6533b825fe1ef96bd1283059

RESEARCH PRODUCT

Physical, chemical and mechanical evolution of the fuel-cladding interface in irradiated PWR fuel rods

subject

description

During the fuel irradiation in nuclear reactor, the fuel-cladding assembly is exposed to several irradiation-induced modifications. The fuel swelling coupled with cladding creep leads to a contact between the fuel and the cladding. The oxygen transport from the UO2 fuel to the zirconium layer induces progressively the Zr-cladding oxidation. This oxidation is initially local with the formation of islets. Then, with the increasing burnup of the fuel, it conducts to a continuous layer of about 8-µm thickness, localized at the fuel-cladding interface. At high burnup, zirconia growths anchor themselves in the periphery of the fuel (which is restructured) leading to pellet/cladding interlocking. The objective of this research work is to provide knowledge on: (1) pellet-cladding interaction at different scales, and (2) the mechanical properties of the pellet-cladding interface. In this objective, physicochemical characterizations by EPMA, FIB-SEM, TEM and EDS, as well as mechanical tests by nanoindentation at the pellet-cladding interface were carried out on fuel rods irradiated in French pressurized water reactors. The microstructure of the zirconia layer is complex because it is composed of different phases (tetragonal and monoclinic), grains of different size and shape (four microstructural zones identified), and is implanted by numerous fission products resulting from nuclear reactions in the fuel. These fission products can be dissolved in the zirconia matrix, condensed, or as metallic precipitates and bubbles, forming complex systems composed of an assembly of different elements. It has been established that U/Zr interdiffusion is limited to the scale of the layer of grains precisely located at the ZrO2|UO2 interface, and that it is necessary for the fuel in contact with the zirconia to be restructured so that the phenomenon of bonding at the pellet-cladding interface occurs. Finally, there was little variation in hardness and elastic modulus within the zirconia layer, despite the heterogeneous microstructure and composition of the layer.