DEVICE FOR DETECTING NEUTRONS WITH IONIZATION CHAMBER AND WITH OPTICAL TRANSDUCTION COMPRISING A PLURALITY OF OPTICAL CAVITIES, EACH ACCOMMODATING THE FREE END OF AN OPTICAL FIBER

Abstract

Device for detecting neutrons with ionization chamber and with optical transduction comprising a plurality of optical cavities, each accommodating the free end of an optical fiber.

The invention relates to a device (1) for detecting neutrons comprising at least one sealed ionization chamber (2) and with optical transduction with a plurality of cavities whose operation is each based on optical transduction using an optical fiber whose free end is within the cavity, which allows multipoint neutron-flux measurement, the measurement points being axially distributed.

Claims

1. A device for detecting neutrons comprising: at least one sealed ionization chamber with optical transduction, which extends along a longitudinal axis (X) and comprises a plurality of optical cavities, wherein each optical cavity accommodates the free end of an optical fiber and comprises at least one inner wall coated at least partially with at least one active material, the optical cavities being filled with a gas that is able to be ionized by an ion arising from the reaction between a neutron and the active material, wherein each optical cavity is delimited by a cylinder that is closed at its longitudinal ends by a closing disk, the lateral inner wall of which is coated at least partially with an active material, the cylinders of the cavities adjoining one another while being centered on the longitudinal axis (X), at least one of the cylinders of the cavities being pierced laterally with an opening, which is designed to allow through one of the optical fibers whose free end is accommodated in an adjacent cavity.

2. The device according to claim 1, comprising: a cylindrical body of central axis (X) delimiting on the inside the ionization chamber and a connection chamber axially adjoining the ionization chamber, the connection chamber being pierced with an opening which is designed to allow through a multifiber optical cable comprising a number of optical fibers at least equal to the number of cavities; a seal-tight passage partition device, arranged between the ionization chamber and the connection chamber, designed to allow through the optical fibers whose free ends are each accommodated in an optical cavity.

3. The device according to claim 2, wherein the seal-tight passage partition device consists of a grouping of optical fibers previously brazed together, the grouping being brazed in turn to the separating wall between the connection chamber and the ionization chamber through which it passes.

4. The device according to claim 1, wherein one of the disks for closing the cavity cylinder is pierced at its center with an opening which is designed to let through the free end of the optical fiber.

5. The device according to claim 1, wherein the ionization chamber comprises a “gathering” region, without any optical cavities, in which the various optical fibers of the cable are gathered together to be distributed on the outside of the optical cavities along the lateral inner wall of the ionization chamber, in a given angular sector, until they pass through the lateral opening in a cavity cylinder.

6. The device according to claim 5, wherein the axial length of the gathering region is greater than or equal to 2 cm.

7. The device according to claim 1, wherein the cylinder of an optical cavity has a diameter greater than or equal to 10 mm and a height greater than or equal to 2 cm.

8. The device according to claim 1, wherein at least one of the optical cavities comprises at least one separating wall arranged, preferably in a plane transverse to the X-axis, so as to measure different neutron spectral indices according to the portion of the cavity on either side of the cavity.

9. A method for operating a device for detecting neutrons according to claim 1, in which the neutron flux is measured simultaneously at multiple points of at least a portion of the plurality of optical cavities.

10. Use of a device for detecting neutrons according to claim 1 for the simultaneous in-line measurement at multiple points of the gamma and neutron flux in a nuclear reactor.

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

12. Use of a device for detecting neutrons according to claim 1 for locating molten fuel elements during or after a serious accident, such as a loss of cooling or power transient.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 is a schematic view illustrating the principle of luminescence caused by the ionization of a gas by an ionizing particle, most commonly called a heavy ion, arising from the reaction between a neutron and an active material.

[0048] FIG. 2 is a schematic view in longitudinal section of a device for detecting neutrons according to the invention.

[0049] FIG. 2A is a view in cross section through plane A of FIG. 2.

[0050] FIG. 3 is a view in longitudinal section of an optical cavity of a device according to one variant embodiment with a separating partition.

DETAILED DESCRIPTION

[0051] FIG. 1 has already been described in the preamble. It will therefore not be discussed in detail in the below.

[0052] FIGS. 2 and 2A show a device 1 for detecting neutrons according to the invention.

[0053] It first comprises a cylindrical body 10 of central axis (X) delimiting on the inside a sealed ionization chamber 2 with optical transduction, and a connection chamber 3 axially adjoining the ionization chamber.

[0054] The connection chamber 3 is pierced with an opening 30 which is designed to allow through a multifiber optical cable 4 comprising a number of optical fibers 40, made of silicon for example.

[0055] The passage for the bundle of fibers 40 through the separating wall between the connection chamber 3 and the ionization chamber 2 is made in such a way that the latter remains sealed.

[0056] Thus, the seal-tight passage device 5 allows the optical fibers 40 through while ensuring the seal.

[0057] The sealed ionization chamber 2 is filled with a pressurized noble gas or mixture of noble gases, this being able to be ionized by an ion arising from the reaction between a neutron and an active material 6, and comprises a plurality of optical cavities 20, which are preferably identical.

[0058] Each optical cavity 20 is delimited by a cylinder that is closed at its longitudinal ends, the lateral inner wall of which is coated at least partially 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 disks 21 for closing the cylinder may also be coated with an active material 6.

[0059] As shown in FIG. 2, the cylinders of the cavities 20 adjoin one another while being centered on the central, X-axis of the cylindrical body 10.

[0060] The cylinders of the cavities 20 are each pierced laterally with an opening 21, through which one of the optical fibers 40 passes.

[0061] The detection chamber 2 comprises, next to the seal-tight passage device 5, a gathering region 22, without any optical cavities, in which the various optical fibers 40 of the cable 4 are gathered together to be distributed on the outside of the optical cavities 20 along the lateral inner wall of the ionization chamber.

[0062] As shown in FIG. 2A, the distribution of the optical fibers 40 is preferably gathered together in a single angular sector parallel to the cylinders of the optical cavities 20 until they pass through a lateral opening 21.

[0063] To route an optical fiber 40 to an optical cavity 20, the disks 23 for closing the cavity cylinder are each pierced at their center with an opening 24, which lets through the free end 41 of the optical fiber 40 so that it is positioned along the central, X-axis in a given cavity 20.

[0064] FIG. 3 illustrates a variant embodiment of an optical cavity 20 consisting of a separating wall 25 arranged arranged in a plane transverse to the X-axis. This wall 25 delimits sub-cavities in which the section of active material 6 is different. Thus, it is possible to measure different neutron spectral indices according to the sub-cavity.

[0065] Other variants and improvements may be envisioned without however departing from the scope of the invention.

[0066] While in the embodiment of FIGS. 2 and 2A the spatial distribution of the optical cavities is an axial alignment with juxtaposition between cavities, other distributions may be envisaged, such as radial distribution of the cavities which might or might not be about a central cavity.

[0067] While in the embodiment of FIGS. 2 and 2A the first cavity 20 is not functional in the sense that no optical fiber free end is accommodated therein, it is of course possible to envisage the contrary.

LIST OF CITED REFERENCES

[0068] [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 [0069] [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. [0070] [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. [0071] [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. [0072] [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.