Cracking and spalling of the oxide layer developed in high-burnup Zircaloy-4 cladding under normal operating conditions in a PWR

Abstract

In this paper, the evolution of cracks from the surface of a thick oxide layer on Zircaloy-4 cladding has been analyzed for the normal operation of pressurized-water reactors. The conditions for the propagation of radial cracks towards the interface, channeling along the axis of the cladding, and the possible subsequent spalling of the oxide have been studied. The analysis was conducted by first calculating how the stresses in the oxide developed during operation, using a numerical model that incorporates multiple mechanisms such as creep, swelling and oxidation. These calculations demonstrate that the circumferential stresses within the oxide increase as oxidation proceeds and as the cladding expands under the effect of fuel swelling. Although the intrinsic growth stresses of the oxide are compressive, tensile stresses can eventually develop in the outer region of the oxide. Within this regime, the energy-release rate for the radial propagation of a crack was determined using the J-integral along with the calculated stress profile. By assuming a suitable value of toughness for the oxide, it is possible to determine the depth to which such a radial crack can grow. A second fracture-mechanics calculation was then conducted to explore the conditions under which such a surface crack can subsequently channel along the axial direction of the cladding. A third fracture-mechanics analysis considered oxide spalling. This showed the possibility of spalling from the radial cracks, which leaves a thin layer of the oxide adhered to the metal that can result in a local cold spot responsible for hydride formation.