Spatial oscillations in neutron flux distribution resulting from xenon reactivity feedback are a matter of concern in large nuclear reactors. If the spatial oscillations in power distribution are not controlled, power density and rate of change of power at some locations in the reactor core may exceed their respective limits causing increase in chances of fuel failure. Hence, during the design stages of any large nuclear reactor, it is essential to identify the existence of spatial instabilities and to design suitable control strategy for regulating the spatial power distribution. This paper presents a method to design and analyze the effect of sliding mode control (SMC) for spatial control of Advanced Heavy Water Reactor (AHWR). The AHWR model considered here is of 90th order with 5 inputs and 18 outputs. In this paper, numerically ill-conditioned system of AHWR is separated into 73rd order `slow' subsystem and 17th order `fast' subsystem and SMC is designed from slow subsystem. Further, using simple linear transformation matrices, SMC for full system is constructed. Also, it is proved that slow subsystem SMC results in a sliding mode motion for full system. Dynamic simulations has been carried out using nodal core model of AHWR to show effectiveness and robustness of proposed method.