Fabry Perot sensor for in-pile nuclear reactor metrology

Optical fibre sensors are attractive devices that can bring substantial advantages over conventional sensing approaches for fission Material Testing Reactors (MTRs), such as high accuracy capabilities with limited intrusiveness and the ability to withstand high temperature. In the framework of the Joint Instrumentation laboratory (JIL), CEA and SCK•CEN have joined their resources to develop, in particular, an OFS prototype with the aim to measure dimensional changes on nuclear materials irradiated in MTRs. We briefly present the objectives and the workplan of that project, in which the first phase addressed an analysis of the different measurement systems considered towards the specific environmental conditions encountered in a fission reactor. Among them, radiation is responsible for the biggest error source through the density change of silica glass due to neutron-induced compaction. The analysis has leaded us to focus mainly on an Extrinsic Fabry Perot design based on low coherence interferometry. As part of the current development, we present the results of table top experiments that allow appreciating the variation with different parameters of the response, especially the modulation of the signal returned. That permits to set partially the design and brings some tolerances data. A home made signal conditioning allows to extract the cavity length and then the change in the dimension of the sample to test.