The performance of Zircaloy nuclear fuel cladding in the environment of the proposed Yucca Mountain (YM) high-level waste (HLW) repository is evaluated. Because the uniform aqueous corrosion/oxidation rate is extremely slow, this evaluation focuses on mechanical failure, localized corrosion, stress corrosion cracking (SCC), hydrogen embrittlement, and initial failure. Mechanical failure is expected to result from (1) disruptive events such as rockfalls from seismicity, faulting, and igneous activities, (2) creep, and (3) splitting by oxidation of the spent fuel (SF) matrix. Effects of chloride ions and radiolysis are evaluated in localized corrosion and SCC. Embrittlement can be caused by delayed-hydride cracking (DHC) and hydride reorientation. Among these cladding failure modes, rockfalls, other disruptive events and initial defects can be important to performance prior to the container failure by corrosion. Confirmatory tests are required to evaluate the susceptibility to splitting by secondary mineral formation, localized corrosion, SCC, and hydride reorientation. Reliable temperature calculations with backfilling are also required. After breach, cladding may still assure slow release of radionuclides through perforations because localized failures may limit the exposure of the SF matrix and may provide high mass-transfer resistance. Failures prior to the repository emplacement arising from reactor operation, pool storage, dry storage, and transportation are also considered.