An experience with the vectorization of the light water reactor transient analysis code RELAP5/MODI on a vector supercomputer FACOM VP-100 (peak speed 250 million floating point operations/s, clock period 7.5 ns) is described. The approach to the vectorization is based on the junction and volume level parallelisms for the hydrodynamic model, and the heat structure and heat mesh levels for the heat transfer model. The VP-100 vectorized code version yields a 2.4 to 2.8 factor speed increase over the FACOM M-380 computer, depending on the number of spatial cells being used. The M-380 is an IBM-type computer with the same speed as the VP-100 in scalar mode.