Mechanical behavior of SiC clad LWR fuels for steady-states and LOCA

Abstract

This paper presents highlights of recent experimental and modelling investigations on mechanical behavior of SiC cladding for both steady-state operations and loss of coolant accidents (LOCA) in light water reactors (LWRs). During the steady-state operation, swelling-induced deformation of SiC is anticipated to cause high tensile stresses in the fission-gas retaining layer of CVD-SiC monolith in a triple layer design. To mitigate it, a duplex SiC cladding design which employs the inner SiCf/SiC composite and the outer CVD-SiC monolith layer has been proposed. The proposed duplex cladding demonstrates remarkably reduced failure rates as it allocates tensile stresses in the inner composite and compressive stresses in the outer monolith. Experimental investigations on thermal shock fracture and oxidation, and simulations on fuel rod mechanical behavior during reflood phase are presented. Oxidation rate of SiC cladding is shown to be a few orders of magnitudes lower than that of zircaloy cladding in simulated LOCA conditions. This implies a diminished influence of oxidation on the structural integrity compared to zircaloy. Water quenching experiments of CVD-SiC demonstrates superior thermal shock tolerance in saturated water. This observation reveals the critical influence of boiling heat transfer modes on thermal shock fracture of a brittle material in water quenching. Subsequent simulation studies on thermal shock fracture analysis of SiC clad fuel rods in the presence of strong thermal-hydraulic interaction in LWR LBLOCA is analyzed with an integrated computation platform of ANSYS-Mechanical and MARS. Remaining technical challenges and key unknowns on SiC cladding developments are discussed and legacies of professor Kazimi on design and safety aspects of SiC clad fuel developments are highlighted.