Optical fibre sensors (OFS) can bring substantial advantages over conventional sensing approaches for in- situ measurement in fission Material Testing Reactors (MTRs) and other nuclear research or industrial facilities: easy remote sensing,
possibilities of multiplexing, passive operation, low sensitivity to electromagnetic interference, compact size, high resolution and accuracy even under high radiation dose and high temperature, once necessary adaptations have been achieved. In this paper firstly we remind the three undesired effects of high level of irradiation on OFS: radiation induced attenuation, radio luminescence and compaction due to fast neutrons.