Cladding metallurgy and fracture behavior during reactivity-initiated accidents at high burnup

Abstract

High-burnup fuel failure during a reactivity-initiated accident has been a subject of safety-related concern. Because of wide variations in cladding metallurgical and simulation test conditions, it has been difficult to understand the complex failure behavior observed in tests in the SPERT, NSRR and CABRI reactors. In this paper, we propose a failure model that is based on temperature-sensitive tensile properties and fracture toughness. The model assumes that dynamic fracture toughness and high-strain-rate tensile properties of high-burnup cladding are sensitive to temperature and exhibit ductile–brittle transition phenomena similar to those of bcc alloys. Significant effects of temperature and shape of the pulse are predicted when a simulated test is conducted near the cladding material's ductile–brittle transition temperature. Temperature dependence of tensile properties and fracture toughness is, in turn, sensitive to cladding microstructural characteristics such as density, distribution and orientation of hydrides; distribution of oxygen in the metallic phase; and irradiation-induced damage. Because all of these characteristics are strongly influenced by corrosion, the key parameters that influence susceptibility to failure are oxide layer thickness and hydriding behavior. Therefore, high-burnup fuel failure is predicted to be more sensitive to local cladding corrosion (e.g. grid span location) than to fuel burnup.