Key Structural Challenges for SiC as Fuel Cladding for LWRs

摘要

Key findings regarding development of SiC as cladding for LWR at the end of 2013 are reviewed. The current knowledge about SiC cladding performance in neutronics, chemical reactions, mechanical behavior, irradiation tolerance, and fuel rod behavior are interpreted in terms of its structural safety. The intent is to identify plausible failure modes of a LWR fuel rod with three-layer SiC cladding. Our conclusion is that it will take certain design constraints to make the SiC cladding highly reliable. The ratio of the thicknesses of the three layers is a major parameter, as well as the effective conductivity of the intermediate SiC_f/SiC composite layer. The inner-most monolith is recommended to remain under compressive stress during steady-state operation to avoid the high susceptibility to brittle fracture. The thicker the overall cladding, the more important are thermal stresses, and thus, the benefit from a mechanical stress standpoint is negated by the detrimental thermal stress during steady-state operation. So far, structural assessment and statistical failure analysis of SiC cladding show reasonable cladding failure probabilities, implying feasibility of the concept. Yet, a few critical unknowns should be settled before the SiC cladding concept earns full credibility as feasible. More information is needed about (1) oxidation tolerance of the fiber composite with cracked matrix, and the impact on cladding mechanical strength, (2) results of advanced statistical treatment of SiC cladding fracture, (3) Weibull modulus and thermal conductivity of SiC cladding under irradiation, and (4) the composite mechanical behavior in a laminated form.