The paper presents the design of spatial control system of a large Pressurized Heavy Water Reactor(PHWR) with the constraint that the closed loop system be stable even if one of the output sensors has failed. Linear state models are obtained corresponding to the normal operating condition and different failure modes of the reactor. Each model is found to possess two time scale property. Application of a standard singular perturbation approach decouples the fast varying modes which are ignored to eliminate the stiffness. Thus a reduced model is obtained for each operating mode of the reactor. Convex programming is used to search for an output injection gain which stabilizes all reduced models simultaneously. This output injection gain is then realized simultaneously in all operating modes by a controller based on periodic output feedback. © 1999 EUCA.