Neutron detector comprising optical fission chamber with reflective optical cavity and detection head coupled to optical fission chamber comprising optical lens and optical fiber coupler

Abstract

To provide a neutron detector comprising an optical fission chamber with a reflective optical cavity, and a detection head coupled to the optical fission chamber comprising an optical lens and an optical fiber coupler.SOLUTION: The invention relates to a neutron detector (1) comprising at least one ionization chamber (2) sealed tight and referred to as an OFC, which is an acronym of an optical fission chamber. Each chamber executes optical transduction and comprises an optical cavity. The optical cavity operates on the basis of the optical transduction. A window and an optical lens as an optical interface are built in at one end of the optical cavity in its longitudinal direction. A mirror for reflecting photons traveling in a direction away from the window is built in at the other end of the optical cavity in its longitudinal direction.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates in particular to the field of measuring instruments for nuclear fission reactors and nuclear fusion reactors.

[0002] More particularly, the present invention relates to a nuclear fission ionization chamber neutron detector, and more particularly to a so-called optical nuclear fission ionization chamber, i.e., a nuclear fission ionization chamber that converts a neutron signal into an optical signal.

[0003] An object of the present invention is to provide such an optical nuclear fission ionization chamber with an improved signal-to-noise ratio for neutron flux measurement.

Background Art

[0004] The operation of a nuclear reactor involves strict requirements regarding the tracking of multiple operating parameters, regardless of whether the purpose is power generation or research.

[0005] Among these, the thermal output generated is one of the important parameters. This is directly correlated with the neutron flux near or inside the vessel. Therefore, an increase in the neutron flux results in an increase in the power level of the nuclear reactor.

[0006] There are various techniques for measuring neutron flux, which are classified under the umbrella of neutron detectors.

[0007] A nuclear reactor may have multiple neutron detectors installed.

[0008] These detectors can be classified into two categories: active detectors, i.e., detectors that require a bias voltage in the detection zone to collect and transmit information related to the detection of neutrons, and passive detectors that do not require a bias voltage in the detection zone.

[0009] To perform neutron measurements inside a nuclear reactor, the commonly used active neutron detectors are nuclear fission ionization chambers or boron deposition chambers, and the nuclear fission ionization chambers or boron deposition chambers operate based on the principle of converting a neutron flux into an electrical signal conventionally. This electrical conversion is carried out by a pair of electrodes biased at several hundred volts.

[0010] Figure 1 shows a nuclear fission ionization chamber or a boron deposition chamber 1. This chamber or box 1 is defined by a conductive hollow cylinder 10 with its ends hermetically closed, and this conductive hollow cylinder 10 forms the cathode. On the central axis X of this conductive hollow cylinder 10, a smaller-diameter cylinder 11 is positioned, and this cylinder 11 forms the anode. The periphery of the anode 11 is coated with a deposit 12 of a material that strongly interacts with neutrons, such as uranium 235 or boron 10 for example. The chamber or box 1 is further filled with a gas. When the deposit 12 interacts with neutrons, heavy charged particles are released into the gas of the chamber or box, and thereby the energy of these particles is transferred to the gas. These particles are decelerated by the ionization of the atoms of the gas filled in the chamber or box. Under the influence of an electric field generated by applying a bias to the chamber or box, electrons move in the direction of the anode 11, that is, in the direction of the positive potential of the chamber or box. An electrical signal is generated by this charge collection, and the measured value of the neutron flux can be estimated from this electrical signal.

[0011] Therefore, in this type of nuclear fission ionization chamber 1, in a situation where the bias of the chamber is accidentally lost, the measurement of the neutron flux becomes impossible.

[0012] Also, when measuring in a nuclear reactor, the only passive detector currently used in the industry is a type of detector called a collectron or a self-powered neutron detector (SPND). In a collectron, an electron-emitting substance (β decay) generates an electric current, which is transmitted via a cable to a measuring device. Certain types of collectrons require stabilization of the emitter, which can take up to 30 minutes, but there remains the problem of transmitting low currents over long distances. Specifically, the same is true for the case of a fission ionization chamber, where the signal generally has to be transmitted through zones where electromagnetic interference (caused by pumps, magnets, motors, etc.) is likely to occur.

[0013] To obtain a usable signal, electromagnetic shielding of the transmission line is required. This means using bulky high-resistant cables, which creates space constraints.

[0014] Therefore, for any reason, whether safety or space related, the neutron detectors currently used, especially for measuring neutron flux inside a nuclear reactor, are insufficient.

[0015] New methods corresponding to the elimination of electrical conversion have been proposed. See, for example, publications [1] - [5] and patent FR3125135B1. In principle, this new method performs optical conversion by collecting photons generated inside a box where ionization occurs (hereinafter referred to as an ionization chamber).

[0016] These fission ionization chambers are called OFC, which is the acronym for optical fission chamber. Since they are passive detectors, it is possible to solve the problem of dependence on a power source and the problem of bulkiness of the electrical cables for signal transmission mentioned above.

[0017] Therefore, the OFC converts neutron signals into optical signals. Specifically, when the gas is ionized by heavy ions generated by the reaction between neutrons and active substances such as boron and uranium, an electron shower is generated, resulting in excitation and subsequent de-excitation in a wide spectral range from ultraviolet to mid-infrared. In Figure 2, this effect is schematically shown.

[0018] Next, the light emission thus generated is collected by an optical fiber designed to have resistance to radiation, thereby effectively limiting the spectral region to be used. Specifically, silica optical fibers with a pure SiO2 core are excellent in resistance to radiation and very little attenuate optical signals with near-infrared wavelengths typically in the range of 800 to 1000 nm, typically several dB / km. See [5].

[0019] Regarding the conversion of the optical signal into an electrical signal, it is carried out outside the reactor vessel by one or more transducers such as, for example, a photodiode, a silicon-based photomultiplier tube, or a camera.

[0020] Therefore, these one or more transducers or detection modules convert the optical signal into an electrical pulse, and the measured value of the neutron flux is estimated from this electrical pulse. All components of the OFC-based measurement system will be described later.

[0021] In the following Table 1, the fission ionization chamber and the prior art optical fission ionization chamber (OFC) can be compared according to various criteria.

[0022]

Table 1

[0023] It is clear from Table 1 that improving the measurement accuracy of the neutron flux is the key to unlocking the potential of the OFC (optical fission ionization chamber).

[0024] An important feature of the OFC is the signal-to-noise ratio. The higher this ratio, the higher the measurement accuracy.

[0025] Regarding the measurement of neutron flux by an optical fission ionization chamber in a nuclear reactor, the mechanisms causing the signal and noise intensities have been investigated by various studies.

[0026] Regarding the signal intensity, when a fission ionization chamber of a given shape is provided, the photon collection efficiency, that is, the value obtained by dividing the number of photons collected by the fiber by the number of de-excited photons emitted, is obtained by Equation 1 below.

[0027]

Equation

[0028] Here, - r chamber is the inner diameter of the chamber, - r fibre is the radius of the fiber, - θ max is the maximum angle at which light can be transmitted to the fiber, - g(θ) and f(r) represent the angular distribution and radial distribution (cm 2 ) calculated for the surface located on the opposite side of the fiber, respectively.

[0029] Since the function f(r) is substantially constant over the radius of the fiber, the collection efficiency is proportional to the square of the radius of the fiber. Based on this principle, increasing the radius of the fiber to increase the collection efficiency and thus the accuracy of the OFC has been proposed by multiple authors ([6], [7]).

[0030] In practice, since the noise is also proportional to the radius of the fiber, this solution of increasing the radius of the fiber becomes insufficient due to the γ-ray flux present during the measurement in the nuclear reactor.

[0031] Specifically, ionization is induced in the fiber by the γ-rays of the nuclear reactor, causing electrons to be emitted at a speed faster than c / n. Here, c is the speed of light, i.e., the propagation speed of electromagnetic waves, and n is the refractive index of the fiber.

[0032] The movement of these electrons in the fiber generates the emission of visible electromagnetic radiation called Cherenkov radiation. Since this parasitic light is superimposed on the collected optical signal, the noise increases and the signal-to-noise ratio decreases significantly.

[0033] According to the research and measurements conducted by the present inventor, it has been found that this Cherenkov radiation is the main noise source of the optical fission ionization chamber (OFC).

[0034] Specifically, the Frank-Tamm formula shows that, as a first approximation, the intensity of Cherenkov radiation changes according to Equation 2 below.

[0035]

Equation

[0036] Here, - r fibre is the diameter of the fiber, - L irrad is the length of the irradiated fiber.

[0037] In the measurement of the nuclear reactor where the optical fiber is exposed to ionizing radiation and both the signal and the noise are proportional to the square of the radius of the fiber, the signal-to-noise ratio does not depend on the radius of the fiber.

[0038] Nevertheless, in order to increase the signal-to-noise ratio of the optical fission ionization chamber, the authors of publication [6] propose compensating for the efficiency reduction caused by the reduction of the solid angle by using a reflective mirror for a short optical cavity and a lens for a long optical cavity.

[0039] Furthermore, FIG. 9 of [6] proposes to increase the collection efficiency by using an optical fiber with a higher numerical aperture.

[0040] In considering the Cherenkov effect in an approach for optimizing the signal-to-noise ratio, the inventor believes that this solution is not appropriate. Specifically, when following the proposal of [6], the proportion of Cherenkov photons guided by the optical fiber also increases. Refer to FIG. 3 cited from publication [8].

[0041] As a result, none of the methods proposed to date for improving the measurement accuracy of a neutron beam by an optical fission ionization chamber (OFC) are promising in that they increase the signal intensity but also increase the optical noise or Cherenkov noise at the same time.

Prior Art Documents

Patent Documents

[0042]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0043] Therefore, it is necessary to achieve either increasing the intensity of the measurement signal without increasing the noise or increasing the intensity of the measurement signal and reducing the noise by improving an optical fission ionization chamber (OFC) capable of measuring a neutron beam, particularly in a nuclear reactor.

[0044] An object of the present invention is to at least partially satisfy this need.

Means for Solving the Problems

[0045] To achieve this object, one subject of the present invention is a neutron detector comprising the following, namely At least one sealed ionization chamber in which optical conversion takes place, called an optical fission ionization chamber (OFC), each chamber extending along a longitudinal axis (X), each chamber A sealed hollow body that defines an optical cavity therein and has at least one inner wall at least partially coated with at least one fissionable material layer, the optical cavity being filled with a gas, the gas preferably being under pressure and being ionizable by ions resulting from the reaction between neutrons and the fissionable material, the sealed hollow body A window disposed at one of the longitudinal ends of the hollow body and configured to seal the optical cavity An optical lens fixed to the window by adhesion or integrally formed with the window and configured to focus photons received by the window, and A mirror disposed at the other longitudinal end, opposite the longitudinal end of the hollow body where the window is disposed, the mirror being configured to reflect photons towards the window Comprising at least one sealed ionization chamber A detection head comprising an optical fiber coupler, the optical fiber coupler having a plurality of optical fibers as inputs, the plurality of optical fibers being disposed in the focal plane of the lens, and the optical fiber coupler having a single optical fiber as an output, the optical signals received by the input being combined in the single optical fiber, the detector head Comprising a neutron detector.

[0046] Advantageously, the OFC is axially symmetric in shape and has a central axis (X). Thus, the hollow body of the OFC is preferably a cylinder, a sphere, or a cone.

[0047] Advantageously, the surface density ρmax of the fissionable material is 2 mg / cm 2The following applies. The surface area of the hollow body increases as the mass of the fissile material to be deposited increases. To maximize the energy imparted to the gas, the diameter of the hollow body of the box is preferably at least equal to the path length of the fission fragments. However, this value depends on the filling gas and its pressure P. Therefore, for a given mass of fissile material and a given gas pressure in the optical cavity, it is advantageous for the dimensions R and H of the hollow body to be the dimensions estimated from Equation 3 below.

[0048] [Number]

[0049] where - R is the lateral dimension of the hollow body, i.e., the radius in the case of a cylinder, - H is the axial dimension of the hollow body, i.e., the length in the case of a cylinder, - Range(P) is the distance that fission fragments can travel through a given gas at pressure P, - m fissile is the mass of the fissile deposit, - ρ max is the surface density of the fissile material.

[0050] Advantageously, the fissile material is selected from boron 10, lithium 7, any isotope of uranium 238, plutonium, and neptunium

[0051] Preferably, the material for making the window is silica-based. The silica window has high transmission efficiency and good radiation resistance. The thickness e determines the ability to withstand the internal pressure of the gas P, as shown by Equation 4 below.

[0052] [Number]

[0053] where E SiO2is the Young's modulus of silica, and R is the radius of the window.

[0054] The radius R of the window is substantially equal to the radius of the hollow body.

[0055] According to an advantageous variant of the embodiment, the optical lens is a Fresnel lens. The advantage of using a Fresnel lens is that the mass of the material, especially silica, which is required to be used for manufacturing the lens, is reduced, and thus the intensity of the Cherenkov radiation emitted from the optical components (window and lens) is reduced. In an optical fiber with a numerical aperture NA, the radius R of the lens is the radius of the window, and the thickness of the lens is a characteristic given by the manufacturer.

[0056] Advantageously, the focal length f' of the lens satisfies the following equation 5.

[0057]

Equation

[0058] Preferably, advantageously, the gas filling the optical cavity is a noble gas selected from helium, neon, argon, krypton, xenon, or a mixture thereof.

[0059] The optical cavity is preferably under pressure, typically under a pressure of several bar.

[0060] According to an advantageous embodiment, this detector comprises at least one spacer arranged between the window and the hollow body and / or between the mirror and the hollow body. Each spacer serves as a physical separator between the deposit of the fissionable material and one of the optical components (mirror or window), thereby preventing these optical components from becoming cloudy prematurely as a result of the collision of the fission fragments.

[0061] Preferably, the axial dimension of this spacer is greater than or equal to the path length of the light fission fragments (LFFs) passing through the gas in the optical cavity or the ions with the maximum range in the gas. LFFs are emitted following the interaction of neutrons with the atoms of the fissile deposit. Two fission fragments are emitted by fission, but only the light fission fragments are considered when setting the dimensions. In the case of the fission of uranium 235, an example of an LFF can be a particle with atomic mass A = 95 and initial kinetic energy E = 100 MeV.

[0062] Specifically, the path length of the LFF is always longer than that of other fission products. To increase the collection efficiency, preferably, the inner surface of the spacer is polished to enable light reflection.

[0063] The radius of the spacer is equal to the radius of the hollow body of the OFC.

[0064] Table 2 below shows the path lengths of LFFs in an optical cavity filled with various noble gases at pressures of 1 bar and 5 bar.

[0065]

Table 2

[0066] It is clear from Table 2 that it is preferable to use a gas with a higher atomic number to reduce the height of the spacer and improve the collection efficiency of the OFC.

[0067] According to a variant of the advantageous configuration, the detection head is fixed, preferably screwed, to the hollow body of the box. The neutron detector is a single, compact, and easy-to-handle object.

[0068] Accordingly, the present invention consists essentially of a sealed optical fission ionization chamber (OFC) and a neutron detector comprising an optical cavity, which optical cavity operates based on light conversion, a window as an optical interface and an optical lens being incorporated at one longitudinal end thereof, and a mirror for reflecting photons moving away from the window being incorporated at the other longitudinal end thereof.

[0069] An optical fiber coupler is appropriately disposed in the focal plane of the lens in order to expand the collection area of the emission signal from the exit of the lens without accompanying a permanent increase in the volume of the optical fiber under irradiation which becomes a noise source as a result of Cherenkov radiation.

[0070] Preferably, the deposition of the reflective layer forming the mirror is adjusted according to the measurement wavelength. Specifically, the light reflection coefficient depends on the wavelength of the incident photons. Further, the most common substrate material, NBK-7, must be avoided. Specifically, this NBK-7 contains B10, and B10 causes premature degradation of the mirror by the (n,α) reaction ([9]). For the reflective layer, materials such as silver and gold are preferred because their reflectance is high and remains constant over a wide range of wavelengths and particularly in the near infrared.

[0071] Since the window and the optical lens are joined by adhesion or integrally formed as a single part, these two optical components are subject to mechanical stress together. Therefore, depending on the thickness of the lens, it is possible to apply a part of the mechanical stress to the lens and make the window thinner. By doing so, the mass of the irradiated silica is reduced, and thereby the noise caused by Cherenkov radiation emitted in these optical components can be reduced.

[0072] The neutron detector with minimized dimensions according to the present invention can be configured to withstand a high-temperature and high-radiation environment such as that found inside an operating nuclear reactor.

[0073] Finally, the detector according to the present invention enables the realization of an excellent signal-to-noise ratio when measuring a neutron flux.

[0074] The present invention has numerous applications, among which the following can be mentioned. - Online measurement of neutron flux in a nuclear reactor. - Characterization and monitoring of neutron flux at locations other than nuclear reactors (experimental reactors or power plants). - Location determination of molten fuel elements during or after a major incident (loss of cooling, and possibly temporary loss of power). - Location determination of the encapsulation stopper of colloidal plutonium, particularly in a chemical treatment process. - Neutron measurements on a neutron beam for the purpose of beam stability, or neutron measurements for the purpose of measuring the time of flight on these same lines.

[0075] Other advantages and features will become clearer by reading the following detailed but non - limiting description given by way of example with reference to the following drawings.

Brief Description of the Drawings

[0076]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0077] FIGS. 1 and 2 have already been described in the preamble. Therefore, they will not be described hereinafter.

[0078] FIG. 3 shows a neutron detector 1 according to the present invention.

[0079] First, the neutron detector 1 includes a sealed ionization chamber 2 in which optical conversion is carried out. This chamber is called an OFC, which is the acronym for an optical nuclear fission ionization chamber, and extends around the central axis (X).

[0080] This OFC 2 includes a sealed hollow cylinder 20 having a length H and a diameter φ corresponding to 2R, and defines an optical cavity 21 inside it.

[0081] The inner wall 22 of the body is coated with a layer of fissionable material such as uranium 235, uranium 238, or boron 10.

[0082] The optical cavity 21 is filled with at least one noble gas, which is preferably placed under pressure P and can be ionized by ions resulting from the reaction of neutrons with the fissionable material. This gas can be selected from helium, neon, argon, krypton, xenon, or mixtures thereof.

[0083] The window 23 is arranged at one of the longitudinal ends of the hollow body 20 and is configured to seal the optical cavity. Further, a spacer 26 is arranged between the window 23 and the hollow body 20 to prevent the window 23 from clouding prematurely as a result of the impact of fission fragments.

[0084] The optical lens 24, preferably a Fresnel lens, is fixed to the window by adhesion or integrally formed with the window and is configured to focus the photons received by the window 23.

[0085] The mirror 25 is arranged at the other longitudinal end, which is the end opposite to the longitudinal end of the hollow body where the window is arranged. This mirror 25 is configured to reflect photons towards the window. Further, a spacer 27 is arranged between the mirror 25 and the hollow body 20 to prevent the mirror 25 from clouding prematurely as a result of the impact of fission fragments. The spacers 26 and 27 have a diameter φ equal to the diameter of the hollow cylinder 20. These diameters may be the same as each other, and the axial dimension H1 may be the same as H2.

[0086] The neutron detector 1 further comprises a detection head 3.

[0087] This head 3 comprises an optical fiber coupler 30 with a plurality of optical fibers 31 as inputs, and the optical fibers are arranged in the focal plane F of the lens 24. Also, this head 3 comprises a single optical fiber 32 as an output, and the optical signals received by the input are combined in this optical fiber.

[0088] The inventor used a software package known by the acronym PHITS (Particle and Heavy-Ion Transport code System) to set various dimensions and perform various calculations. This software is an MCNP simulator (MCNP is short for Monte Carlo N-Particle Transport), that is, a numerical simulation software platform that models nuclear physics processes using the Monte Carlo method. This general-purpose PHITS software package was developed in the context of joint research between the Japan Atomic Energy Agency (JAEA) and several other research institutes around the world.

[0089] To verify the shapes of various components of the neutron detector 1 according to the present invention shown in FIG. 3, particularly the shapes of the optical components, the inventor compared this shape with the shape of the OFC detector 1' used as a prototype in the laboratory.

[0090] In FIG. 5, such a detector 1' is schematically shown. This detector 1' does not include any of a mirror, an optical lens, or an optical fiber coupler. In fact, the detector 1' only includes a window 23, and this window 23 is separated from the hollow cylindrical body 20 that defines the cavity 21 by a spacer 26.

[0091] A single optical fiber 31 is directly attached to the window 23 along the X-axis.

[0092] To quantify the influence of the optical focusing system, calculations of the angular distribution and radial distribution on the surface of the window 23 were performed for the shape of the neutron detector 1 according to FIG. 3 and the shape of the neutron detector 1' according to FIG. 5 as a comparative example.

[0093] The graphs of FIGS. 4 and 6 show the calculation results of the angular distribution g(θ) of photons passing through the surface of the window 23 for each of the neutron detectors 1 and 1'. These calculations were performed using the PHITS software package.

[0094] Note that for these two detectors 1, 1', the light source is modeled as being uniform, extended, and isotropic.

[0095] Table 3 below shows the dimensions of the detectors 1, 1' to be considered.

[0096] [Table 3]

[0097] According to the evaluation of these results, by modifying the shape of the hollow body 20 of the OFC chamber and adding the mirror 25, it can be seen that the ratio (P window ) of photons passing through the surface of the window increases by a factor of 25. This value is a conservative estimate as it does not take into account the improvement due to the reflection of photons from the polished wall portion.

[0098] FIG. 7 shows two neutron detectors 1 as shown in FIG. 3 favorably installed at two positions P1, P2, and these neutron detectors are mounted on the reflector 100 inside the irradiation chamber in the nuclear reactor.

[0099] Since the radial and angular distributions of photons colliding with the window 23 are known, it is possible to estimate the same distributions at the exit of the window by ray matrix analysis and thus estimate the collection efficiency obtained as a result. This model assumes that the paraxial approximation is valid, that is, the angle of the incident light ray is small. This applies to more than 60% of the photons in the case of the OFC detector 1 having the optimized dimensions described above.

[0100] The inventor compared three neutron detectors having different optical components. That is, - A neutron detector 1' having a single sealed window as shown in FIG. 5, - The neutron detector 1 according to the present invention, in which a window 23 and a thick optical lens 24 as shown in FIG. 3 are mounted, - The neutron detector 1 according to the present invention, in which a window 23 and a Fresnel lens 24 as shown in FIG. 3 are mounted were compared.

[0101] In the following Table 4, the transfer matrices of each of these three detectors are shown.

[0102] In Table 4, - L is the distance between the optical component (window with or without an optical lens) and the optical fiber, - R is the radius of curvature of the thick lens, - f' is the focal length of the Fresnel lens, - t is the thickness of the window and the thick lens set to be equivalent to 2 mm, - n1 is the refractive index of air, - n2 is the refractive index of silica, - θ and r are randomly drawn on the distribution calculated by the PHITS software package.

[0103] [Table 4]

[0104] According to the calculation simulation, it is shown that the collection efficiency in the case of a single window is lower than that of the optical system by the combination of the window and the optical lens. This is reasonable on the premise that the acceptance cone of this single window occupies only a small part of the emission volume.

[0105] According to this calculation, it is shown that the collection efficiency increases up to 1.01 times in the case of the optical system combining the window and the thick optical lens, and up to 25 times in the case of the optical system combining the window and the Fresnel lens.

[0106] By using an optical system combining a window and an optical lens, the focal length and the distance between the lens and the fiber can be changed.

[0107] This efficiency sensitivity profile clearly shows that in the case of an optical system combining a window and a lens, the maximum efficiency is achieved when the optical fiber is placed at the image focus F of the lens.

[0108] In contrast, when using a non-thin lens, the collection efficiency drops by as much as 40%.

[0109] Figure 8 shows the collection efficiency for the case of an optical system combining a window and a Fresnel lens.

[0110] The inventor verified the effect of the optical fiber coupler 30 and optimized it with respect to two parameters, namely the number of optical fibers to be coupled and the ratio x of the length of the optical fiber after coupling to the length of the optical fiber before coupling.

[0111] Assuming that the radius of curvature of the optical fiber is not excessively large, the inventor believes that x can be changed between 0.2 and 0.8.

[0112] The inventor calculated the changes in the signal δs and the noise δb explained by Equation 6 as follows by comparing a neutron detector according to the present invention, i.e., a neutron detector having an optical fiber coupler, with a detector not having a coupler.

[0113]

Equation

[0114] Here, N is the number of optical fibers through which the light entering the coupler passes.

[0115] Figure 9 shows the change in the signal-to-noise ratio realized using the optical fiber coupler according to the present invention.

[0116] Therefore, through computational simulations, the advantages of the presence of various optical components and optical fiber couplers in the OFC neutron detector 1 could be demonstrated. This is because these components enable the maximization of the light intensity of the image of the light source.

[0117] Figure 10 shows the axial and radial distributions of the energy of the light source passing through the neutron detector 1 according to the present invention.

[0118] The shape of this light source depends on several parameters, such as the shape of the optical cavity, the filling gas of the optical cavity, the pressure of the filling gas, and the properties of the fissionable materials selected from uranium 235, boron 10, lithium 7, etc.

[0119] Due to the complexity of the light source combined with the optical components of the detector 1, it becomes impossible to calculate the image of the light source. Since the image of this light source in the detection surface F is not known, a plurality of optical fibers arranged as they are can ensure that the location where the image of the light source is the brightest is occupied. This additional degree of freedom enables further optimization of light collection and, thus, improvement of the signal-to-noise ratio.

[0120] The OFC neutron detector 1 described above has a signal collection efficiency and a signal-to-noise ratio that are dozens of times higher than those of the prior art OFC detectors.

[0121] As can be understood from the above, this improvement is due to the appropriately added optical components (mirrors, windows, optical lenses), optical fiber couplers, optimization of their shapes, and selection of parameters such as the number of optical fibers through which the light entering the coupler passes and the ratio of the lengths of the fibers that are coupled and those that are not coupled.

[0122] Of course, these results of the preliminary analysis carried out using the PHITS software package can also be found by using a specialized optical calculation code.

[0123] Other changes and improvements may also be possible without departing from the scope of the present invention.

[0124] (References) [1]: M. Lamotte, G. De Izarra, C. Jammes, "Heavy-ions induced scintillation experiments," J. Instrum., 14 (09) (2019), p. C09024, https: / / doi.org / 10.1088 / 1748-0221 / 14 / 09 / C09024 [2]: M. Lamotte, G. De Izarra, C. Jammes, "Development and first use of an experimental device for fission-induced spectrometry applied to neutron flux monitoring", Nucl. Instrum. Methods Phys. Res. A953 (2020), p. 163236, https: / / doi.org / 10.1016 / j.nima.2019.163236. [3]: M. Lamotte, G. De Izarra, C. Jammes, "Design and irradiation test of an innovative optical ionization chamber technology", Nucl. Instrum. Methods Phys. Res. A968 (2020), p.163945, https: / / doi.org / 10.1016j.nima.2020.163945. [4]: M. Lamotte, G. De Izarra, C. Jammes, SCENA: "A simulation tool for radiation-induced gas scintillation", Nucl. Instrum. Methods Phys. Res. A982 (2020), p. 164576, https: / / doi.org / 10.1016 / j.nima.2020.164576. [5]: Cheymol G., Long H., Villard J.-F., Brichard B., "High level gamma and neutron irradiation of silica optical fibers in CEA OSIRIS nuclear reactor", IEEE Trans. Nucl. Sci., 55 (4) (2008), pp. 2252-2258. [6]: Goburnov, B., "Perspectives d'utilisation de convertisseurs optiques nucleaires pour l'enregistrement de flux de photons dans les reacteurs nucleaires" [Prospects of using nuclear optical converters to record photon flux in nuclear reactors]. Russian Federal Nuclear Centre. (2015). [7]: Lamotte, M., "Etude du signal optique des chambres a fission et evaluation de son exploitation pour un systeme de mesure neutronique d'un reacteur de generation IV" [Study of the optical signals of fission chambers and evaluation of applicability to a neutron - measuring system for a Generation IV reactor]. Doctoral thesis of the University of Grenoble. (2021). [8]: Brichard, B. "Fiber - optic gamma - flux monitoring in a fission reactor by means of Cerenkov radiation". Measuring Science Technology - vol 18, 3257 - 3262. [9]: Wirtenson, R. H. "Radiation Induced Darkening of the Optical Element in the Startracker camera." Technical report of Lawrence Livermore National Laboratory. (2007)

Explanation of symbols

[0125] 1 Fission ionization chamber or boron - deposited chamber, OFC neutron detector 2 Sealed ionization chamber 3 Detection head 10 Conductive hollow cylinder 11 Smaller - diameter cylinder, anode 12 Deposits 20 Hollow body, hollow cylindrical body, hollow cylinder, sealed hollow cylinder 21 Optical cavity 22 Inner wall 23 Window 24 Optical lens, Fresnel lens 25 Mirror 26 Spacer 27 Spacer 30 Optical fiber coupler 31 Optical fiber 32 Optical fiber 100 Reflector

Claims

1. A neutron detector (1), comprising: At least one sealed ionization chamber (2) in which the photoconversion takes place, called OFC, an acronym for Optical Fission Chamber, each chamber extending along a longitudinal axis (X) and each chamber having: A sealed hollow body (20), the sealed hollow body (20) defining an optical cavity (21) therein and comprising at least one inner wall (22) at least partially coated with at least one layer of fissile material, the optical cavity being filled with a gas, the gas being preferably under pressure and capable of being ionized by ions resulting from a reaction between neutrons and the fissile material; a window (23) arranged at one of the longitudinal ends of the hollow body and configured to seal the optical cavity; an optical lens (24) adhesively secured to or integrally formed with the window and configured to focus photons received by the window; and a mirror (25) arranged at the other longitudinal end of the hollow body opposite the longitudinal end at which the window is arranged, the mirror (25) being configured to reflect photons towards the window; At least one sealed ionization chamber (2), a detector head (3) comprising a fiber optic coupler (30) having a number of optical fibers (31) as inputs, the optical fibers being arranged in a focal plane of the lens, the fiber optic coupler (30) having a single optical fiber (32) as output, the optical signals received by the inputs being summed in the single optical fiber; A neutron detector (1).

2. 2. The neutron detector (1) according to claim 1, wherein the OFC is axisymmetric in shape and has a central axis (X) corresponding to the longitudinal extension axis of the ionization chamber.

3. The neutron detector (1) according to claim 2, wherein the hollow body of the OFC is a cylinder, a sphere or a cone.

4. The surface density ρmax of the fissile material is 2 mg / cm ２ A neutron detector (1) according to any one of claims 1 to 3, wherein:

5. 5. The neutron detector (1) according to any one of claims 1 to 4, wherein the fissile material is selected from boron-10, lithium-7, isotopes of uranium-238, plutonium, and neptunium.

6. The neutron detector (1) according to any one of the preceding claims, wherein the gas is selected from helium, neon, argon, krypton, xenon, or mixtures thereof.

7. The neutron detector (1) according to any one of the preceding claims, wherein the material for making the window is silica-based.

8. The neutron detector (1) according to any one of the preceding claims, wherein the optical lens is a Fresnel lens.

9. 9. The neutron detector (1) according to any one of the preceding claims, comprising at least one spacer arranged between the window and the hollow body and / or at least one spacer arranged between the mirror and the hollow body.

10. 10. The neutron detector (1) of claim 9, wherein the axial dimension of the spacer is equal to or greater than a path length of light fission fragments (LFF) passing through the gas of the optical cavity.

11. Neutron detector (1) according to any one of the preceding claims, wherein the detector head is fixed, preferably screwed, relative to the hollow body of the box.

12. Use of a device for detecting neutrons according to any one of claims 1 to 11 for characterising and tracking neutron flux in a nuclear reactor.

13. 12. Use of a device for detecting neutrons according to any one of claims 1 to 11 for locating a melted fuel element during or after a severe incident, such as loss of cooling or temporary power loss.

14. Use of a device for detecting neutrons according to any one of claims 1 to 11 for localizing containment stoppers, in particular of colloidal plutonium, in a chemical treatment process.

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