A two-dimensional numerical analysis has been carried out to study the phenomenon of thermal striping in a prototype fast breeder reactor using a 10-jet water model that represents a row of the reactor core consisting of fuel and blanket zones. The above-core structures in the reactor are modeled with a porous lattice plate and solid core cover plate. The Reynolds stress model is used for simulating the turbulence characteristics of jet mixing phenomena. When the ratio of hot jet velocity to cold jet velocity is equal to 1, maximum fluctuations of temperature have been observed. Also the temperature fluctuations reduced gradually beyond a hot jet to cold jet velocity ratio of 1.0. The lattice plate is found to be more prone to thermal striping as compared to the core cover plate.