Dimensional measurements under high radiation with optical fibre sensors based on white light interferometry - report on irradiation tests

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. Then we present two types of sensor that we develop: elongation Fabry Perot sensor - jointly with SCK·CEN - and Michelson type displacement sensor; they both rely on white light interferometry to retrieve the desired measurement. We report the results of irradiation of Fabry-Perot sensors in the core of BR2 material testing reactor in Mol (Belgium), under intense neutron-gamma flux and at high temperature. Six Fabry Perot fibre optics sensors are mounted on a support insensitive to radiation. The objective is to test the survival and the drift of the sensors. The temperature is maintained steady at 200°C during 22 days then the temperature is increased up to 390°C. Among five sensors built according to nominal scheme, four are still alive at the end of the cycle and three show a low drift of 1 to 4 μm. These results show a clear progress compared to the previous irradiation and make it possible to consider the use of these sensors in real tests of material in MTR reactor. However, improvement in the robustness of the interfaces is still necessary. The next part is devoted to the measurement of displacements perpendicular to the direction of the lead-in fibre, of interest for small room environments where the fibre cannot make a 90° turn. The optical scheme is based on the- Michelson interferometer. The fibre and signal conditioner are the same as used with the Fabry Perot extensometer. The sensor is expected to be applied for the measurement of variations in the diameter of the fuel cladding during exposure in a research reactor. Three sensors arranged on a circle, within 120° from each other, must measure displacements of the external surface of the cladding with an error lower than 10 μm, over a 0.5 mm range. The irradiation device considered is in development for the Jules Horowitz Reactor (RJH) under construction in Cadarache (France). We have developed and tested up to 400°C the optical module which will be inserted in the mechanical part comprising the mechanical probe. With a bit larger room available, we also develop a device capable of measuring displacement, under high nuclear flux in the vicinity of the core of Sodium Fast Reactor. The objective could be to sense more or less directly the deformation of the core support structure and core movement. The uncertainty expected is near 1 μm with a range of more than 10 mm.