Experimental programs on fuel behavior during simulated reactivity-initiated-accident (RIA) conditions at the Nuclear Safety Research Reactor (NSRR) in Japan and the CABRI test reactor in France appear to indicate that cladding failures may occur at enthalpy values lower than would be expected. Results from two experiments designated as HBO-1 in NSRR and REP Na-1 in CABRI indicate that the occurrence of fuel failure is strongly influenced by corrosion of cladding in the tested fuels. However, data had been limited to fuel rods with conventional (1.5% Sn) Zircaloy-4 cladding. Results are described from newly conducted NSRR experiments, TK test series, for 38 to 50 MWd/kg U pressurized water reactor fuels with low-tin (1.3% Sn) Zircaloy-4 cladding, and anticipated processes of fuel behavior during the transient are discussed.