SAKE 1 Experiment - Fibre Optic Compaction Measurement and Fibre Bragg Gratings Testing under High Fast Neutrons Fluence

In the framework of the development by CEA and SCK•CEN of a Fabry Perot Sensor (FPS) able to measure dimensional changes in Material Testing Reactor (MTR), the first goal of the SAKE 1 irradiation was to measure the linear compaction of single mode fibres under high fast neutron fluence. Indeed, the compaction of the fiber which forms one side of the Fabry Perot cavity in the modified sensor design [1], may generate a noticeable measurement error. Primak [2] measured 1% compaction for bulk silica after 2×1019 nfast/cm². An accurate measurement of the linear compaction of the fibre optic associated with the sensor operation in MTR will permit to predict the radiation-induced drift and optimize its design. In the SAKE1 experiment approximately fifty uncoated fibre tips (length: 30 to 50 mm) prepared using several different fiber types have been set in three different capsules, which were exposed in the SCK•CEN BR2 reactor (Mol Belgium). The irradiation lasted 22 days leading to a total fast (E > 1 MeV) fluence of 3 to 5×1019 nfast/cm², depending on the capsule. The temperature during irradiation was 291°C, which corresponds to the condition of the intended FPS use. A precise measurement of each fiber tip length was made before the irradiation and compared to the post irradiation measurement highlighting a decrease of the fibres' length. The results of the length measurements will be presented. With a good precision, they give around 0.25% of linear compaction for the fibres used to make the FPS. The scale of the changes is independent on the capsule, which means that the effect saturated even at the lowest dose. In the prospect of performing distributed temperature measurement in MTR, some Bragg gratings have been irradiated in the same capsules. Four gratings were provided by the University of Mons, Belgium and four others were made by the LabHC. All the gratings were written in radiation tolerant fibres, but those from Saint-Etienne underwent an additional treatment with a procedure enhancing their radiation resistance to ionizing radiations [3,4]. We have made a special mounting allowing to test the reflexion and the transmission of the gratings on fibre samples cut down to 30 to 50 mm. The comparison of measurements made before and after the irradiation, at the same temperature, allowed to measure the loss in reflection as well as the Bragg wavelength drift. The results will be presented; they are quite promising for some of the investigated gratings. [1] G. Cheymol, A. Gusarov, S. Gaillot, C. Destouches and N. Caron, « Dimensional Measurements Under High Radiation With Optical Fibre Sensors Based on White Light Interferometry - Report on Irradiation Tests"; Nuclear Science, IEEE Transactions on (Volume:61 , Issue: 4 ) [2] W.Primak, "Fast neutron induced changes in quartz and vitreous silica", Phys. Rev.B 110(6), 1240-1254 (1958) [3] A. Morana, S. Girard, E. Marin, C. Marcandella, P. Paillet, J. Perisse, J.-R. Mace, A. Boukenter, M.Cannas and Y. Ouerdane, "Radiation tolerant fiber Bragg gratings for high temperature monitoring at MGy dose levels", Optics Letters, Vol. 39, No. 18, September 15, 2014 [4] AREVA-LabHC pending patent, "Procede de fabrication d'une fibre optique traitee pour capteur de temperature resistant aux radiations", deposited Dec. 16th 2013, n°13 62691