Study of Fiber Bragg Grating Samples Exposed to High Fast Neutron Fluences

G. Cheymol (1) , L. Remy (1) , A. Gusarov (2) , D. Kinet (3) , P. Mégret (3) , Guillaume Laffont (4) , Thomas Blanchet (4, 5) , A. Morana (5) , E. Marin (5) , S. Girard (5)

1 SEARS - Service d'études analytiques et de réactivité des surfaces
2 SCK-CEN - Centre d'Etude de l'Energie Nucléaire
3 Service d'Electromagnétisme et de Télécommunications [Mons]
4 LCFO - Laboratoire Capteurs Fibres Optiques
5 LHC - Laboratoire Hubert Curien

Fiber Bragg grating (FBG) sensors are expected to provide valuable data in extreme radiation environments associated with nuclear research reactors. However, when the fast neutron fluence reaches 10^18 -10^19 n/cm 2 , the radiation-induced changes in the material density and refractive index may drastically bias the measurements. This paper evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 10^19 n/cm2 (E 1 MeV). We studied the responses of FBGs manufactured by three different laboratories in the same singlemode optical fiber but using different inscription conditions. The Bragg wavelength and the reflectivity were measured before and after irradiation thanks to a dedicated mounting. For nearly all FBGs, the Bragg peak remains visible after the irradiation while the radiation-induced Bragg wavelength shifts (RI-BWSs) vary from a few picometers (equivalent temperature error <;1 °C) to nearly 1 nm (~100 °C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractive-index variation and compaction observed for bare fiber samples through other experimental techniques. Our results show that by using specific hardening techniques, the FBG-based temperature measurements in a nuclear research reactor experiment may become feasible.

Keywords: fiber Bragg grating sensors; refractive index; reflectivity; optical variables measurement; optical fibre testing; neutron flux; Temperature measurement; gamma; fiber Bragg grating; neutron; optical fiber sensor; extreme radiation environments; high fast neutron fluences; nuclear research reactors; fast neutron fluence; radiation-induced changes; material density; single-mode optical fiber; radiation-induced Bragg wavelength shifts; inscription conditions; high radiation-induced Bragg wavelength shifts; FBG based temperature measurements; nuclear research reactor experiment; compaction; huge refractive-index variation; FBG inscription conditions; radiation effect; optical fibers; instrumentation; Distributed and Quasi-distributed sensing

Domains: Optics / Photonic; Nuclear Experiment [nucl-ex]

G. Cheymol, L. Remy, A. Gusarov, D. Kinet, P. Mégret, et al.. Study of Fiber Bragg Grating Samples Exposed to High Fast Neutron Fluences. IEEE Transactions on Nuclear Science, 2018, 65 (9), pp.2494 - 2501. ⟨10.1109/TNS.2018.2820505⟩. ⟨ujm-01926002⟩