Cracking and Relocation of Mixed-Oxide Fast-Reactor Fuel

Abstract

A computer graphics technique was used to measure cracks and other features of mixed-oxide fuel ceramographs to provide information useful for the modeling of fuel cracking and fragment movement. These measurements provided qualitative and quantitative information in several areas: crack formation and fuel-fragment movement, fuel-cladding gap size, crack size, crack orientation, radial distribution of crack porosity, and change in fuel volume (referred to as total fuel swelling) as a function of oxygen-to-metal ratio (O/M) and burnup. Examination of the ceramographs indicated that a crack starts on a free surface and propagates until it reaches another free surface. Thus, the first crack extends from one side of the fuel to the other, and succeeding cracks terminate on existing cracks or on the fuel surface. While crack formation was found to be independent of O/M, differences in crack healing at moderate power (19 kW/m) and high burnup (12 at.%) lead to a predominance of radial cracks for high O/M (--1.96) fuel and both radial and circumferential cracks for low O/M (--1.92) fuel. The different effects of circumferential and radial cracks on fuel behavior produce smaller postirradiation fuel-cladding gaps and larger cracks in the lower O/M fuel pins. Fuel swelling at intermediate burnup more » (--8 at.%) is independent of O/M, but at high burnup (--12 at.%) lower O/M fuel swells more. This swelling behavior may be related to a similar O/M dependence of retained fission gas. « less