A device for detecting neutrons having an ionization chamber that utilizes optical conversion and includes multiple optical cavities, each housing the free ends of optical fibers.

Abstract

The present invention relates to an apparatus (1) for detecting neutrons, the apparatus comprising at least one sealed ionization chamber (2) utilizing optical transduction, the ionization chamber having a plurality of cavities, each cavity being operated based on optical transduction using an optical fiber with a free end within the cavity, which allows for multi-point neutron flux measurement, wherein the measurement points are axially distributed.

Description

Technical Field

[0001] This invention relates to the field of instrumentation, and in particular to instruments used in nuclear fission and fusion reactors.

[0002] More specifically, the present invention relates to ionization chamber neutron detectors, particularly neutron detectors that operate over decades of measurements and are capable of adapting to stringent installation constraints (e.g., sub-centimeter sleeve and fiber optic cable duct diameters). Background Technology

[0003] Operating a reactor, whether for power generation or research, requires stringent requirements in monitoring multiple operating parameters. Among these, the generated thermal power is a key parameter. This is directly related to the neutron flux near or inside the reactor vessel. Therefore, increasing the neutron flux leads to an increase in the reactor's power level.

[0004] Various techniques exist for measuring neutron flux, which are collectively categorized under neutron detectors. Their development must consider several constraints.

[0005] The most important of these is the installation constraint. For example, these geometric constraints are the diameter of the conduit (such as a sleeve or fiber optic cable conduit).

[0006] To limit the number of detectors that need to be introduced into the core, operating over decades of measurements is a significant advantage.

[0007] Thus, a single detector, instead of the usual three detectors, allows for tracking of neutron flux from startup to full power.

[0008] Another constraint relates to signal integrity. In the case of electrical transmission (known as the only type currently in use), shielded coaxial fiber optic cables must be used to prevent electromagnetic interference or potential crosstalk problems between the signal transmission lines themselves.

[0009] Furthermore, the high levels of radiation and temperatures exceeding 300°C within the reactor vessel severely limit the types of neutron detectors that can be used.

[0010] Therefore, detectors from ionization chambers are favored, as they are known to be robust under the extreme conditions of such environments. In this particular context, detectors such as scintillators or silicon-based solid-state detectors are far less reliable.

[0011] Current ionization chambers, such as boron-coated ionization chambers or fission chambers, typically operate on the principle of converting neutron flux into an electrical signal. This electrical conversion is performed by one or more pairs of electrodes biased at hundreds of volts. The number of ion pairs collected by these electrodes is at most equal to the number of ion pairs produced by the incident radiation, and depends on the type and kinetic energy of the incident particles.

[0012] As already mentioned, in order to obtain a usable signal, electrical conversion requires the transmission line to be electromagnetically shielded.

[0013] Furthermore, under strict spatial constraints, electrical conversion is not very satisfactory in terms of the number of achievable measurement points. Specifically, if the problem is to distribute multiple measurement points along a cylindrical channel with a diameter of 15 mm and a length of 1 m, then assuming a maximum detector diameter of 10 mm, this solution does not allow for more than two measurement points due to the transmission line diameter (coaxial optical cable with a diameter of at least 6 mm).

[0014] Patent application WO 2017 / 027679 discloses a multi-point neutron detector, which the applicant commercially refers to as a "Micropocket Fission Detector" (abbreviated as MPFD). This MPFD is rendered inoperable due to poor signal transmission performance: electric field interference from the transmission lines and inductive and capacitive crosstalk between these identical transmission lines occur.

[0015] The inventors have proposed a new approach that differs from electrical conversion: see, for example, references [1] through [5]. The principle is to perform optical conversion by collecting photons generated in an ionization chamber.

[0016] Therefore, this solution is based on converting neutron signals into optical signals. Specifically, when a gas is ionized by heavy ions produced by a reaction between neutrons and an active material (such as a fissile material), an electron cascade occurs, leading to excitation and then de-excitation. The spectrum ranges from ultraviolet light (approximately 80 nm) to mid-infrared light (approximately 2000 nm and 3000 nm), accompanied by significant luminescence caused by the emission of atoms from the same gas in the infrared and near-infrared regions. This effect is... Figure 1 It is shown schematically in the middle.

[0017] This luminescence is then collected by optical fibers designed to withstand radiation, which effectively limits the spectral range to be used. Specifically, it has been shown that the irradiated silica fiber attenuates very little, typically a few dB / km, and the wavelength of the light signal is in the near-infrared range, typically between 800 nm and 1100 nm: see reference [5].

[0018] As for converting optical signals into electrical signals, this is done outside the nuclear reactor vessel using one or more transducers (such as photodiodes, silicon photomultiplier tubes, or cameras).

[0019] That said, while the aforementioned references [1] through [5] provide proof of the idea of ​​using passive optical methods to track neutron flux, they do not offer any suggestions for addressing the need for compact multi-point measurement devices.

[0020] Therefore, there is a need for a neutron and / or gamma detection device that allows for simultaneous coaxial measurement of neutron flux at multiple points while advantageously complying with potentially stringent space constraints. Summary of the Invention

[0021] To achieve this, one aspect of the present invention is a device for detecting neutrons, comprising at least one sealed ionization chamber utilizing optical conversion, the ionization chamber extending along a longitudinal axis (X) and comprising a plurality of optical cavities, each optical cavity accommodating the free end of an optical fiber and comprising at least one inner wall at least partially coated with at least one active material, the optical cavities being filled with a gas, preferably a pressurized gas, capable of being ionized by ions generated by a reaction between neutrons and the active material, each optical cavity being defined by a cylinder, the cylinder being closed at its longitudinal end by a sealing disk, the transverse inner wall of the optical cavity being at least partially coated with the active material, the cylinders of the optical cavities being adjacent to each other and centered on the longitudinal axis (X), at least one of the cylinders of the optical cavities being transversely perforated with an opening designed to allow one of the optical fibers to pass through, the free end of which is accommodated in the adjacent optical cavity.

[0022] The ionization chamber is preferably pressure-tight, typically withstands pressures of several bar. The filling gas is advantageously an inert gas (e.g., argon) or a mixture of inert gases.

[0023] The active material can be a fission element or boron.

[0024] According to an advantageous embodiment, the device includes:

[0025] - A cylindrical body with a central axis (X) that internally defines an ionization chamber and an axially adjacent connecting chamber of the ionization chamber. The connecting chamber is perforated with an opening designed to allow a multi-core optical cable to pass through, the multi-core optical cable comprising multiple optical fibers in a number at least equal to the number of optical cavities.

[0026] - A sealed channel separation device is arranged between the ionization chamber and the connection chamber and is designed to allow the free ends of optical fibers, each housed in an optical cavity, to pass through.

[0027] The connecting chamber allows the multi-core optical cable to exit the detector and ensures robust mechanical rigidity at the junction between the multi-core optical cable and the ionization chamber. This connecting chamber does not need to be airtight relative to external pressure, and in this case, sealing is not required.

[0028] It is manufactured in a manner that allows a bundle of optical fibers, formed by passing multiple optical fibers, through the walls separating the connecting chamber and the ionization chamber, thereby keeping the ionization chamber sealed.

[0029] An advantageous variant of the sealed channel device includes a set of optical fibers pre-brazed together, which are then brazed to a partition wall between the connection chamber and the ionization chamber, through which the set of optical fibers pass.

[0030] Therefore, according to this variant, the optical fibers are first brazed together, and then the brazed set of optical fibers (brazed assembly) is brazed to the partition wall between the connection chamber and the ionization chamber. For operating temperatures up to 700°C, silver, copper, or aluminum-based brazing is preferred. Tin-lead based solder can withstand temperatures up to 300°C.

[0031] By employing a process that involves introducing the optical fiber through a transverse channel opening in an optical cavity adjacent to the optical cavity housing the free end of the fiber, the radius of curvature of the fiber is advantageously taken into account. Therefore, this optimal process adheres to the mechanical constraints of the optical fiber related to its minimum radius of curvature (between 15 mm and 30 mm).

[0032] According to this variant, one of the disks of the cylinder used to enclose the optical cavity has an opening at its center, which is designed to allow the free end of the optical fiber to pass through.

[0033] Centering the free end of the optical fiber maximizes photon collection. Specifically, the cone of acceptance of the fiber thus covers the volume of the optical cavity housing the fiber to the greatest extent possible. Advantageously, the coverage of the cone of acceptance can be increased by placing a spherical lens at the end of the fiber. In other words, this lens at the tip of the free end of the fiber improves the fiber's collection efficiency. The optical tip forming the lens can, for example, be soldered to the end of the fiber. See, for example: https: / / www.thorlabs.com / newgrouppage9.cfm?objectgroup_id=13120.

[0034] According to another advantageous variant, the ionization chamber includes a “gathering” region without any optical cavity, in which the individual optical fibers of the optical cable are gathered together to be distributed outside the optical cavity in a given angular sector along the transverse inner wall of the ionization chamber until these fibers pass through a transverse opening in the cylinder of the optical cavity.

[0035] Preferably, the axial length of the aggregation region is greater than or equal to 2 cm.

[0036] Therefore, in order to reach the individual optical cavities, the optical fibers are clustered together in the clustering region to form an optical fiber bundle. The axial length of the clustering region allows for compliance with the constraints caused by the radius of curvature for each optical fiber.

[0037] Advantageously, the cylinder of the optical cavity has a diameter greater than or equal to 10 mm and a height greater than or equal to 2 cm. These dimensions allow for compliance with constraints related to the manufacturability of the detector, as well as constraints arising from the minimum radius of curvature of the optical fiber.

[0038] According to another advantageous embodiment, at least one optical cavity includes at least one partition wall, which is preferably arranged in a plane transverse to the axis X, so as to measure different neutron spectral indices depending on the portion of the optical cavity on either side. In other words, dividing the optical cavity defines different effective portions for one or more active materials, which allows neutron spectral index measurements to be distributed at multiple points.

[0039] Therefore, the present invention mainly includes a device for detecting neutrons, the device comprising a sealed ionization chamber having multiple cavities, each cavity being operated based on optical conversion using an optical fiber with its free end within the cavity, which allows for multi-point neutron flux measurement with the measurement points axially distributed.

[0040] The size of the cavity and the arrangement of the optical fibers can make the device very compact.

[0041] The free end of the optical fiber should be able to collect as many photons as possible generated within the optical cavity. Therefore, the positioning of the free end of the optical fiber should be precise. Collection efficiency decreases with increasing distance from the entrance wall of the optical cavity. Ideally, the end of the optical fiber should be in the same plane as and abut against the entrance wall of the optical cavity to achieve 100% collection efficiency. However, due to the mechanical retention of the optical fiber and the avoidance of truncating the optical cone (solid angle), it is preferable to position the free end of the optical fiber approximately 1 mm from the entrance aperture (opening) within the optical cavity. Therefore, preferably, the free end of the optical fiber is arranged within the optical cavity from which the fiber exits, at a distance between 1 mm and 2 mm from the opening, more preferably between 1 mm and 1.5 mm.

[0042] The miniaturized neutron detector according to the present invention is suitable for withstanding high-temperature, high-radiation environments, such as those inside a running nuclear reactor.

[0043] Finally, the detector according to the invention enables optimal compliance with the comprehensive constraints it may face, while maintaining an excellent signal-to-noise ratio for multi-point neutron flux measurements.

[0044] Another subject of the invention is a method for operating the apparatus for detecting neutrons as described above, wherein neutron flux is simultaneously measured at multiple points at at least a portion of a plurality of optical cavities.

[0045] This invention has many applications, among which the following applications may be mentioned:

[0046] - Simultaneous coaxial measurement of gamma and neutron flux at multiple points in a nuclear reactor;

[0047] - Not only characterizing and tracking neutron flux in nuclear reactors, whether experimental or power reactors;

[0048] - Positioning molten fuel elements during or after a severe accident (cooling failure or power transient);

[0049] - Locate the regulating blockages in the chemical processing, especially those formed by colloidal plutonium.

[0050] Other advantages and features will become clearer and more apparent when reading the detailed but non-limiting description given in illustrative manner with reference to the following figures. Attached Figure Description

[0051] Figure 1 This is a schematic diagram illustrating the principle of luminescence caused by ionized gas produced by the reaction between neutrons and active materials (commonly known as heavy ions).

[0052] Figure 2 This is a longitudinal cross-sectional schematic diagram of a device for detecting neutrons according to the present invention.

[0053] Figure 2A Through Figure 2 A cross-sectional view of section A.

[0054] Figure 3 This is a longitudinal cross-sectional view of the optical cavity of a device with a separation septum according to a variant embodiment. Detailed Implementation

[0055] Figure 1 As already described in the background section, it will not be discussed in detail below.

[0056] Figure 2 and Figure 2A A device 1 for detecting neutrons according to the present invention is shown.

[0057] The device 1 first includes a cylindrical body 10 with a central axis (X), which internally defines a sealed ionization chamber 2 utilizing optical conversion and a connecting chamber 3 axially adjacent to the ionization chamber.

[0058] The connection chamber 3 is perforated with an opening 30, which is designed to allow a multi-core optical cable 4 to pass through, the multi-core optical cable 4 comprising, for example, multiple optical fibers 40 made of silicon.

[0059] The fiber bundle, formed by multiple optical fibers 40, is manufactured in a manner that keeps the ionization chamber 2 sealed by passing through a channel in the partition wall between the connecting chamber 3 and the ionization chamber 2. Therefore, the sealed channel device 5 allows the optical fibers 40 to pass through while ensuring a seal.

[0060] The sealed ionization chamber 2 is filled with a pressurized inert gas or a mixture of inert gases, which can be ionized by ions generated by the reaction between neutrons and the active material 6, and the ionization chamber 2 includes a plurality of optical cavities 20, which are preferably identical.

[0061] Each optical cavity 20 is defined by a cylinder closed at its longitudinal end, the transverse inner wall of which is at least partially coated with an active material 6, which may be a fissile element or boron. For example, the cylinder of the optical cavity may be made of stainless steel. The disk 23 used to close the cylinder may also be coated with the active material 6.

[0062] like Figure 2 As shown, the cylinders of the optical cavity 20 are adjacent to each other, and are centered on the central axis X of the cylindrical body 10.

[0063] Each of the cylinders in the optical cavity 20 is transversely perforated with an opening 21 through which one of the optical fibers 40 passes.

[0064] The ionization chamber 2 includes a gathering region 22 without any optical cavity after the sealed channel device 5. The individual optical fibers 40 of the optical cable 4 are gathered together in the gathering region 22 to be distributed outside the optical cavity 20 along the transverse inner wall of the ionization chamber.

[0065] like Figure 2A As shown, the distribution of optical fibers 40 is preferably concentrated in a single corner sector of a cylinder parallel to the optical cavity 20 until they pass through the transverse opening 21.

[0066] In order to guide the optical fiber 40 into the optical cavity 20, the cylindrical disks 23 used to close the optical cavity each have an opening 24 at their center, which allows the free end 41 of the optical fiber 40 to pass through, thereby positioning it along the central axis X in the given optical cavity 20.

[0067] Figure 3 A variant embodiment of the optical cavity 20 is shown, which includes a partition wall 25 arranged in a plane transverse to the central axis X. The partition wall 25 defines partially different sub-cavities in which the active material 6 resides. Therefore, different neutron spectral indices can be measured depending on the sub-cavity.

[0068] However, other variations and modifications are conceivable without departing from the scope of the invention.

[0069] Although Figure 2 and Figure 2AIn one implementation, the spatial distribution of the optical cavities is axially aligned side-by-side, but other distributions are conceivable, such as radial distribution of the optical cavities, which may or may not surround the central optical cavity.

[0070] Although Figure 2 and Figure 2A In one implementation, the first optical cavity 20 does not function as long as it does not contain the free end of the optical fiber, but the opposite is certainly conceivable.

[0071] References

[0072] [1]: M.Lamotte, G.De Izarra, C.Jammes, "Heavy-ions induced scintillationexperiments," J.Instrum., 14(09)(2019), p.C09024, https: / / doi.org / 10.1088 / 1748-0221 / 14 / 09 / C09024;

[0073] [2]: M.Lamotte, G.De Izarra, C.Jammes, "Development and first use of anexperimental device for fission-induced spectrometry applied to neutron fluxmonitoring", Nucl.Instrum.Methods Phys.Res.A953(2020),p.163236,https: / / doi.org / 10.1016 / j.nima.2019.163236;

[0074] [3]: M.Lamotte, G.De Izarra, C.Jammes, "Design and irradiation test of aninnovative optical ionization chamber technology", Nucl.Instrum.MethodsPhys.Res.A968(2020), p.163945, https: / / doi.org / 10.1016j.nima.2020.163945;

[0075] [4]:M.Lamotte,G.De Izarra,C.Jammes,SCENA:"A simulation tool forradiation-induced gas scintillation",Nucl.Instrum.Methods Phys.Res.A982(2020), p.164576;

[0076] [5]:Cheymol G.,Long H.,Villard J.-F.,Brichard B.,"High Level Gammaand Neutron Irradiation of Silica Optical Fibers in CEA OSIRIS Nuclear Reactor",IEEE Trans.Nucl.Sci.,55(4)(2008),pp.2252-2258.

Claims

1. A device (1) for detecting neutrons, said device (1) comprising at least one sealed ionization chamber (2) with optical conversion, said ionization chamber (2) extending along a longitudinal axis (X) and comprising a plurality of optical cavities (20), each optical cavity housing a free end (41) of an optical fiber (40) and comprising at least one internal wall at least partially coated with at least one active material, said optical cavities being filled with a gas capable of being ionized by ions generated by a reaction between neutrons and said active material, each optical cavity being defined by a cylinder closed at its longitudinal ends by closing discs, the transverse internal walls of said optical cavities being at least partially coated with active material, the cylinders of said optical cavities being contiguous to each other while being centered on said longitudinal axis (X), at least one of the cylinders of said optical cavities being transversely pierced with an opening (21) designed to allow the passage of one of said optical fibers, the free end of which is housed in an adjacent optical cavity.

2. The device (1) according to claim 1, comprising: - a cylindrical body (10) having a central axis (X), said cylindrical body internally defining said ionization chamber and a connecting chamber (3) axially contiguous to said ionization chamber, said connecting chamber being pierced with an opening designed to allow the passage of a multicore optical cable (4) comprising a number of optical fibers at least equal to the number of said optical cavities; - airtight passage separation means (5) arranged between said ionization chamber and said connecting chamber and designed to allow the passage of said optical fibers, the free ends of which are each housed in said optical cavities.

3. The device (1) according to claim 2, wherein Said airtight passage separation means (5) comprise a group of optical fibers pre-soldered together, said group of optical fibers being in turn soldered to a separation wall between said connecting chamber and said ionization chamber, wherein said group of optical fibers passes through said separation wall.

4. The device (1) according to claim 1, wherein One of said discs closing the cylinders of said optical cavities is pierced in its center with an opening (24) designed to let the free ends of said optical fibers pass.

5. The device (1) according to claim 1, wherein Said ionization chamber comprises a gathering area (22) without any optical cavity, in which the individual optical fibers of the optical cable are gathered together to be distributed outside said optical cavities in a given angular sector along the transverse internal walls of said ionization chamber, until said individual optical fibers pass through the transverse openings in the cylinders of said optical cavities.

6. The device (1) according to claim 5, wherein The axial length of said gathering area is greater than or equal to 2 cm.

7. The device (1) according to claim 1, wherein The cylinders of said optical cavities have a diameter greater than or equal to 10 mm and a height greater than or equal to 2 cm.

8. The device (1) according to claim 1, wherein At least one of said optical cavities comprises at least one separation wall (25) arranged to measure different neutron spectral indices depending on the portion of said optical cavity on either side thereof.

9. A method for operating a device (1) for detecting neutrons according to claim 1, wherein The neutron flux is measured simultaneously at a plurality of points of at least a portion of said optical cavities.

10. Use of the device for detecting neutrons according to claim 1 for simultaneously coaxially measuring the gamma and neutron flux in a nuclear reactor at a plurality of points.

11. Use of the device for detecting neutrons according to claim 1 for characterizing and tracking the neutron flux in a nuclear reactor.

12. Use of the device for detecting neutrons according to claim 1 for locating molten fuel elements during or after a severe accident.

13. Use of the device for detecting neutrons according to claim 1 for locating regulation blockages in chemical treatment processes.

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