Spallation of oxide from stainless steel AGR nuclear fuel cladding: mechanisms and consequences

Abstract

Abstract20Cr–25Ni–Nb stabilised stainless steel is used in the advanced gas cooled reactors (AGR) as cladding for the UO2 fuel. Extensive studies have been made and are continuing in an attempt to understand the spallation of surface oxide from this steel and in this paper recent progress is surveyed. In particular, it is shown how the decohesion of oxide can be thought to occur when the strain energy in the oxide layer, resulting from differential thermal contraction during cooling, attains a critical value. This value can be equated to the fracture energy of the oxide/metal interface. With continued cooling, the quantity of spall increases as an increasing fraction of the surface oxide reaches the fracture condition. The intrinsic distribution in the oxide strain energy required to cause the observed spallation kinetics is derived. The paper is concluded with a consideration of the consequences of such spallation. First, the release of radioactive particles of oxide into the primary circuit can cause co...