[en] Irradiation experiments were conducted at the Nuclear Safety Research Reactor (NSRR) to examine the effects of fuel rod support conditions on light water reactor fuel behavior during a reactivity initiated accident (RIA). Two rod-support conditions, (1) both-end fixing condition and (2) top-end fixing bottom-end free condition, were employed in the experiments. Comparison of the results between these experiments and the standard experiments (bottom-end fixing top-end free condition) led to the following conclusions. (1) Compared to one-end fixing condition, a fuel rod may bend significantly and consequently may become vulnerable to cracking of the cladding at lower energy deposition under both-end fixing condition, because longitudinal displacement of a fuel rod due to temperature change during transient is restrained. However, cracks do not penetrate the cladding wall if the cladding has residual ductility because cracks are stopped at ductile layer. Fuel failure caused by through-wall cracks in the cladding is not likely to occur until the cladding loses ductility, which occurs at around 260 cal/g.UO2. Fuel failure threshold under both-end fixing condition is identical with that under one-end fixing condition. (2) Concerning the cause of incipient fuel failure (brittle fracture of cladding by through-wall cracks) under one-end fixing condition, bending moment which may act on the cladding due to the vibration of a fuel rod during transient does not cause fuel failure. (author)