Influence of Outer Zirconia Transient Cracking and Spalling on Thermomechanical Behaviour of High Burnup Fuel Rod Submitted to RIA

Abstract

The French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN) conducted the REP-Na tests in the CABRI reactor within the framework of its research program on nuclear fuel safety. These tests were devoted to the study of Reactivity Initiated Accident (RIA). Cracking and spalling of the fuel rod zirconia layer were observed after several REP-Na tests. Sometimes, an ovalisation of the rod after the RIA transient is also observed. Metallographic examinations showed that the outer and inner zirconia cracks are regularly spaced and that the crack density is linked to the clad plastic hoop strain. An analogy with brittle thin film layering a ductile substrate submitted to a tensile test is made and helps to understand this specific phenomenon. A numerical simulation evaluates the thermo-mechanical behaviour of the rod, including the zirconia influence during a RIA. This work make it possible both to identify the spalling process and to clarify the preferential spalling along the less corroded azimuths for several tests. The influence of the transient spalling on the boiling crisis occurrence in PWR condition is finally addressed. © Atomic Energy Society of Japan.