentioned above, the fast fission nuclide used in the reaction section 56 has a feature that it has a large fission cross section in the fast neutron region.

[0066] As mentioned above, there are two cases where the fission cross section in the fast neutron region is large:

[0067] The first case is when, in the neutral energy spectrum of the fission cross section of the nuclide, the fission cross section σff in the fast neutron region is larger than the fission cross section σfth in the thermal neutron region and the epithermal neutron region.

[0068] The fission reaction is proportional to the product of the cross section and the neutron flux. Therefore, in this case, the ratio of the neutron flux Φth in the thermal neutron region to the neutron flux Φf in the fast neutron region of the neutron energy spectrum can be tolerated up to a certain level.

[0069] The second case is when the fission cross section σff of the nuclide in the fast neutron region is equal to or greater than the fission cross section of uranium-235 (U-235) in the fast neutron region.

[0070] The fission cross section σff in the fast neutron region must be large to a certain extent. Even if a current output is obtained from nuclear fission in the fast neutron region, if the current output is small, a sufficient S / N ratio cannot be obtained by signal processing alone.

[0071] 8 is a graph showing the fission cross section of a first example of a fast fissionable material in the reaction section 56 of the fast neutron detector 50 according to the embodiment. The horizontal axis represents neutron energy (eV), and the vertical axis represents the reaction cross section (barns).

[0072] F N The curve indicated by F is the cross section of the fission reaction of neptunium-237 (Np-237), the fast fission material in the reaction section 56. U The curves indicated by and F B The curve indicated by F is shown for comparison. U The curve shown is the cross section of the fission reaction of uranium 235 (U-235). B The curve shown is the cross section for the (n,α) reaction of boron-10 (B-10), which is not a fissile material.

[0073] F for the cross section of the (n,α) reaction of B-10 B The curve shown by changes almost linearly in the logarithmic horizontal and vertical axes from the thermal neutron region to the fast neutron region. Specifically, when the neutron energy increases by two orders of magnitude, the reaction cross section decreases by one order of magnitude, i.e., it shows a 1 / v characteristic that is inversely proportional to the neutron velocity v.

[0074] F for the fission cross section of U-235 U The curve shown by shows almost 1 / v characteristics from the thermal neutron region to part of the epithermal neutron region. U The curve indicated by has a fission cross section higher than the 1 / v characteristic from part of the epithermal neutron region to the fast neutron region.

[0075] F for fission cross section of Np-237 N The curve shown by shows almost 1 / v characteristics from the thermal neutron region to part of the epithermal neutron region, but the value is four orders of magnitude lower than the fission cross section of U-235. N The curve shown by is about two orders of magnitude larger in the fast neutron region, and is comparable to the fission cross section of U-235.

[0076] Regarding the neutron energy spectrum of the fission cross section, we have previously discussed two cases: the first case in which the fission cross section σff in the fast neutron region is larger than the fission cross section σfth in the thermal and epithermal neutron regions, and the second case in which the fission cross section σff in the fast neutron region is equal to or larger than the fission cross section of U-235 in the fast neutron region.

[0077] The case of Np-237 shown in FIG. 8 has the characteristics of both the first and second cases.

[0078] Fig. 9 is a graph showing the fission cross section of a second example of a fast fissionable material in the reaction section 56 of the fast neutron detector 50 according to the embodiment. In Fig. 9, uranium-238 (U-238) is shown as the second example. Fig. 9 is adapted from a diagram included in JENDL-4.0 (Non-Patent Document 3).

[0079] In Figure 9, the elastic scattering cross section σes is shown as a dashed line, the inelastic scattering cross section σies is shown as a dotted line, the fission cross section σf is shown as a dashed line, and the neutron capture cross section σc is shown as a dashed line for U-238. For ease of identification, the fission cross section σf curve is indicated by the symbol F.

[0080] The fission cross section σf shown by the dashed-dotted curve F in Figure 9 shows a nearly 1 / v characteristic from the thermal neutron region to part of the epithermal neutron region, but is seven orders of magnitude lower than the fission cross section of U-235. On the other hand, the fission cross section σf shown by the curve F increases to about 1 barn in the fast neutron region, becoming a value similar to that of U-235. Thus, the case of U-238 shown in Figure 9 also has the characteristics of both the first and second cases.

[0081] While Np-237 and U-238 have been exemplified above as fast fissionable materials for the reaction section 56, thorium-232 (Th-232) can also be used. Note that, compared with Np-237 and U-238, the ratio of the fission cross section in the fast neutron region to the fission cross section in the thermal neutron region is smaller for Th-232. Therefore, when using Th-232, the thickness of the high-temperature thermal neutron shielding section 57 must be greater than when using Np-237 or U-238.

[0082] <Summary of the operation of the embodiment> As described above, in the fast neutron detector 50 according to this embodiment, ambient neutrons first enter the high-temperature thermal neutron shielding section 57, and thermal neutrons in the thermal neutron region are captured by the high-temperature thermal neutron shielding section 57. As a result, the neutrons that pass through the high-temperature thermal neutron shielding section 57 and enter the interior of the casing 11 are mostly fast neutrons. The neutrons that enter the interior of the casing 11 reach the reaction section 56 provided within the casing 11. The reaction section 56 contains fast fissionable material with a large fission cross section in the fast neutron region. Therefore, the fast neutrons that enter the reaction section 56 efficiently cause a fission reaction. An electric field is formed between the first electrode and the second electrode within the casing 11 in which the internal gas 13 is sealed. The internal gas 13 is ionized by fission fragments produced by the fission reaction. This ionized gas moves to one of the electrodes in the electric field, causing a current to flow through the coaxial cable 21 and the lead wire 31, which is detected by the ammeter 33.

[0083] Here, the elements constituting the fast neutron detector 50, including the high-temperature thermal neutron shielding portion 57, have melting points higher than the temperature (environmental temperature) of the location where the fast neutron detector 50 is installed. As a result, the fast neutron detector 50 according to this embodiment can maintain its integrity and selectively detect fast neutrons even in a high-temperature environment.

[0084] <Comparison with conventional examples> 10 is a conceptual graph showing an example of neutron energy spectra before and after passing through the thermal neutron shielding portion 17, for explaining the operation of a conventional example of the fast neutron detector 10. The horizontal axis represents neutron energy (eV), and the vertical axis represents neutron flux Φn (arbitrary scale).

[0085] In the conventional example, neutrons at the location where the fast neutron detector 10 is installed first flow into the thermal neutron shielding portion 17. In the thermal neutron shielding portion 17, neutrons in the thermal neutron region in particular are captured by the thermal neutron shielding portion 17, and the thermal neutron flux decreases. As a result, the neutrons that pass through the thermal neutron shielding portion 17 become mainly neutrons in the fast neutron region, as shown in FIG.

[0086] 11 is a conceptual graph showing an example of the neutron energy spectrum before and after passing through the neutron moderation section 18, for explaining the operation of a conventional example of the fast neutron detector 10. The horizontal axis represents neutron energy (eV), and the vertical axis represents neutron flux Φn (arbitrary scale).

[0087] In the neutron moderation section 18, the energy of the fast neutrons is reduced to the energy in the thermal neutron region and epithermal neutron region through repeated elastic scattering and inelastic scattering. In other words, the neutrons that pass through the neutron moderation section 18 and flow into the housing 11 mainly become thermal neutrons and epithermal neutrons.

[0088] The reaction section 16 in the housing 11 contains fissile nuclides such as U-235 that have a large fission cross section in the thermal neutron region, so that thermal and epithermal neutrons efficiently cause nuclear reactions.

[0089] Incidentally, some of the components constituting such a conventional fast neutron detector 10 are made of materials that cannot withstand high temperatures, making it difficult to perform measurements in a high-temperature environment.

[0090] As described above, the fast neutron detector 50 according to this embodiment enables measurement even in high-temperature environments that cannot be measured with the conventional fast neutron detector 10. Furthermore, it enables fast neutron detection using a single detector, without using a complex system such as the fast neutron detection method in the second example of the prior art described in the background art section.

[0091] According to the embodiment described above, it is possible to provide a fast neutron detector 50 that can selectively detect fast neutrons even in a high-temperature environment.

[0092] [Other embodiments] Although the embodiments of the present invention have been described above, they are presented as examples and are not intended to limit the scope of the invention. Furthermore, features of each embodiment may be combined. Furthermore, the embodiments may be implemented in various other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit of the invention. The embodiments and their modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. [Explanation of symbols]

[0093] 10...fast neutron detector, 10a...fission counter tube, 11...casing, 11a...tube section, 11b...first end, 11c...second end, 11s...internal space, 12a, 12b...insulator, 13...internal gas, 15...central conductor section, 16...reaction section, 16a...fissionable nuclide, 17...thermal neutron shielding section, 18...neutron moderator section, 21...coaxial cable, 21a...inner conductor, 21b...outer conductor, 30...detector circuit, 31...lead wire, 32...DC power supply, 33...ammeter, 35...signal processing section, 50...fast neutron detector, 50a...fission counter tube, 56...reaction section, 56a...fast fissionable nuclide, 57...high-temperature thermal neutron shielding section, 60...fast neutron detection device

Claims

1. a housing having a tube portion extending in a longitudinal direction and a first end portion and a second end portion closing both ends of the tube portion; a reaction section provided on the inner surface of the tubular section and containing fast fissionable nuclides; an internal gas sealed in the internal space of the housing and ionized by fission fragments produced by the fission reaction of the fast fissionable nuclides; a central conductor portion that is housed in the internal space of the housing, is electrically insulated from the tubular portion, and extends in the longitudinal direction; a high-temperature thermal neutron shielding section including a thermal neutron capture nuclide and arranged so as to cover the radial outside of the tube section; A fast neutron detector comprising: A fast neutron detector characterized in that the high-temperature thermal neutron shielding portion has a melting point higher than the environmental temperature at a position where the fast neutron detector is installed.

2. 2. The fast neutron detector according to claim 1, wherein the high-temperature thermal neutron shielding portion contains an isotope of rhodium or gadolinium as the thermal neutron capture nuclide.

3. 2. The fast neutron detector according to claim 1, wherein the high-temperature thermal neutron shielding portion is an alloy of platinum and rhodium.

4. 2. The fast neutron detector according to claim 1, wherein the high-temperature thermal neutron shielding portion is made of gadolinium oxide.

5. 5. The fast neutron detector according to claim 1, wherein the fast fissionable nuclide is at least one of neptunium-237, uranium-238, and thorium-232.

6. A fast neutron detector according to claim 1; a coaxial cable attached to the first end and having an outer conductor and an inner conductor; an ammeter for detecting a current flowing through the coaxial cable; a signal processing unit that processes a signal of the current detected by the ammeter; A fast neutron detection device comprising:

7. the central conductor portion is connected to the inner conductor; the second end is connected to the outer conductor; 7. The fast neutron detection device according to claim 6.

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