Optical measurements of nuclear fuel rod claddings

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In nuclear power reactors, the extreme environment makes appear various physical, mechanical and physico-chemical phenomena on the fuel pellets (UO2) and their cladding (zirconium alloy). These phenomena initiate several mechanisms inherent to the pellet-cladding interaction (ridges and cracks), leading to the deformation of the outside fuel cladding surface. In the case of high power transients, these mechanisms can cause the failure of the cladding. Thus, dynamical characterization of these deformations is of high interest to assume safety in nuclear power reactors.

In order to understand these mechanisms, the LAMIR laboratory (CEA Cadarache) is working on the development of a compact facility able to reproduce the extreme conditions (i.e. high temperature gradients) undergone by the fuel pellets in the reactors. This group wants to complete its equipment with a non destructive tool allowing the on-line detection and quantification of the deformations. Accounting for all the specifications related to this environment (magnetic fields, vibrations, gas flows, scattering surfaces, etc.), measuring micrometric displacements is quite challenging and rules out most of classical techniques usually employed for non destructive testing. In this way, we are working on a Michelson-type interferometer, adapted to the on-line quantification of these deformations. This is a phase-shifting speckle interferometer designed for out-of-plane measurements, associated with image processing for the reconstruction of the surface shape evolution in highly adverse conditions.