Compaction in Optical Fibres and Fibre Bragg Gratings Under Nuclear Reactor High Neutron and Gamma 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 SAKE 1 (Smirnof extention-Additional Key-tests on Elongation of glass fibres) campaign was performed. Its first goal was to measure the fiber linear compaction under high fast neutron fluence. This compaction is known to cause a noticeable measurement error of a FPS through a radiation-induced drift and an accurate quantification of this effect is required to optimize the FPS design. To achieve this, an innovative approach was used. Ad hoc samples of various fiber types have been prepared and exposed in the SCK.CEN BR2 reactor (Mol Belgium). After 22 days of irradiation at 291°C a total fast (E > 1MeV) fluence of 3 to 5 × 10
¹⁹
n
_fast
/cm
²
, depending on the sample location, was accumulated. A precise measurement of each fibre sample length was made before the irradiation and compared to the post irradiation measurement highlighting a decrease of the fibres' length corresponding to about 0.25% of linear compaction. The amplitude of the observed changes is independent of the neutron fluence, which indicates that the compaction effect may saturate below the lowest considered fluence. A second goal was to perform preliminary evaluation of the feasibility to implement distributed temperature measurement in MTR using fibre Bragg gratings (FBG). Several FBG written in radiation hard fibres using a femtosecond laser have been elaborated with a procedure allowing to enhance their radiation tolerance. First promising results on their vulnerability to these extreme conditions are discussed.