Abstract
Pressurized Water Reactor (PWR) fuel rods are made of ceramic pellets (UO2,(U,Pu)O2 or gadolinium fuel) assembled in a zirconium alloy cladding tube. By design, an initial gap, filled with helium, exists between these two elements. However during irradiation this gap decreases gradually, on the one hand, owing to a variation in cladding diameter, due to creepdown caused by pressure from the coolant, and, on the other hand, increased pellet diameter, due to thermal expansion, and swelling. In hot conditions, during the second or third cycle of irradiation, the pellet/cladding gap is closed. However, during the return to cooler conditions, the gap can reopen. At a high burnup (generally beyond the 3rd cycle of irradiation) an inner zirconia layer of the order of 10 to 15 μm is developed by oxidation leading to a chemical bonding between the pellet and the cladding. This bonding layer may contribute to a non-reopening of the pellet-cladding gap.Currently, only destructive examinations, after cutting fuel rods, allow the visualization of this area, however, they require a preliminary preparation of the samples in a hot cell. This limits the number of tests and measurements on the fuel rods. In this context, the Institute of Electronic and Systems of Montpellier University (IES - UMR 5214 CNRS ), in collaboration with the Alternative Energies and Atomic Energy Commission (CEA), Electricité de France (EDF) and Framatome, is developing a high frequency acoustic microscope adapted to the control and imaging of the pellet/cladding interface by taking into account the complexity of the structure's cladding which has a tubular form. Because the geometrical, chemical and mechanical nature of the contact interface is neither axially nor radially homogeneous, the ultrasonic system must allow the acquisition of 2D images of this interface by means of controlled displacements of the sample rod along both its axis and its circumference. The final objective of the designed acoustic microscope is to be introduced in hot cells.