Fiber optics for metrology in nuclear research reactors: Applications to dimensional measurements

In-situ measurement is a key issue for advanced irradiation programs in nuclear research reactors, especially in Material Testing Reactors (MTR). In that prospect, Optical Fiber Sensors (OFS) can feature unique intrinsic properties that bring substantial advantages over conventional sensing approaches. One of the objectives of the Joint Instrumentation Laboratory (JIL), gathering SCK̇CEN (Belgium) and CEA (France) resources and know-how, is to develop Optical Fiber Sensors (OFS) for metrology under high radiation. A preliminary condition to the development of OFS is to ensure that the Radiation Induced Absorption (RIA) of the light in the fiber does not exceed a pre-defined threshold. We report the results of a long lasting irradiation experiment, at higher dose than previously reported: up to a fast neutron fluence of 10 ²⁰ n/cm ² and an ionising dose of 16 GGy, carried out on various fibers, including hollow core photonic band gap (PBG) fibers, singlemode (SM) and multimode (MM) fibers. If we consider that RIA measurements losses < 10 dB are acceptable for the development of OFS in harsh environments such as MTRs, it can be stated that multimode and single mode fibers exist that fulfill this criteria, as far as the most favourable spectral region, in the 800 to 1100/1200 nm wavelength range is considered. Possible applications of OFS sensors in the vicinity of research reactors core are presented. We focus then on the project of dimensional measurement, based mainly on low coherence interferometry with an Extrinsic Fabry Perot. We report some first irradiation experiments on commercial sensors, then we present the adapted schemes designed to avoid the consequences of the radiation induced silica compaction. On that basis, we have set the configuration, built some first prototypes in metallic capillaries and we have worked on the anchoring of the sensor on a sample. We present the prototypes developed and the first results of testing accuracy. The irradiation of some sensors anchored on a stainless steel plate is planed in 2009 in the BR2 reactor facility of SCK̇CEN in Mol/Belgium using a dedicated irradiation rig. The purpose of this experiment will be to assess, in a first step, the survivability and the level of signal drift under high radiation when a stable strain is applied on the sensor.