During 2001-2004, research was performed to develop test data on the embrittlement of niobium-bearing (Zr-1%Nb) cladding of the VVER type under loss-of-coolant accident (LOCA) conditions. Procedures were developed and validated to determine the zero ductility threshold. Pre-test and post-test examinations included weight gain and hydrogen content measurements, preparation of metallographic samples, and examination of samples using optical microscopy, scanning electron microscopy and transmission electron microscopy. Sensitivity of the zero ductility threshold to heating and cooling rates was determined. Oxidation kinetics and ductility threshold were measured for the standard E110 alloy, six variants with different impurities, Zircaloy, and irradiated E110. Oxidation temperatures were varied from 800-1200 C, and mechanical (ring compression) testing temperatures were varied from 20-300 C. It was concluded that (a) the current type of E110 cladding has an optimal microstructure, (b) oxidation and ductility of the oxidized cladding are very sensitive to microchemical composition and surface finish, (c) the use of sponge zirconium for fabrication of cladding tubes provides a significant reduction of the oxidation rate and an increase in the zero ductility threshold, and (d) additional improvement in oxidation and ductility can be achieved by polishing the cladding surface.