The paper presents a novel method to design a spatial control system based on multirate output feedback robust sliding mode control for a large pressurized heavy water reactor (PHWR). The non-linear model of PHWR including xenon and iodine dynamics is characterized by 70 state variables and 14 inputs and outputs each. Linear nodal model is obtained by linearizing the non-linear dynamic equations of the reactor about the full power operating point. A multirate output feedback based discrete sliding mode spatial controller is designed for the linearized model of the reactor. The proposed control method does not require state information of the system for feedback purposes and hence may be easier to implement. From simulation of the non-linear model of the reactor in representative transients, the proposed control scheme is found to be superior to other methods.